How do I prevent these problems?
Although there is no silver bullet for preventing these problems, carefully evaluating materials before construction starts will indicate potential problems and help develop guidelines on what to change if problems occur. The evaluation tests should be conducted over the range of likely temperatures and using the range of likely material quantities and admixture dosage rates.
Conduct preconstruction tests to evaluate the proposed system’s sensitivity to variations in materials composition and the environment. This will allow you to select alternative materials in advance or to prepare action plans to be implemented if such changes occur in the field. The preconstruction testing also will provide calibration between field- and laboratory-based tests, and offer guidance on appropriate limits for the materials to be used and conditions likely to be encountered.
Before conducting any physical tests, review the chemistry of the reactive materials. Fine c ementitious materials with high C3A or low sulfate contents (or both) may be at risk, as will fly ashes with high calcium oxide contents. Sugar and triethanolamine-based water-reducing admixtures may increase the risk of problems, especially if the concrete is to be placed at high temperatures.
Paste- and mortar-based laboratory tests, including the minislump test and the ASTM C 359 mortar stiffening test, will indicate whether aluminate-based incompatibilities are occurring. Tests that flag silicate reaction problems in paste and mortar include parallel-plate rheology, setting time, and isothermal calorimetry. If the paste and mortar tests indicate potential problems, then concrete mixtures should be made and tested for slump loss, semiadiabatic temperature curve, and setting time.
If problems are still likely in the field, then adjust any of the following: supplementary cementitious material (SCM) type, source, or quantity; chemical admixture type or dosage; batching sequence; and mix temperature. If time and budget allow, a series of mixtures can be run to indicate the range of variability that can be accommodated. The best corrective action can then be implemented when field problems occur or appear likely.
Field tests during construction should aim to confirm that the materials being delivered are uniform and similar to those used in the preconstruction tests. Significant variations indicated by control charts will flag that the mixture is not performing in the same way that it has previously, and that changes to mix proportions or construction practices may be necessary. Field tests would include monitoring chemical reports for the delivered reactive materials, measuring and tracking for concrete slump, slump loss at different times after mixing, semiadiabatic temperature curve, and setting time. These results then can be compared with the preconstruction data and monitored for drift.
Not all laboratories perform all of these tests, and some tests are more expensive than others. The decision about which suite of tests to run is largely governed by the balance of costs and risks. A large, high-profile project with significant penalties will require more tests than a small urban repair.
— Peter C. Taylor, Ph.D., PE, is a principal engineer and manager of the Materials Consulting practice for CTL-Group, Skokie, Ill. He served as project manager for FHWA-HRT-06-080, Identifying Incompatible Combinations of Concrete Materials and was the principal author of the FHWA TechBrief on which this article is based. The full report on the research project is available in printed form at the FHWA Product Distribution Center by e-mail to report.center@fhwa.dot.gov, by fax to 301.577.1421, or by phone to 301-577-0818. An electronic copy is available at the Turner-Fairbank Highway Research Center Web site. To download the report, go to www.tfhrc.gov.
