THE USE OF RECYCLED-CONCRETE AGGREGATE FROM CONCRETE EXHIBITING ALKALI-SILICA REACTIVITY

An investigation was made into precautions that were needed to prevent expansive alkali-silica reactivity (ASR) when ASR-affected concrete is recycled as coarse aggregate in new concrete. Cements with alkali levels of 0.50%, 0.75%, and 1.00% equivalent Na2O were used with highly reactive fine and coarse aggregate in original concretes that were recycled at ages of two months and at about one-half maximum expansion and near maximum expansion. New concretes containing recycled concrete as coarse aggregate were made using innocuous fine aggregate and cements with 0.50% or 1.00% cement alkali, with and without fly ash. Test storage in all cases was over water in sealed containers held at 38 degrees C (100 degrees F). Results indicated that excessive expansions due to ASR can develop in the new concrete containing the recycled concrete, particularly when low alkali cement was used in the original concrete and high alkali cement was used in the new concrete. Low lime ASTM Class F fly ash used at a 20% mass replacement level for cement, for the most part, reduced expansions to safe levels. Observations of pavement concrete indicate successful control of potential ASR in ASR-affected concrete recycled as aggregate in which fly ash is included in the concrete mixture.