InSights

The Progression of High Strength Concrete

By Kevin A. MacDonald, FACI

The definition of high strength concrete continues to change. This change occurs as the art of achieving a particular strength is reduced to practice, and the structural requirements push at the edge with needs for higher strength. One such example is the CN Tower in Toronto, with its required strength in 1976 of 5000 psi. At that time, this was difficult to achieve. Today 5000 psi concrete is routinely used and produced without special precautions. In the manufacturing of high strength concrete, there are significant differences from those seen in practice just a few years ago. Cementitious components and content, admixtures aggregates and curing have changed. Once the province of high cement contents and silica fume, much of the developments over the last decade have revolved around a better understanding of, and attention paid to, the microstructure of the concrete. High strength concrete can be modeled as a three phase system – the paste, the aggregate and the interface between them (Figures 1 and 2). By taking this approach, an engineered composite material can be designed. Actions taken to modify the interfacial transition zone between the aggregate and paste have increased the load transfer between the paste and aggregate; thereby increasing the strength of the concrete. It is the action of meta-kaolin, silica fume and other finely divided materials in modifying this interfacial transition zone that originally led to significant increases in strength. These materials were once used at high replacement levels, frequently greater than 10%. While high strength was often achieved, the workability and susceptibility to fracture of the concrete were problems that ultimately limited the strength. In modern high strength concrete, blends of smaller quantities and fractions of silica fume result in large increases in strength without compromising the ability of the mixtures to be placed. In many cases, ternary or even quaternary blends of pozzolanic material with Portland cement are seen in practice. One of the reasons for these blends is that the heat generated during the hydration process can cause residual stress within the paste, and reduce the strength of the concrete. While these types of effects can readily be removed in metals by annealing, no such process is

Figure 1: Thin section of concrete showing microstructure of paste. Red arrows are typical flyash particles. Figure 2: Thin section of concrete showing microstructure of paste. Note the relatively low degree of hydration of the slag particles (red arrows).

available for concrete. The curing process must be engineered to control the hydration reaction so residual stresses are minimized. Temperature monitoring or other devices are used to track the progress and monitor the reaction that produces the binder. The literature at the turn of the 20th century often referred to the curing process as annealing. While having very little to do with the concept of heat treating of metals, this curing, when performed properly, is a critical factor in the performance of higher strength concrete. Strengths up to 20,000 psi have been realized using pozzolanic materials, dispersants, limestone modified cements and careful attention to aggregate materials selection. The limestone acts to nucleate the reaction and reduces the quantity of unhydrated-cement. Admixture technology has progressed. Stabilizing admixtures and dispersants with a low affinity for the solid surface, where a large fraction of dispersant remains in solution, has allowed mixtures to be held in a state of “suspended animation” while the concrete is placed. They can perform predictably, allowing scheduling of the construction process. Retarders, dispersants and stabilizers will increase the strength. One of the difficulties that the designer of high strength concrete mixtures encounters is the ability to have workable concrete with very low water/cement ratios. Use of modern high-efficiency dispersants (super plasticizers) has led to observed autogeneous drying of materials due to hydration or vaporation due to high temperature. As a consequence, some very low water/cement ratio concrete have shown good performance in the laboratory but poor performance in the larger structural members, where the heat of hydration is not as readily dissipated and where the sample is not immersed in water for 28 or even 56 days. These limits are advancing by innovations such as the use of lightweight aggregate for internal curing and steel whiskers to distribute stresses. Aggregate materials are no different. As the paste strength increases, and the interfacial transition zone densifies, the strength of the aggregate or the presence of fractures therein become a limiting factor. Reducing the maximum particle size and carefully selecting the geological origin of the materials can lead to significant improvements in strength. Recognition by the design and construction team that the concrete strength does not need to be achieved at seven or 28 or even 56 days, but only as the structure is loaded, allows mixtures that have relatively low cement contents and very high pozzolanic replacement to achieve compressive strengths in excess of 15,000 psi. Care must be taken in the production of high strength concrete in order to ensure that the performance of the concrete in situ is what was intended in the design. Raw materials, batching and handling at the plant and at the installation site must be controlled. Without an understanding of the importance of high strength concrete at the production plant and by the individual vehicle drivers, it will be more difficult to achieve performance of the structure.▪ Kevin A. MacDonald, FACI, is President and Principal Engineer for Beton Consulting Engineers LLC, Mendota Heights, MN. He was named a Fellow of ACI in 2004. Kevin can be reached at kmacdonald@betonconsulting.com.

