The correct proportioning of the raw materials is critical to the production of quality roller-compacted concrete (RCC) mixes. The mix design process should not be approached as one of trial and error, but rather a systematic procedure based on the aggregates, water, and cementitious materials used in the mix. This knowledge of the ingredients is coupled with the construction requirements and specifications for the intended project in order to ensure a roller-compacted concrete (RCC) mix that meets the design and performance objectives.
There currently exists several methods for proportioning roller-compacted concrete mixes for pavements; however, there is not one commonly accepted method. The main RCC proportioning methods include those based on concrete consistency testing, the solid suspension model, the optimal paste volume method, and soil compaction testing. Whichever method is employed, the goal is to produce an roller-compacted concrete mixture that has sufficient paste volume to coat the aggregates in the mix and to fill in the voids between them.
Regardless of which proportioning method is used, it is important that a roller-compacted concrete mixture meet the following requirements:
the fine and coarse aggregates should be chosen to achieve the required density and to provide for a smooth, tight surface the moisture content should be such that the mix is dry enough to support the weight of a vibratory roller yet wet enough to ensure an even distribution of the cement paste the cementitious materials used should meet the required design strength requirements at minimal cost.
Because Roller-Compacted Concrete (RCC) uses aggregate sizes often found in conventional concrete, a Ready Mixed Concrete (RMC) producer will probably discover the necessary coarse and fine aggregates for RCC already stored in existing bins or stockpiles. However, the blending of aggregates will be different than what the producer is used to with conventional concrete.
Coarse aggregates consist of crushed or uncrushed gravel or crushed stone while the fine aggregates consist of natural sand, manufactured sand, or a combination of the two. Crushed aggregates typically work better in roller-compacted concrete mixes due to the sharp interlocking edges of the particles, which help to reduce segregation, provide higher strengths, and better aggregate interlock at joints and cracks. Because approximately 80 percent of the volume of a high-quality roller-compacted concrete mix is comprised of coarse and fine aggregates, they should be evaluated as to their durability through standard physical property testing such as those outlined in ASTM C 33.
The American Concrete Institute (ACI) has established aggregate gradation limits that have produced quality roller-compacted concrete pavement mixtures. These ACI gradation limits effectively allow the use of blends of standard size stone, most commonly #67’s, #7’s, #8’s, and #89’s, along with sand, to be used in roller-compacted concrete pavement mixes.
Both ACI and the Portland Cement Association (PCA) recommend the use of dense, well-graded blends with a nominal maximum size aggregate (NMSA) not to exceed ¾ inch in order to help minimize segregation and produce a smooth finished surface. Gap-graded mixes that are dominated by two or three aggregate sizes are not desirable for roller-compacted concrete. Additionally, the recommended gradation calls for a content of fine particles (two to eight percent passing the #200 (75 µm) sieve) that is typically higher than that of conventional concrete. This eliminates the need for washed aggregates in many cases and produces a mix that is stable during rolling.
In cases where washed aggregates are being used, it may be difficult to meet the specification for to two to eight percent fine particles. In cases like this, fly ash can be added to the mix to provide the desired fines content. These fines provide lubrication that helps to distribute the paste throughout the mix. However, these fines need to be non-plastic with their Plasticity Index (PI) not to exceed four.
In many cases, aggregates used in typical highway construction will also meet the roller-compacted concrete gradation requirements mentioned above. Graded aggregate base material, crusher run material, and aggregates for Hot-Mix Asphalt (HMA) paving mixes can be used with little or no modification in roller-compacted concrete mixes.
Roller-compacted concrete (RCC) is a zero-slump concrete that has been used successfully for more than 30 years in all types of climates. Pavement applications vary from heavy-duty intermodal yards, to streets and local roads, and commercial parking areas. Among the many advantages of roller-compacted concrete is its ability to resist frost attack and deicer salt-scaling.
Several studies have been conducted that confirm the ability of roller-compacted concrete to resist damage due to freeze-thaw conditions and deicer salts. In addition to laboratory testing, condition surveys have been made of existing roller-compacted concrete pavement projects, many located in harsh freeze-thaw climates, to evaluate the long-term performance of RCC. On the basis of these past studies, it can be stated that the construction of a frost- and deicer salt-scaling–resistant RCC pavement is common. As with any pavement, good construction practices (including sufficient compaction and proper curing) are required, and the use of supplementary cementitious materials also appears to provide benefit.
