title: Ultra high performance fibre reinforced concrete paving flags
Creators: name: Trung Le, Thanh
Publisher: name: University of Liverpool Repository
Year: 2023

description: A literature survey on Ultra High Performance Fibre Reinforced Concrete (UHPFRC) reveals that the applications until now have remained limited. This is because although it has proved to have outstanding mechanical properties such. as the compressive strength of 150-250 MPa and flexural strength of 25-50 MPa, UHPFRC has a highcosting issue as well as shortage of mix design procedure and of structural design guidelines. Therefore, the following works are carried out in this PhD study. The mix design of UHPFRC is first studied. A mix design procedure based on the 'Excess Paste Model' is proposed to' design the preliminary mix proportions. The superplasticiser dosage and excess paste volume significantly influence the workability as well as the mechanical properties of UHPFRC. Optimisation of superplasticiser dosage and excess paste volume leads to the optimum mix proportions by reducing the water-binder ratio and finally enabling achievement of high strength. The by-products of industry such as Ground Granulated Blast-Furnace Slag (GGBS), Pulverised Fuel Ash (PFA), and glass cullet are studied to replace cement and silica sand in UHPFRC. These replacements help to reduce the cost. The replacement of cement by GGBS and PFA also lessens the environmental impact caused by the cement producing industry. The sustainability of UHPFRC is therefore improved. The mechanical properties of UHPFRC are investigated experimentally by testing cube specimens for compressive strength and prism specimens for flexural strength, fracture energy and flexural toughness. There is no significant difference of compressive strengths between two sizes of specimens, Le. 50 mm cube and 100 mm cube, but a considerable difference of flexural strengths between 40x40x160 mm specimens and 100x1 00x350 mm specimens is found. The origin of this size effect issue on flexural strength is therefore investigated by using different loading arrangements, extracting small specimens from large specimens, and various sizes and shapes of specimens. Two factors are found to contribute to this size effect. These are the boundary surface layer and the ratio of the total crack-surface area to the cross section area of beam specimens. Paving flags with the lower half of thickness comprising UHPFRC with 2.0% fibres are fabricated as an outcome product of the studies on mix proportions and mechanical properties. These 400x200x30 mm UHPFRC paving flags are cast using a 'Hydraulic press' technique replicating a factory's typical casting procedure. The flexural strength and fracture energy of UHPFRC paving flags are then examined to compare with those of ordinary concrete paving flags. These flexural strengths and fracture energies are used as input parameters for finite element analysis (FEA) models of pavements using these paving flags. The structural behaviour of pavements using ordinary concrete paving flags and pavements using UHPFRC paving flags positioned on sand bedding and sub-base layers are studied using both laboratory experiments and FEA models. The FEA model of UHPFRC pavement is then used to predict the structural behaviour when the thickness of flag, the thickness and elastic modulus of sub-base layer vary. The following novel and major outcomes from this PhD study contribute to the development of UHPFRC: Â• Systematic investigations of mix proportions and mechanical properties of UHPFRC help the users to select the raw materials, mix proportions and curing regime suitably for the structures required. The results on the effect of specimen size on the flexural behaviour of UHPFRC suggest that UHPFRC large-scale beams should be trialied carefully before use; Â• An application of UHPFRC for very high performance crack-resistant paving flags using in pedestrian pavements has been studied by both experimental and FEA methods. The potential benefits obtained from using UHPFRC paving flags compared with ordinary concrete paving flags include: (1) UHPFRC paving flags can be made thinner and lighter, resulting in the reduction of health and safety concerns during handling and placing and also the reduction in transportation costs; (2) increase of pavement service life and lead to reduced maintenance costs; (3) reduction of liability claims arising from uneven pavements.