ADAPT Corporation

Web: www.adaptsoft.com Product: ADAPT-ABI Description: Easy-to-use and practical software for 4D construction phase analysis of any segmentally constructed bridge. ABI calculates time-dependent behavior of concrete, tendons, and stays. Particularly well suited for balanced cantilever and spliced girder bridges. Handles geometry, camber, and stress control during construction and reports service load design values.

Bentley Systems

Web: www.bentley.com Product: LEAP Bridge Description: A powerful modeling and analysis solution for small to medium-sized concrete bridges of all types: precast, cast-in-place, reinforced, and post-tensioned. This comprehensive bridge information modeling (BrIM) system offers a synthesis of geometric modeling, substructure, superstructure analysis and design, and load rating in a single, information-rich environment.

Product: RM Bridge

Description: A comprehensive software for structural design and analysis. Used worldwide by engineering firms, government organizations, contractors, and project consortia, RM Bridge sets the standard for both routine and signature bridge design and delivery.

Cintec Reinforcement Systems

Web: www.cintec.com Product: Archtec Description: Reinforcement of masonry arch bridges to bring them up to code.

CTS Cement Manufacturing Corporation

Web: www.ctscement.com Product: Rapid Set® Low-P™ Cement and Type-K Cement Shrinkage Compensating Concrete

Description: Complete bridge deck overlays faster with Rapid Set Low-P cement. Get better quality, lasting performance and an in-place cost less than Portland cement concrete. Type-K Cement Shrinkage-Compensating Concrete has been used in over 800 bridge decks with reduced permeability, excellent durability, virtually no cracks and increased concrete life cycle.

Devco Software, Inc.

Web: www.devcosoftware.com Product: LGBEAMER v8 Description: Analyze and design cold-formed cee, channel and zee sections. Uniform, concentrated, partial span and axial loads. Single and multi-member designs. 2007 NASPEC, including the 2010 Supplement (2013 IBC) compliant. ProTools include shearwalls, framed openings, X-braces, joists and rafters.

Fabreeka International, Inc.

Web: www.fabreeka.com Product: Expansion Bearing Description: Slide and expansion bearings for bridge deck.

Hayward Baker Inc.

Web: www.HaywardBaker.com Product: Geotechnical Construction

Description: A complete range of geotechnical construction techniques for build or design-build solutions for new bridge construction and remediation of existing bridges, specializing in deep foundations, foundation rehabilitation, and ground improvement. Old and new bridges benefit from decades of advancement of geotechnical construction techniques. LARSA, Inc.

Web: www.Larsa4D.com Product: LARSA 4D Description: Advanced software for the design and analysis of bridges and structures, LARSA 4D addresses the specialized needs for a variety of bridge types including cable-stayed, suspension, and curved. Standard in leading U.S. firms for bridge design and construction analysis, and continues to lead innovation in analysis and support.

MDX Software

Web: www.mdxsoftware.com Product: MDX Software Description: Steel bridge design and rating software according to AASHTO, ASD, LFD, LRFD, LRFR specs.

Pile Dynamics, Inc.

Web: www.pile.com/pdi Product: Systems for QA-QC of Bridge Foundations

Description: Pile Driving Analyzer® (Dynamic Load Testing, Pile Driving Monitoring) Pile Integrity Tester Thermal Integrity Profiler (for cast-in-place foundations) GRLWEAP (Pile Driving Simulation/Wave Equation Analysis) PIR (ACIP/CFA Automated Monitoring Equipment); Cross-Hole Analyzer (Single/Crosshole Sonic Logging); SPT Analyzer (SPT hammers Calibration); E-Saximeter (Blow Count, Driving Logs, Energy Measurements) LITE (Steel Pile Depth).