Good quality conventional concrete can be quite resistant to frost and deicer salt-scaling if it is properly air-entrained. Interestingly, even though RCC has a very low water content and paste content, making it difficult to entrain air uniformly throughout the mixture, both laboratory and field studies have shown acceptable performance of non-air entrained RCC when exposed to freezing-thawing and deicer salts. Where air entrainment has been attempted, results have showed success using a high energy mixer and a higher dosage of air-entraining admixture.
Measurements made on samples extracted from roller-compacted concrete field sections indicate that it is possible to roller-compacted concrete pavement with adequate portland cement content, that is well-mixed, placed to the specified density, and properly cured, appears to be very resistant to the effects of freezing and thawing, and deicing salt.
Similar to conventional concrete, RCC must be consolidated or in the case of roller-compacted concrete (RCC), compacted in order to achieve the desired performance characteristics. The degree of compaction of RCC has a direct role on its ultimate strength and durability. Because of roller-compacted concrete’s very dry consistency and reduced workability, adequate compaction of roller-compacted concrete can be more difficult to achieve than with conventional concrete. Compaction of RCC depends upon many variables including materials used, mixture proportions, mixing, transporting and placement methods used, compaction equipment, lift thickness, and time of compaction.
The best performance characteristics are obtained when the roller-compacted concrete is reasonably free of segregation and compacted throughout the entire lift at, or close to, maximum density. Research studies including the one below have shown that the strength of RCC drops appreciably as the density drops.
In obtaining the specified density, it’s important to recognize that delays in compaction, segregation of material, inadequate compaction equipment, too thick of lifts, and insufficient water in the mixture are some of the issues that may lead to reduced density and subsequent strength loss. On the other hand, over-compaction can lead to a weaken roller-compacted concrete surface by loosening of the material directly under the roller.
The recommended approach is to determine an optimum rolling pattern that will result in the specified minimum density being met in the least amount of time and passes. This can be determined either during a trial placement or early in the construction process. Additional rolling that will not result in an increased density should be avoided.
For occasional passenger cars traveling short distances, as soon as rolling is complete or final density is achieved and a curing compound has been applied.
For traffic beyond occasional cars, as soon as RCC reaches adequate strength, typically between 2,000 and 2,500 psi.
Roller-compacted concrete has enough load carrying capacity to support occasional light vehicle traffic (such as a car entering or leaving a driveway) immediately following placement. This load carrying capacity is due to the compaction process, which creates friction between the confined particles (aggregate interlock) of the relatively dry mixture and allows for the occasional light vehicle to be placed on the roller-compacted concrete without damaging or disrupting the in-place material. However, traffic beyond the occasional light vehicle is not recommended until the RCC has achieved adequate compressive strength—typically between 2,000 and 2,500 psi.
The question then becomes: how long would it take roller-compacted concrete to reach this compressive strength? Similar to conventional concrete, roller-compacted concrete (RCC) mixtures can be designed to achieve high early strength or to gain strength at a normal pace until a compressive strength of about 2,500 psi is reached at about seven days. Designers are encouraged to consider the requirements for each application and design an economical roller-compacted concrete mixture to meet those requirements. Once the aggregates for the project are selected, mix design parameters affecting strength gain profile of RCC include type and quantity of cement (and supplementary cementitious materials, if used), and admixtures, if used.
For instance, a pavement for a parking lot at an automotive manufacturing plant may require opening for traffic a few weeks after construction. Whereas, a roller-compacted concrete pavement built to replace a failed pavement on a major route may require reaching 2,500 psi compressive strength and be opened for traffic within one or two days from placement. Different mixes of roller-compacted concrete can be designed to meet the different requirements of the two applications. It is therefore recommended that users determine how soon the pavement needs to be opened for traffic, and then design the most cost effective pavement section and roller-compacted concrete mixture meeting the requirement. In North America, RCC pavements have been typically opened for traffic as early as one day and as late as several weeks from placement, depending on the application.
Credit: Portland Cement Association