Investigation into the engineering properties of recycled red clay brick aggregates and crumb rubber mixtures for pavement construction
Large volumes of Construction & Demolition (C&D) waste and End-Of-Life rubber tyres produced every year cause a huge environmental problem. Both bricks and rubber wastes are non-biodegradable in nature and pose a threat if not adequately and safely disposed of. On the other hand, these materials have good engineering properties that could be beneficially used in civil engineering, e.g. in construction of pavement layers in road works. Brick rubble is a major component of C&D, constituting approximately 7 - 10% of the total C&D waste generated on construction sites. In this study, recycled brick aggregates (RBA) crushed from red clay bricks picked from a demolition site in Świętochłowice (Silesia Region in Poland) and crumb tyre rubber (granulated) were tested to determine their engineering properties with the aim of utilizing them as unbound pavement material. Physical and mechanical tests such as particle size distribution, particle density, Proctor test, water absorption and California bearing ratio were conducted on the recycled brick aggregates. The compressive strength of the whole bricks was determined as well. Recycled brick aggregate – tyre rubber blends were prepared with varying percentages of tyre rubber (2%, 5% and 10% of the total weight of the dry mix) to determine the effect of this addition on the physical and mechanical properties of RBA. To present the varying moisture content regimes of pavement layers in various field applications, both unsoaked and soaked samples were investigated for California bearing ratio, swelling and moisture content. The experimental results indicated that RBA behaved like self-draining material with the bleeding water content of approximately 15% and its dry density did not largely depend on the moisture content. Compaction of materials in the Proctor moulds caused RBA to break and the breakage increased with the compaction energy. The RBA0 remained multi graded with Cu ≥ 26.7 and Cc ≤ 1.16 and were RBA2 (25.0 and 1.26), RBA5 (22.1 and 1.25) and RBA10 (28.9 and 2.45) respectively after modified Proctor energy compaction. Addition of recycled rubber to RBA greatly enhanced protection against crushing of RBA during compaction which increased by approximately 46% as rubber content increased to 10%. The CBR test values of only RBA were 87.8% (unsoaked) prepared with 2-5 mm size rubber mixture and 106.1% (unsoaked) prepared with 0.5-2 mm size rubber mixture which increased to 141.3% and 126.0% after 4 days of soaking in water. Blending of tyre rubber and RBA drastically reduced the CBR values in all samples by over 60%, even with the rubber content as little as 2% which was not consistent with results presented in literature for soil-rubber mixtures. Based on the strength values (CBR) limits of the subbase and base layers of unbound granular materials in the roads specifications (i.e. Overseas Road Note 31), RBA alone meet the specifications. However, RBA-tyre rubber blends do not meet the minimum requirements for being used as unbound pavement materials.