Long-term loss of tension stiffening of concrete containing recycled aggregate and steel fibres

This paper presents experimental results on the long-term loss of tension stiffening of concrete containing recycled aggregate and steel fibres. Washed construction and demolition wastes (CDW) size 20 mm were used as a replacement for the natural coarse aggregate, and hooked-end steel fibres (DRAMIX 3D 65/35BG) were added to the mixes. Square section prisms of concrete (120x120x1200 mm) reinforced with a central steel bar were prepared and cast with different replacement percentages of recycled aggregate and volume fractions of steel fibres. A direct tensile load was applied to the steel bar and the measurements of the strain on the concrete surfaces were then recorded for a period of 35 days to assess the long-term loss of tension stiffening. The results indicated that the substitution of NA by RA increased the loss of tension stiffening over time up to 12.9%. However, the addition of steel fibres proved to be highly beneficial; the inclusion of 0.5% and 1.0% steel fibre content to the NC and RAC reduced the loss up to 9.8% and 20.4%, respectively. According to the obtained results, modifications to the Eurocode2 method for predicting the long-term deflection of reinforced concrete beams were proposed to be implicated in the calculation steps for this type of concrete.