POSTEN Engineering Systems

Web: www.postensoft.com Product: POSTEN Multistory Description: With sophisticated post-tensioning algorithms, only POSTEN automatically & efficiently designs the tendons and rebar (no hours of fiddling with drapes). The only Post-tensioned concrete software that includes design of moment frames, seismic & wind, columns, torsion, and truly sustainable design (with LEED documentation).

RISA Technologies

Web: www.risa.com Product: RISA-3D Description: With RISA-3D’s versatile modeling environment and intuitive graphic interface, you can model any structure from bridges to buildings in minutes. Get the most out of your model with advanced features such as moving loads, dynamic analysis, and over 40 design codes. Structural design has never been so thorough or easy!

S-FRAME Software Inc.

Web: www.s-frame.com Product: S-STEEL Design Description: Use S-STEEL to design your steel structures – all or partial. Code check and auto design for both strength and serviceability. S-STEEL supports composite beam design, staged construction, numerous steel design codes and a large choice of optimization criteria and constraints. Comprehensive design reports include equations, clause references and interactive graphics.

Product: S-FRAME Analysis

Description: A powerful yet easy-to-use 4D structural analysis and design environment. Featuring linear and advanced nonlinear analysis capabilities, multiple design code support, a variety of material models, flexible load combination methods and fully integrated steel design, reinforced concrete section design and foundation design tools. Simpson Strong-Tie

Web: www.strongtie.com Product: Repair, Protection and Strengthening Systems for Concrete and Masonry

Description: Simpson Strong-Tie offers repair, protection and strengthening systems for concrete and masonry, which complements its anchor systems offering. The line includes products for: general concrete repair; crack repair; nonstructural injection; joint sealing; performance coating, and specialty products. The product guide is available at website.

Product: Anchoring and Fastening Products

Description: Simpson Strong-Tie offers anchoring and fastening products for concrete and masonry, including: codelisted solutions; cold-formed steel connectors; direct fastening systems; general purpose anchors and fasteners; restoration products; carbide drill bits, and accessories. For the new Product Guide visit the website.

Strand7 Pty Ltd

Web: www.strand7.com Product: Strand7 Description: An advanced, general purpose, FEA system used worldwide by engineers, designers, and analysts for a wide range of structural analysis applications. Preprocessing, solvers (linear and nonlinear static and dynamic capabilities) and post-processing. Features include staged construction, quasistatic solver for shrinkage and creep/relaxation problems.

StructurePoint

Web: www.StructurePoint.org Product: Concrete Design Software Description: spColumn is widely used for design and investigation of bridge piers, shear walls as well as typical framing elements in structures including slenderness effects and biaxial loading. Can also investigate irregular sections with any reinforcement layout that are impossible to find on design charts or to do by hand.

Western Wood Structures

Web: www.westernwoodstructures.com Product: Timber Bridges Description: Timber bridges are naturally beautiful, durable and cost-effective. Western Wood Structures designs and supplies vehicular, pedestrian, golf course, park and residential bridges, using the highest quality, pressure-treated, glulam timber. Experience our enduring commitment to quality, achieved through our performance as an on-time designer, supplier and erector of timber bridges.

Wheeler

Web: www.wheeler-con.com Product: Panel-Lam Description: Shop manufactured Timber Bridge Kits suitable for vehicular and recreation applications.

Williams Form Engineering Corp.

Web: www.williamsform.com Product: All-Thread-Bars Description: Williams Form Engineering Corporation has been providing threaded steel bars and accessories to the bridge construction industry for over 90 years. Williams’ pre-stressing/post tensioning 150 KSI All-Thread-Bars are high tensile steel bars available in seven diameters from 1 to 3 inches with guaranteed tensile strengths to 969 kips.

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