Roller compacted concrete -RCC- test strips

Step 1: Well-graded aggregates required

Many first-time concrete producers use readily available crusher-run materials, which are generally too variable to produce consistent RCC.

Plotting aggregates on a coarseness factor chart is useful (see figure on page 19). For larger aggregates, the mix should plot in Zone II. For the small maximum coarse aggregate sizes used in today’s mixes, plot the mix in Zone III.

Most specifications recommend a minimum two coarse aggregates blend, typically smaller than 3/4-inch top size, as well as ASTM C33 natural concrete sand or manufactured sand. This new American Concrete Paving Association specification is an excellent guideline.

The concrete and engineering industries want to know what water-cementitious materials (w/cm) ratio to specify. The answer is simple: You don’t need to.

First, RCC gets 35% to 40% of water content from free moisture in the aggregates, compared to 25% for conventional mixtures.

Second, proper moisture content is both a strength and a density issue. Mixes are delivered in dump trucks, so you can’t tweak them after batching. If water content is too far above or below optimal, the mixture will be strong enough but lack the required density — and you won’t be able to fix that. RCC pavements without proper density are subject to issues such as surface raveling and freeze-thaw vulnerability.

Thus, water volume is based on the amount needed to achieve the required compaction. The concrete producer or RCC contractor sends all mix materials to a lab that performs a modified proctor according to ASTM D1557 to identify the optimal water content (a percentage of the total dry weight of all 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.

When the mix design has been finalized, test a trial batch by molding cylinders according to ASTM C1435. Mold enough to plot a history curve, which will be useful when comparing strength gain to tests taken from the test strip.

Currently, there is no ASTM method for molding RCC beams. Breaking cylinders in split tensile may be useful for determining bending strength.

Next page: Step 2-4