Concrete footings in a Bag

Faster Footings with Fabric Forms

by Rick Fearn, President, Fastfoot Industries Ltd.*

Fabric forms offer an economical method of

placing concrete footings quickly and accurately Using fabric to form concrete is a proven technology: concrete-filled fabrics are used to prevent erosion on riverbeds and embankments and to add cover around existing wharf pilings.

A new application for forming concrete with fabric makes it possible to build strip or pad footings similar to those obtained with conventional forms (see Fig. 1). The results equal or exceed those of conventional forming.

Advantages of Fabric Forming Fabric is superior to conventional forms (lumber, plastic, metal) for many reasons:

Fig. 1. Fabric forms allow rapid and economical placement of footings. (69275)

• lighter, less expensive, and installed faster than lumber or plywood • adapts to the ground on which it sits,

CT002 — July 2000 Vol. 21 No. 2

Contents Faster Footings with Fabric Forms Pavement Durability: A Case Study What’s New in Petrography Concrete-Related Associations

including handling grade changes easily (see Figs. 2 and 3) • can double as a dampproofing membrane • saves trees • kicker boards for wall alignment are part of the footing forms Fabric-formed footings are suitable for both residential and commercial construction. Since

Designing Durable Concrete New Literature

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commercial footings are typically larger, the labor and material savings on these

While fabric-formed footings do differ in shape

jobs become more significant than on residential projects.

from traditional footings, they meet critical depth and width requirements of building codes in the

Footing Design

U.S.A. and Canada.

From an engineering standpoint, the critical measurements of a footing are the

Concrete for the Footing

contact width at the ground and the footing thickness at the faces of the wall. The fabric is marked with width adjustment lines. These lines ensure that the desired

There are no special requirements for the con-

footing contact width is

crete used with this system. All mixtures are com-

achieved when the fabric is

patible with the fabric. ACI 332, Residential

properly filled. A single width

Concrete, recommends the following specifica-

of fabric allows a footing

tions (based on a maximum coarse aggregate size

height up to 460 mm (18 in.).

of 19 mm [3/4 in.]):

Tall footings—up to 1320 mm (52 in.) high—have also been built with this system. The bulbous shape of fabric-formed footings satisfies shear and bending moment stresses across the footing width (see Fig. 4). The bulbous shape also provides a built-in safety

• minimum strength of 17 MPa (2500 psi) for foundations not exposed to weather • maximum slump of 150 ± 25 mm (6 ± 1 in.)

• at least 340 kg/m3 (574 lb/cu yd) cementitious material • if used, air content of 6 ± 1.5% (air is not required for non-exposed members) • where sulfates are a concern, maximum water-cementitious materials ratio = 0.50 for moderate exposure and w/cm = 0.45 for severe exposures

Two Systems Available: Strip Footings and Pads Fig. 2. A wooden bulkhead is used to step the footing at grade changes. (69276) Fig. 3. Multiple steps in footings are quick and easy with fabric forms.

(69277)

The backbone of this system is a fully adjustable steel support bracket called a yoke. To form the strip footings, these brackets hold 2 x 4s (or aluminum ledges) in position above the ground. The 2 x 4 brackets slide along the cross bar and are carefully positioned, as they will become the kicker boards for the wall forms (see Fig. 4). Fabric is then stapled into position on the 2 x 4s and filled with concrete to form the footing. Although you need a level surface to place wood forms, the fabric easily handles bumps and uneven surfaces—saving time.

factor: settlement of the footing into the ground increases the footing

The strip fabric system has significant advantages

contact area.

over conventional lumber: • fast installation – up to 30 m (100 lineal ft) of

The shape has another benefit: it encourages drainage away from the joint with the base of the wall (see Fig. 4). Unlike conventionally formed footings, which are flat across the top (in cross-section), the fabric-formed footings are sloped away from the sides of the wall they support , thereby preventing the pooling of water, and minimizing the potential of leakage inside the foundation.

