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What to Do With Crushed, Returned Concrete

What to Do With Crushed, Returned Concrete

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    A ready-mix truck discharges returned concrete at the plant.

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    A crusher produces crushed concrete aggregate.

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    Crushed concrete aggregate of different strength classes is stored at the NRMCA research laboratory. (Red is 3000 psi, black is 5000 psi, and gray is 1000 psi.)

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    Crushed concrete aggregate is stored at the ready-mix plant.

In all three of the steps while discharging the concrete, the driver should take precautions to avoiding using water to clean the truck at the location where he discharges the concrete. Drivers should clean trucks at the wash-out pit.

The producer should test the concrete containing CCA for a wide range of properties that are important for the application. If CCA will be used, the producer should adopt quality control measures while producing the CCA. The CCA pile should be kept moist, as the CCA should ideally be maintained at a level greater than the saturated surface dry condition.

CCA characterization studies such as absorption and relative density (specific gravity) are recommended weekly. When high amounts of CCA are used, the producer should watch for slump retention issues and conduct comparative measurement of air content by the pressure meter (C 231) and the volumetric method (C 173); if the results agree, use ASTM C 231.

Guidance for the engineer

The ACI 318 Building Code for Structural Concrete (Section 3.3.1) and ACI 301 Reference Specification for Structural Concrete require that concrete aggregates conform to ASTM C 33. ASTM C 33 is also referenced in ASTM C 94 and in AIA MasterSpec, which is the basis of specifications in most design firms.

It is clear that ASTM C 33 permits using CCA. There should be no restriction in its use if the concrete meets the requirements of a project in most concrete applications. The design professional can choose a more conservative approach in limiting its use to non-structural or less critical applications related to loads or durability.

Based on the results, it seems that using CCA in the “as received” condition can be permitted for most applications to a limit of 10% by weight of the total aggregate. Engineers who are unsure can request additional data on service records or test results that will do no harm to the concrete.

In non-structural applications, provided the concrete producer does further processing such as isolating the returned concrete of greater than 3000 psi, the producer could be allowed to use CCA in the as-received condition up to 30% by weight of total aggregate. In non-structural applications, if the producer just used the coarse fraction of the CCA, it could be allowed to replace all of the virgin coarse aggregate with coarse fraction of CCA.

Codes in many European countries accept using 20% of crushed coarse concrete aggregates in structural concrete. In light of that, for structural concrete applications, only the coarse CCA (cumulative material retained on the No. 4 sieve) should be used at 10% by weight of total aggregate. Therefore, the recommendation for structural concrete is more conservative.

In all of the above situations, the concrete should still meet all the performance requirements for that application. For increased acceptance of CCA, the ASTM C 94 Standard Specification for Ready Mixed Concrete should include a recommended provision that crushed concrete aggregate can be used to a limit of 10% of the total aggregate weight.

You can dowload the complete reports from the 20-month study at the following Web sites: www.rmc-foundation.org and at www.nrmca.org.

Karthik H. Obla is managing director, research and materials engineering, at NRMCA. This article is an excerpt from NRMCA's Concrete In Focus magazine.