Q. We are about to start our mix design approval for next season. We have been in a market where cement availability has been tight, so we are considering including more ingredients in our designs. How do you suggest we begin our material selection process for our concrete mix designs?
A. Today's concrete mixes are more complex than ever before, according to Peter Taylor, principal engineer and manager of the Materials Consulting practice for CTLGroup, Skokie, Ill. Producers are combining a wider range of cements, supplementary cementitious materials, and chemical admixtures to produce economical concretes that can meet exact performance requirements.
Unfortunately, constituent materials sometimes interact in unexpected ways that adversely affect setting time, workability, and strength development. Among the more common problems that result are premature stiffening, excessive cracking, and poor air-void systems.
Taylor recently completed a Federal Highway Administration study of material incompatibility issues and developed a testing protocol to prevent problems before construction.
“Incompatibility” of concrete materials was defined as interactions between otherwise acceptable materials that result in unexpected or unacceptable performance. The most common problems involve premature stiffening (rapid slump loss) and erratic setting of concrete mixtures (flash set, false set, or delayed setting and strength gain) along with an increased risk of cracking and unacceptable air void systems. Proper consolidation, finishing, texturing, and curing also can be disrupted.
Uncontrolled stiffening and setting can cause serious problems with concrete pavement construction and with other types of flatwork and structures (bridge decks, for example) where the timing of finishing and texturing is critical. These problems may not be noticeable in formed concrete structural elements as long as the concrete is workable enough to be consolidated in place. However, for pavements and structures, rapid stiffening may lead to honeycombing along with incomplete consolidation.
The study's aim was to develop a protocol to assess whether a given combination of materials for pavement concrete would likely exhibit such incompatibility in a given environment.
Many mechanisms and effects contribute to incompatibility. The mechanisms are complex and interrelated, and often are temperature-related. Some test methods indicate the risk of problems in the first 30 minutes because of aluminate and sulfate balance issues. Other tests detect later silicate hydration problems. Others assess other signs of distress.
The protocol provides as much information as possible before construction, including calibration of the more sensitive laboratory tests with equivalent field tests, using materials likely to be used in the field, under environmental conditions likely to be encountered in the field.
Base the extent of preconstruction and field testing on the availability of equipment and the relative cost of testing versus the potential cost of a failure.
An example is determining setting time, which can be measured by any of six different techniques. Select from among these different techniques based on other project requirements and conditions.
A relatively simple suite of field tests, conducted regularly, can provide reassurance that the concrete mixture is performing satisfactorily or warn of undesirable variability or incompatibility.
Preventing these problems
Quality control managers should carefully evaluate materials before construction starts. “This will indicate potential problems and help develop guidelines on what to change if problems occur,” says Taylor. The evaluation tests should be conducted over the range of likely temperatures and using the range of likely material quantities and admixture dosage rates.
He also advises labs to 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,” says Taylor. 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 cementitious materials with high C3A or low sulfate contents (or both) may be a risk, as will fly ashes with high calcium oxide contents. Sugar and triethanolamine-based water-reducing admixtures may also increase the risk of problems. This is especially true if the concrete is placed at high temperatures.
He suggests performing a paste- and mortar-based laboratory test, including the minislump test and the ASTM C 359 mortar stiffening test.
The result 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, Taylor suggests adjusting any of the following: supplementary cementitious material type, source, or quantity; chemical admixture type or dosage; batching sequence; and mix temperature. A series of mixtures also 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 practices may be necessary.
Peter Taylor, Ph.D., PE was 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 is available at the FHWA by e-mailing firstname.lastname@example.org,or telephone 301-577-0818. An electronic copy is available at the Turner-Fairbank Highway Research Center Web site at www.tfhrc.gov.