Concrete e book by PG Designs

Concrete: On solid ground

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Contents Part One Harnessing the All-in-one Aspects of Tilt-up

By Ross Monsour, CET

Part Two Specifying Recycled Concrete Aggregate

By Uwe Schuetz, PhD, and R. Doug Hooton, PhD, P.Eng.

Part Three Concrete Floors: Flatness vs. Smoothness

By Keith Robinson, FCSC, RSW, LEED AP

Part Four Building More Durable Balconies

By Gavin Lobo, P.Eng., Sally Thompson, P.Eng., and John Kosednar, P.Eng.

Part Five Specifying Concrete Repair By Alexander M. Vaysburd, FACI and BenoÃ®t Bissonnette, FACI

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Part One Harnessing the All-in-one Aspects of Tilt-up

BY ROSS MONSOUR, CET

Ross Monsour, CET, is director of marketing at Concrete Ontario. In the past, he was a residential construction advisor with Canadian Home Builders’ Association (CHBA) and has worked under contracts with the Natural Resources Canada (NRCan) R-2000 Home Program and National Research Council’s Industrial Research Assistance Program (NRC-IRAP). Monsour’s expertise ranges from troubleshooting residential buildings to codes and standards development. He is a sitting member of the National Building Code Standing Committee on Small Buildings and Housing. Monsour has been in the industry for more than 30 years. He can be contacted via e-mail at rmonsour@rmcao.org.

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Photos courtesy SiteCast Construction

Harnessing the All-in-one Aspects of Tilt-up Constructing buildings requires the skills and knowledge to meet the demands of speed of construction, energy efficiency, resiliency to environmental conditions, cost efficiency, and longterm durability. The tilt-up concrete method, which speaks to all these attributes, has steadily grown since the 1940s due in large part to the development of the mobile crane and various advancements in ready-mixed concrete.

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Photos courtesy Concrete Ontario

On the left, a tilt-up concrete panel is being raised into place—notice the bracing is pre-attached beforehand. On the right, an insulated tilt-up panel can be seen with the bracing in place.

Thomas Edison was one of the early pioneers of the tilt-up system, building a New Jersey village in 1908 in an attempt to minimize the labour requirements for the site. This effort to find a simpler way to build, was ironically derailed with the advent of World War II, as the economy no longer required labour savings—an injection of public funds intended to drive the war effort and put everybody to work. Tilt-up has come a long way from those days with the refinements and advancements in technology, materials, systems, and better understanding of building science elements.

Anatomy of a tilt-up system In tilt-up, the concrete panels are cast on the structure’s floor slab or, depending on the building’s footprint, on an adjacent slab. A bond breaker is used between the casting slab and the panel’s exterior. The outside cladding is placed first on the slab, and then the panel is built from the exterior to the interior depending on the design requirements. This allows for insulation to be encased in the building panel. The lifting hardware is placed in the wall before the casting of the concrete. Sufficient time must be allowed for the concrete to reach its structural lifting strength. Then, the panels are lifted into place where they are tied together. The junction of the roof and wall panels has an edge-to-edge junction; it provides a continuous layer of insulation that eliminates thermal bridging. Another option for the construction of the panels is in a precast location offsite. The only issue with this method is transportation to the site becomes another design factor, and can have an impact on the construction timing. However, an advantage of offsite casting is it can be done inside, away from the elements. This still requires proper setup time of the concrete, and may also limit the potential size of panels both from a transport and facility perspective. Design considerations for tilt-up construction go beyond the detailing and construction of the panels. The layout of the panels on the slab must be engineered so sequential placement is crucial when they are lifted into place. Another major consideration is the location and loading capacity of the cranes required for a project. These elements affect the speed and placement of the panels onsite.

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Photos courtesy SiteCast Construction

Example of different tilt-up finishes. Appearance is not only a matter of style—it can also be for corporate branding.

Safety is a major consideration. As with any site, crews must be trained in the placing and lifting of panels. Individual panels can reach as high as 27 m (90 ft), and weigh in at 150 tonnes. Once the panels are lifted into place, they require temporary bracing until tied into the building’s structural elements. The exterior design of panels can accommodate various claddings. Decorative architectural finishes can provide other options, including a wide array of colours that can be added directly to the concrete mix, or textured paints that may be applied to the surfaces. The panels can be designed to use extensive surface textures by using formliners and conventional forming materials to produce any surface effect required. Exposed aggregate and mechanical tooling surface treatments can be incorporated in panels for the desired architectural finish. Using a brick inlay system offers the appearance of masonry construction for the finished building.

Determining whether tilt-up concrete delivery is right for a project Applications for tilt-up have varied over time, with the initial use limited to warehouses and commercial facilities—the construction method was not originally seen as

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At left, an aerial shot of a mall being assembled with tilt-up concrete. The photo above shows the reassembly of a school’s porta-pac—the tilt-up delivery method offers flexibility with respect to building portability.

particularly flexible. The move to more energy-efficient buildings has brought this technology into a much broader series of potential uses. Today, applications cover a wide range, such as single- and multi-storey retail, schools, or generally any project where the owner-developer retains ownership of the building, and is seeking the operating savings and maintenance benefits outlined in this article. The selection on whether a tilt-up system is right for a particular project can be assessed by looking at the needs of the design. The first consideration around every decision is the budget; looking at the owner’s operating and maintenance costs will make an impact on this decision. Another determining factor in using tilt-up is the site’s layout must allow for sufficient room to cast the panels, whether they are cast on the slab footprint of the building or on the surrounding construction site. All owner developers require the job to be constructed in the shortest possible time frame, consideration for buildings such as high schools or seasonally sensitive developments can make use of the speed of construction that tilt-up offers.

Speed of construction The panels are constructed from the exterior to the interior, with the crews completing the panels before being craned into place. This reduces the logistical time of co-ordinating the trades to build the envelope and results in significant savings of construction time.

Energy efficiency The tilt-up system provides an airtight barrier with the continuity of the walls and roof panels. Floor junctions are in the interior of the building assisting in the continuous air barrier assembly. The insulated tilt-up panel consists of two concrete layers with insulation between, providing not only good thermal resistance, but also the positive impact of thermal mass for the building’s energy efficiency.1

Site security As moisture and fire resistance are inherent concrete properties there is reduced potential for vandalism damage during the construction process.

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Portability The construction and attachment of the panels allows for the dismantling and moving of the building to another location. Although this is presently not normal for most Canadian buildings, the potential for moving schools could have a positive impact on society’s future economic demands with changing demographics. For example, in urban areas, some inner-core schools are being closed, with new ones built out in the developing areas. The savings afforded by transporting buildings from location to location to follow demand could be substantial.

Other advantages Architects and designers want the flexibility to express their own signature for the building—with a wide range of finishes, tilt-up assemblies can provide specific esthetic features. Different formliners placed on the slab to emboss any design directly into the concrete can be used.2 This embossing can be textured, coloured, or painted. Finishes such as brick masonry, with the option for large glazing areas, can also be incorporated into the design. Another area of concern is the long-term sustainability and durability of current building and infrastructure systems. Building certification programs have had success raising awareness in the design community, and tilt-up provides the necessary features and benefits to address those concerns. Concrete itself has many properties attracting designers seeking durability. Use of local materials reduces carbon emissions for transport, and concrete is recyclable. Further, having panels constructed onsite minimizes material waste, reducing impact on local landfills. Additionally, the combination of an insulated airtight envelope and the thermal mass can allow for reduced carbon footprint and potentially lower operating costs over the life of the building. Thermal mass allows more stable control of building temperature, which reduces the overall design load for heating and mechanical equipment. The design versatility of tiltup can create a smaller footprint during the construction of the building, minimizing the land usage because panels can be cast on the interior slab. The glazing can be designed to maximize the solar gain or minimize the cooling loads on a building using natural daylighting as it uses concrete as the structural component in the panels.

Examples from the field An example of the energy savings achieved with tilt-up in a high school—Des Sentiers in Orléans, Ont.—has been verified through one of the major tilt-up contractors in North America, Ottawa-based SiteCast Construction. The actual energy savings for hydro was 23 and 38 per cent less for natural gas in comparison to a similar school built at the same time. This is attributed to the airtightness detailing of the tilt-up panels when combined with the thermal mass properties. Further, the school also showed the tilt-up building was completed three months ahead of the standard construction for the school board. The tilt-up panel minimizes the need for scheduling trades at each stage of construction as complete panels are built from the slab up and reduces time spent co-ordinating the standard construction

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With tilt-up concrete work, safety is an important consideration. Crews must be trained in the placing and lifting of panels that can reach up to 27 m (90 ft) and weigh up to 150 tonnes.

of the wall. This has created an extended market in high schools and portables for the tilt-up system. Another example of SiteCast’s contractor’s tilt-up development work was SimonCalloway’s first Canadian mall outlet in Toronto. There were eight buildings covering approximatley 39,950 m2 (430,000 sf) of floor area, which resulted in 389 tilt-up panels covering almost 20,440 m2 (220,000 sf) of panel area. Reasons cited for opting for tilt-up included: • structural panels allow for unobstructed show, office, and retail floors; • exterior finish enables corporate identity to be projected on building; • construction time savings; and • safety and security of the durable panels during the operation of the building.

