Flexural Behaviour of Reinforced Concrete Beams with Recycled Aggregates and Steel Fibres

Abstract

The use of recycled aggregate concrete is on the increase but the remnant of mortar clinging to the aggregate, typically lightweight and porous, is responsible for its downsides when compared to normal concrete. This presence of mortar makes the recycled aggregate (RA) a heterogeneous substance (as opposed to the homogeneity of the natural aggregate). Consequently, the mix design method appropriate to RA reuse for structural purposes, has long been debated. In this work, conventional and unconventional mix design methods were investigated to determine the flexural performance of reinforced concrete beams produced with RA obtained from a precast waste concrete and steel fibres. Both experimental and numerical investigations were employed in this research. The experimental study involved: characterization of the aggregates, design of the concrete mixes, set of trial mixes to attain mixtures of comparable workability and to optimize steel fibre content appropriate to the design methods, and the production, curing, and testing of concrete specimens for mechanical and durability properties. Two conventional methods—absolute volume method given by the American Concrete Institute (ACI) and the British Department of Environment (DoE) guidelines—were examined experimentally for the production of recycled aggregate concrete. The rationale was to ascertain which of the two traditional methods would manage resources better without compromising strength all the same. Eventually, five principal mixes were developed and investigated experimentally as follows: natural aggregate concrete (NAC), recycled aggregate concrete (RAC) composed of 100% RA, steel fibre-reinforced recycled aggregates concrete (SFRRAC) made of 100% RA and 1% steel fibre content by volume of concrete, blended aggregate concrete (BAC) which consisted of 60% RA and 40% normal aggregates, and steel fibre-reinforced blended aggregate concrete (SFRBAC) constituting of the BAC mix with 0.5% steel fibre volume ratio. Whereas the NAC, RAC, and SFRRAC mixes were designed using the ACI approach, the BAC and SFRBAC mixes were proportioned according to the extended Equivalent mortar volume (EMV) technique. Then, the flexural behaviour of the reinforced concrete beams was simulated using commercial ANSYS Mechanical APDL 2019 R1 software, with the aid of finite element model, for the numerical investigation. The results of the experiments showed that the oven-dry specific gravity of recycled fine aggregates (RFA) and recycled coarse aggregates (RCA) were respectively 23% and 12% lower than those of their corresponding natural aggregates. The average water absorption capacity of RFA and RCA were 13% and 5.3%, while those of their comparable natural aggregates were 1.0% and 0.8%, respectively. The mortar content of the RCA was 52%. Comparatively, the ACI mix design method used about 11% lesser amount of cement to produce RAC of a higher strength than its DoE counterpart. With over 28% reduction in cement content, the BAC mix produced a concrete of comparable compressive strength to that of the NAC mix and of a higher compressive strength relative to those of RAC and SFRRAC mixes. In spite of mix design methods, all concretes composed of RA showed a substantially higher tensile splitting strength than the conventional concrete. The flexural strength of the concrete mixes was not dependent on both RA and RA content. The flexural behaviour of the tested beams revealed that the SFRRAC had the most load bearing capacity. Although both BAC and NAC beams gave the same load capacity, the former showed a fewer crack, the least crack width (at the fracture), and visually the least deflected at failure compared to the other beams. The greatest effect of steel fibres was on the tensile splitting strength, moment capacity, and ductile failure mode of the SFRRAC beams. Finally, all the concretes containing RA and prepared in the conventional way, exhibited a higher water absorption capacity up to a degree of 49% than the normal concrete. Nevertheless, the use of the EMV procedure reduced this gap to just 8.8%. This research necessitates that the ACI and DoE mix design methods be revised for concrete containing RA. This is essential to encourage the use of the actual properties of the RA (particularly their water absorption capacity) instead of adopting the tables of values and curves developed from the results of the experiments conducted using natural aggregates. Also, the extended application of the EMV mix design technique has, in this research, been proven adequate for the production of recycled aggregate concrete fit for structural purposes.

Date of Award	Sept 2021
Original language	English
Awarding Institution	Coventry University
Supervisor	Messaoud Saidani (Supervisor), Adegoke Olubanwo (Supervisor), Essie Ganjian (Supervisor) & Mark Tyrer (Supervisor)