Roller-Compacted Concrete (RCC) FAQs

RCC FAQs

• Are sawed contraction joints needed for roller-compacted concrete pavements?

  Sawed contraction joints for controlling random cracking are not required in roller-compacted concrete (RCC) pavements due to the fact that  roller-compacted concrete has lower water and cement contents than conventional concrete, and thus shrinkage is reduced. Random shrinkage cracks typically occur at 20- to 60-foot intervals, depending on the RCC properties and pavement thickness.

  The primary reason for sawing joints in  roller-compacted concrete pavements is to reduce or prevent random cracking. On certain projects where efficiency of aggregate interlock or increased load transfer at the joints is critical, joints are sawed to minimize crack openings through reduced saw joint spacing, compared to longer spacing of random cracks. Widely spaced random cracks can have wider then desired crack width. The improved aggregate interlock increases load transfer across the joint. Furthermore, reducing random cracking is sometimes desired for aesthetic reasons.

  When used, transverse sawed joints are typically spaced at intervals of 15 to 20 feet for pavements less than eight inches thick, and three to four times (in feet) the pavement thickness (in inches) for pavements eight inches thick or greater.

  

  Cutting joint with early-entry saw

   

  Because the longitudinal loading of RCC pavements is different than the transverse loading and causes more of a hinge action, the spacing of longitudinal joints is typically smaller than the spacing of transverse joints. For large paved surfaces, such as industrial sites, a square jointing pattern is preferred. The spacing is normally 15 to 20 feet for pavements less than eight inches and two and a half times the pavement thickness (in feet) for pavements eight inches thick or greater.

  As with conventional concrete, the timing of saw cuts is based on the prevention of raveling and random cracking. Sawing should begin as soon as the concrete is hard enough to withstand spalling damage caused by sawing operations. For increased load transfer through aggregate interlock, the depth of the saw cuts should not exceed 1/4 of the pavement depth. Thin early-entry saws are being used more frequently because of the speed and convenience they offer. Sawing can begin within one to four hours after final compaction. The sawcut depth for early-entry sawing ranges from one to 1.25 inches regardless of the pavement thickness.

• Do you need to cure roller-compacted concrete?

  As with conventional concrete, curing is very important for roller-compacted concrete (RCC). However, RCC has no bleed water, so the main concern is drying. At least three negative things will happen if RCC is allowed to dry: 1) the concrete will experience drying shrinkage which will lead to cracking, 2) the cement will not continue to hydrate which will result in lower strengths and less durability, especially at the surface, and 3) dusting of the surface is more prevalent. 
  
  To keep roller-compacted concrete from drying the surface should be kept moist for seven days, or until a curing compound is applied. The surface should be gently moistened with water from the time compaction is complete. Curing compounds conforming to ASTM C 309 which are used for conventional concrete can be used for RCC. However, because RCC has a more open texture surface than conventional concrete, the curing compound application rates are 1.5 to two times the application rates used for conventional concrete. (See figure) It is good practice to apply the curing compound in two coats with the second coat applied perpendicular to the first.

  Other curing techniques such as plastic sheeting and wet burlap are not commonly used for roller-compacted concrete pavements because of the large coverage area; however, for small areas these methods have proved successful. If the RCC is going to be surfaced with asphalt, a bituminous prime coat will also serve as a good curing compound to seal in the moisture. Before placing the RCC it is also important to moisten the base or subgrade material immediately beneath the concrete so that moisture from the concrete is not drawn into the subgrade.

• Does an asphalt surface contribute structurally to roller-compacted concrete (RCC)?

  Historically, asphalt surfacing that has been placed on roller-compacted concrete (RCC) has not provided any structural contribution in the design of such a pavement structure. For one it is difficult to quantify the asphalt surface contribution through the use of traditional pavement design methods such as the AASHTO 1993 “Guide for the Design of Pavement Structures”. Roller-compacted concrete  is not a pavement even considered in the ’93 Guide. Secondly, the robust structural nature of roller-compacted concrete lends itself to carrying the bulk of the load even when an asphalt surface is applied. With this said, it is possible to quantify the minimal contribution of asphalt surfacing by mechanistic-based methods.
  
  Knowing that roller-compacted concrete thickness is primarily based on a fatigue cracking model that is controlled by the stress ratio within the RCC slab, one can use mechanistic methods to predict the asphalt surface contribution. The stress ratio is defined as:
  
  

   

  The modulus of rupture is a known RCC material property. By the use of layered elastic analysis or finite element methods one can predict the critical tensile stress at the bottom of theroller-compacted concrete slab. Asphalt surfacing can be introduced as a layer in one of the methods and essentially its contribution to the structure can be quantified using accepted assumptions. An example of using the layered elastic method can be found in PCA publication, Thickness Design of a Roller-Compacted Concrete Composite Pavement System, PL633.