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Concrete Technology Today / July 2000

footing forms per hour • accurate – 6-mm (1/4-in.) precision in aligning wall forms • lightweight – on a square area basis, fabric is 300 times lighter than wood

• less labor – no ground leveling or trenching, no stakes, minimal stripping, no secondary wall layout • no concrete leakage, no concrete contamination with the ground • environmentally friendly – system minimizes lumber consumption and yokes are reusable • cheaper – fabric is about 5% of the cost of wood

Installation Method The system—ideally suited to forming footings

Fig. 4. Cross section of footing shows the unique bulbous shape. The rounded top

where accurate wall location is important—is

promotes drainage away from the wall.

compatible with all common types of wall forms, such as insulated concrete forms (ICFs), steel, aluminum, and plywood. A typical strip footing installation requires a few easy steps: Step 1 – Lay out the 2 x 4s The 2 x 4s are laid out on the ground in position of the wall forms beginning with the outside face. Corner brackets join the corners and union brackets (or 460-mm [18-in.] pieces of 1 x 4)

the foundation, using a 150-mm (6-in.) overlap at all joints to prevent concrete leakage. Nail the centerline of the fabric to the ground to ensure the footing is centered under the wall. Staple the fabric to the 2 x 4s with a tacker-stapler.

Step 4 – Install steel reinforcement After the fabric is stapled to the 2 x 4s, suspend individual steel bars from the yokes using tie wire. For more complex reinforcement, attach only one side of the fabric, slide the steel into place, then staple the second edge of the fabric to the boards.

join the butt ends. Lay the inside 2 x 4s with a

Step 5 – Place concrete

spacer using the outside ones as a guide, again

Concrete placement into the fabric form is usually done with a pump or from a

joining corners and butt joints.

chute. Placing concrete in the direction of the fabric overlaps prevents leakage. To

Step 2 – Install the support braces (yokes) With the legs in the contracted position, lay the yokes around the perimeter of the foundation at

consolidate, give the fabric form a light kick for more fluid mixes or use a spud vibrator, especially if the footing contains reinforcement. Trowel the concrete to the bottom of the 2 x 4s to create the base of the walls.

2- to 3-m (6- to 10-ft) spacings. (Deep footings

Step 6 – Remove the yokes

increase the load on the fabric and require clos-

If the 2 x 4s are used as kicker boards for wall alignment, nail them to the green

er yoke spacing to prevent excess deflection of

concrete prior to removing the yokes. The wall forms are then placed between

the 2 x 4s.) Attach 2 x 4s to the yoke with a 50-

the 2 x 4s.

mm (2-in.) nail. Use a laser or builders level to lift the yoke to the proper grade. Then slide the 2

Pad Footings

x 4 hangers along the cross bar until they are located in the correct position for the wall forms.

Step 3 – Install the fabric Fabric comes in bundles 37 m (120 ft) long, and in standard and wide widths, 1575 and 1880 mm (62 and 74 in.). The accordion C-fold enables one-person installation.

Sometimes, pad footings are needed instead of strip footings. Self-supporting fabric bags are used to form concrete pad footings. These bags are available in sizes from squares 460 to 1220 mm (18 to 48 in.) as well as custom sizes. Zippered bags enable placement of steel reinforcement and are available in sizes greater than 760 mm (30 in.).

The bags are labeled to indicate the contact width and height of the formed con-

Place a bag bundle on the ground near a corner.

crete. Proper bag filling will form a concrete footing pad with the correct length,

Slit the bag, pull the fabric under the yokes to

width, and height. If the bag is underfilled, the height will be less than indicated,

the next corner, and cut the fabric at 45 degrees

and the contact width will be larger. Conversely, if the bag is overfilled, the contact

(see Fig. 5). Continue around the perimeter of

width will be less than indicated, and the height will be greater.

Concrete Technology Today / July 2000

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C O N C R E T E

T E C H N O L O G Y

T O D A Y

Pavement Durability: A Case Study Study reveals causes for deterioration of Iowa concrete pavements

The National Institute of Standards and Technology (NIST) recently studied premature deterioration of several concrete pavements in Iowa. Several previous studies had provided conflicting opinions as to the cause of the deterioration. The NIST study’s aim was to provide guidance to prevent similar problems in future construction.

Fig. 5. The fabric is quickly installed by laying it at one corner and opening the bag, pulling the fabric under the 2 x 4s.