Conclusion Tilt-up has been used across Canada with the markets in the Western and Atlantic provinces more developed than Ontario. This might be related to the U.S. influence on these markets, with some of the big-box stores south of the border employing tilt-up as their main form of construction. Ontario has been slow to move in this direction and the technology remains the choice of a few owner/developers. However, this may change in the future as development continues to allow larger panels, more artistic finishes, and potentially high R-value vacuum panels incorporated into the wall system.3

Notes 1

For more on thermal mass, see the article, “Mass Appeal-Energy savings through concrete’s thermal mass,” by Andy Vizer, P.Eng., LEED AP, in the September 2007 issue of Construction Canada. Visit www.constructioncanada.net and select “Archives.” 2 Additional information on the visual possibilities of these tools can be found in “Function Meets Esthetics: Using Architectural Concrete Formliners,” by Ray Clark and Bob Fedchyshyn in the May 2013 issue of Construction Canada. 3 Those looking for more information, can consult the Tilt-up Concrete Association (TCA), which provides training and certification for this type of construction throughout North America. Its site, www.tilt-up.org, provides additional resources.

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Part Two Specifying Recycled Concrete Aggregate

BY UWE SCHUETZ, PHD, AND R. DOUG HOOTON, PHD, P.ENG.

Uwe Schuetz, PhD, is the director of product development and materials for Holcim (Canada). He is responsible for the co-ordination of all quality- and product-related aspects within the cement, aggregate, and concrete product lines. Before moving to Canada in 2004, Schutz worked for more than eight years as a senior consultant in various departments at the Holcim headquarters in Switzerland, supporting group companies in Asia, Europe, and the Americas. He can be contacted via e-mail at uwe.schutz@holcim.com. R. Doug Hooton, PhD, P. Eng., is the Natural Sciences and Engineering Research Council of Canada/Cement Association of Canada (NSERC/CAC) Industrial Research Chair in Concrete Durability and Sustainability for the University of Torontoâ&#x20AC;&#x2122;s (U of Tâ&#x20AC;&#x2122;s) Department of Civil Engineering. Over the last 38 years, his research has resulted in more than 200 publications on the durability of cementitious materials in concrete, as well as in the development of performance specifications for concrete. Hooton is a member of numerous Canadian Standards Association (CSA), American Concrete Institute (ACI), ASTM, and International Union of Laboratories and Experts in Construction Materials, Systems, and Structures (RILEM) technical committees. He can be reached at d.hooton@utoronto.ca.

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Photo courtesy Doug Hooton, University of Toronto

Specifying Recycled Concrete Aggregate Putting old rock to new use In seemingly every sector, from government to real estate development to retail, project owners are looking to make their construction sites greener. The movement toward more sustainable construction practices is neither trend nor fad; it is a functional philosophy responding to both a heightened public consciousness of the impact of human activities on the planet and the added value of doing â&#x20AC;&#x2DC;the right thing.â&#x20AC;&#x2122;

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Recycled crushed aggregate trials are underway on a sidewalk at the Highway 10-401 interchange. The Ontario Ministry of Transportation (MTO) already accepts recycled concrete aggregate (RCA) that meets Granular A specifications. Granular A, as specified in the Ontario Provincial Standard Specifications (OPSS) 1010, is a densegraded aggregate intended for use as a granular base within the pavement structure, granular shouldering, and backfill. In trials currently underway at the Highway 10-401 interchange, the MTO, a concrete provider, and the University of Toronto (U of T) are jointly testing concretes made with hardened returned crushed concrete (RCC) on a 100-m (328-ft) sidewalk. A 15 per cent and a 25 per cent RCA mix, both meeting the Granular A requirements,

Photos courtesy Ministry of Transportation

Walking the Talk on the Sidewalk

are being evaluated. To date, RCA concretes have performed equivalently to product using virgin aggregate, with the 25 per cent RCA mix maintaining slump slightly better than the concrete with 15 per cent recycled material.

The use of recycled or reclaimed materials is one of the ways builders and specifiers are achieving a stronger sustainability scorecard for their projects. One such example, recycled concrete aggregate (RCA), is gaining significant traction in several European countries and in Japan. Now, demand is also on the rise in Canada and the United States. The global growth in interest and use of this material can be attributed to its considerable environmental and practical advantages. This article examines RCA and its impact on various construction projects and sitework.

Aggregate 101 Aggregate is found in many types of structures and building projects, including roads, bridges, buildings, and dams. It is mostly used for concrete, asphalt pavement, and road bases, but other applications include ornamental landscaping, erosion control, and manufactured concrete products (e.g. brick, block, paving stone, pipe, and tunnel liners). Further, aggregate is employed in: • water purification; • sandpaper; • crayons; • plastics; • chewing gum; • paper;

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• sugar; • rubber; • fertilizers; • glass; and • ceramics. In concrete, virgin aggregate (e.g. sand, gravel, crushed stone, and lightweight aggregate) occupies approximately 60 to 75 per cent of the volume. This aggregate needs to be clean, and free of absorbed chemicals or coatings of clay or other fine materials that could cause concrete deterioration. Aggregate properties such as gradation, maximum size, unit weight, particle shape, surface texture, and moisture content (MC) significantly impact workability and pumpability of plastic concrete, as well as its durability, strength, thermal properties, and density once hardened. Virgin aggregate has to be mined from surface deposits of sand and gravel or by blasting and crushing rock in quarries. As urban areas expand, it is more difficult to obtain permits for new pits and quarries, requiring aggregate to be transported from increasing distances.

What is recycled concrete aggregate? There are three main categories of RCA that are either used as aggregate in new concrete or as granular base material.1

Construction and demolition waste (CDW) CDW consists of materials arising from activities such as the construction or demolition of buildings and civil infrastructure, road planing, and maintenance. Some examples of CDW include: • metals; • glass; • solvents; • gypsum; • brick; • wood; and • concrete, which may or may not be contaminated with small amounts of other demolition materials.

Reclaimed concrete material (RCM) RCM is a generic term for after-use, hardened, hydraulic-cement concrete that has been obtained from variable sources (e.g. sidewalks and concrete roads) for use as a construction material.

Returned crushed concrete (RCC) RCC is unused concrete material obtained from plastic concrete that has been returned directly to the ready-mixed concrete plant, allowed to harden, and processed by crushing. It can be used for the same applications as CDW and RCM, but since it has never left the mixing truck, it is cleaner and it has a well-known origin—thus, it is more readily accepted for use in fresh concrete.

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Returned crushed concrete (RCC) production at an Ontario facility.

The pluses and minuses of RCA Recycled concrete aggregate is a viable choice for various applications and offers numerous environmental and efficiency benefits. As with any material, the maximum benefit is obtained only when it is used for the right applications.

Advantage #1: availability Arguably the single biggest edge of RCA compared to virgin aggregate is its availability and proximity to virtually any market. Construction demolition is prevalent in all major urban centres, ensuring a supply of readily accessible material. If a structure is deconstructed and the resulting CDW is used for suitable applications onsite, the owner reduces the need to purchase and transport virgin aggregate while also saving the hauling, dumping, and landfill costs for these materials. In Canada, there were more than 3.3 million tonnes of CDW generated in 2002.2 In 2010, the United States generated 104 million tonnes, while Europe generated more than 500 tonnes in 2009–2010.3 (These numbers cover all materials falling under the CDW classification, including concrete.)

Michigan Department of Transportation In Michigan, the use of recycled concrete aggregate (RCA) is permitted in the ‘aggregate’ section of the 2003 Standard Specifications of Construction for coarse aggregate for portland cement in various road applications. It is also allowed as coarse aggregate in hot-mix asphalt and as dense-graded aggregate for base and surface course, shoulders, approaches, and patching. Enabled by this standard, the Michigan Department of Transportation (MDOT) has adopted a strategy of “recycling and reusing concrete aggregate if it enhances or equals the performance of virgin material in the final product.” As a result, it has built 26 projects with 1046 lane km (650 lane mi) of portland cement concrete (PCC) pavements using recycled concrete aggregate. The Detroit Metro region is also currently using RCA as a base material on two projects. For more on the engineering, economical, and environmental aspects of MDOT’s use of RCA across the state, visit www.fhwa.dot.gov/pavement/ recycling/rcami.cfm.

