The deterioration law of recycled concrete under the combined effects of freeze-thaw and sulfate attack

The freeze-thaw of the 3% Na2SO4, 5% Na2SO4 and 10% Na2SO4 (mass fraction) solutions and water was tested using the quick freezing method for recycled concrete, with a water-binder ratio of 0.45 and a recycled aggregate content of 30%. The damage-deterioration law of recycled concrete was studied from the aspect of changes in the weight of recycled concrete, relative dynamic elasticity modulus, loss of compressive strength, thickness of the damage depth, and compressive strength of the damaged layer concrete. Combined with scanning electron microscopy, the composite damage mechanism of recycled concrete under sulfate and the freeze-thaw cycles was analyzed. The results revealed that the relative dynamic elasticity modulus of recycled concrete presents three stages: slow declining stage, rapidly declining stage, and accelerated declining stage. Compressive strength exhibited a slow declining stage, and accelerated the loss of two stages. With the increase in freezing and thawing times, the ultrasonic velocity in the damaged layer of recycled concrete decreases, the thickness of the damaged layer increases, the compressive strength of the recycled concrete in the damaged layer decreases, and the damage degree of the recycled concrete increases. The freeze-thaw damage of recycled concrete in the 5% Na2SO4 solution was serious, and the frost resistance was the worst. In the 3% Na2SO4 solution, the degradation degree of recycled concrete in the early stage of the freeze-thaw cycle was smaller than that of water, and the degradation degree of recycled concrete was more than that of water after 200 cycles of freezing and thawing. The deterioration of recycled concrete in the 10% Na2SO4 solution was greater than that of the 3% Na2SO4 solution and water. Through theoretical analysis and experimental data regression, the freeze-thaw damage model of recycled concrete with different sulfate concentration is established. (C) 2018 Elsevier Ltd. All rights reserved.