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Innovative use of recycled materials in reinforced concrete beams
[Thesis]. Manchester, UK: The University of Manchester; 2020.
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Abstract
Despite extensive research studies, construction demolition waste (CDW) and worn-out tyres of motor vehicles are still not fully reused and are hence disposed of in ways that are damaging to the environment. . There have been a number of investigations into manufacturing concrete using recycled aggregates from CDW and crumb rubber. However, it has not been possible to make recycled aggregate concrete with the-same performance as standard natural aggregate concrete (NAC) without incurring extra costs. Production of natural coarse aggregates is environmentally damaging and also costly. With surging demand for concrete worldwide, and to respond to the call of sustainable development, it is vital that the enormous quantities of CDW and tyres with high embodied carbon content are used in structural applications without compromising cost and performance. This thesis focuses on two ways of doing so: by finding alternative ways of improving the mechanical properties of recycled aggregate concrete to achieve those of concrete using natural aggregates, and to find new ways of using recycled aggregate concrete where the demand on the mechanical properties of concrete can be easily met by recycled aggregate concrete. Regarding the former, the research investigates the feasibility of adding a tiny amount of graphene (109g) to a cubic metre of recycled aggregate concrete. The 109g of graphene is equivalent to 0.01% (optimized concentration) of the combined weight of cement and sand of the design concrete mix. Regarding the latter, this research considers using recycled aggregate concrete in the tension zone of reinforced concrete beams. Mechanical property tests have been carried out on recycled aggregate and crumb rubber concrete, with or without graphene. In the first part of this research, comparisons were made between concrete using 10mm size uncrushed quartzite stones (NAC mix, C40), recycled aggregate concrete (RAC) and rubber recycled aggregate concrete mix (RRAC). The slump test results show RAC suffered a 73% reduction in slump value compared to the NAC mix. However, the slump value of RAC was increased by 72% by introducing an additional amount of water (4.21% of recycled aggregate weight measured from a water absorption test) or by 75% using DARACEM 215 superplasticizers (1% of cement weight). As expected, the compressive and tensile strengths, elastic modulus and ultrasonic pulse velocity of the recycled aggregate concrete were lower than those of the NAC mix, by 14.9%, 14.8% , 4.3% and 7.4% respectively. Introducing crumb rubber (by 5%, 10%, 15% and 20% of recycled aggregate weight) into the recycled aggregate concrete further decreased its workability and mechanical properties with or without superplasticizers. In the interest of workability, this research recommends limiting the weight of rubber to not more than 10% of the recycled aggregate weight. With 5% and 10% rubber, the compressive strengths of RRAC were 32.8% and 47.1% of the NAC mix using uncrushed 10mm quartzite aggregates. Inclusion of rubber crumb seemed to help the ductility of RRAC as observed by the slower rate of stress reduction in the descending branch of the measured stress strain curve. The second part of the mechanical property investigation explores the feasibility of using graphene to improve the mechanical properties of recycled aggregate concrete and rubber recycled aggregate concrete with 10% of crumb rubber content. Graphene concentrations of 0.01%, 0.02%, 0.05% and 0.1% of the combined weight of cement and sand and of different sizes (5um, 10um and 20um) were incorporated into recycled aggregate concrete and natural aggregate concrete. Except where G5 0.02% was used (graphene size 5µm, 0.02% by weight) which resulted in a modest 12.2% increase in compressive strength compared to the recycled aggregate concrete with rubber, including graphene decreased the compressive strength of rubber recycled aggregate concrete. This was attributed to the inability of graphene to enhance the low bonding strength between rubber particles and cement matrix. Due to the excessive amount of dust in the recycled aggregates, adding graphene to recycled aggregate concrete without rubber did not improve the mechanical properties of recycled aggregate concrete. The key to improving the mechanical properties of recycled aggregate concrete is to enhance the bonding at the interface between the cement and aggregates. To do so, the recycled aggregates were washed in further tests. By adding 0.01% (2.2g) of G10 graphene to the