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Advantage #2: smaller environmental and social footprint The most obvious environmental benefit of specifying RCA is it proportionally reduces the amount of virgin aggregate that must be mined. This is no small consideration and it will likely become an even more significant factor going forward as social and environmental concerns rise over developing new quarries, while aggregate reserves are on the decline. In Ontario, for example, future aggregate requirements have been estimated at about 186 million tonnes annually for the next 20 years—a 13 per cent increase compared to the past. The licensing of replacement aggregate reserves has not kept pace with this growing demand, and this gap will result in a 2.5:1 consumption replacement ratio between 1991 and 2013 alone.4 In addition to the benefits in terms of efficiency and economics, the close-to-market availability of recycled concrete aggregate also offers environmental and social advantages. Proximity to most project sites conceivably precludes transportation of thousands of truckloads of materials on local roads and highways. The implications of this traffic decrease include: • less congestion (and wear and tear) on roadways; • reduced traffic noise; • less dust related to the passage of trucks on gravel service roads from quarries; and • lower carbon dioxide (CO2) emissions related to transportation. Again, the proof is in the numbers—an average ready-mix plant annually uses about 150,000 tonnes of aggregate, requiring delivery from nearly 4000 trucks every year. Specifying RCA on a project does more than just give a new life to old concrete; it also keeps it out of the wastestream. The environmental impact can be considerable—2.8 million tonnes of CDW were landfilled in Canada in 2002.5

Advantage #3: LEED potential To demonstrate a commitment to sustainability, a growing number of private-sector owners and government organizations are aiming for Leadership in Energy and Environmental Design (LEED) certification of their construction projects. LEED particularly rewards the reduction of waste at a product’s source. The use of RCA can help earn the following credits: • Materials and Resources (MR) Credit 2, Construction Waste Management; • MR Credit 4, Recycled Content; and • MR Credit 5, Regional Materials.

Technical challenges While RCA is a versatile alternative to virgin aggregate, it is not without its challenges and special requirements. It is perfect for use in road base applications; but it should not be employed for architectural concrete due to potential discrepancies in the final material’s surface colour. In the case of RCM/CDW, the chemical and physical properties vary more when compared to natural aggregate depending on the amount of attached mortar, or exposure of the concrete to foreign materials and chemicals during its lifecycle, processing, and storage.

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Photo courtesy Doug Hooton, University of Toronto

Recycled concrete aggregate (RCA) is being used in the construction of the new aprons of Pearson Airport’s Terminal 1 in Toronto.

When used to replace up to 20 per cent of the coarse aggregate in fresh concrete mixtures, comparable performance and durability are attained with RCA. Still, when intended for fresh concrete, the recycled concrete aggregate has to be pre-soaked so as to not use the mix water and alter the end performance, as most RCA tends to have higher absorption values than natural aggregate.

External restrictions The current limited use of recycled concrete aggregate in Canada is not only related to its properties or performance, but also due to ambiguous characterization of the product in national, provincial, or municipal material standards and building codes. Some specifiers completely omit it from their lists while others simply seem to lack full understanding of its capabilities, imposing unduly severe restrictions. The Canadian Standards Association (CSA), for example, addresses RCA very loosely. A note to clause 4.2.3.1 in CSA A23.1-09, Concrete Materials and Methods of Concrete Construction/Methods of Test for Concrete, advises particular attention be paid to deleterious substances and physical properties, etc. Additionally, it states testing frequency may need to be increased to daily depending on the RCA source and variability. Notes are, of course, not mandatory to meeting the standard, and are really just ‘additional information.’ At the same time, however, CSA allows use of RCA as long as the resulting product meets its performance standards. Other standards organizations prohibit the use of RCA altogether for use in fresh concrete. The Ontario Provincial Standard Specifications (OPSS) preclude the use of recycled concrete as an aggregate in concrete works. Additionally, OPSS 1001, Material Specification for Aggregates–Granular A, B, M, and Select Subgrade Material, does not allow the blending of aggregate products to improve physical properties. While some authorities and project managers do not specify RCA at all, or only for certain projects or applications, others have such exacting specifications that they render the choice of recycled material all but impossible.

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For example, the specifier for a current Elementary Teachers’ Federation of Ontario (ETFO) project requires any RCA used in the cast-in-place concrete should be from a single-strength class—a robust 40 MPa that is not a very commonly used building material. Since adequate quantities of recycled material may have to be sourced from multiple locations with different original strength specifications, such a tight restriction effectively eliminates RCA as a viable option for gathering points toward the project’s LEED certification. The City of Toronto also has standards in place that preclude the use of RCA for appropriate applications like unshrinkable backfill, commonly known as ‘u-fill.’ The city’s current specifications do not mention recycled aggregate as an alternative to virgin aggregate, thereby eliminating it by omission.

RCA in the field RCA has been proven to offer similar or even improved properties for road base/ sub-base due to characteristics like better compactability. In Europe, concrete containing between 10 and 20 per cent good-quality RCA is demonstrating a performance level equivalent to concrete made with virgin aggregate. While some differences have been noted in terms of higher porosity and material variability, the concrete has shown similar workability and strength development, with only certain limitations regarding durability. In North America, RCA use is also on the rise. Departments of Transportation (DOTs) in Canada and the United States are choosing recycled concrete for various road applications with great success. A joint research program by Purdue University and the Indiana Department of Transportation (IDOT) to help decrease the state’s infrastructure costs by 10 to 20 per cent has led to the development of over 400 concrete mixes containing RCA.6 In Michigan, recycled concrete has been a favoured material in more than two dozen MDOT projects since 1983. Canadian specifiers and building organizations are also coming to appreciate the potential of RCA, which, up to this point, has been used mainly as granular material for road base. In fact, the Ministry of Natural Resources (MNR) of Ontario 2011 report, “State of the Aggregate Resource in Ontario Study, Consolidated Report,” identified more than a 100 per cent increase in annual use of recycled aggregate from 1991 to 2006, from 6 million to 13 million tonnes. Greater Toronto Area (GTA) construction industry associations such as the Toronto and Area Road Builders Association (TARBA), Ontario Hot Mix Producers Association (OHMPA), Ontario Road Builders Association (ORBA), and Ontario Stone, Sand, and Gravel Association (OSSGA) are lobbying local municipalities to accept RCA as a green alternative to virgin aggregate for specific applications. Despite their efforts and the proven properties of RCA, many municipalities remain resistant. (Potential reasons for this hesitancy include a lack of information or experience with projects using the recycled material, reluctance to take responsibility for changing current specifications, or different views on long-term experiences.) On the other hand, Toronto Pearson International Airport has identified RCA as a material of choice, specifying it as a base for the new aprons at Terminal 1 and using concrete recycled from the demolished Terminal 2 and its parking garage. The Ministry

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Going Green While Going for Gold The London Organising Committee of the Olympic and Paralympic Games aims to make the 2012 Summer Games the most sustainable in the event’s history. In fact, sustainability was a key component of its bid, as outlined in the 2012 “Sustainability Plan: Toward a One Planet 2012.”* Their approach encompasses five main areas of sustainable development, including waste minimization and reuse. RCA is among the favoured materials, with 25 per cent (by weight) recycled and/or secondary 20- to 5-mm (0.8- to 0.2-in.) aggregate planned for construction of venues and park-wide infrastructure. * For more, visit www.london2012.com/documents/locog-publications/london2012-sustainability-plan.pdf.

of Transportation of Ontario (MTO), which already uses RCA for road base and other applications, is conducting trials to expand its use of recycled materials in different concrete applications. Aggregate manufacturers are also making RCA more readily available to serve the growing industrial and consumer demand for green products in Canada. A green concrete mix containing 60 per cent RCA is being offered by one manufacturer for the DIY market, while another company is even branding recycled concrete aggregate.

Conclusion Recycled concrete aggregate is a readily available, flexible, and sustainable product that has a proven track record as a high-performance material for sub-base and other road construction projects. Up to a certain replacement level, it is also a viable substitute for virgin aggregate in concrete for an increasing number of applications. Governments, standards authorities, industry associations, and builders in a wide range of sectors and in more and more parts of the world are taking notice of the performance properties and environmental benefits of RCA.

