Some concretes exhibit undesirable characteris- tics because of incompatibility among different concrete materials. One definition of an incom- patibility is when combinations of acceptable materials (all materials individually meet the specifications) interact in an undesirable or un- expected way. Undesirable characteristics are:
1. Early loss of workability (early stiffening), 2. Delayed set (retardation),
3. Early-age cracking due to excessive au- togenous and drying shrinkage of con- crete, and
4. Lack of a proper air-void system.
Chapter 7 – Concrete Mixture
These incompatibility-related problems affect construction productivity and long-term pave- ment performance. As concrete mixtures be- come more complex with the use of SCMs and combinations of chemical admixtures, the likeli- hood of incompatibility among cementitious ma- terials and admixtures increases with the number of ingredients added to the mixture. The problem is compounded because:
1. Factors that result in incompatibility among various cementitious materials and admixtures are not well known.
2. Material incompatibility may be induced by temperature changes. Therefore, trial batches need to be tested at the ex- tremes of temperature anticipated at the project site.
Steps to minimize incompatibility problems in- clude the following:
1. All admixtures used on the project need to be from the same manufacturer to en- sure compatibility among them. Do not exceed the recommended dosages.
2. Ensure all cementitious materials meet project specifications and/or the require- ments of appropriate ASTM standards.
PAVING ALERT
The Case of Class C Fly Ash:
Certain Class C fly ash sources that conform to ASTM standards may be detrimental to the performance of the concrete, causing premature stiffening of the fresh concrete, thermal cracking, and/or reduced sul- fate resistance. Class C fly ash is generally not effective in controlling expansions due to ASR. The po- tential for early stiffening when combined with specific cement sources, or in the presence of certain water reducers and in hot weather, can be verified as follows:
• Test the concrete by making trial batches at the highest temperature anticipated.
• Verify that slump loss is not too rapid for the conditions of the job and that setting times are accept- able.
• If the concrete loses slump too rapidly, consider reducing the dosage of fly ash, using a different fly ash, using a different cement, or using a different water reducer.
This early stiffening is a classic material incompatibility related to the sulfate and aluminate balance in the cementitious materials. If the CaO content of the fly ash is greater than 10% as indicated by mill cer- tifications provided by the fly ash supplier, incompatibility testing should be performed on the proposed mixture to determine if C3A contributed by the fly ash will result in acceptable workability properties.
In addition to material incompatibilities, research has shown that some SCMs can retard expansion for a brief time before the expansion accelerates in the standard ASTM C1567 protocol. Some SCMs (e.g., Class C fly ash) can actually exacerbate expansion when used at typical dosage levels (i.e., 25% mass replacement of cement). Therefore, it is recommended that Class C fly ash not be used for airfield paving project where potentially reactive aggregates may be used or the pavement will be exposed to pavement deicing chemicals.
It is also advisable to have hot and cool weather mixture designs in locations where seasonal dif- ferences in temperature are usually significant.
Concrete mixture designs proposed for use on airfield projects should be evaluated for potential incompatibilities. Detailed guidance for incom- patibility testing can be found in FHWA’s Report FHWA-HRT-06-080, “Identifying Incompatible Combinations of Concrete Materials: Volume II—Test Protocol”and the National Concrete Pavement Technology (CP Tech) Center’s,
“Testing Guide for Implementing Concrete Paving Quality Control Procedures.” A summary of suggested incompatibility testing strategies in- cludes the following procedures:
• Preliminary combined chemical analysis based on mill certifications of the cementi- tious material sources – recommended guidelines can be found in both referenced documents.
• Foam drainage testing to evaluate air void stability.
QUALITY ALERT
Incompatibility Manifestations:
Early Stiffening of Concrete– Early stiffening occurs when there are not enough sulfates in solution at the right time to control the hydration of the aluminates. The early stiffening leads to a loss of workability, as indicated by a loss of slump. Workability loss leads to difficulties in concrete placement and consoli- dation. The tendency to stiffen early may be attributed not only to the individual cementitious materials, but also to interactions among the various cementitious materials and the chemical admixtures and am- bient temperatures.
Problematic combinations often include cements with relatively low sulfate contents or with sulfates available only in forms that are not readily soluble. While such cements may perform satisfactorily alone, they may be prone to early stiffening if used with water-reducing admixtures containing lignosulfonates and/or triethanolamine (TEA). Combinations of such cements with Class C fly ashes having high alu- mina contents may also result in early stiffening. In hot weather these effects are more pronounced.
Retarded Concrete– Although not a common phenomenon, some paving projects occasionally experi- ence concrete setting problems. At these projects, setting may be delayed by a few hours to more than 12 hours. A consequence of this problem is the inability to perform joint sawing in a timely manner, lead- ing to uncontrolled cracking.
Concrete Shrinkage– Premature cracking in concrete can be caused by a host of factors. Shrinkage can occur in the fresh (plastic) or hardened concrete. Plastic shrinkage cracking results as water rapidly evaporates from the surface of the fresh concrete. Cracking may also occur somewhat later in the life of the pavement due to excessive autogenous and drying shrinkage.
Concrete Air Void System– Problems related to the use of certain air-entraining agents include:
1. Accumulations of air voids around the aggregate particles, leading to strength loss.
2. A poor quality air void system that affects long-term freeze-thaw durability of the hardened con- crete.
Chapter 7 – Concrete Mixture
• Stiffening (ASTM C359) at differing tem- peratures.
• Temperature development curves of the mixture.