Strength and durability characteristics of recycled aggregate concrete

Recycling demolished concrete as an alternative source for the production of new concrete can help solve the growing waste disposal crisis and the problem of depleted natural aggregates. However, to make such recycling feasible, the strength and durability of the recycled aggregate concrete (RAC) must also be assured. This study was an attempt to provide that assurance by conducting an extensive investigation regarding the strength and frost resistance of RAC. The following three phases were carried out:

( 1 ) Three replacement ratios (0.50, 0.75, 1.00) of the recycled aggregate were considered to determine their effects on the physical properties of the RAC and to obtain an acceptable quality of concrete with maximum utilization of recycled aggregate in place of natural aggregate.

( 2 ) Four cases for both RAC and natural aggregate concrete (NAC) were considered to determine the main factors that affect the freeze-thaw durability of both concretes:

(a) Using a medium water -cement ratio of 0.47;

(b) Increasing the fly-ash content from 14 percent to 28 percent;

(c) Lowering the water-cement ratio from 0.47 to 0.29 ;

(d) Using air entrainment (5 percent air content).

( 3 ) Three different types of recycled aggregates were derived from three highly used structural grade concretes (3,000, 4,000, and 5,000 psi) to determine the effects of various original concrete strengths on the new strength of the RAC. Regression analyses were performed to establish the relationships between the various properties of the RAC and the NAG obtained in this study, and the best fit equations were compared to those usually used by the ACI Code. From the strength point of view, the recycled aggregate compared well with the natural aggregate in most cases and, therefore, could be considered for various potential applications. From the frost resistance point of view, the recycled aggregate had negative effects on the durability performance of the RAC. However, with the use of mineral and chemical admixtures, particularly air entrainment and fly-ash , the RAC could be as durable as the NAC. Some of the relations commonly used for the prediction of the concrete properties of the NAC cannot be used for the RAC. Therefore, some alternative relations were proposed for the prediction of the physical properties of the RAC.