Modern concrete mixtures are designed to fulfill many functions on a project. Traditionally, engineers are most concerned with the compressive strength for design purposes and contractors are most concerned with slump for placement reasons. As concrete technology continues to develop, the use of modern concrete mixtures become more intricate and complicated, as other engineering properties are specified and new materials are available to help meet those engineering performance characteristics. Recent developments in chemical admixtures and supplementary cementitious materials (SCMs) can help solve many problems when these new materials are used properly. Sometimes these developments can conflict with one another, or new materials may interact in unpredictable ways, which can place a contractor in unfamiliar territory. To avoid problems with concrete placements, and to take advantage of what new technology may have to offer, it helps to know what to expect when working with modern concrete mixtures and how they impact the concrete contractor.
What makes a mixture “difficult”?
Concrete mixtures should not be difficult! Placement conditions may be challenging, or achieving the desired engineering properties may be difficult, but having the right concrete mixture is the solution, not the problem.
There are many placement conditions that are considered difficult. Long duration placements, like drilled shafts or tunnel liners, are difficult because they may require concrete to remain fluid for a long period of time. Mass placements can be difficult because they require careful consideration of internal temperature rise and temperature differences, and necessitate special treatment like extreme precooling, cooling pipes, and insulation. Sometimes a placement can be very restrictive or congested, requiring self-consolidating concrete (SCC). Special engineering properties, such as shrinkage or cracking limitations, can be difficult to predict without past experience.
In each of these scenarios, knowing how to handle a difficult placement condition with the right admixtures can become an advantage for a contractor. Having the knowledge to handle each type of situation, and knowing the pitfalls of modern concrete mixtures, can lead to strategic advantages and cost savings.
Long placement durations
New chemical admixtures are available to help manage the difficulties of long concrete placements. Hydration stabilizers are classified according to ASTM C 494 as either Type B (retarding) or Type D (water reducing and retarding) admixtures. Even though they are required to meet the same specification as conventional retarders, they differ chemically, adsorbing on the surface of cementitious materials and forming a coating that delays the cement hydration process. They are effective on both the silicate and the aluminate phases of cement hydration, unlike conventional retarders. As such, they often provide more uniform and predictable delay in set and typically do not cause concrete to exhibit the early stiffening and crusting associated with conventional retarders. Originally developed to treat wash water for reuse, they can put concrete “to sleep” permanently at high dosage rates. In the dosage range typical for delaying set (2 to 4 ounces/cwt), they can improve both early and later age compressive strength, often attributed to more complete cement hydration.
Polycarboxylate-based high-range water-reducing agents are now commonly available from most concrete admixture suppliers. When long-term fluidity is needed, the selection of a high-range product that provides slump retention is important. New products are available that provide only slump retention without increasing slump. These products can be used to enhance slump life for long placements. With any new product, trial batch testing is needed to dial in the correct dosage. For long placements where pumpability is a concern, a pumping trial is needed to troubleshoot the mixture proportions. Remember that aggregate gradation and coarse aggregate content, not just the maximum aggregate size, play important roles in pumpability.
Perform early compressive strength tests to determine adequate time for form stripping, especially when using hydration stabilizers. Keep in mind that curing temperature is crucial. Even a 10° F difference in curing temperature can affect early strengths by 50% or more. Using the maturity method for monitoring in place strengths will improve the prediction and allow for faster stripping times. A delay in setting will also change the window for finishing, so planning for the right finishing time and adequate protection of the plastic concrete will avoid problems associated with longer setting.
Mass concrete is defined by ACI as “any volume of concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat from hydration.” These considerations apply to many seemingly typical placements in bridges and buildings, not just dams. Mass concrete placements often require special concrete mixtures to reduce the potential for thermal cracking associated with high temperatures. Using high SCM percentages will reduce temperature rise, and often saves the contractor money by reducing the costly requirements for insulation, cooling pipes, or pre-cooling concrete.
Development of mass concrete requires careful consideration of the available materials. The cement and SCMs that are available should be analyzed for potential compatibility issues. Replacement of 50% to 75% of portland cement is common if the right combination of materials is available. Consideration of early strength is important when using high cement replacement percentages. The effect of temperature on early strength may be more pronounced for high SCM contents, so monitoring with maturity loggers can be especially helpful in avoiding problems with early strength. Selection of clean well-graded aggregates is another key component to development of mass concrete mixtures.