Notes 1

This definition comes from the Ontario Ministry of Transportation (MTO) Materials Engineering and Research Office’s “Supporting the Thematic Strategy on Waste Prevention and Recycling Service Request Five Under Contract Env.G.4/ Fra/2008/011225,” released in October 2010. 2 This comes from the 2005 Statistics Canada report, Human Activity and the Environment. 3 For more info, visit www.wastebusinessjournal.com/news/wbj20101005A.htm, as well as www.biois.com/en/menu-en/expertise-en/assess/new-a/construction-wastemanagement-in-europe.html. 4 See note 3’s first URL. 5 See note 2. 6 This information comes from 2011 Ministry of Natural Resources (MNR) of Ontario report, “State of the Aggregate Resource in Ontario Study, Consolidated Report,” which was published in 2011.

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Part Three Concrete Floors: Flatness vs. Smoothness

BY KEITH ROBINSON, FCSC, RSW, LEED AP

Keith Robinson, FCSC, RSW, LEED AP, has worked as a specifications writer since 1981, and is currently an associate at DIALOG in Edmonton. The immediate past-president of Construction Specifications Canada’s (CSC’s) executive council, he sits on several standards review committees for ASTM and the National Fire Protection Association (NFPA). Robinson is also a contributor to the Terrazzo, Tile, and Marble Association of Canada’s TTMAC Specification Guide 09 30 00 Tile Installation Manual and works closely with the Concrete Floor Contractors Association (CFCA) to address specification requirements for floor flatness and levelness. He can be reached at krobinson@dialogdesign.ca.

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Photos courtesy Apex Granite and Tile

Concrete Floors: Flatness vs. Smoothness Identifying differences between structural, esthetic, and functional tolerances There is a great deal of confusion in the world of flat concrete floor surfaces. On one side of the discussion are the people who place concrete floors—the concrete floor finishers. As a Work Results-based component of the final construction, they obtain guidance about what is or is not ‘good’ concrete floor finishing from reference standards such as Canadian Standards Association (CSA) A23.1, Concrete Materials and Methods of Concrete Construction, and CSA A23.2, Test Methods and Standard Practices for Concrete, and from the American Concrete Institute (ACI) Manual of Practice documents ACI 117, Specifications for Tolerances of Concrete Construction and Materials, and ACI 302.1R, Guide for Floor and Slab Construction.

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A successful finished tile project requires an understanding of who is responsible for what when it comes to tolerances in concrete slabs and floor finishes.

In the world of concrete floor finishers, the results of their work are easily quantified through measurement criteria contained in ASTM E1155, Standard Test Method for Determining Floor Flatness (FF) and Floor Levelness (FL) Numbers, and ASTM E1486, Standard Test Method for Determining Floor Tolerances Using Waviness, Wheel Path, and Levelling Criteria. The referenced standards and measurement criteria are supported by the Concrete Floor Contractors Association of Canada (CFCA). The group holds its members accountable for providing concrete finishing to the CSA A23.1 requirements for overall F-number tolerances for flatness and levelness. Based solely on these requirements, everything in the world of concrete floor finishers appears perfectly controlled and achievable. On the other side of this discussion, however, there are those who install floor finishes. These flooring installers include a wide range of trades ranging from soft surfaces such as carpeting to hard surfaces such as tile. This group is as diligent about its Work Results as the concrete floor finishers, and also has a set of binding standards, provided from the various associations and manufacturers governing the components of the work for which they are responsible. Flooring installers are governed by the same CSA and ACI reference standards and ASTM testing criteria listed for the concrete floor finishers, but are usually asked to meet more restrictive measurement requirements. This difference in standards is where a problem starts to become apparent. It raises the question, who is responsible for achieving required floor tolerances for installed finishes? This author has been around for a few years, and remembers a time when construction practices differed from those commonly accepted today as being standard.

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To understand what is different now compared to then, we need to explore applicable tolerances, and perhaps become more tolerant of the differences encountered between the level of concrete finishing provided by the concrete floor finishers, and the expectations of the flooring installer. However, we also need to understand the changes to floor finish products that are altering the expectations for reasonable floor flatness and levelness used in the past.

The structural tolerance Table 12 of CSA A23.1 (shown in Figure 1, on page 25) states clearly what we can expect for slab and floor finishes. It includes an exception on levelness for elevated slabs— tolerances for FL do not apply for that installation condition. CFCA supports the tolerance requirements of CSA A23.1 within its guide specifications. However, do specifiers and flooring installers understand what constitutes an FF20 or FF25 floor finish? As explanation, FF indicates how well the concrete floor finisher worked the surface—more effort usually results in better overall flatness. The FL metric indicates how skillfully the side forms were set by the contractor and where the concrete was struck-off. The FL number has nothing to do with the concrete floor finisher’s workmanship or skill level in producing the specified FF number—in other words, placing concrete and finishing concrete are different Work Results.1 There is one more sticking point: FF and FL are measured within days of concrete placement, and before removal of shoring for suspended slabs, and concrete is a natural material that continues to change as it cures. Most changes occur within the first 30 days, although they continue for four to six months or longer depending on humidity and temperature conditions during the curing period. Drying shrinkage and curling modifies the surface profile to a much greater extent than the concrete floor finishers can account for within their Work Result. Curling has become more common with the use of high perm rating under slab moisture mitigation membranes where the concrete mix design and reinforcing have not been modified to account for the improved floor flatness requirements for finish materials. The esthetic and functional tolerances listed in this article are not integral to the structural design, and as such will not be accounted for unless specifically identified by the architect or interior designer.

The esthetic tolerance Floor finishing materials manufacturers establish tolerances for flatness based in part on what makes their products look good. A typical manufacturer’s requirement will state a required tolerance of a 3-mm (1/8-in.) gap measured under a 3-m (10-ft) long straightedge. The tolerance does not have a direct correlation to the FF and FL tolerances established to meet structural requirements, with the additional concern the FL does not (and should not) apply to the finishes. The materials are not affected by the levelness component of the structural tolerance. Essentially, the straightedge measurement is not a practical tolerance guide for concrete floor construction. When it comes to thin and flexible floor materials, the concern is more about the actual waviness of the substrate—more peaks means a less esthetic appearance for the applied floor finishes. The flooring manufacturers are describing the smoothness and evenness of the floor, not the flatness or levelness.

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Figure 1

Class

Examples

Recommended Procedures

Overall F-number FF

"Conventional" slab on grade and elevatwed floors

Hand screeded and steel trowel finished

15*

"Flat" slab on grade and elevated floors or surfaces with thin applied finishes.

Advanced hand or mechanical screeding, pan floating, and steel trowel finished

20**

"Very Flat" slab on grade floors

Specialized materials advanced hand or mechanical screeding, pan floating, and steel trowel finished

"Extremely Flat" slab on grade floors

Specialized materials, advanced mechanical screeding, large pan float, highway straightedge, and steel trowel finished

Specialized surfaces including automatic guided vehicles and air pallet systems

Specialized materials and methods of concrete construction

Canadian Standards Association (CSA) A23.1, Concrete Materials and Methods of Concrete Construction, states clearly what can be expected for slab and floor finishes. It includes an exception on levelness for elevated slabs.

The tolerances for flatness and levelness (i.e. FF and FL) are used improperly by specifiers in an attempt to describe an expectation for the concrete floor finisher to which they can relate; specifiers unintentionally create unrealistic tolerances for conventional concrete floor finishes. As an example, based on the manufacturersâ&#x20AC;&#x2122; 3-mm straightedge gap measurement criteria, it is possible to achieve an FF20, FF25, FF50, or even an FF150. The floor flatness number is a statistical measurement that takes into account the relative waviness of the concrete floor. Waviness in concrete is omnidirectional, meaning the peaks and valleys do not align in parallel rows as they would if you were watching waves crashing against a beach. Based on the statistical model supported by the FF approach; the 3-mm gap measurement counts the numbers of peaks and troughs measured along the 3-m straightedge, and it is the number of observed gaps that count toward the floor flatness rating along several repeatable lines of measurement established by ASTM 1155. Fewer valleys mean a higher FF measurement. Four troughs under the 3-m straightedge is equivalent to FF20 whereas one trough provides FF150. By the same statistical measurement, a floor having two 8-mm (5/16 in.) troughs under the 3-m straightedge would also achieve the same FF20 rating, but probably would not be acceptable as an esthetic tolerance.

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The FF result is important, but does not tell the whole story without the understanding of the 3-mm gap limitation. Specifying a higher FF number results in a more esthetically pleasing expectation. The closer to true planar perfection the specifier sets as a project requirement, the higher the cost will be to achieve that level of perfection. If true perfection is specified, the expectation may not be practically achievable using concrete placing methods only.

