Pore pressure development in hybrid fibre-reinforced high strength concrete at elevated temperatures

The present experimental work investigates the build-up of pore pressure at different depths of High Strength Concrete (HSC) and Hybrid-Fibre-Reinforced High Strength Concrete (HFRHSC) when exposed to different heating rates. First, the effect of the measurement technique on maximum pore pressures measured was evaluated. The pressure measurement technique which utilised a sintered metal and silicon oil was found to be the most effective technique for pore pressure measurement. Pore pressure measurements carried out showed that addition of polypropylene fibres is very effective in mitigation of spalling and build-up of pore pressure inside heated HSC. Addition of steel fibres plays some role in pore pressure reduction at relatively higher pressures in deeper regions of concrete during fast heating. Pore pressure development is highly influenced by the rate of heating with fast heating leading to higher pore pressures in the deeper regions of concrete compared to slow heating.     

Residual strength of hybrid-fiber-reinforced high-strength concrete after exposure to high temperatures

Residual strengths of high-strength concrete (HSC) and hybrid-fiber-reinforced high-strength concrete (HFRHSC) after exposure to high temperatures were investigated in the paper. The results showed that normal HSC is prone to spalling after exposure to high temperatures, and its first spalling occurs when the temperature approaches 400 °C. For HSC reinforced by high melting point fibers, the first spalling occurs when the temperature reaches to approximately 800 °C, while there is no spalling during exposing to high temperatures for HSC reinforced by polypropylene (PP) fiber with a low melting point. Mixing high melting point fiber (i.e., carbon or steel fiber) with low melting point fiber (i.e., PP fiber) HSC greatly improves the properties of HSC after exposure to high temperatures.     

Revisiting the protected paste volume concept for internal curing of high-strength concretes

Internal curing of high-strength concrete has been the subject of extensive research for the last decade. The concept of protected paste volume has been one of the most significant theoretical approaches to internal curing. In this paper, the applicability of the protected paste volume concept to internal curing is re-evaluated in view of recent experimental evidence. It is shown, that the concept of protected paste volume and recommendation to limit the spacing factor to approximately 200 μm, cannot be extended to internal curing of high-strength concrete, since the distance of penetration of the internal curing water into the surrounding matrix depends mainly on the availability of internal curing water to the surrounding cementitious matrix. The pore structure of LWA and the size of SAP particles seem to have a marked influence on the availability of internal curing water and thus are factors of greater importance than the spacing factor.     

Thermal and mechanical properties of fiber reinforced high performance self-consolidating concrete at elevated temperatures

This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.     

Internal curing of high-performance concrete with pre-soaked fine lightweight aggregate for prevention of autogenous shrinkage cracking

The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m³ of saturated LWA in HPC, providing 27 kg/m³ of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.     

An experimental approach for the analysis of early-age behaviour of high-performance concrete structures under restrained shrinkage

This paper presents a systematic approach for the experimental testing and analysis of the early-age thermo-mechanical behaviour of large prismatic high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint in order to enable the characterisation of high-performance concrete specimens without premature termination of the test in case of significant restrained shrinkage. The instrumentation system can measure the strains and other parameters from the setting time with high accuracy and reliability. The analysis method takes into account the temperature effects on the measured properties and provides equations to determine the time-evolution of shrinkage, thermal expansion, stiffness and creep of the concrete. Results from the testing of 200 × 200 × 1000 mm specimens made with a 0.34 water-cement ratio concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

Evolution of penetration resistance in fresh concrete

The objective of this research was to examine the setting of concrete through its penetration resistance; an experimental device, especially developed for this purpose was used, which consists of a system that lets a sphere fall on concrete from a certain height and then measures the depth of the crater. Forty-five samples were made with four different types of cement varying its quantity, water-cement ratio, at ambient temperature and humidity. A semiempirical model which explains the penetration resistance evolution in fresh concrete was proposed and experimentally demonstrated. The results are compared with the consolidation curves for soils represented by the logarithmic method. The proposed setting time was defined as the elapsed time between the placement of the concrete and the time when the depth of the crater is 18% of the initial one.     

The scratch test for strength and fracture toughness determination of oil well cements cured at high temperature and pressure

Recent advances in scratch test analysis provide new ways to relate measured scratch test properties not only to strength properties but fracture properties of materials as well. Herein, we present an application of such tools to oil well cements cured at high temperatures and pressures. We find a concurrent increase of strength and toughness of different oil well cement baseline formulations which we relate to the water-to-binder ratio for a series of cementitious materials prepared with cement and silica flour. The scratch test thus emerges as a self-consistent technique for both cohesive–frictional strength and fracture properties that is highly reproducible, almost non-destructive, and not more sophisticated than classical compression tests, which makes this ‘old’ test highly attractive for performance-based field applications.     

Experimental study on the failure mechanism of recycled concrete

In this study, Young's modulus, strength, and peak strain of recycled concrete under both compressive and tensile loading were experimentally studied to understand its failure mechanism. Due to the different colors of natural aggregates, old hardened mortar, new hardened mortar, and interfacial transition zone (ITZ), the quantity and the distribution of each phase were analyzed by images processing and analysis of cut sections. With the tests, the failure processes and crack situation of the recycled concrete under tensile and compressive loadings were illustrated. It was found that some mechanical properties of recycled concrete are similar to those of mortar, for instance, lower Young's modulus, higher peak strain and more brittleness, due to a larger volume content of both new and old hardened mortar. When compared with old hardened mortar, new hardened mortar has more significant influence on both the strength and the Young's modulus of recycled concrete.     