• How does hot weather affect the construction of roller-compacted concrete pavements?

  There are two factors that should be considered when evaluating hot-weather construction of roller-compacted concrete (RCC) pavements: ultimate strength and workability.

  Ultimate Strength

  The optimal curing temperature for concrete is from 50 to 70 degrees Fahrenheit. When concrete is cured at temperatures above 80 degrees Fahrenheit the early strengths (one, three, seven days) are higher than concrete cured at normal temperatures. However, ultimate strength is reduced. Concrete cured at 90 to 105 degrees Fahrenheit will see 28-day strengths reduced five to 15 percent, respectively (Refs 1,2), compared to curing at 73 degrees Fahrenheit. 
  These strength reductions are related to the temperature of curing, not the temperature at placement. With roller-compacted concrete pavements there is a large surface area compared to the concrete thickness, so heat of hydration is not a significant concern. However, the higher placement temperatures will increase evaporative losses, and with the very dry consistency of roller-compacted concrete rapid surface drying and subsequent surface dusting can be an issue during hot weather placement. The use of water curing to keep the roller-compacted concrete surface moist will help to reduce evaporative losses and ensure a strong, durable surface, in addition to reducing the curing temperature.

  Construction specifications for roller-compacted concrete dams often require that the concrete mix temperature not exceed 80 degrees Fahrenheit (Ref 3). This is to reduce the chance that cracking might occur because of the difference in temperature between the concrete and the ambient air during curing. Methods for reducing the concrete temperature for mass concrete placement include using chilled water, ice chips, cooled aggregate, night placement and liquid nitrogen in extreme cases. The problem with relying on chilled water to cool the RCC is that, unlike conventional concrete, there is generally insufficient water in the mix to make a significant impact on lowering the concrete temperature.

  Since heat of hydration is not a concern with RCC pavements, a better approach to reduce the temperature of the concrete mix is to cool the coarse aggregate either by shading the aggregate piles or sprinkling the piles with water. The water sprinkling approach also aids in the mixing operation by providing moist aggregate which helps assure a more uniform, consistent mixture.

  Workability

  Hot temperatures will make the concrete less workable and more difficult to place and compact, resulting in a poorer quality final product. High temperatures lead to higher rates of moisture evaporation, which is very important to monitor with RCC because there is so little moisture in the concrete. As temperatures increase from 70 to 90 degrees Fahrenheit, the time to initial set and final set are reduced by 20 to 30 percent (Ref 4).

  When placing roller-compacted concrete during hot weather, it will be to the contractor’s advantage to keep the concrete as cool as possible during placement and compaction. As ambient air temperature increases beyond 90 degrees Fahrenheit, the time allowed from time of mixing to completion of compaction should be reduced accordingly (for example, from 60 minutes to 30 to 45 minutes). To compensate for moisture loss during hauling and placement, additional mix water can be added at the plant. For long haul times, consideration should be given to the use of hydration-stabilizing admixtures to provide more workability time.
   

  References

  1. Burg, Ronald G., The Influence of Casting and Curing Temperature on the Properties of Fresh and Hardened Concrete, Research and Development Bulletin, RD113, Portland Cement Association, 1996

  2. Klieger, Paul, Effect of Mixing and Curing Temperature of Concrete Strength, Research Department Bulletin, RX103, Portland Cement Association, 1958.

  3. Verbeck, George J., and Helmuth, R. A., “Structures and Physical Properties of Cement Pastes,”Proceedings, Fifth International Symposium on the Chemistry of Cement, Vol III, The Cement Association of Japan, Tokyo, 1968.

  4. Compacted Concrete for Embankment Armoring and Spillway Projects, EB124, Portland Cement Association Publication, 2000.
     
     

  5. Report on Roller-Compacted Mass Concrete, American Concrete Institute, 2011.

• How soon can an RCC pavement be opened for traffic?

  1. 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.
  2. For traffic beyond occasional cars, as soon as RCC reaches adequate strength, typically between 2,000 and 2,500 psi.

  

  RCC pavement on US-78 in Aiken, South Carolina.

   

  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. 
  
  

  

  RCC pavement on US-78 in Aiken, SC was opened for traffic one to two days after placement

  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.

• I know that roller-compacted concrete uses the same basic ingredients as conventional concrete – aggregates, water, and cement – but are they mixed together in the same proportions?

  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

• Is roller-compacted concrete durable in freeze-thaw environments?

  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.

  

   

  RCC for Snow Storage Pad in Denver, Colorado.airport

   

  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 conRroller-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.
  
  For more information on the frost durability of RCC pavements, please refer to PCA publications Frost Durability of Roller-Compacted Pavements and Frost Durability of Roller-Compacted Concrete Pavements:Research Synopsis.

