Road Science Tutorial
The mortar content of the final RCA product is key to a successful application. “Crushed concrete will come down to either large-size coarse aggregate particles, or the ‘glue’ that holds them together, the mortar, which is the cement paste plus sand particles and fly ash or any other admixture,” Snyder told Better Roads.
How much mortar should come off the RCA depends on the ultimate use of the RCA, Snyder said. “It really depends on the application,” he said. “RCA needs to be treated as an engineered material. If you want to maximize reclamation efficiency, so you are reclaiming as much mortar as possible to be as ‘green’ as possible, there are certain types of crushing processes that will remove less mortar. Then you will need to take the presence of that added mortar into account in designing the application.”
For example, concrete mix designs may need to be modified to compensate for the higher absorption capacity of the RCA, and base course gradations need to be selected with consideration of the higher abrasion or degradation properties of the material.
“If you are considering the use of high mortar-content RCA in an unbound foundation layer, there may be more potential for leaching of calcium hydroxide, which is a byproduct of the hydration of cement,” Snyder said. “This will result in a high-pH runoff at first, and perhaps the collection of calcium carbonate precipitate in your drain pipes or filter fabric. So you will want to reduce RCA mortar content for this application, use a daylighted base instead of pipe drains, or use the material in an undrained layer instead.”
If the goal is to reclaim as much mortar as possible, and the high mortar-content RCA is going into a concrete mix, it will result in high absorption and lower specific gravity for the aggregate, Snyder said. “Perhaps there will be higher shrinkage as well,” he said. “Thus you may need to shorten up your joint spacing, adjust water content, or use additional fly ash or water reducers to adjust the mix design. You need to test and understand the properties of the RCA so that you can adjust your designs to achieve the performance you want.”
The other end of the spectrum is reclaiming less material while producing an RCA that is as close to natural aggregate as possible. “For that you may use an impact crusher, making an effort to remove as much mortar as possible,” Snyder said. “The mortar can be recycled separately into an undrained base layer, or stabilization layer. The processed RCA with minimal mortar can be placed into a new mix application, without too many mix design adjustments in that case, as you are essentially reusing existing aggregate.”
The production processes might be dictated by the engineer who decides how the material will be used, Snyder said. If the contractor wants to pick the material up and reuse it in the same pavement structure, the selected use of the RCA within that pavement structure will dictate the required handling, crushing and any post-crushing treatments (like washing or air-blowing or other beneficiation) of the material.
Dealing with ASR Concrete
Alkali-silica reactivity, called ASR, is the bane of concrete. Formerly thought to afflict only concrete made with western aggregates, it’s now thought that the potential for ASR-prone aggregates exist in every state.
ASR is a chemical reaction that occurs between alkalis contributed primarily by cement, and a reactive form of silica from reactive aggregate, which forms an alkali/silica gel. Under the right conditions – particularly enough available moisture – the gel will expand and produce stresses and damage in the concrete.
Over time, this expanding ASR gel exerts tremendous internal pressure that can lead to cracking of the concrete. This cracking can provide pathways for potentially deleterious materials such as water, sulfates and chlorides to the interior of the concrete matrix, which in turn can lead to serious durability issues such as freeze/thaw damage, sulfate attack or steel corrosion.
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