Effects of stress during heating on strength and stiffness of concrete at elevated temperature

Concrete in structures exposed to high temperatures is practically always heated under stress. Yet, there are few experimental studies in which the concrete was heated under stress and then loaded to the peak, and most of these were performed under uniaxial compression. This paper reports on an experimental study of the effects of different heat–load regimes on the stress–strain behaviour of partially sealed concrete under multiaxial compression, at elevated temperature. The specimens were first heated (stressed/unstressed), then loaded to the peak in multiaxial compression. In contrast with previous experimental research, the results show that concrete heated under relatively low compressive stress has lower strength and stiffness than concrete heated without load. The results suggest that the presence of stress during first heating produces a specific damage, which could be the cause for a major component of the load induced thermal strain (LITS) in concrete.     

The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement

The purpose of this study was to investigate the influence that replacing natural coarse aggregate with recycled concrete aggregate (RCA) has on concrete bond strength with reinforcing steel. Two sources of RCA were used along with one natural aggregate source. Numerous aggregate properties were measured for all aggregate sources. Two types of concrete mixture proportions were developed replacing 100% of the natural aggregate with RCA. The first type maintained the same water–cement ratios while the second type was designed to achieve the same compressive strengths. Beam-end specimens were tested to determine the relative bond strength of RCA and natural aggregate concrete. On average, natural aggregate concrete specimens had bond strengths that were 9 to 19% higher than the equivalent RCA specimens. Bond strength and the aggregate crushing value seemed to correlate well for all concrete types.     

Properties of fresh and hardened concrete

The present paper reviews the literature related to the properties of fresh and hardened concrete published after the previous (12th) International Congress on the Chemistry of Cement held in Montreal in 2007.Workability and fundamental rheological properties, reversible and non-reversible evolution, thixotropy, slump loss, setting time, bleeding, segregation and practical issues related to formwork filling and pressure, are addressed among the properties of fresh concrete.Among hardened concrete properties compressive strength and other mechanical and physical properties of hardened concrete, such as tensile strength, elastic properties, shrinkage, creep, cracking resistance, electrical, thermal, transport and other properties are covered. Testing, interpretation, modeling and prediction of properties are addressed, as well as correlation with properties of fresh concrete and durability, effects of special binders, recycled and natural aggregates, fiber reinforcement, mineral and chemical admixtures. Special attention is given to the properties of hardened lightweight and self-compacting concrete.     

Hydration and microstructure of ultra high performance concrete incorporating rice husk ash

Rice husk ash (RHA) and silica fume (SF) have a similar chemical composition and a very high specific surface area, but RHA is not an ultra-fine material like SF. The high specific surface area of RHA originates from its internal porosity. For this reason RHA can be expected to behave differently from SF in terms of the hydration and the resulting microstructure of concrete. This still remains unclear in Ultra High Performance Concrete (UHPC). The objective of this research was to study the effect of RHA on the hydration and microstructure development of UHPC. The results are compared to those obtained with a control sample and a sample made with SF.The results show that the addition of RHA can increase the degree of cement hydration in UHPC at later ages. RHA can also refine the pore structure of UHPC and reduce the Ca(OH)₂ content, but less significantly than SF. The thickness of the interface transition zone (ITZ) between sand particles and cement matrix of all samples is very small at the age of 28 days. The compressive strength of the sample made with RHA after 7 days was higher than that of the control sample and the sample made with SF. It is suggested that the porous structure of RHA and the uptake of water in this porous structure results in a kind of is attributable to the internal water curing of the RHA modified mixture.     

Mechanical and fracture properties of cement-based bi-materials after thermal cycling

This study investigates the effect of thermal cycles on the fracture properties of the cement-based bi-materials. Sixty eight cubes were exposed to a varied number of 24-hour thermal cycles ranging from 0 to 90 and subsequently were tested in a wedge splitting configuration. The mechanical and fracture properties of normal strength and high strength concretes are substantially improved after 30 thermal cycles, but less so after 90 thermal cycles both in isolation and when bonded to an ultra high-performance fibre-reinforced cement-based composite.     

Biomineralization,strength and cytocompatibility improvement of bredigite scaffolds through doping/coating

Coprecipitation-derived, sacrificial polymeric (urethane) foam-fabricated bredigite (Ca₇MgSi₄O₁₆) scaffolds were processed by individual and combined treatments of fluoride doping and poly (lactic-co-glycolic acid) (PLGA) coating and then studied in terms of structure, mechanical strength, bioactivity and cell biocompatibility in vitro. According to scanning electron microscopy and Archimedes porosimetry, the geometrical characteristics of pores for all the scaffolds are in the appropriate range for hard tissue regeneration applications. The apatite-formation ability of the samples immersed in a simulated body fluid is improved by doping for both the bare and coated conditions, based on microscopic and energy-dispersive X-ray spectroscopic analyses. Both the treatments advantageously buffer physiological pH changes imposed due to the fast bioresorption of the ceramic. Also, the biodegradable PLGA coating typically enhances the compressive strength of the scaffolds, which is critical for bone tissue engineering. In accordance with the MTT assay on osteoblast-like cells (MG-63) cultures, both the processes individually enhance the cell viability, while the highest improvement is obtained for the combined application of them. It is finally concluded that fluoride doping and PLGA coating are impressive approaches to improve the bioperformance of bredigite-based scaffolds.