Search

The converter steel slag is a by-product during the steel-making process, which usually acts as an inert ingredient in construction due to the relatively low reactivity. However, its mineral composition results in a high reactivity in a CO2 rich environment. This study reveals the modification of converter steel slag by an ambient CO2 pretreatment, and the application of modified converter steel slag as the supplementary cementitious material (SCM) in the design of eco-friendly ultra-high performance concrete (UHPC) by using a particle packing model. The results show that converter steel slag after ambient CO2 pretreatment is feasible to be used in eco-friendly UHPC design, which achieves a relative higher compressive strength over 150 MPa with the cement substitution from 15% to 45%, compared to non-carbonated steel slag. The ambient CO2 pretreatment can modify the physical and chemical properties of converter steel slag particles, which produces a rough and porous surface of slag particles because of the precipitation of calcium carbonate and amorphous silica gel as carbonation products. Consequently, the incorporation of carbonated steel slag improves the cement hydration, enhance the formation of ettringite and C-S-H, while densify the microstructure compared to non-carbonated slag in eco-friendly UHPCs. The leaching of potentially hazardous elements, e.g. Cr and V from carbonated steel slag can be efficiently reduced below legal limits in UHPC mixtures.

This study investigates the synergistic effect of steel fibres and coarse aggregates on impact behaviour of ultra-high performance fibre reinforced concrete (UHPFRC). UHPFRC matrices with a low cement content and maximum aggregate sizes of 8 mm and 25 mm are designed by using a particle packing model. Three types of steel fibres (13 mm short straight, 30 mm medium hook-ended and 60 mm long 5D) are studied in terms of the utilization efficiencies. The results show that UHPFRC with coarser aggregates tends to have a lower cement consumption but slightly weaker mechanical strength, and the largest aggregate size is suggested to be no more than 25 mm considering the reduction on flexural toughness and impact resistance. The medium and long fibres contribute to an excellent deflection/strain hardening behaviour instead of short ones. A preferential synergistic effect on impact and flexural properties is observed between the medium fibres and the finer aggregates, while the longer fibres are more compatible to the coarser aggregates. The length of steel fibre is recommended between 2 and 5 times the maximum aggregate size. The flexural strength controls the impact resistance under low-energy impact loadings, and flexural toughness determines it under relatively high-energy (beyond energy threshold) impact loadings. Keywords Synergistic effectUltra-high performance fibre reinforced concreteImpact resistanceFlexural propertySteel fibreCoarse aggregate

The present paper studies the ballistic performance of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) applying multi-layered concept against the 7.62 mm projectile at 840 m/s. Coarse basalt aggregates are incorporated in the UHPFRC under the premise of reducing the cement powder consumption and taking advantages of their superior ballistic resistance. We found that the designed triple-layered UHPFRC 16a1s(40)-8a1s(10)-16a1s(40) achieves a superior impact resistance compared to the single-layered reference, with a 32% reduction of the penetration depth. The improved resistance of the triple-layered UHPFRC is associated with the multiple effects of the coarse aggregate, the layer interface, the fibers direction in the thin middle layer, and the edge confinement of the rear layer. Moreover, a new analytical model is proposed to predict the penetration depth in the multi-layered UHFRC, which can take the varying mechanical properties of the layered targets into consideration. The results from this study shed light on understanding the ballistic performance of layered UHPFRC, and promote its application in protective constructions.

This paper aims to optimize high-volume limestone powder in sustainable ultra-high performance concrete (UHPC), and characterize its roles on plasticization effect, hydration kinetics, microstructure and hardened properties. The spread flow, hydration products, compressive strength, porosity and pore structure, shrinkage, embedded CO2 emission and unit cost are investigated with different substitution levels of binders by limestone powder, varying from 0 to 80 vol%. Results show that replacing high volume of binders by limestone powder is an efficient way to develop eco-friendly and low-cost UHPC. Limestone powder shows a positive mineral plasticization effect that should be considered in designing UHPC. The degree of secondary pozzolanic hydration is more intensive than C3S/C2S hydration, which can enhance the later-age strength development potential. An appropriate content of limestone powder can contribute to a higher strength, denser pore structure, diminished total free shrinkage and higher sustainability efficiency. The optimum content of limestone powder appears to be 50 vol% of the total powder content in UHPC.

