Q. We plan to bid on some state bridge deck work planned in our area. Our district engineer is open to allowing a high-performance mix that will help mitigate the possibility of undue cracking.
Our lab is analyzing several mixes with standard features such as strength, set time, and chloride resistance. But we have been struggling with how to demonstrate a concrete mix's performance regarding crack potential. What is an acceptable test method to use to compare different concrete mixes?
A. Bridge engineers recognize that concrete shrinkage is a major contributor to bridge deck cracking. As a result, there are several research projects underway to assess how to best solve the problem. Notable efforts are underway at the Indiana Department of Transportation, the University of Kansas, and the Colorado Department of Transportation.
One commonly accepted method used to compare concrete mixes for potential shrinkage is the ASTM C 157 standard test method, Test for Length Change of Hardened Cement Mortar and Concrete. This test is also known as AASHTO T 160.
This test allows technicians to determine length changes of a laboratory-made concrete specimen. Lab technicians expose the sample to the controlled conditions of temperature and moisture, thus eliminating any other cause for expansion, such as externally applied forces or ambient temperature changes. The ASTM subcommittee which oversees the procedure reports that the method provides useful information for products that require testing under nonstandard mixing, placing, handling, and curing conditions.
In his recent article published in the HPC Bridge Views newsletter, Jerzy Zemajtis, a senior engineer at CTL-Group, reports that ASTM C 157 is useful in comparing different concrete mixes when the specimens have the same original size. He outlines how the procedure has been conducted on bridge deck mix design research.
Zemajtis does provide a word of caution. The researcher warns that the length change measured on the test prisms under the constant environmental conditions does not equal the shrinkage on the bridge deck. He advises that field conditions such as deck thickness, internal restraint from reinforcement, and deck curing all affect the shrinkage of the deck.
One way to keep current on the applied research underway on the topic of concrete bridge construction is to subscribe to HPC Bridge Views, published jointly by the Federal Highway Administration and the National Concrete Bridge Council. It provides relevant, reliable information on all aspects of high-performance concrete's application in bridge construction.
This bimonthly electronic version replaces the mailed newsletter first published in Jan./Feb. 1999. You can register to receive HPC Bridge Views at www.cement.org/bridges.
Also, bridge deck design is one session scheduled for discussion at the 2008 Concrete Bridge Conference, May 4–7, at the Hyatt Regency in St. Louis. To learn more and to register, visit www.nationalconcretebridge.org.
We invite questions from our readers on any aspect of concrete mix design, production, testing, or troubleshooting. New Approved ASTM Standards
There have been several approvals to updates of important ASTM standards affecting concrete producers in the last few months.
C1227 - Standard Specification for Precast Concrete Septic Tanks has been revised to C1227-08. This specification covers design requirements, manufacturing practices, and performance requirements for monolithic or sectional precast concrete septic tanks.
C231 - Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method has been revised to C231-08. The test determines the air content of freshly mixed concrete, exclusive of any air that may exist inside voids within aggregate particles.
To view the document summary page on these and other standards, visit ASTM's Web site at www.astm.org. You can quickly access the information by entering the standard designation number in the site's search engine. Or you can contact ASTM's customer service by telephoning 610-832-9585, or e-mailing firstname.lastname@example.org.