The functional tolerance Functional tolerances for resilient floor coverings on concrete floor substrates are established by ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring. This standard requires the concrete to be smooth enough to prevent irregularities, roughness, or other defects from telegraphing through resilient flooring products. It also requires a flatness to within an equivalent of a 4-mm (5/32 in.) gap measured along a 3-m (10-ft) straightedge with no gap measurement greater than 0.8 mm (1/32 in.) within 300 mm (12 in.). This requirement fits within the expectations of FF20 to FF25, and the esthetic tolerances expected by many flooring products manufacturers. The primary difference between ASTM F710 and ASTM E1155 is the functional tolerance is measuring the relative waviness of the floor. Using the omnidirectional shortspan measurement addresses the typical concerns of using FF20 or FF25 where the installed gap tolerance is greater than the 3-mm (1/8-in.) required by flooring manufacturersâ&#x20AC;&#x201D;a floor that is acceptable by the statistical structural tolerance may not meet the esthetic or functional tolerance required by the flooring installers. Hard-surface flooring materials require a higher degree of flatness, often approaching true planarity to avoid installation problems such as lippage between large tile products. The larger the hard surface material is, the more restrictive the F number becomes. The Terrazzo, Tile, and Marble Association of Canada (TTMAC) has found stone and ceramic tile products with any dimension larger than 400 mm (16 in.), or any tile installation requiring joints 3 mm (1/8 in.) and less, are particularly problematic when installed on conventionally achieved FF20 and FF25 floor substrates, and recommend FF50 to FF60 as a more appropriate tolerance. In addition to the floor flatness criteria, there is also a structural deflection concern wrongly associated with floor levelness. Suspended floors deflect and all slabs can change shape significantly in the first few days of placement, which is the reason flatness is measured within 72 hours and before shoring is removed from suspended slabs. The deflection occurring after shoring is removed can be significant enough to produce a sharp drop-off at the structural lines of support and the field area of the floor space. This drop-off can lead to significant lippage concerns with large-bodied tile installations as the tiles are placed across the crest of the deflection. To counteract this concern, specifiers often set requirements for L/600 to L/720 slab deflection restrictions that are not economically feasible to attain from a structural costing point of view. Changes to the reinforcing can help limit the sharpness of the drop-off; however, in reality, the flooring installers will be dealing with L/360 and tiling installation details need to account for the change in floor slopes caused by deflections.

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Photo courtesy Mapei

Application of overlay on prepared floor surfaces.

Identification of the problem The root of the problem is the differences between the three types of tolerances; we use the structural tolerance to define the esthetic tolerance, but are looking for a different measurable functional tolerance result. A smooth and even concrete finish satisfies the requirement for the esthetic concern of not telegraphing concrete imperfections through flexible floorcoverings. This is achievable using the FF20 and FF25 floor substrates. Too often, defects attributable to the concrete floor finisher through (and appropriate references to) patching and levelling within the Work Results that are specified in Division 03−Concrete for work done by the concrete floor finishers are not being addressed. Corrections and repairs made by the concrete floor finishers address only the structural tolerances, and not the esthetic or functional tolerances. Defects can also occur as a result of weather conditions—for example, rainwater dilution of the surface paste can lead to powdery residues, exposed aggregates, pitted surfaces, or loosely bonded surfaces. Additionally, there are consequences of multi-floor construction that leave remnants of previously installed temporary construction such as crane blockouts and screed points. As a result, flooring installers are often made responsible for repairing or correcting these concrete fossils, which is different than the spot patching and smoothing they are required to do as a part of their Work Result covered by Division 09−Finishes.

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Finding a solution In the historical sense to which this author previously alluded, floor preparation on suspended slabs often entailed installation of a topping material. This was called a two-course pour and covered a multitude of floor problems including crane block-outs, overworked surfaces, rain damage, and other types of imperfections leading to flooring installation issues. CFCA addresses the requirements for limitations of floor flatness established by CSA A23.1, and states tolerances greater than FF35 are only practical through the use of improved mix design and reinforcing, by the use of deferred application of cementitious floor toppings, or incorporating advanced (i.e. non-standard) methods of construction. Installation of cementitious overlays is not the responsibility of the concrete floor finisher. The logical conclusion is the flooring installers have this responsibility since the increased tolerances are a requirement associated with the products they install. As a practical solution to most of the improved floor flatness problems, toppings are the ideal solution. They are also a good idea for most suspended slabs in multi-storey buildings to maintain a consistent, smooth, and even finish that is required by most flooring manufacturers.

Incorporating floor topping into the specification The different types of toppings for specific installation requirements can be recommended by a number of manufacturers of cementitious and non-cementitious products. The goal for the specifier is to identify a range of products that capture any compatibility issues inherent between toppings and the substrates that they are applied to, and the finishing materials placed over them. Trusting product representatives and using them as intelligent resources is important; this way, the specification writer can focus on co-ordinating the construction documentation. MasterFormat has a placeholder at Section 09 05 61â&#x20AC;&#x201C;Common Work Results for Flooring Preparation. The specification starts by indicating the expected floor flatness as either FF20 or FF25. Keeping in mind concrete floors will continue to change shape between the time of placement and the time of finishing, there needs to be a mechanism that allows for changed site conditions, either by cash allowance or unit price adjustment.2 Having money put to one side can greatly improve the co-ordination between the structural tolerance and the esthetic or practical tolerances. One could specify a fixed price to bring the floors from the initial structural requirement of FF25 to the functional requirement of FF50, and use the cash allowance or unit prices to adjust if the actual conditions encountered are different than those expected.

Conclusion Putting the responsibility for improved floor smoothness and evenness to the flooring installers takes a lot of the guesswork out of the equation with regards to the end result required for the project, and places contract risk for this component into the scope of those best able to address and correct the work. Multiple Work Results in Division 09 can make reference to the Common Work Results specification, meaning each flooring installer has control over his or her own work and is in a more responsible position for co-ordinating the â&#x20AC;&#x2DC;gap conditionsâ&#x20AC;&#x2122; where different works abut one another.

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Photo courtesy DIALOG

The above image shows rain damage to a slab.

This is a new approach to an old idea. Change is necessary, but in this case a return to the way things used to be done is an improvement to the entirety of concrete flooring community.

Notes 1 In brief, floor flatness and levelness are statistical measurements describing structural tolerances for slope and concrete floor finish for slabs. FF and FL are referenced by CSA A23.1 and A23.2, and ACI 117. Additionally, ACI 302.1R references FF and FL by typical usage classification. ASTM E1155 has been adopted as the governing standard for measuring variations in surface tolerances for structural components; however, there are other measurement standards such as ASTM F710 or DIN 18202, Tolerances in Building Construction−Buildings, that use a straightedge measurement to determine the relative smoothness of the slab. There is no direct correlation between the straightedge and statistical tolerance measurements, and is where the concern for smoothness required by finish installers has not been addressed by the standards used for concrete placement. 2 The cash allowance or unit price adjustment is not used to correct construction deficiencies of the concrete floor finishers; they are still required to meet the specified structural tolerances. Further, the allowance is not in place to pay for the floor preparation by Division 09—that cost is included in the preparation work to bring the substrate from FF25 to FF50. The allowance is in place to account for the natural variation between that will occur from the as constructed condition measured three days after concrete placement and the time applied floor finishes will actually be placed, and is only expended for the difference. This controls the monetary risk of each contributor to the quality of the concrete substrate, and costs are allocated based on each individual’s ability to bear that risk—including the owner.

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Part Four Building More Durable Balconies

BY GAVIN LOBO, P.ENG., SALLY THOMPSON, P.ENG., AND JOHN KOSEDNAR, P.ENG.

Gavin Lobo, P.Eng., is a project manager with WSP, a consulting engineering firm with seven offices across Canada. He specializes in the repair of reinforced concrete structures as well as building envelope elements, including roofs and windows.

Sally Thompson, P.Eng., is a capital planning specialist at Synergy Partners, with over 20 years of experience delivering property condition assessments, reserve fund studies, performance audits, and building renewal. She sits on the Tarion Condo Task Force and has been heavily consulted as an expert in the public consultations on the Condominium Act over the last three years. Thompson can be reached at sthompson@synergypartners.ca. John Kosednar, P.Eng., is a managing principal at WSP. As a corrosion specialist, he oversees the evaluation and repair of reinforced concrete structures, as well as other restoration projects on various building types.

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All images courtesy WSP

Building More Durable Balconies Balcony slab repairs are perhaps the most disruptive project that can take place in a multiresidential building or hotel. Vibrations from the jackhammers that must be used to remove concrete can travel through the entire building, making it unbearable for residents who are home during the process.

