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Volume Changes of Concrete
Thursday, October 15, 2009 - 12-1pm CDT

If concrete were free of any restraints to deform, normal volume changes would be of little consequence. However, concrete is restrained by foundations, subgrades, reinforcement, or connecting members, and significant stresses can develop. This one-hour webinar will cover the variables that affect volume changes and how to minimize high stresses and cracking. More information and purchase.

How to Minimize Early-Age Cracking

Early-age cracking can be a significant problem in concrete. Early age for concrete is the first seven days starting with final set, which is when the concrete has obtained a benchmark level of stiffness. During this time, concrete undergoes a significant amount of volume change caused by many variables, such as the hydration reaction (chemical shrinkage), water content (drying shrinkage and swelling), and temperature changes (thermal dilation).

Volume changes in concrete will drive tensile stress development when they are restrained, which is the case with most concrete. Tensile stresses are forces trying to pull apart the concrete and are opposite from compressive stresses. Cracks can develop when the tensile stress exceeds the tensile strength. While concrete is strong in compression, the tensile strength is generally only 10% of the compressive strength. At early ages, this strength is still developing while stresses are generated by volume changes. Controlling the variables that affect volume change can minimize cracking and create a higher quality concrete placement. More on early-age cracking.

Concrete Sustainability Hub Launched at MIT

To address the sustainability and environmental implications of the use of concrete as the backbone of our housing, schools, hospitals and other built infrastructure, including highways, tunnels, airports and rail systems, the Massachusetts Institute of Technology (MIT) has announced the creation of the Concrete Sustainability Hub, a research center established at MIT in collaboration with the Portland Cement Association and Ready Mixed Concrete (RMC) Research & Education Foundation.

The Concrete Sustainability Hub (CSH), established with the goal of accelerating emerging breakthroughs in concrete science and engineering and transferring that science into practice, will provide $10 million of sponsored research funding during the next five years. Researchers from MIT’s School of Engineering, School of Architecture and Planning and Sloan School of Management are expected to participate in the CSH’s research activities. More on the Concrete Sustainability Hub.

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Isolation/Expansion Joints: Why and Where They are Used

Isolation joints are used to relieve flexural stresses due to vertical movement of slab-on-grade applications that adjoin fixed foundation elements such as columns, building or machinery foundations, bridge abutments, light standards, drop inlets, and so on. In addition, an isolation joint may be used in slabs that require a change in contraction joint layout, which would create T intersections. Expansion joints are used primarily to relieve stress due to confinement of a slab. More on isolation/expansion joints.

Q: What causes concrete slabs to curl/warp, and how can this mechanism be minimized?

A: Concrete slabs curl due to the effects of temperature variation between opposing surfaces of the slab. Concrete slabs warp due the effects of moisture variation between opposing surfaces of the slab. Slab on grade applications are typically too thin to be significantly affected by temperature variations that cause curling, however variations in moisture commonly change the surface profile of slab-on-grade installations leading to warping. Slabs design with appropriate thickness using quality materials, good placement practices, properly jointed and cured will achieve sufficient strength to minimize slab warping. More on curling and warping of slabs.

New Standard Test Method for Heat of Hydration: ASTM C1702-09

ASTM International’s Subcommittee C01.26 has developed a new standard: ASTM C1702-09, Test Method for Measurement of Heat of Hydration of Hydraulic Cementitious Materials Using Isothermal Conduction Calorimetry, was published in August 2009. The new method provides a means of determining the heat of hydration of hydraulic cements at ages up to 7 days. This method holds promise for possible replacement of ASTM C186 in specifications. ASTM C1702 is less hazardous to perform and is expected to be less variable than the older method (research is ongoing). For more information, visit the ASTM website for C1702.

Stress-Strain Properties of Concrete at Elevated Temperatures

Researchers at the University of Notre Dame recently completed a study on the compressive stress-strain behavior of North American concrete under elevated temperatures from fire. A database was developed on the temperature-dependent properties of concrete from previous experimental research. Using this database, regression relationships were proposed for the concrete strength, elastic modulus, strain at peak stress, ultimate strain, and stress-strain behavior, including the aggregate type (calcareous, siliceous, light-weight), heating test type (unstressed, stressed, residual), and room temperature strength (with both normal-strength and high-strength materials) as parameters. In comparison with previous relationships, the proposed relationships are based on a larger dataset. Furthermore, unlike previous models, creep deformations are not included in the proposed models, resulting in significantly smaller temperature-dependent concrete strains. This allows the proposed relationships to provide a baseline to which creep strains could be explicitly added through a time-dependent procedure. Download the report.

Tools for Concrete Thinking
PCA Concrete Technology Catalog has recently been updated. Each product in this catalog is linked to the PCA Bookstore for ease in ordering. To download your copy, click here.

2007-2009 PCA Research and Technical Reports
This listing is updated monthly and is a compilation of completed publications from PCA's Research and Technical Services programs for the past two years. Reports are listed by title and code number. There are over 125 research reports referenced in this edition. Click to download.

Get Free Admission to World of Concrete 2010
Exhibits: February 2-5, 2010
Seminars: February 1-5, 2010
Once again, PCA offers free registration and a discount on seminars to the World of Concrete in Las Vegas, Nev. You can save as much as $65 on exhibit hall entry fees and $30 on each course you sign up for. Register now.

Greenbuild Registration Now Open
It’s not too late to register for the preeminent event on sustainable development – the Greenbuild International Conference and Expo. The green gathering is scheduled for November 11-13, 2009 in Phoenix, Ariz. Information about the Master Speaker series, volunteer opportunities, the conference schedule, registration links, and more are online.

ICRI 2009 Fall Convention
The ICRI 2009 Fall Convention focusing on flooring issues will be held October 21-23, 2009 in Tempe, Arizona. More information.

ACI Fall Meetings
The Louisiana Chapter of ACI will be hosting the Fall 2009 ACI Convention in New Orleans on November 8-12, 2009 at the Marriott New Orleans. A special Concrete Sustainability Forum will be held prior to the meeting on Saturday, November 7, 2009. More information.

ASTM C01 and C09 Committee Week December 2009
ASTM International’s Committee C09 on Concrete and Concrete Aggregates and Committee C01 on Cement will meet in their semi-annual Committee Week meeting at the Hyatt Regency Atlanta, in Atlanta, Ga., December 6 to 11. For more information, visit the ASTM website for Committee C09 or Committee C01.

Complete listing of industry conferences.