In producing economical concretes to meet exacting performance requirements, complex mixes have been designed that combine a wider range of cements, supplementary
In producing economical concretes to meet exacting performance requirements, complex mixes have been designed that combine a wider range of cements, supplementary cementitious materials (SCM), and chemical admixtures than ever before. As a result, constituent materials sometimes interact in unexpected ways that adversely affect setting time, workability, and strength development. Premature stiffening, excessive cracking, and poor air-void systems are common conditions arising from incompatibility among mix components.
Multiple complications are related to hydration, the series of nonreversible chemical reactions with water by which hydraulic cementitious systems stiffen, set and harden. Of the two aluminate compounds contained in portland cement, C3A and C4AF, the latter does not contribute significantly to system performance, while C3A reacts rapidly when mixed with water and generates a large amount of heat, unless the reaction is controlled by the presence of sulfate. An uncontrolled reaction of C3A with water due to insufficient sulfate in solution for the amount of C3A involved may lead to flash (or permanent) set.
Calcium sulfate is added to cement as gypsum (CSH2) during grinding to control the initial C3A reaction. During grinding, some gypsum is dehydrated to form plaster (CSH1/2). Though the amount of dehydration is typically controlled by the mill to provide optimum cement performance, an incorrect amount of dehydration can result in false (temporary) set. Moreover, use of a fly ash containing C3A may cause flash set or rapid stiffening due to insufficient sulfate to control its hydration. Some Type A water-reducing admixtures also may influence the balance between C3A and sulfates, as they tend to accelerate C3A hydration. Likewise, increasing temperatures accelerate chemical reactions and heighten the risk of uncontrolled stiffening if marginally balanced materials are in use. Other contributors to potential risks are finely ground cements, high alkali content in the system, and low water-to-cementitious materials ratios.
Additionally, a product of silicate (C2S and C3S) hydration in cement is calcium silicate hydrate (CSH) Û the primary contributor to concrete strength, durability, and heat of hydration. Starting two to four hours after mixing as calcium reaches supersaturation in the mix solution, silicate reactions lead to setting and strength gain. If too much calcium has been consumed during earlier, uncontrolled C3A reactions, then setting may be delayed. Potentially adding to the delay as well, Type A water-reducing admixtures that accelerate C3A reactions may retard silicate reactions. Low temperatures also will slow the hydration process. Thus, both accelerated C3A (uncontrolled stiffening) and delayed silicate reactions (delayed setting) can occur in the same mix.
To prevent such problems before construction begins, Skokie, Ill.-based engineering and construction technology consultant CTLGroup developed a seven-test protocol based on its recently completed Federal Highway Administration-funded study of material incompatibility issues. Research kicked off in the late 1990s with several goals:
- Gain a clearer understanding of the chemistry of reactive materials in concrete
- Develop a preconstruction laboratory testing regimen to 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 on site
Project Manager Peter Taylor, a CTLGroup principal engineer and manager of materials consulting, reports that concrete mix material compatibility issues began to surface in the 1990s, as some mix designs incorporated more components with new chemical properties. The research into materials incompatibility has implications for all concrete applications, he contends, but the problems we’re addressing have been most apparent in pavement projects, which are extremely sensitive to the workability and stiffness of the mixture.
Often, concrete mixtures that contain incompatible materials present serious difficulties during placement and finishing, and yet meet strength requirements, Taylor adds. In one extreme example, a concrete containing cement, fly ash, blast furnace slag, and two chemical admixtures was to be placed on a 90-degree day. The mixture stiffened to zero-slump before mixing was complete, but then didn’t set for three days. Even so, its 28-day strength was normal.
Research results demonstrated that properties of both plastic and hardened concrete can be changed by even slight adjustments in mix design or environmental conditions. The preconstruction test protocol allows for evaluation of this sensitivity in a particular mixture, facilitating the selection in advance of alternative materials or action plans to be implemented if such changes are observed in the field.
Suppliers of cement, SCMs, and chemical admixtures can use the protocol to test their products in various combinations and conditions in order to establish or confirm their compatibility and performance characteristics. For producers and contractors, the protocol provides tools to help prevent the occurrence of problems as well as guidance on alleviating those that do occur.
Developed to provide as much information as possible during the preconstruction phase, the protocol includes calibration of more sensitive central laboratory tests with equivalent field tests using materials to be employed on site and under environmental conditions likely to be encountered in the field. 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 potential incompatibility. Making up the protocol are the following tests: foam index, foam drainage, unit weight, slump loss, semiadiabatic temperature monitoring, setting time, and chemistry of reactive materials.