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Removing balcony slab edges to repair corroded reinforcing is one of the most disruptive construction projects for a residential occupancy. This is an easily prevented problem with minor attention to detail during design and construction.

For hotels with balconies, repairs can also lead to significant loss of revenue. While the hotel rooms themselves become unusable during the day, the noise from the repairs also affects meeting rooms, restaurants, and other income-generating facilities. Deferring repairs is rarely an option due to the risk of falling concrete. However, what if design/construction professionals could prevent—or at least, minimize— balcony deterioration in the first place?

Carbonation: The most common cause of balcony slab edge failure Most Canadian high-rise buildings are built using conventionally reinforced castin-place concrete construction. Fresh concrete is highly alkaline, which makes the embedded reinforcing steel immune to corrosion because a ‘passive’ layer is formed on the steel’s surface. In effect, it acts as a protective layer against corrosion. The chief culprit contributing to the deterioration of reinforced concrete balconies is ‘carbonation.’ This natural process, which involves interaction between the concrete and carbon dioxide in the air, reduces the concrete’s alkalinity. The process advances over time from the exposed surface inward. When this ‘carbonation front’ reaches the depth of the embedded reinforcing steel, the passive layer is destroyed, and the steel is now susceptible to corrosion.

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With the concrete removed, it is clear the lower steel corroded due to carbonation of the surrounding concrete. In contrast, the upper steel is in better condition—the top surface of the slab is protected from carbonation by repeated wetting from precipitation.

As the steel corrodes, it expands and causes the overlying concrete to work loose. This loose concrete is, of course, at risk of falling from the balcony slabs and therefore presents a serious safety risk. Carbonation is more of an issue on the undersides of slabs than it is on their vertical surfaces or topsides. This is because the top surface and vertical edge are more frequently wet from precipitation. This moisture in the concrete actually helps protect against carbonation by effectively blocking the concrete pores.

What can be done in order to minimize balcony failure? Design/construction professionals can help reduce the risk of these failures by specifying durable materials, properly detailing the slab edges, and enforcing the ultimate design.

Specify and install durable concrete Carbonation is a self-limiting process. As concrete carbonates, it becomes less porous, eventually limiting its own exposure to carbon dioxide in the air. In other words, the speed at which the ‘carbonation front’ progresses into the concrete slows down over time. With high-quality concrete, even in a building 30 or more years old, the depth of carbonation should not exceed 5 or 10 mm (1/5 to 2/5 in.). With reasonable-quality concrete, the carbonation depths can reach perhaps 20 or 25 mm (4/5 to 1 in.). Only very-poor-quality concrete is susceptible to greater depths.

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Figure 1

To prevent carbonation from reaching the steel, care must be taken to ensure no steel is placed in the bottom 40 mm (1 3/5 in.) of concrete, with particular attention paid to the zone around the drip slot (shown in red).

Using high-quality concrete reduces the total depth of carbonation for the life of the building. At a minimum, Class F-1 air-entrained concrete should be specifiedâ&#x20AC;&#x201D;this material is designed for exposure to freezing and thawing. Class C-1, designed for exposure to chlorides (which, like carbonation, cause embedded reinforcing steel to corrode) would be even more durable. Air-entrainment is a feature of both F-1 and C-1 concreteâ&#x20AC;&#x201D;it distributes minute air bubbles throughout the concrete. These bubbles provide the space to accommodate the expansion that naturally occurs when water freezes. Non-air-entrained concrete specified for the interior floor slabs (which are not subject to freeze-thaw or chlorides) should not be inadvertently extended into the balcony slab areas. The proper concrete, designed for exposure, must be used when pouring the balcony slabs.

Carefully detail the slab edges As indicated, carbonation most strongly impacts the undersides of slabs. For example, on a 30-year-old balcony, the carbonated layer of concrete might be 20 mm (4/5 in.) deep on the underside, 10 mm deep on the vertical edge, and only 5 mm deep on the top surface. ASTM A23.1, Concrete Materials and Methods of Concrete Construction, which is referenced by the National Building Code of Canada (NBC) requires the rebar on balconies to have 40 mm (1 3/5 in.) of concrete cover between the steel and the exposed surface for concrete exposed to the exterior. So if carbonation normally only reaches depths of about 20 mm, why are balcony slab edges falling apart?

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To Steel or Not to Steel There are two basic balcony configurations: those with cantilevered slabs and those with slabs supported at the edges by shear walls. Slabs falling under the first category rely on the top matt of reinforcing steel for structural capacity. Slabs supported at the edges span side to side and require concrete that is reinforced with steel in the bottom of the slab. In theory, therefore, balcony-slab-edge deterioration should really only be an issue on side-supported slabs. In reality, corrosion also occurs in cantilevered slabs. This is because designers and forming contractors often put ‘temperature’ steel—whose purpose is to prevent cracking rather than provide structural capacity—right near the outside bottom edge of even cantilevered slabs. To avoid corrosion in cantilevered slabs, designers should consider placing this temperature steel further back from the edge of the slab, or closer to the midline of the slab to increase the cover over the steel, particularly near the drip slot. This minor change in design and construction practice could save each building hundreds of thousands of dollars in repair work and all the disruption that goes along with it.

As is often the case, the answer to this question is an unintended consequence of another normal detail on a balcony slab: the drip slot. The drip slot functions to prevent rainwater that spills over the slab edge face from running back and staining the slab’s underside. The drip slot is placed on the underside of the balcony at the outside edge. It is typically formed using a piece of wood laid in the formwork for the slab. This displaces concrete, which locally reduces the material’s depth over the rebar. The drip slot location usually lines up directly below and near reinforcing steel running along the slab’s bottom edge—this creates the perfect opportunity for carbonation to reach the steel, initiate corrosion, and cause the slabs to delaminate (Figure 1, page 34). The drip slot may seem like a minor issue because it is so small relative to the slab’s size. Nevertheless, it is, in essence, the Achilles heel of balcony slab construction. This small area of reduced concrete cover causes the carbonation front to reach the steel, which then results in the corrosion that eventually requires rebuilding of the balcony slab edges. To help prevent this, designers should ensure the drip slot is clearly indicated on construction drawings, with all necessary dimensions for slot location and size. Details should indicate the minimum distance of reinforcing bars to the drip slot as well as to all exposed concrete faces. The drip slot design should also clearly allow for adequate concrete cover to the adjacent steel. This cover should be at least 40 mm (1 3/5 in.), taking care not to allow any unintended reduction at drip slots. This may require the structural engineer to slightly modify the steel’s placement in this location.

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This photo shows the drip slot running parallel to the slab edge. The steel placed too close to the drip slot has corroded, and the concrete has fallen off. If the steel had been placed higher in the slab or further towards the building wall, this failure would not have occurred. Building maintenance personnel should be alert to fine cracks in the drip slot as these are typically a warning that loose concrete is developing. For safety reasons it should be removed before it falls, potentially causing injury.

As a belt-and-suspenders approach, designers might also consider specifying that any rebar running parallel to (and just above) the drip slot be epoxy-coated. While this will not prevent carbonation, it still helps protect the steel should, by chance, it get installed without the intended cover in isolated locations.

Enforce the design During construction, care should be taken to position the reinforcing steel and form the drip slots as indicated in the improved design details. It will help if the formwork contractors understand why this particular detail is important. This way, they will be partners in preventionâ&#x20AC;&#x201D;the installers will know the care they take with their work will save future owners money, and residents and guests a lot of disruption.

Conclusion As an industry, architects, structural designers, and contractors must step up and work together to build durable concrete balconies that will not be susceptible to carbonation-induced corrosion. Making even these few minor changes when designing and forming balcony slab edges can go a long way toward preventing deterioration to concrete balconies. More importantly, these changes can be made with almost no additional cost to the construction.

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Part Five Specifying Concrete Repair

BY ALEXANDER M. VAYSBURD, FACI, AND BENOÎT BISSONNETTE, FACI

Alexander M. Vaysburd, FACI, is the principal of Vaycon Consulting, a Baltimore, Md.-based firm specializing in concrete and concrete repair technology. He is a Fellow of the American Concrete Institute (ACI) and a member of various ACI and International Concrete Repair Institute (ICRI) committees. Vaysburd also serves as an associate professor at Laval University in Québec City. He can be contacted via e-mail at avaysburd@structural.net. Benoît Bissonnette, FACI, is a professor in the department of civil engineering at Laval University, and a member of the Research Centre on Concrete Infrastructure (CRIB). A Fellow of ACI, he is part of various ACI, ICRI, and International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM) technical committees. Bissonnette can be reached at benoit.bissonnette@gci.ulaval.ca.

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Photo © BigStockPhoto.com/Golkin Oleg

Specifying Concrete Repair Good concrete repair is not a bandage ‘fix’ for a structure in trouble—rather, it is a complex system that consists of numerous engineering tasks (Figure 1, page 39). Designing and specifying concrete repair has unique needs differing from new construction. Thus, the specifications must serve as action plans or roadmaps for the project’s engineer, contractor, and quality controller.

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Image courtesy Structural

Figure 1

Flow chart of a concrete repair project.

Unfortunately, when it comes to the durability of repaired concrete structures, project documentation is not always adequate. For example, one shortcoming constantly repeated in specifications is the frequent reference to the “direction” of the engineer or architect. The authors came across a specification concerning a material for an industrial plant repair project that simply read: The patch repair material’s durability shall be as directed by the engineer. The subjective character of such specifications can make appropriate bidding impossible. Additionally, such references are useless because they stem from either not knowing what should be required, or from a refusal to make an effort to study and

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Images courtesy Vaycon Consulting

Good concrete repair must be more than a bandage, holding together components and hoping for the best.

analyze the issue to determine a specification. It is an unfortunate fact some “directing” engineers can be literal to a degree the specifier never intended. Many specifications for repairs are a mixture of referenced standards and ‘cut-andpaste’ clauses from previous projects, recycled with little thought. Achieving durability should be found in carefully considered specifications for a particular project. However, this does not appear to be the case—widespread durability problems have led to extensive repair of previous repairs and, in some cases, eventual replacement of structural members.1

Durability considerations There are several durability-related factors playing into concrete repair specifications.

Compressive strength There is a common misconception higher-strength cement-based materials are necessary in severe environments for enhanced durability. However, this way of thinking does not take into account the realization a stronger and stiffer material is more likely to crack because the higher modulus of elasticity increases the tensile stress arising from shrinkage and other restrained volume changes. Unfortunately, many specifiers blindly opt for so-called high-strength (i.e. ‘highperformance’) cementitious materials containing ingredients such as silica fume or high-range water-reducing admixtures (HWRAs), which can also have negative side effects on performance of repair materials. While useful in certain applications, some components can also have negative side effects when it comes to using them for repair work. The terms ‘high-performance’ and ‘high-strength’ are often used as synonyms. However, high compressive strength is not an indication of improved durability and performance—in fact, the increase in strength may be gained at durability’s expense.

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In reviewing 120 North American projects, the authors could not find a specification for a repair material with a compressive strength of approximately 20 MPa (3000 psi), even when the strength of the existing substrate was of such magnitude. The rationale seems to be the belief a 55-MPa (8000-psi) ‘high-performance’ material will always be more crack-resistant and achieve better durability. The major fault of any material is not a matter of strength or stiffness, but rather a lack of resistance to crack initiation and propagation. The Victorians had high-strength cast iron, but its brittleness led to many failures. They soon realized ductility was the essence of safe structures and began using lower strength, ductile mild steel. In many cases, it makes sense to follow their lead and pay more attention to deformability and crack resistance of cementitious repair materials. Another critical problem in many specifications is the often unjustifiable requirement for high early-strength repair materials. This may be necessary for some special applications, but for typical repairs it creates a greater potential for higher shrinkage and cracking. Long-term durability is primarily achieved by dimensional stability, not by high earlystrength. Accelerated gain in strength generally comes with more self-stress from drying shrinkage, autogenous shrinkage, and thermal contraction. The rate of strength gain, in addition to the total degree of hydration, has a significant effect on cementitious materials’ pore structure, micro- and macro-cracking, and transport (i.e. permeability) properties. Accelerated strength gain has also been known to result in lower ultimate strength. However, the effect on durability is generally overlooked. At a ‘normal’ rate of strength gain (i.e. three days for 50 per cent ultimate strength, seven days for 70 per cent, or 28 days for 100 per cent), hydration products have sufficient time to diffuse throughout the cement matrix and precipitate uniformly. At accelerated rates, hydration is so much faster than the diffusion process that most products remain static near the cement grains, leaving the interstitial space relatively open. These relatively dense deposits of hydration products surrounding (and sometimes encapsulating) the cement grain serve as diffusion obstacles to water and hydration products. Therefore, further hydration is hindered, producing a much more open pore structure than that of comparable materials with a normal rate of hydration. Hence, strength gain acceleration in cementitious materials generally has a negative effect on their transport properties. Based on this analysis, it can be concluded that for concrete and other cementitious materials—especially those exposed to severe environments—the rate of strength gain is critical to durability. Materials with slow strength gain (e.g. those containing fly ash or slag) might perform more satisfactorily under these conditions. Repair materials with acceptable minimum early-strength properties should be used. If practical, their compressive strength should be specified at a stage later than the traditional 28 days. It should not be in excess of what is necessary for load-carrying purposes. The ‘specified’ strength values should be kept at levels similar to the actual ‘in-place’ compressive strengths.

Cement and aggregates When ready-mixed concrete is specified as a repair material, the ‘more-cement-is-better’ rule tends to wrongly prevail. Any attempt to produce durable cement-based material

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Data courtesy the authors’ “Roadmap for Improvement of Crack Resistance of Repair Materials” in Applicator (vol. 28, no. 2; 2006).

Figure 2 Parameter

Effect Major

Drying shrinkage

Modulus of elasticity

Creep

Moderate

Minor

Compressive strength

Early strength

Paste content

Cement content and type

Aggregate content, type, and size

Co-efficient of thermal expansion

Water-cementitious materials ratio

Accelerating admixtures

Plasticizers

Silica fume

Fly ash

Slag

Water content

Slump (within typical ranges)

Charting the material’s sensitivity to cracking control parameters.

comes up against a dilemma. If a small amount of cement is added, the material is relatively crack-resistant, but permeable. If a large amount of cement is mixed into the concrete, the material becomes stronger and more impermeable, but less crack-resistant. In fact, if cement is added until extremely low permeability is achieved, the material becomes more brittle and has much less creep relaxation to sustain high tensile stresses induced by drying and autogenous shrinkage. In other words, it is impermeable between the cracks, but in the end, its true permeability can become substantially higher than the lower-strength material. Therefore, durability cannot practically be achieved between the extremes of either too little or too much cement. One of the main reasons for more extensive cracking and the reduced durability of ‘high-performance’ concrete and other cementitious materials is these materials have higher cement contents, higher paste volumes, higher moduli of elasticity, and lower creep. The specifications that were reviewed by the authors followed standard material manufacturers’ recommendations, such as the need for incorporating aggregates in thicker repairs: When thickness of the repair exceeds 50 mm (2 in.), the repair mortar should be extended with 10-mm (3⁄8-in.) coarse aggregate.

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Figure 3 Charge passed (coulombs)

Chloride ion penetrability

>4000

High

2,000-4,000

Moderate

1,000-2,000

Low

100-1,000

Very low

>100

Negligible

Chloride ion penetrability based on charge passed—ASTM C1202.

The specifications also require the same aggregate quantity be used, regardless of the material composition and repair specifics (e.g. thickness, spacing of reinforcing steel, and clearance from reinforcement to the bottom of repair cavity). A crack-resistant, ‘durable’ repair material should not have a deficiency of any aggregate particle size. The adequate aggregate size distribution minimizes void content, as the incrementally small particles fill these spaces. The goal is to pack as much aggregate into the material mixture as practically possible, thereby reducing the amount of paste needed to fill the voids between particles.

Drying shrinkage Drying shrinkage of a concrete repair material is one of the major factors influencing the overall repair durability. However, not a single limitation for shrinkage was found in the specifications of concrete as a repair material in the cases studied. Pre-packaged repair materials may be limited to certain shrinkage values, but without any indication to what age of the material and test conditions this is to be applied, such requirements are useless. Of equal concern is the current myth that specifying low water-to-cementitious materials (w/cm) ratios reduces shrinkage. A low w/cm ratio may increase strength and density, but it is unlikely to reduce ultimate shrinkage (i.e. self-desiccation shrinkage and drying shrinkage). For given constituents, it is not the w/c ratio, but the total water and paste content of the mixture that has the greatest influence on the material’s shrinkage and cracking potential.2 Cement paste acts as a binder, filler, and finishing aid. However, it is also the phase undergoing shrinkage in concrete. Unrestrained neat cement paste can shrink four to five times more than concrete prepared with the same paste. Therefore, any reduction in paste quantity will make the greatest contribution to reducing shrinkage and cracking, along with improving durability (as far as adequate consolidation can be achieved). Often, ‘high-performance’ concrete possessing a w/cm ratio of about 0.25 is unnecessarily specified. In so doing, designers unintentionally create an epidemic outbreak of self-desiccation and cracking. Water-reducing admixtures are quite effective in modifying some concrete properties, but they may not necessarily reduce the amount of shrinkage. Sometimes, the opposite is true. Unfortunately, it appears that ASTM C494, Standard Specification for Chemical Admixtures in Concrete—which allows 35 per cent more shrinkage in test specimens

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Quality control of a concrete repair. Pull-off tests can measure the adhesive strength of applied coatings such as plastic coatings, concrete coats, and mortars and plasters.

with the admixture than that of control specimens—is too often disregarded. This means using low w/c and WRAs to keep the necessary workability may not always result in the expected shrinkage decrease. Instead, it can actually end up causing an increase in shrinkage and result in cracking. Specifications that unintentionally increase the shrinkage can lead the concrete to experience severe cracking.3 Figure 2 (page 42) illustrates some of the material properties critical for a low-cracking, durable material.

Permeability One of the fundamental factors influencing the initiation and the extent of damage to reinforced concrete is its permeation characteristics. The movement of moisture— which can contain aggressive agents—is fundamental to the repaired structure’s durability. This transport mechanism can produce detrimental physical, chemical, and electro-chemical effects. Specification of chloride permeability limits based on ASTM C1202, Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration, is a regular practice in North America, and almost all specifications limit the repair material’s permeability based on this standard. During this test, crack-free specimens are formed in the laboratory or extracted in the field. According to ASTM C1202, the permeability of 400 coulombs is ‘very good,’ while 4000 is ‘very bad’ (Figure 3, page 43). Of course, the material’s micropermeability has to be considered and limited—but only after macropermeability issues are successfully addressed by specifying an allowable drying shrinkage value. Specifications for cement-based materials with requirements for durability must set up criteria for drying shrinkage. The time to corrosion is first controlled by the transport of aggressive agents on the macrostructural level, and then on the microstructural level. Aggressive agents in the vicinity are ignoring diffusion and instead taking the path of least resistance—the network of cracks and microcracks.

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Figure 4

A Technical Factors

Environmental Factors

Client Needs

Economic Factors

Functional Factors

B Function structural strengthening protection safety esthetics

Geometry

Material

Method

Figure 4a shows the factors to be addressed in a repair project. In Figure 4b, the factors to be considered in material specifications are demonstrated.

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The primary significance of deformations caused by moisture-related effects in cementitious materials is whether their interaction will lead to cracking. Here, the magnitude of the restrained shrinkage strain is the most important to be specified. Linking the two aspects of permeability (i.e. macro and micro) is a measure of the cement-based material as it is—its so-called protective character. There can be little doubt macropermeability is perhaps the most important of all. Factors to be analyzed by the repair project’s engineer/specifier are presented in Figure 4 (page 45). The most critical engineering issues to be addressed to achieve durability are collectively called ‘compatibility factors.’ The meaning of compatibility in concrete repair composite systems relates to a balance of the physical, chemical, and electrochemical properties and deformations between the system components (e.g. existing substrate, repair, and transition zone between them). This balance ensures the whole system can withstand stresses induced by restrained volume changes, chemical, and electrochemical effects without premature deterioration or distress over a designed period.4 Figure 5 (page 47) summarizes the most important factors to be considered in compatibility analysis.

Corrosion-inhibitors Many specifications call for use of corrosion-inhibitors in concrete repairs. While these admixtures appear to offer added protection against corrosion in newly constructed concrete structures, there are some concerns and uncertainties related to their use in repairs. In other words, they can become an aspect of the problem, rather than a solution. When a corrosion-inhibitor is added to the repair material, the local nature of the repair does not address the whole structure’s predicament if chlorides or carbonation are widespread. Even when the local repair contains the necessary concentration of the inhibitor, it can become a clean (i.e. non-corroding) cathodic area that stimulates increased corrosion around it, causing a ‘ring effect.’ In numerous cases, repair procedures of this type have resulted in early cracking and spalling in the original concrete adjacent to the repair. Another concern is how to maintain the inhibitor’s necessary concentration in the repair area. It is likely the inhibitor will spread beyond this point and migrate with water and other ions, causing the effective concentration to be reduced. The inhibitor solution can in fact move in response to concentration, moisture, and temperature gradients occurring between different parts of the structure. Both moisture and temperature gradients determine the transport of water and other agents, via water, in the repair system. This flow can be significant when the structure is subjected to wetting and drying. It is also more than likely chloride ions from chloridecontaminated existing concrete will move into the repair phase by the aforementioned transport mechanism (Figure 6, page 49).

Prescriptive or performance? Specifications can be either prescriptive- or performance-based—there are many discussions underway regarding which better serves the intended purpose. Theoretically, the performance concept is ideal. However, considering the status of the concrete repair industry, it can be unsuitable because of the inadequate knowledge of those involved

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PPhoto Ph h t ÂŠB BigStockPhoto.com/Tanantornanutra i St kPh kPh kP h t //TT t t

Figure 5

Compatibility factors and properties are shown here.

and the lack of evaluative techniques for some aspects of performance, especially in terms of the durability. Simply put, performance can be specified by way of satisfying a particular test. Some attempts to develop performance tests in the concrete repair field are now underway, but their practical reliability has yet to be determined and their application has still not been implemented. With respect to the performance of repair materials, the situation is somewhat improved in that at least certain characteristics can be ensured. Nevertheless, many other behavioural repair characteristics, such as electrochemical activities, are largely unknown and difficult to predict. Caution needs to be exercised in establishing performance requirements, especially for repairing corrosion-related damage on structures subject to chlorides and marine environments.5 The performance approach may be applicable where the potential future performance is understood. However, this remains a challenge for repaired structures, as there is no proven link between lab-based performance test methods and actual in-situ performance. Each issue, step, and requirement must be specified and controlled as performed. The actual myth of performance specifications for concrete repair projects might have risen from the assumption the contractor knows more about the achievement of durability than the engineer. Detailed guide specifications are needed in which the engineer, contractor, and inspector are given guidance in not only the â&#x20AC;&#x2DC;how,â&#x20AC;&#x2122; but also

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A concrete situation in distress in need of repair. Note the condition of the reinforcing bars.

the ‘why.’ This allows for a real analysis of the situation and, ultimately, for a suitable decision to be made on how to proceed. The contractor should also be given direction concerning materials, methods, and equipment to be employed, unless he or she can demonstrate equal or better results by other means.

Conclusion The engineering community involved in the design and implementation of concrete repair projects must recognize and accept the fact the specification documentation is not a formality, but rather a critically important engineering guideline to fulfil durability requirements. Specification writing is a complex task requiring extensive knowledge of science, engineering, and in-situ practice. It also entails a considerable standard of responsibility on the part of the professional working with it. Engineers have become accustomed to accepting heavy responsibilities. According to the Babylonians’ ancient Code of Hammurabi, if a builder made a house and the house collapsed and caused the death of its owner, the builder was put to death. While the authors would not propose quite so harsh a measure for premature failure of a repair, they would nonetheless assert the industry must accept the responsibility for its shortcomings and strive forward to improve concrete repair projects.

Notes 1

The authors acknowledge the assistance of Peter H. Emmons (CEO of concrete repair/strengthening firm, Structural and author of Concrete Repair and Maintenance Illustrated) and Christopher C. Brown (president of the Conproco Corp., a U.S. repair

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Image courtesy Structural

Figure 6 Substrate concrete chloride contaminated Cementitious repair material with corrosion-inhibitor

An illustration of the penetration of chloride ions from contaminated substrate into repair with added corrosion-inhibitor.

and restoration materials manufacturing company). Both were co-authors in an earlier iteration of this article. 2 See S.H. Kosmatka and W.C. Famarege’s Design and Control of Concrete Mixtures (Portland Cement Association [PCA]), 1998). 3 This sentiment (and others in this article) may not be shared by everyone, as it hits at the crux of the controversy. 4 See P.H. Emmons, A.M Vaysburd, and J.E. McDonald’s “A Rational Approach to Durable Concrete Repairs,” published in Concrete International (vol. 15, no. 9; 1993). 5 See B.N. Sharp’s “Performance Specifications for Coastal Structures: Limits and Limitations,” Concrete in the Services of Mankind, Concrete for Infrastructures and Utilities (1996). See also J.R. Mackechnie and M.G. Alexander’s “A Rational Design Approach for Durable Marine Concrete Structures” in JSA Inst. Civil Eng. (vol. 39, no. 1; 1997).

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