The densely graded aggregates used in Roller Compacted Concrete mixtures help the concrete achieve high levels of compressive strength. The low w/cm of Roller Compacted Concrete mixtures produces a low-porosity cement matrix that also contributes to the high compressive strength of the concrete. Every mixture proportion has an optimum moisture content at which it achieves the maximum dry density.
This density most often provides the maximum strength. The compressive strength of Roller Compacted Concrete is comparable to that of conventional concrete, typically ranging from 4,000 to 6,000 psi. Some projects have reached compressive strengths higher than 7,000 psi; however, practical construction and cost considerations would likely specify increased thickness rather than strengths of this nature.
photo courtesy of Andale Ready Mix Central/Andale Paving, Inc.
Flexural strength is directly related to the density and compressive strength of the concrete mixture. In properly constructed Roller Compacted Concrete pavements, the aggregates are densely packed and minimize the development of fatigue cracking. The density of the paste and the strength of its bond to the aggregate particles are high due to its low w/cm ratio. As a result, the flexural strength of Roller Compacted Concrete, depending on the mix design, is generally high—ranging from 500 to 1,000 psi.
Based on beams and cores obtained from a test section, the relationship between the compressive and flexural strengths of Roller Compacted Concrete appears to be similar to that for conventional concrete and can be represented by the equation below:
fr = C fc
fr = flexural strength (third-point loading), psi
fc = compressive strength, psi
C = a constant between 9 and 11 depending on actual Roller Compacted Concrete mix
Modulus of Elasticity
Modulus of elasticity represents a material’s propensity to undergo reversible elastic deformation in response to a stress. Limited tests on Roller Compacted Concrete cores obtained from a full-scale test section indicate that Roller Compacted Concrete modulus of elasticity values are similar to or slightly higher than those of conventional concrete when the mixes have similar cement contents.
The modulus of elasticity expresses the ratio between the applied stress and strain, as shown below:
E = modulus of elasticity (psi)
σ = stress (psi)
ε = strain (in./in.)
Fatigue failure occurs when a material is subjected to repeated stresses. While the stress caused by a single load is not greater than the strength of the material (and therefore will not cause the material to fail), repetition of these loads will wear on the material over time and eventually result in fatigue failure.
Because the critical stresses in Roller Compacted Concrete pavements are flexural, fatigue due to flexural stress is used for thickness design. Stress ratio, as used in fatigue relationships, is the ratio of flexural stress to flexural strength.
Stress Ratio =
Although testing to evaluate the fatigue behavior of Roller Compacted Concrete has been limited, results indicate that the fatigue behavior of Roller Compacted Concrete is similar to that of conventional concrete.
Roller Compacted Concrete pavements have been successfully used for many years in northern climates where they are frequently subjected to freeze-thaw conditions. Studies in the US and Canada have shown that Roller Compacted Concrete pavements have performed well for over 30 years. Much of Roller Compacted Concrete’s freeze-thaw durability can be attributed to its low moisture content and high density.
The following factors should be taken into consideration when designing an Roller Compacted Concrete pavement for freeze-thaw resistance:
Material selection for freeze-thaw durable Roller Compacted Concrete is similar to that for conventional concrete.
Selection of a dense, well-graded aggregate ensures optimum compaction and maximum density in the field.
Adequate cement content is necessary to provide sufficient strength. Experience has shown that maintaining a w/cm ratio below 0.40 plays a key role in assuring necessary strength and reduced permeability.
Proper compaction of Roller Compacted Concrete, targeting highest possible density, provides the greatest potential for high strength and low permeability.
Proper curing is critical to Roller Compacted Concrete’s freeze-thaw resistance.
Inclusion of air-entraining admixtures can have a positive effect on Roller Compacted Concrete’s freeze-thaw durability. Tests have shown that very little entrained air is necessary for adequate protection against frost-induced microcracking and deicing salt scaling. It appears that good air void spacing, rather than actual air content, is a larger factor in Roller Compacted Concrete freeze-thaw resistance.
It is important to note that different air-entraining admixtures will require different mixing energies and times to establish a sufficiently stable air void system. Therefore compatibility of the admixture with the mixer type is essential. Additionally, due to the lower water and cement contents of the Roller Compacted Concrete mix, the required dosage of air-entraining admixture may be larger for Roller Compacted Concrete than for conventional concrete.
It should also be noted that non-air entrained Roller Compacted Concrete pavements have been shown to experience excellent freeze-thaw resistance, provided those factors listed above – sound aggregates, adequate cement content, sufficient compaction and proper curing – are incorporated into the design and construction process.
The permeability of Roller Compacted Concrete pavement is largely dependent on the voids in the compacted Roller Compacted Concrete and the porosity of the mortar matrix. Therefore, it is most affected by mixture proportioning, method of placement and degree of compaction. Permeability of hardened Roller Compacted Concrete is comparable to that of conventional, though it may be slightly more permeable due to entrapped voids.