Credit: Christopher Tull
A paving machine spreads and compacts roller-compacted concrete at a jobsite.
Whether it’s football, baseball, or any other sport, isn’t it interesting that the best players seem to be the ones who are willing to continually prepare? For example, you can usually find an all-pro quarterback double- and triple-checking items to make sure everything is good to go before game time.
Good roller-compacted concrete (RCC) contractors are the same. No matter how successful they have been in the past, they will install a test strip prior to each project to verify the process. The following information can be learned via the test strip:
- The concrete production and construction techniques will produce the same (within tolerances) results as those of the submitted mix.
- The construction techniques will produce the proper thickness.
- The test strip will determine the amount of roll down.
- The aesthetics of RCC are reviewed to see that the owner’s expectations are met.
- Any issues in the process are worked out before in-place work has begun.
The following steps can assist you in planning for an RCC test strip.
Step 1: Mixture Development
RCC is a unique material. Technically, the science of RCC exists in the overlapping region of soil mechanics and concrete materials.
Proper mix development of RCC is critical to a successful project. This mix must consist of well-graded aggregates. The Portland Cement Association provides a recommended gradation in its guide specification. Many first-time RCC manufacturers will try readily available crush and run materials for the aggregate. While these aggregates can be successfully used, the variability of the crush and run materials is probably too high to produce consistent RCC. Your concrete background will be helpful here. Plotting the aggregates on a coarseness factor chart is useful (see Figure 1 below). For larger aggregates, the mix should plot in Zone II. For the small maximum coarse aggregate sizes used in many of today’s mixes, the mix should plot in Zone III.
Credit: Christopher Tull
The test strip's density and moisture content are monitored via a nuclear gage calibrated to the modified proctor of the approved mix.
The concrete and engineering industries want to know what water-cementitious materials ratio to specify. The answer is simple: Do not specify a w/cm. The amount of water required is based on the amount needed for the required compaction. The materials (including the cementitous) are sent to a lab where a modified proctor is performed. This test will identify the optimal water content (a percentage of the total dry weight of all of the materials) that corresponds with the maximum density. Concrete technicians must have the skills to mathematically convert the optimal water content into SSD (saturated surface dry) batch weights for a computerized concrete batch panel or pull weights for manual batching.
Having the correct moisture content (within tolerances) is not only a strength issue — it is a density issue. RCC is delivered in dump trucks so there is no opportunity to correct the water content after batching. If the moisture content is too far above or below optimal, the mixture will likely have adequate strength, but it will not be physically possible to obtain the required density. RCC pavements without proper density are subject to issues such as surface raveling and freeze-thaw durability.
It will quickly become apparent that a substantial amount of the required water will come from free moisture in the aggregates. A conventional mixture receives about 25% from free water in the aggregates, while the RCC mixture can receive 35% to 40% of its total water from the aggregates.
When the mix design has been finalized and the optimal moisture content has been identified, a trial batch can be mixed and the potential strength of the mixture obtained by molding cylinders according to ASTM C1435, “Molding Roller Compacted Concrete in Cylinder Molds Using a Vibrating Hammer.” Currently, there is no ASTM method for molding RCC beams. Breaking cylinders in split tensile may be useful for determing bending strength.
Mold enough cylinders so that a strength history curve can be plotted. This will be useful when comparing strength gain to tests taken from the test strip.