This study investigates the influence of key parameters on in-service bullet impact resistance of ultra-high performance fibre reinforced concrete (UHPFRC), with the aim to provide design guidance for the engineering applications. The effects of steel fibre type and dosage, matrix strength, coarse basalt aggregates, and target thickness are researched by subjecting the UHPFRC to a 7.62 mm bullet shooting with velocities of 843–926 m/s. The results show that the UHPFRC, designed by using a particle packing model with compressive strength around 150 MPa, is appropriate to develop protective elements considering both anti-penetration performance and cost-efficiency. The 13 mm short straight steel fibres show better anti-penetration than the 30 mm hook-ended ones, and the optimum volume dosage is approximately 2% by considering both the penetration and crack inhibition. Introducing coarse basalt aggregates with the particle size up to 25 mm into UHPFRC reduces the powder consumption from 900 kg/m3 to 700 kg/m3, and results in slightly higher mechanical strength and significantly enhanced bullet impact resistance with 14.5% reduction of penetration depth. The safe thicknesses (perforation limit) of the designed UHPFRC slabs are approximately 85 mm and 95 mm to withstand the 7.62 × 51 mm NATO armor-piercing bullet impact under velocity 843 mm/s and 926 mm/s, respectively.

This study aims to investigate the synergistic effect of quaternary blends applying supplementary cementitious materials on sustainable Ultra-high Performance Concrete (UHPC) pastes. The hydration kinetics, pore structures, fresh behaviour, strength, fibre-to-matrix bond, shrinkage and environmental sustainability of 14 UHPC pastes are determined and analysed. The results show that limestone powder contributes to better environmental sustainability and fresh behaviour, but enlarged shrinkage and diminished strength, and application of silica powder is an effective measure to overcome those disadvantages. Slag cement possessing a relatively lower Ca/Si ratio (2.45) is preferred to a lower amount but finer silica (3% nano silica) in the presence of limestone powder, compared to the Portland cement with a higher Ca/Si (3.22) that needs more silica even with coarser particle size (5% micro silica). Quaternary blends with cement-slag-limestone-silica in UHPC pastes have considerable advantage of reducing embedded CO2 emission and and improving sustainability efficiency. Furthermore, positive synergies in term of strength, fibre-to-matrix bond and total free shrinkage are observed in UHPC pastes with quaternary binders compared to binary and ternary ones.

This study proposes a novel concept of two-stage ultra-high performance concrete (TS-UHPC), towards ultra-low binder consumption. The effects of grout and coarse aggregate are investigated and their compatibility is evaluated. Results show that TS-UHPC has a low binder amount (down to 364 kg/m3) and high binder efficiency (up to 0.417 MPa·m3/kg), possessing excellent compressive strength of up to 151.8 MPa at 91 days. Microstructural analysis reveals that grout with a sand-to-powder ratio of 1.0 shows a higher hydration degree, denser structure, and increased later strength. Coarser basalt aggregate tends to slightly lower compressive and splitting tensile strength, 14% and 12% reduction with the maximum size from 8 mm to 25 mm, respectively. The TS-UHPC has an excellent interfacial transition zone that induces a water-permeable porosity of 0.91%–1.32%. New formulas are proposed to describe correlation between compressive and splitting tensile strength of TS-UHPC, and to predict strength of TS-UHPC by grout.

The study aims to improve the flexural behaviors of ultra-high performance fiber reinforced concrete (UHPFRC) by applying the concept of layered-structure. Deterministic criteria for layer cracking and debonding are proposed, formulas to predict the critical load at the first failure stage are developed, and effects of the layer E-modulus and thickness are assessed. Subsequently, double-layered UHPFRC beams are designed and tested under the three-point bending. Mechanical and interfacial properties of the beams are studied. Influences of the bottom layer thickness on the peak flexural load and the flexural energy are then investigated, which presents that a layer thickness ratio of 0.6 gives the optimum load carrying ability and beam flexural energy. The subsequent section discusses the effects of fiber re-arrangement on the flexural performances, revealing that the designed double-layered UHPFRC beam is able to withstand higher flexural load and energy than its single-layered counterpart with the same total fiber content. Moreover, it is exhibited that the peak flexural load is dependent on the fibers in the bottom layer while the flexural energy enhancement is related to fibers in both layers. The layered UHPFRC beam composed of a 40 mm-thick top layer with 0.6% steel fibers and a 60 mm-thick bottom layer with 1.6% fibers is an optimal choice leading to the superior peak flexural load and energy.