Papers by Author: Shi Lang Xu

Abstract: The flexural behavior of reinforced concrete (RC) members strengthened with postpoured Ultra High Toughness Cementitious Composites (UHTCC) was investigated in this paper. The flexural behavior, failure mode and crack propagation during loading process of composite specimens were studied, and their structural behavior was also compared to that of original members. The experimental results showed that post-poured UHTCC materials enhanced flexural bearing capacity and toughness of existing concrete members. And introducing UHTCC material into strengthening enabled the composite specimens sustain the loading at a larger deflection without failure. It also revealed that post-poured UHTCC layer dispersed larger cracks in upper concrete into multiple tightly-spaced fine cracks, which prolonged the appearance of harm surface cracks and improved the durability of existing structures.

Abstract: This paper presents research work relating to design method and mechanical properties of functionally-graded composite (FGC) beams, in which a kind of strain hardening material UHTCC is used as a replacement for the concrete material that surrounds the longitudinal reinforcements. According to mechanical models of materials and a series of assumptions, a simplified design method of such FGC beams is proposed. On the basis of superposition, the determination and the influencing factors of the thickness of UHTCC layer in FGC beams is discussed. Compared with theoretical and experimental results, simplified method shows conservation and guarantees the safety of structures. During the whole flexural loading process, it is found that the opening of cracks in flexural members can be greatly restricted by the introduction of UHTCC. The possibility of steel corrosion therefore can be evidently reduced.

Abstract: The analysis of material variables and curing procedure design are the key techniques of UHPCC. Material variables include cement-water ratio, silica fume content, sand’s content and size, filling powder’s content and type. Based on the objective of compressive strength, UHPCC with compressive strength 180MPa was developed based on the influence analysis of material variables on its performances and the substitution of silica fume by lime stone powder. By the analysis of the influences of early curing method, initial time, duration time and high temperature curing, curing procedure design was proposed, i.e. early water curing at first after 24 hours of casting and then high temperature steam curing. Based the simplified constitutive relation of UHPCC, the ultimate flexural of UHPCC hybrid girder was analyzed. This paper can be a basis of UHPCC-NC hybrid structure analysis.

Abstract: Ultra high toughness cementitious composite (abbreviated as UHTCC) shows significant pseudo strain hardening behavior and offers prominent tension strain ability of more than 3% when subjected to uniaxial tension load. The failure pattern of the UHTCC components exhibits multiple fine cracks with crack width lower than 100μm even corresponding to the ultimate tensile strain state. Four-point bending investigations of reinforced ultra high toughness cementitious composite (RUHTCC) members without web reinforcement have been carried out due to the excellent crack dispersion and strain energy absorption abilities of UHTCC material, aiming at design issues of strictly anti-cracking structures or aseismic design in key parts of structures such as beam column joint when using UHTCC. The moment-curvature curves have been measured and compared with the theoretical analysis proposed before. There is a reasonable agreement between them, especially that both moment and curvature present little difference before yielding. According to experimental results of RUHTCC beams with three different reinforcement ratios, UHTCC can delay yielding of reinforcements and improve load bearing capacity and ductility of structures or components compared with ordinary reinforced concrete beams, then steel products can be saved. The possibility of corrosion should be evidently reduced in respect that RUHTCC can effectively control crack. Accordingly, durability of structures improves by using UHTCC members.

Abstract: Ultra-high toughness cementitious composite (UHTCC) exhibits the pseudo-hardening feature when subjected to tensile load and has high tensile strain capacity of normally up to 3%. Also, UHTCC has a unique cracking behavior. From cracking up to ultimate tensile strain capacity, the crack width in UHTCC could be still kept below 100m. This paper presents the utilization of UHTCC to replace a layer of concrete surrounding the main flexural reinforcement in ordinary RC beam to improve flexural performance especially beam durability as UHTCC displays high toughness and shows multiple fine cracks. Analytical closed-form formulae for flexural capacity, curvature and deformation of UHTCC/RC composite beam derived based on the elastic beam theory is presented first. Subsequently, experimental results of two groups of different reinforcement ratios of UHTCC/RC beams and control RC beams tested under flexural loading to verify the feasibility of analytical formulae as well as to examine the performance improvement of UHTCC/RC composite beam over the control beam is presented. Moment-curvature curves and load-mid span displacement curves for the tested beams are compared with the theoretical analysis. A good agreement between experimental and analytical results is found. The experimental results show that the use of a layer of UHTCC in RC beams can enhance both flexural capacity and ductility. The improvement is not significant with the increase in reinforcement ratio; however, the maximum crack width under service load even in the case of lightly reinforced beams can be limited within 0.1mm.

