Incompatibility —Too Many Good Things?
The variety of materials options and mix proportions possible in concrete allows it to be customized for a wide range of applications and placement and service environments. However, the cementitious materials (cements, fly ashes, slag cements, etc.) and chemical admixtures (accelerators, retarders, water reducers, etc.) are all chemically complex and this can lead to problems. Even when all materials meet and exceed their specification requirements individually, problems can arise under field conditions. These problems might manifest themselves as:
- Early stiffening or excessive retardation, which can lead to workability, placeability, consolidation, and finishing issues
- Early-age cracking due to plastic shrinkage
- Air-void system issues including non-uniformity, low air contents, coalescence of air voids around aggregate, and excessively large voids
Although these problems are relatively rare, the resulting construction delays, performance issues, and loss of confidence in concrete as the preferred construction material are unacceptable. The Federal Highway Administration (FHWA) and Portland Cement Association (PCA) co-sponsored research into these phenomena, with the goal of minimizing or preventing these problems in the field. The project developed relatively simple protocols for evaluating concrete material combinations both pre-construction and during construction.
The pre-construction protocol is suited for testing of new mix designs and evaluating new materials sources. Tests should be performed over the range of temperatures expected in the field and cover the range of mix proportions expected to be used.
Figure 1, from the FHWA TechBrief (see complete reference below), summarizes the pre-construction protocol. It begins with a review of material characteristics that have been implicated in incompatibility issues, for example, cementitious materials' fineness, aluminate, sulfate, and alkali contents. A review of historical properties of the materials being used is valuable as changes in some characteristics can raise a flag that might cause the concrete producer to be wary. Additional testing is suggested to bracket potential problems, for example, some incompatibility issues only appear with certain materials during hot weather. Potential solutions are also recommended in the pre-construction protocol, and testing can confirm which of these solutions will be effective. This allows the problems to be avoided or permits changes to be quickly implemented if problems arise during construction.
There is a separate testing protocol for use during construction that is best suited to identifying issues that arise. The goal of these tests is to assure that materials used are similar to those used in pre-construction testing. Figure 2 suggests routine tests during construction that can be used to monitor performance, and quickly detect potential problems as they arise, allowing a more rapid response, particularly if various solutions have been evaluated with pre-construction testing.
It is important to note that the protocol offers flexibility because not all of these tests are routinely available. Some are relatively more time consuming or costly to run. The decision about which suite of tests to use will be based on engineering judgment based on the costs/benefits and project-specific risks. A high-profile bridge project will obviously require more tests than a neighborhood sidewalk.
TechBrief: Protocol to Identify Incompatible Combinations of Concrete Materials, PCA R&D Serial No. 2897a, Federal Highway Administration, Research, Development, and Technology, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, Virginia, USA 22101-2296, No. FHWA-HRT-06-082, July 2006, 7 pages.
Taylor, Peter C.; Graf, Luis A.; Zemajtis, Jerzy Z.; Johansen, Vagn C.; Kosikowski, Ronald L., and Ferraris, Chara F., Identifying Incompatible Combinations of Concrete Materials: - Volume 1, PCA R&D Serial No. 2897, Federal Highway Administration, 6300 Georgetown Pike, McLean, Virginia, USA, 22101-2296, USA, FHWA-HRT-06-079, August 2006, 159 pages.
Taylor, Peter C.; Graf, Luis A.; Zemajtis, Jerzy Z.; Johansen, Vagn C.; Kosikowski, Ronald L., and Ferraris, Chara F., Identifying Incompatible Combinations of Concrete Materials: - Volume II - Test Protocol, PCA R&D Serial No. 2897b, Federal Highway Administration, 6300 Georgetown Pike, McLean, Virginia, 22101-2296, USA, FHWA-HRT-06-080, August 2006, 87 pages.
Taylor, Peter C.; Graf, Luis A.; Zemajtis, Jerzy Z.; Johansen, Vagn C.; Kosikowski, Ronald L., and Ferraris, Chara F., Identifying Incompatible Combinations of Concrete Materials: Volume III—Additional Appendices, PCA R&D Serial No. SN2897c, Portland Cement Association, Skokie, Illinois, USA, 2008, 89 pages.