Pavement Construction The pavements were non-reinforced, two-lane roads between 200 mm and 300 mm (8 in. to 12 in.) thick, with dowel-reinforced, skewed joints. Though the pavements studied were all

(69278)

constructed in the mid-1980s, they did not share

Installation of Pad Footings

Analysis and Interpretation of Deterioration

Place the bag on the ground in the desired location. A screed stake is usually driven

Three methods were used to determine the cause

into the center of the bag, both to locate the pad and to provide a screed for the

of the deterioration:

common material characteristics.

proposed concrete height. If using a zippered bag, unzip it, place the rebar cage on chairs inside the bag, and zip up.

• a thorough review of construction records and field evaluation of the pavements • a comprehensive petrographic examination

To place concrete in the bag, hold up the edge of the bag, and pump or chute the

of cores for both distressed and non-dis-

concrete inside the bag to the desired height. Kick the bag to level the concrete, or

tressed sections of pavements (see Fig. 1)

lightly vibrate if reinforcement is present.

• computer simulation of various proposed degradation mechanisms at a microstructur-

Good for both business and the environment, fabric forms for concrete footings

al level

offer a sensible option for concrete contractors.

The air-void systems in the deteriorated pavements were "marginal to substandard," according *Fastfoot Industries Ltd., Unit 212, 6333 148th St., Surrey BC, V3S 3C3, Canada,

to guidelines in ACI 212.3R, Chemical

phone: 888.303.3278, fax: 604.501.6090, Internet: www.fastfoot.com

Admixtures for Concrete. The conclusion: the pavements were victims of freeze-thaw damage to mortar and aggregates.

4

Concrete Technology Today / July 2000

the paste during mixing. The high sand content may also have led to difficulty in placing, consolidating, and finishing the pavements, increasing the potential for drying shrinkage. If cracks form as a result, it becomes easier for moisture to enter the concrete interior. Accelerated deterioration is usually associated with moisture.

While the research uncovered the probable cause of damage, it also indicated that alkali-silica reaction (ASR), while present, did not appear to play a major role in the degradation of these pavements. No evidence of delayed ettringite formation (DEF) was found.

Conclusions Fig. 1. A poor air-void system is responsible for deterioration of this pavement. Note the

As a result of this study, two points can be made. First, basic practices are important

large bubble sizes, the clustering of air voids

for concrete durability. This includes establishing an adequate air-void system through

in certain areas, and the lack of any bubbles

proper proportioning, thorough mixing, and good construction techniques. Second,

in some locations. (68887)

when concrete durability issues arise, petrography is a helpful analytical tool.

In addition, the mix design—notably the high

Reference

sand content—may have contributed to the problems. Thorough mixing may be difficult when concrete contains large amounts of sand.

This makes it harder to develop a good air-void

Stutzman, Paul E., D eterioration of Iowa H ighway Concrete Pavement: A Petrographic

Study, IR6399, NIST, www.fire.nist.gov/bfrlpubs, 2000, 84 pages.

system, which is generated by the shearing of

Serial No. 2224a

What’s New in Petrography Already an established science, the petrograph-

To further the efforts of a growing group of individuals using light optical microscopi-

ic (microscopic) study of hardened concrete

cal methods to evaluate concrete, The Society of Concrete Petrographers was formed

and related materials is emerging as an impor-

in February 2000.

tant field in concrete construction. By studying prematurely deteriorated concrete, petrographers have helped identify what makes concrete durable. Recent activities in petrography

By providing a meeting and speaking forum, and even adopting a code of conduct, the group hopes to achieve many goals: • exchange information about methods and techniques of examination

include a book and the formation of a society.

• apply these methods to evaluate concrete and related materials

A 1998 reference, Concrete Petrography: A

• interpret collected data

handbook of investigative techniques, provides

• write efficient and effective reports

help with every aspect of this science, includ-

• increase awareness of an individual’s limitations

ing sample selection, equipment, and test

• advance petrography as a profession

methods. It is published by Arnold and is avail-

• establish qualification requirements for allied associations

able from PCA as LT226.

• maximize information obtainable from concrete

To find out more about the Society, contact any of the following individuals:

Another valuable document, Petrographic

Bernard Erlin, The Erlin Company:

724.539.1800

Methods of Examining H ardened Concrete: A

Mauro Scali, Simpson, Gumpertz & Hager:

781.641.7342

Petrographic Manual, is available at

Robert O’Neill, MicroChem Laboratories:

209.728.8200

www.tfhrc.gov/pavement/pccp/petro.htm.

Derek Cong, W. R. Grace Company:

617.498.4849

Concrete Technology Today /July 2000

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C O N C R E T E

ConcreteRelated Associations Two new non-profit groups will address specialty areas

The Silica Fume Association

T E C H N O L O G Y

T O D A Y

Designing Durable Concrete Durability tests to consider when working with concrete Concrete, the most widely used building material, is both durable and economical in a broad range of environments. Though concrete is a forgiving material, every step of manufacturing concrete influences its properties—from design to mixing to placing and finishing—perhaps more than with any other building product.

What Works Now? What Worked in the Past?

Local practice provides a good starting point for mixture designs. Even though many ingredients have fairly consistent properties, materials must be constantly monitored because there is variation

For decades, silica fume has been

over time. Ongoing testing of materials and mix designs helps reduce concerns about durability.

improving concrete structures, adding

Typical factors of mix designs include cement content, water-cementitious material ratio, and per-

strength, and reducing permeability

meability of hardened concrete. It is also important to assess exposure conditions for things like

beyond cement-only mixtures. To

sulfate levels in soil and water, and exposure to frost.

promote the use of silica fume for infrastructure building and repair, the

Past performance of concrete mixtures is valuable data. It helps to see how materials have per-

Silica Fume Association (SFA) has

formed in similar environments. This could save you time and expense. Unfortunately, this data is

been formed. Contact them at:

not always available. You should run tests if you are uncertain about a material or potential expo-

3927 Old Lee Highway, Suite 100 Fairfax, VA 22030 Phone: 703.934.2937

sure, but how do you decide which tests to run?

Major Durability Topics

Fax: 703.934.0279

The table on the next page guides designers, concrete contractors, and technicians through con-

Internet: www.silicafume.org

crete durability topics. The tests may focus on concrete ingredients, on concrete mixtures, or on

American Shotcrete Association

exposure conditions (soil, water).

Likewise, shotcrete is a specialty

Computer programs for designing concrete mixtures are becoming more sophisticated and

concrete product with definite

widespread. Many of these are accessible through the Internet, often focusing on the "high perfor-

advantages in certain applications.

mance" aspect of concrete, where the high performance refers to durability. Some sites of interest:

Virtual Design

To promote increased use of shotcrete, the American Shotcrete

Partnership for High Performance Concrete Technology http://ciks.dbt.nist.gov/phcpt

Association (ASA) aims to educate

CIKS for High Performance Concrete http://ciks.cbt.nist.gov/~bentz/welcome.html

consumers and practitioners on the

Concrete Optimization Software Tool (COST) http://ciks.dbt.nist.gov/bentz/fhwa

proper installation and benefits of shotcrete.

References

1323 Shepard Drive, Suite D

1. Effects of Substances on Concrete and G uide to Protective Treatments, PCA IS001

Sterling, VA 20165-4428

2. D iagnosis and Control of Alkali-Silica Reactions in Concrete, PCA IS413

Phone: 703.444.9619

3. D esign and Control of Concrete Mixtures, PCA EB001

Fax: 703.450.0119

4. Manual on Control of Air Content in Concrete, PCA EB116

Internet: www.shotcrete.org

5. Significance of Test and Properties of Concrete and Concrete-Making Materials, ASTM STP 169C (available from PCA as LT205)

6

Concrete Technology Today / July 2000

What to Look for Regarding Concrete Durability Durability aspect and potentially affected locations Acid attack — sewers, manholes, industrial facilities

Effect

Cement paste or (calcareous) aggregates are dissolved, matrix deteriorates

Typical prevention methods Low w/cm Low cement content Special cements or aggregates Sacrificial cover Coatings

Measurement criteria*

Test designations

Depth of attack and rate of disintegration Ref. 1

No standard test: see Ref. 1 for effect of particular acid. If attack is expected, run tests with concrete and acid (at expected strength), or use recommended protective measures

Change of length of mortar bars or concrete prisms (aggregate-cement combinations) Mineral composition of aggregate (aggregate characteristics/behavior) Refs. 2 and 5

ASTM C 295, Practice for Petrographic Examination of Aggregates for Concrete ASTM C 856, Practice for Petrographic Examination of Hardened Concrete ASTM C 1260, Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method) ASTM C 1293, Test Method for Concrete Aggregates by Determination of Length Change of Concrete due to Alkali-Silica Reaction

Alkali-silica reactivity (ASR) — any concrete in North America, mostly exterior, but also, moist interior exposures

Alkali hydroxides react with reactive aggregate to form gel, which grows and expands within the concrete, resulting in cracking

Addition of pozzolans or slags Blended cements Low alkali loading of concrete

Corrosion (deicer and seawater) — marine environments, bridges, and pavements

Concrete spalling Reinforcement corrosion

Low w/cm (max. 0.40) Permeability Addition of pozzolans Depth of cover or corrosion inhibitors Refs. 1and 5 Maintain proper cover over reinforcement

D-cracking — any pavement made with susceptible aggregate in North America

Aggregates take up water, then crack after repeated exposure to freezing temperatures

Decrease aggregate particle size or choose other aggregates

Freeze-thaw testing of ASTM C 666, Test Method for Resistance saturated aggregates of Concrete to Rapid Freezing and Monitor length change, Thawing mass loss, and relative dynamic modulus Ref. 3

Freeze-thaw Concrete matrix exposure — freezing cracks or surface exposures, especially scales where deicers are used on structures (pavements and bridges). Also, cold marine environments

Specify and maintain recommended air contents (5% to 8%)

Total air content (fresh or hardened concrete) Air-void system (of hardened concrete) Freeze-thaw testing to monitor length change, mass loss, and relative dynamic modulus Ref. 4

ASTM C 173, Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method ASTM C 231, Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method ASTM C 457, Practice for Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete ASTM C 666, Test Method for Resistance of Concrete to Rapid Freezing and Thawing ASTM C 672, Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals

Sulfate exposure — soils in (predominantly) Western U.S. Also, seawater and some industrial environments

Make low permeability, usually by keeping low w/cm (0.40 to 0.50) Use sulfate-resistant cement Types II, V, MS, or HS

Expansion of concrete samples in sulfate solutions Sulfate level of soils or water Refs. 1 and 5

ASTM C 1012, Test Method for Length Change of Hydraulic-Cement Mortars Exposed to Sulfate Solution ASTM D 516, Test Methods for Sulfate Ion in Water

Sulfate ions react with cement paste in an expansive process

ASTM C 1202, Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration AASHTO T 259, Resistance of Concrete to Chloride Ion Penetration Inspect cover over steel, should meet building codes

*Consult local practices, ASTM, ACI, and other standards for limits. Consult Ref. numbers in this column as a starting point.

Concrete Technology Today / July 2000

7

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CONCRETE TECHNOLOGY Today New Literature The following publications are now available. To purchase them in the United States contact Portland Cement Association, Customer Service, P. O. Box 726, Skokie, IL 60076-0726; telephone 800.868.6733, fax 847.966.9666, or Internet www.portcement.org (fax and Web available 24 hours/7 days a week). In Canada please direct requests to the nearest regional office of the Canadian Portland Cement Association (Halifax, Montreal, Toronto, and Vancouver).

Durability of Concrete, LT237 This 68-page Transportation Research Circular (No. 494) gives a general overview of how to make durable concrete. It addresses material selection, proportioning, construction practices, specifications, testing, and case studies. Published by the Transportation Research Board in 1999.

Best Sellers CD, CD007 An electronic compilation of the most popular publications from PCA and ACPA. Developed originally for distribution to university students and professors, this college literature CD contains references needed to design and build with concrete. Call Customer Service or visit our Web site for a list of the over two dozen publications contained on this CD.

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by PROFESSIONAL PERSONNEL who are competent to evaluate the significance and limitations of the information provided herein, and who will accept total responsibility for the application of this information. The Portland Cement Association DISCLAIMS any and all RESPONSIBILITY and LIABILITY for the accuracy of and the application of the information contained in this publication to the full extent permitted by law.

PUBLISHER’S NOTE: Intended for decisionmakers associated with the design, construction, and maintenance of concrete projects, Concrete Technology Today is published triannually by the Construction Information Services department of the Portland Cement Association.

Our purpose is to highlight practical uses of concrete technology. If there are topics readers would like discussed in future issues, please let us know. Items from this newsletter may be reprinted in other publications subject to prior permission from the Association. For the benefit of our readers, we occasionally publish articles on products. This does not imply PCA endorsement.

Direct all correspondence to Jamie Farny, Editor Phone: 847.966.6200 Fax: 847.966.8389 E-mail: jamie_farny@portcement.org

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