A simple method to address the high water absorption of recycled aggregates in cementitious mixes

Abstract

The high water absorption of recycled concrete aggregates (RCA) can cause a significant reduction in the flow properties of cement-based materials in relation to mixes produced with natural aggregate. In order to compensate for this adverse effect, two extreme and simple approaches based on the water absorption of RCA in pure water are often used: using RCA in a saturated surface-dry (SSD) state or adding extra water based on the 24-hour water absorption of RCA in pure water (WA24h). However, these approaches can result in a significant drop in mix’s strength due to excessive water being used. Therefore, this study proposes a simple method to accurately evaluate the water absorption of RCA in cement paste (WAP) based on the variation in the mortar’s apparent density resulting from the RCA absorption of water from the cement paste. This study also investigates the effects of the aforementioned two approaches on the mortars’ performance. Recycled concrete sand (RCS) was adopted as the object of the study. The results show that, after an initial increase, the WAP of RCS stabilizes at 24 min. WAP also increases with the mortar’s W/C and WA24h, and decreases with the aggregate’s volume fraction. In addition, the maximum water absorption of RCS in cement paste (WAP_30min) is lower than WA24h, and the WAP at 6 min (WAP_6min) can reach 80% of the WAP_30min. A campaign of mortars was produced using the WAP_30min as previously assessed. It is found that the flowability of mortar prepared with RCS is poorer than that of mortar prepared with natural sand at the same effective W/C, but their strength is higher. Using RCS in a SSD state or adding extra water based on the WA24h of RCS can compensate for the flowability loss of the mortars to some extent, but at the expense of an excessive reduction in mechanical performance. Based on the results of this study, the authors suggest that the mix design of mortars prepared with RCA can be improved using WAP, namely adjusting the initial W/C to balance flowability and mechanical properties.