Selection should start with the largest aggregate suitable for the placement based on the spacing requirements. A blend of aggregate sizes that provides the most uniform gradation will reduce the void space between aggregates, which will reduce the necessary amount of cement paste for workability. By reducing cement, you reduce the heat generated during hydration. A mass concrete mixture can also take advantage of modern admixtures such as a polycarboxylate-based high-range water-reducer to obtain flowability without using an excessive amount of cementitious material. Flowable mass concrete with less than 500 pounds/cubic yard of total cementitious material has been successfully developed for many projects around the country. Contractors can realize cost savings not only from the reduction or elimination of cooling pipes, but also from reducing portland cement and faster placement methods such as pumping.
Self-consolidating concrete (SCC)
SCC is high-performance concrete designed to flow into formwork under its own weight, without mechanical agitation or vibration. It has many advantages for projects with tight or congested formwork or rebar, or when it is necessary to speed up the construction process. When specified on a project, there are many possible pitfalls a contractor should avoid. Specifications sometimes avoid the terminology SCC and instead refer to highly flowable concrete. As a result, the specialized testing that is recommended by ACI Committee 237 may be overlooked, leading to mixture development that is incomplete.
The SCC mixture development process can be challenging due to the vast range of properties that are possible with SCC. The selection of materials and admixtures is of primary importance. Avoid problems by seeking aggregates that are clean and well-graded and then proceed with careful trial batching and testing. The testing program should always include a quantitative assessment of segregation, such as ASTM C 1712. Visual observations are useful, but should not be used for acceptance. A laboratory program to assess the robustness (sensitivity to water) is also recommended. Changes can be made to SCC, such as adding SCMs or viscosity-modifying admixture (VMA) that thicken the cement paste and improve cohesiveness while maintaining flowability. Having a robust SCC mixture and using tests for segregation will enable the contractor to avoid problems during construction.
Shrinkage and cracking limits
Many projects have started using shrinkage limitations in specifications to reduce cracking or other problems associated with volume change. The restrained ring shrinkage test (ASTM C 1581) is also sometimes used, which measures the relative cracking probability. Trying to meet specifications with these test methods can be frustrating for contractors because cracking is difficult to predict in concrete. It depends not only on shrinkage, but also on tensile creep relaxation and tensile strength. Selecting the right mixture proportions to meet a shrinkage or cracking specification is not trivial. Cracking in the restrained ring test often occurs due to early age volume changes, which are not measured in traditional test methods, such as ASTM C 157. Early age volume change is problematic in concrete mixtures that have a relatively high cementitious materials content and a low water-to-cementitious materials ratio. Some specifications may have requirements that conflict with early volume change and cracking risk, such as high strength or low chloride ion penetrability to enhance service life. Such requirements often result in concretes with low w/cm and high cementitious content.
When facing a challenge such as early cracking, there are new shrinkage mitigation techniques that should be employed. Shrinkage reducing admixtures (SRAs) have been available since the late 1980s and the latest formulations are fairly predictable and easy to use. SRA reduces the shrinkage of concrete by modifying the surface tension of the cement pore solution. By suppressing the driving force for shrinkage, the measured shrinkage is often reduced by 50% or more. These products can be relatively expensive, but may help reduce overall cost by reducing the time needed for extensive laboratory qualification testing or repair of cracks. A potential drawback can be their effect on the air content, so trial batch testing should be conducted to ensure that a sufficient air-void system develops.
Another shrinkage mitigation strategy developed more recently is the use of saturated lightweight aggregate (SLA) fines for internal curing. SLA is typically added in small amounts, making up approximately 6% to 10% of the concrete mixture by volume. A newly approved ASTM standard specification, ASTM C 1761, establishes the criteria for the properties of the lightweight aggregate product. The small addition of lightweight material does little to change the overall density or porosity of the concrete, but it will provide a reservoir of water that is available for hydration. In particular, low w/cm concrete that experiences self-desiccation will benefit from this additional water source. Early shrinkage is reduced due to the presence of curing water and over time, strength and durability are enhanced due to the additional hydration of cement.
Avoid problems with:
- Long duration placements by using the latest in admixture technology to delay setting or enhance slump retention.
- Mass concrete by using high SCM replacement levels, well-graded aggregates, and trial batch testing for early strength.
- SCC by using well graded aggregates, a careful mixture development process, and thorough testing including laboratory robustness and field quality.
- Shrinkage and cracking limitations by using shrinkage mitigation strategies such as shrinkage reducing admixtures or saturated lightweight aggregates.
Matthew D’Ambrosia, Ph.D., PE, is a materials consulting engineer at CTL Group where he specializes in solving difficult problems for the construction industry. He has been a member of ACI for almost 15 years and serves as a member of several committees.