Getting it together: A new research project is working on procedures for identifying compatible combinations of concrete materials - Brief Article

Public Roads, July-August, 2002 by Shiraz D. Tayabji

Properly designed and constructed concrete pavements can provide 20 to 25 years of initial service life without significant maintenance. In recent years, however, cases of early-age problems and premature deterioration have resulted from use of incompatible materials. Highway professionals have noted instances of early loss of workability (early stiffening), delayed set (retardation), early-age cracking due to excessive shrinkage, and lack of proper air voids. These problems affect long term performance and even construction productivity.

As concrete mix designs become more complex, the likelihood of incompatibility among materials increases with the number of ingredients added to the mix. The problem is compounded because not much is known about the factors that lead to incompatibility, and tests are lacking to determine the susceptibility of materials combinations to distress mechanisms.

Construction Technology Laboratories, Inc., was tasked with developing reliable tests to identify incompatible combinations that adversely affect fresh and hardened concrete at early ages. The materials include cement, supplementary cementitious materials, chemical admixtures, and aggregates.

"The project is intended to develop practical tests and tools for concrete producers, contractors, and State department of transportation personnel to use in mixture evaluation for adequate air entrainment, setting characteristics, and early-age cracking," says Dr. Colin Lobo, vice president of engineering at The National Ready Mixed Concrete Association and task leader for the project.

Because of the complexity of concrete, predicting incompatibility from tests of the individual materials is impossible. Combinations must be tested using methods that simulate conditions at job sites. Once test procedures are refined, the research team for the incompatibility of concrete materials project will develop protocols and guidelines for evaluation of materials for specific jobs. Some of these protocols are already established standards, while others are in the advanced stages of development.

The results will enable concrete producers and users to have greater confidence in the use of portland cement concrete for pavements and avoid use of marginal concrete for paving. The materials can be prequalified in laboratories to prevent incompatibility problems in the field, costly errors, and construction delays.

Early Stiffening

Loss of workability may be due to false set or early stiffening. Excessive calcium sulfate in the form of plaster in the cement results in false set whereas the uncontrolled early hydration of alluminates in the cementitious materials may cause early stiffening. Continued mixing of the concrete can overcome false set, whereas early stiffening is not reversible. Standard test methods for early stiffening of portland cement often are unreliable and do not incorporate consideration of supplementary cementing materials or chemical admixtures. The poor reliability of the test methods has been attributed to the mixing intensity specified in these test methods.

Early stiffening in the cement phase of concrete leads to loss of workability, as indicated by loss of slump. Loss of workability leads to difficulties in concrete placement and consolidation. When concrete is hard to place, contractors may add additional water, reducing both strength and durability and increasing the potential for shrinkage and cracking. The addition of admixtures (such as high-range water reducers) improves workability without these negative effects but adds to the cost of the concrete. High-range water reducers may retard setting, depending on the amount used. The tendency to early stiffening may be attributed not only to the individual cementitious materials, but also to interactions among the various cementitious materials and the chemical admixtures.

Retarded Concrete

From time to time, some paving projects experience problems with concrete setting being delayed by a few hours to more than 12. Although retardation is not a common phenomenon, the consequence is the inability to perform joint sawing in a timely manner, leading to unplanned cracking.

A common cause for delayed set is the incompatibility between the water reducer and cementitious materials, compounded frequently by cool weather conditions during placement of the concrete. The effect of the admixture dosage on cement hydration for a given placement temperature needs to be considered during the mix design. The admixture dosage needs to be optimized, and any excess that can lead to extended setting and poor development of early-age strength should be avoided.

Setting is identified with a certain stage in the development of the coagulational-crystallization network, while the process of hardening denotes the development of the much stronger and irreversible crystalline structure. Retarders, such as gypsum, and surface active agents, such as calcium lignosulphanate, influence the rate of formation of the coagulational structure and the speed and form of the crystallization of the alluminate hydration products. The retarders may combine with certain cement constituents to form insoluble metal organic complexes, which coat the cement grains and retard hydration.