  For more information on the factors affecting durability of concrete pavements, their deterioration mechanisms, and their mitigation options, please reference Chapter 11 of the new 15th edition of PCA's Design and Control of Concrete Mixtures, EB001.

• What is the proper joint spacing for roller-compacted concrete pavements?

  There have been many roller-compacted concrete (RCC) pavement projects that have been built without contraction joints and have performed satisfactorily. In particular, industrial pavements where appearance is not a major concern and the design incorporates the random cracking, contraction joints have been eliminated for economic reasons. Random cracks will typically form from about 30 feet  to more than 60 feet apart. Since it is impractical to install dowel bars in RCC, the load transfer is provided through aggregate interlock. Crack openings that form randomly at long spacings tend to be wide and consequently will provide less aggregate interlock than closely spaced cracks.

  For projects where crack control is important, contraction joints are used to control the location of cracking in the concrete. Joint spacing should follow similar concepts as used for conventional concrete pavement. Because roller-compacted concrete has less shrinkage than conventional concrete, the control joints can be spaced further apart. Proper joint spacing depends on pavement thickness, concrete and subbase properties, aggregate type and climatic conditions.

  Current practice for roller-compacted concrete pavements is to space control joints from 20 to 30 feet. As a rule of thumb, transverse joint spacing should be about 40 times the pavement thickness with a maximum spacing of 30 feet. The reason for the 30 foot maximum spacing is to ensure that the shrinkage cracks are narrow enough to provide adequate load transfer.

  It is also important to keep the slabs as square as possible. Transverse joint spacing should not exceed 125 to 150 percent of the longitudinal joint spacing. Joints should be sawn as soon as the concrete has obtained adequate strength to resist raveling of the joint edges. Early entry saws work best for this type of work, and can often be used within two to three hours after compaction. The depth of the saw cut should be at least one-fourth the thickness of the slab (D/4) and have a minimum width of 1/8 inch.
  
  

  Cutting joint for RCC pavement at Honda facility in Alabama

• What is the solar reflectance of roller-compacted concrete?

  Until recently, no testing had been performed to determine the solar reflectance index (SRI) of roller-compacted concrete (RCC) pavements. However, a recent report prepared by the CTLGroup, Skokie, Illinois shows that the SRI of RCC pavements is similar to SRI of conventional concrete. Both conventional concrete and RCC pavements provide high solar reflectance, which help mitigate the urban heat island effects.

  

  The solar reflectance index of conventional concrete pavements decreases with age from about 38 when new to about 30 at ages beyond 10 years. Whereas, the SRI of asphalt pavements increases with age from about seven when new to about 20 at the age of eight years. Asphalt surface layers are typically replaced with new materials every six to nine years, which results in an SRI of 20 or less during the entire service life of an asphalt pavement.

  To determine the solar reflectance index of two roller-compacted concrete parking area pavements in Tennessee, three core specimens were obtained from each pavement. At the time of sampling, Pavement 1 and Pavement 2 were about two and 10 years old, respectively. Both RCC mixes contained #57 stone, manufactured limestone sand, 400 lbs/yd³ portland cement, and about 125 lbs/yd³ Class F fly ash. The cores surfaces were tested in accordance with ASTM C1549-04, Standard Test Method for Determination of Solar Reflectance near Ambient Temperature Using a Portable Solar Reflectometer. The SRI of each set of three specimens was calculated in accordance with ASTM E1980-01, Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces. The SRIs for Pavement One and Pavement Two were 36 and 38, respectively, which are very similar to the SRI of new conventional concrete pavements.

  Although the data is limited and additional SRI evaluations of RCC pavements covering different regions of the country and different applications are needed, it is believed that the SRI of all new RCC pavements would meet the minimum of 29 needed to earn a point toward the sustainable sites credit of the LEED rating system.

• What types of aggregates can be used to create a quality roller-compacted concrete mix for pavements?

  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.

• Why is compaction of roller-compacted concrete so important?

  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. (Figure 1).

  
  
  

   Figure 1. Strength vs. density for various RCC mixtures (Schrader)

   

  Data from the figure indicates that for compacted densities between 97 to 100 percent of theoretical air-free density (TAFD), the compressive strength of RCC showed little variation. However, as the density drops below 96 percent, significant strength loss occurs. This is why most specifications require a minimum density of at least 98 percent of maximum density, based on a modified Proctor density (MPD). (Note: 98 percent of MPD is not equivalent to 98 percent of TAFD because there are some voids – typically 0.5 to two percent - in the MPD).

  
  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.

  Reference:

  Schrader, E.K., “Roller-Compacted Concrete for Dams – State of the Art”, International Conference on Advances in Concrete Technology, Athens, Greece, May 1992.