washed recycled aggregate concrete, the compressive and tensile strengths were enhanced by 43.9% and 24.1% respectively to reach values of 39.14MPa and 3.76MPa which are similar to those of C40 NAC with values of 42.MPa and 3.77MPa respectively. Tests were carried out to obtain the bond strength between rubber recycled aggregate concrete and deformed steel rebar. The bond strength of recycled aggregate concrete without rubber particles was 16.3% higher than that of the NAC. This confirms the results of other investigators and can be attributed to the better interlocking resistance due to the more irregular shapes of recycled aggregates compared to the uncrushed 10mm quartzite stones. Adding crumb rubber particles decreased the rebar pull-out bond strength, due to a lack of resistance of the rubber particles. However, if the rebar bond strength is normalised to the strength of concrete, according to then incorporating rubber particles would not adversely affect the bond strength of recycled aggregate concrete. In regular reinforced concrete beams or slabs under bending, the tension strength of concrete is ignored in designs according to EC2, thus providing an opportunity to use recycled aggregate concrete in the tension zone of such members. This would necessitate a two-layer construction in which concrete with natural aggregates is used in the compression zone to provide the required compressive strength. The two layers of concrete are not cast monolithically. It is important that the concrete interface has sufficient shear strength to prevent interfacial shear failure. The slant shear test was used to determine the interfacial shear strength in this work. In this research, two time lags between casting of the two different layers, of 4 and 24 hours, were investigated. The two-layer concrete cast in the 4-hour time interval behaved monolithically while those cast at the 24-hour cast time interval failed at the interface with an interfacial strength of about 1.5MPa for all tests. For the particular types of concrete examined here, it is recommended to cast the two layers of concrete within a time limit of 4 hours in order to reduce the cost of construction by avoiding provisions of extra shear links that may be necessary for preventing interfacial shear failure between the two layers of concrete. Experimental, numerical and analytical investigations have been carried out to investigate the feasibility of using concrete made with recycled aggregate and crumb rubber in the tension area of reinforced concrete beams under bending and shear, and to propose design recommendations. In the beams, the top layer (1/3rd) of concrete, mainly in compression, is a higher grade using uncrushed 10mm quartzite stones, and the bottom (2/3rd) layer, in tension, is a lower grade using rubber recycled aggregate concrete. A total of 8 beam tests were conducted, to investigate the effects of changing NAC and recycled concrete strength and, top and bottom layer thickness ratio and shear span to depth ratio. The results confirm that the two-layer beam achieves the same bending resistance as the regular reinforced concrete beam made entirely of the higher grade concrete. The flexural bending resistance of the two layer beam can be calculated in exactly the same way as for the concrete beam made entirely of the higher grade concrete. In beams without shear reinforcement for investigation of shear resistance, the two-layer beams attained lower shear resistance than the regular concrete beams. This was attributed to the unzipping effect: once the lower strength concrete has failed in shear, it loses its shear resistance and transfers the shear force from the failed lower strength concrete to the natural aggregate concrete in compression. Further FE simulations, using ABAQUS, reveal that the top layer higher grade concrete plays no role in influencing the beam’s shear resistance. Therefore, when calculating the shear resistance of concrete of the two-layer beam, the lower concrete grade should be used. However, since the shear force in reinforced concrete beams is mainly resisted by shear links according to EC2, the lower shear resistance of rubber recycled aggregate concrete has very minor implications on beam shear resistance, as demonstrated by representative design cases. Modification of recycled aggregate concrete by adding a small amount of graphene (0.01% of the combined weight of cement and sand) and the proposed two-layer beam construction facilitate the wider adoption of recycled aggregate concrete in structural applications to partially replace natural aggregate concrete (NAC).
Keyword(s)
Bending resistance; Crumb rubber; Graphene concrete; Recycled aggregates; Rubber recycled aggregate concrete; Shear resistance; Two layer concrete beams