Today's concrete mixes are more complex than ever, combining a wider range of cements, supplementary cementitious materials (SCM), and chemical admixtures to produce economical concretes that can meet exacting performance requirements. Unfortunately, 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.
CTLGroup has recently completed a comprehensive Federal Highway Administration (FHWA) study of material incompatibility issues and developed a testing protocol to prevent problems before construction. The comprehensive research project, which began in 1998–99, had several goals:
- Gain a clearer understanding of the chemistry of reactive materials in concrete
- Develop a preconstruction laboratory testing regimen that could detect problematic interactions and incompatible mix designs
- Correlate lab and field test methods to facilitate quality assurance during construction
- Recommend field tests to confirm concrete quality and allow for needed adjustments onsite
What is incompatibility?
In the context of the FHWA study, “incompatibility” of concrete materials was defined as interactions between otherwise acceptable materials that result in unexpected or unacceptable performance. The most common problems included premature stiffening (rapid slump loss) and erratic setting of concrete mixtures (flash set, false set, or delayed setting and strength gain) which increased risk of cracking and unacceptable air-void systems. Proper consolidation, finishing, texturing, and curing also can be disrupted.
Uncontrolled stiffening and setting of concrete can cause serious problems with concrete pavement construction and with other types of flatwork and structures (such as bridge decks) 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 can be consolidated in place; however, for both pavements and structures, rapid stiffening may lead to honeycombing and incomplete consolidation.
The aim of the FHWA study was to develop a protocol for users to assess whether a certain combination of materials for pavement concrete would be incompatible in a given environment.
Many mechanisms and effects contribute to incompatibility. The mechanisms are complex and interrelated, and frequently they are temperature-related. This means there is no simple way to reliably measure the risk of incompatibility. No one test method is ideal for everything. Some tests can identify problems in the first 30 minutes because of aluminate and sulfate balance issues. Other tests detect later silicate hydration problems, while still others assess other signs of distress.
The protocol was developed to provide 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 and under environmental conditions likely to be encountered in the field. The work may also include preparing alternative mix proportions and practices, to accommodate changes in either the environment or in the source of the materials. Field tests could be based on the more rugged tests that are regularly conducted, primarily to monitor the uniformity of the materials and the final mixture.
Most of the tests used in this work have some value, so the extent of pre-construction and field testing should be based on the availability of equipment and the relative cost of testing versus the potential cost of a failure. A typical example is determining setting time, which can be measured by any of six different techniques. You should 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 variables or potential incompatibility.
The following tests make up this protocol:
- Foam index
- Foam drainage
- Unit weight
- Slump loss
- Semiadiabatic temperature monitoring
- Setting time
- Chemistry of reactive materials