Strength and Durability Performance of Ultra-High-Performance Cementitious Composite Enhanced with Carbon Nanofibres

Ultra-high-performance concrete (UHPC) is known for exhibiting excellent strength and a more durable matrix compared to conventional concrete. Typically UHPC consists of carefully selected constituent materials of: ultrafine graded sand, silica fume (or other alternative binders), steel micro-fibres, and ordinary (or special blends) Portland cement. This study was initiated to synthesize a new blend of Ultra-high-performance cementitious composite (UHPCC) with compressive strength higher than 150 MPa under normal curing conditions, and to investigate the influence of two different sources of carbon nanofibres (CNF) on its strength, durability and microstructure properties. Several mix designs with different CNF percentages were designed, optimised and analysed to obtain the optimal proportion for the UHPCC, and their strength development were monitored up to 28 days. Subsequently, the durability performance of the selected UHPCC mixes were characterised though the rapid chloride permeability, Mercury Intrusion Porosimetry and water penetration tests. It was found that a stable dispersion with low CNF percentage (0.06%wt) was able to improve the water penetration, lower the rapid chloride permeability and reduced the pore sizes of the UHPCC matrix. The overall findings of the research assert that a stable dispersion of CNF contributes in positive effect on the strength and durability characteristics of UHPCC, and is feasible to enhance overall microstructure and contribute to a denser matrix. This study is part of the larger research programme to synthesize an innovative UHPCC mix for specialised applications for structures under impulsive loadings

Synthesis of Ultra-High Performance Cementitious Composite incorporating Carbon Nanotubes

Ultra High-Performance Concrete (UHPC) is a type of special concrete developed to meet the demand for niche applications in the industry, where this type of concrete comes with enhanced durability and superior mechanical characteristics in comparison to conventional normal- and high-strength concrete. However, UHPC has its drawbacks in terms of lower tensile strength ratio and brittleness. Nanomaterials such as carbon nanotubes (CNT) with their superior mechanical properties are potential candidates to act as nano-reinforcement in Ultra High-Performance Cementitious Composite (UHPCC) matrix, to create a more denser and ductile UHPCC system. However, prior to arriving at the “desired” UHPCC mix design, attention has to be paid to the process of dispersing the CNT into the fresh composite mix since the dispersion of CNT in cement-based material is a challenge due to their agglomerating behaviour. This paper presents on the synthesis of UHPCC mix design which optimizes on its packing density of its constituent materials. The influence of different dispersion methods of CNT on the mechanical strength and microstructure of UHPCC are also reported. It was found that samples reinforced with CNT exhibit higher compressive and tensile strengths and denser microstructure compared to control samples without CNT

Bond strength of reinforcing steel embedded in fly ash-based geopolymer concrete

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel