There are significant environmental benefits of recycling waste concrete and reusing it as aggregate for structural concrete, but the use of Recycled Aggregate Concrete (RAC) is yet limited to non-structural applications such as road sub-base. Widespread application of RAC in areas such as seismic design requires an improved knowledge of RAC behavior under multiaxial state of stresses.

The main objective of this research is the characterization of seismic properties of RAC by developing a stress-strain model which can reasonably describe the behavior under both unconfined and confined conditions. An extensive experimental program, including testing of several plain RAC cylinders as well as reinforced RAC columns, was conducted. There are numerous variables influencing the behavior of confined RAC, creating unlimited experimental possibilities, so the tests parameters were chosen to be limited to square columns with normal strength RAC and rectilinear tie configurations.

Stress-strain curves were obtained for several 4 inches by 8 inches RAC cylinders with compressive strength from 2.5 ksi to 7.5 ksi. Based on the experimental results, a new model for stress-strain behavior of plain RAC was developed. Up to an axial strain of 0.0025, a nonlinear equation is considered for the ascending branch, which is primarily a function of compressive strength and elastic modulus, while the descending branch is a straight line and is a function of compressive strength. A sustaining branch was proposed where the plain RAC is capable of sustaining 10% of the compressive strength.

44 reinforced RAC columns, 10 inches by 10 inches in section and 32 inches in height with nine different combination of tie patterns and spacings were tested under quasi-static (1.6 10-5 per in.) and dynamic (1.6 10-2 per in.) axial straining rates. The columns with a volumetric ratio of ties of 1.5% more were capable of sustaining the load than the columns with lower volumetric ratios. Under the fast straining rate, the columns showed about 27% increase in strength. The columns with sufficient and well-distributed lateral confinement did not also show any stiffness degradation under cyclic loading.

Special attention was paid to recording the axial strains at which vertical cracks were developing, as well as the strains at which the cover was no longer effective. These strains were indicative of the gradual transition of axial stresses from cover to the core. Based on these strains and contributions of concrete and longitudinal steel in for each column, a new one-of-a-kind stress-strain model for confined RAC was proposed. The model is comprised of an ascending-transition-descending structure would be suitable to define the behavior. The ascending branch proposed for plain RAC is applicable for confined RAC as well. The transition zone starts at a strain of 0.0020 and ends at a strain where the cover is completely ineffective. The descending branch is a straight line and primarily a function of the lateral reinforcement. The variables required to define the curve were described in terms of the compressive strength of plain RAC, the yield strength of steel tie and the volumetric ratio of the confinement steel.

The proposed models for plain and confined RAC were examined under the flexural loading condition using two identical RAC beams. The nonlinear loaddeformation of the beam predicted using the proposed RAC models agreed with the experimental results to within 10%.