Abstract: As cement-based composite, concrete can be properly represented by three phases in microstructure: cement paste, aggregate as well as interfacial transition zone between them. As the matrix compositions of concrete, fracture properties of cement paste and mortar have great influence on fracture performance of concrete, and fracture energy is an important parameter for concrete non-linear fracture mechanics research. Therefore, three-point bending beams of cement paste and mortar with different sizes and strengths were tested. A complete load-deflection (P-δ) curve was directly obtained, and a fit was made for the tail of the P-δ curve using power and exponential function, respectively. And then the fitting results of the two function were compared. It was found that with the increase of specimen size the influence of tail curve on fracture energy is decrease，and considering the influence of tail curve, fracture energy of cement paste and mortar are size-independent. The discussion on the obtained results was made.

Abstract: For micro-fiber reinforced strain-hardening cementitious materials, in addition to the basic characteristics and mechanical properties of fiber and interfacial properties between fiber and matrix, mechanical properties of matrix such as strength and crack resistance are essential parameters for material design, too. Therefore, the fracture properties of cement paste and mortar which are two most basal cementitious materials were studied, using three-point bending beams of which strength and depth are varied. Complete load versus crack mouth opening displacement (P-CMOD) curve directly obtained, and double-K fracture parameters ini Ic K and un Ic K were subsequently determined. The initial cracking load Pini was determined using resistant strain gauges. The results show that an apparent stable crack propagation before unstable failure was observed both in cement paste and in mortar. For cement paste, due to the influence of shrinkage crack, the divergence of the unstable fracture toughness un Ic K is more evident than initial fracture toughness ini Ic K .

Abstract: The apparent size effect of the specific fracture energy of concrete according to the RILEM procedure has been confirmed by numerous published works. The paper offers an explanation for this size effect by considering the specimen boundary influence on local fracture energy over the ligament length, which is closely associated with the measured fracture energy of concrete. To address this boundary influence, boundary affected length is introduced, over which local fracture energy is different from that in the bulk far away from the surface of the specimen. Based on previous work, a continuous smooth function is hypothesized to simulate the distribution of local fracture energy. At the same time, the model established was compared to the existing models, i.e. Perturbed Ligament Model (PLM) and Bilinear Model (BLM). Some test results from wedge splitting specimen in the literature were used to verify these three models. The results show that the true fracture energy of concrete, irrespective of the specimen size, could be obtained from the measured values directly from RILEM, and is less sensitive to determination approach. The predicted boundary affected length when the crack reaches the specimen surface is more close to the value of the perturbation length in PLM.

Abstract: Using the double-edge notched geometry proposed by Xu and Reinhardt recently, the dimension of 200 mm×200 mm×100mm concrete cube specimens, of which the crack length are 10 mm, 20 mm, 30mm, 40mm, 50mm respectively, are designed to experimentally measure mode II fracture toughness KIIC of concrete. For almost all specimens, typical shear fracture features i.e. approximately 0º initial cracking angle as well the following crack forwards propagation along the direction of ligament is phenomenally observed. This fact strongly confirms that this double-edge notched geometry is validly and capable of being utilized as a mode II fracture geometry to evaluate mode II fracture behavior. Then, from the discontinuity point of the measured load-displacement plot, the critical shear fracture load Pc is determined and the corresponding mode II fracture toughness KIIC is also calculated using the formula developed by Xu and Reinhardt. The computed results show that KIIC has no dependency on initial crack length, about 3.36MPa·m1/2 for the tested specimens.

Abstract: Crack width is a significant parameter for assessing service life of reinforced concrete structures in chloride-laden environments. Corrosion-induced concrete cracking is a predominant causal factor influencing premature degradation of reinforced concrete structures, incurring considerable costs for repairs and inconvenience to the public due to interruptions. This gives rise to the need for accurate prediction of crack width in order to achieve cost-effectiveness in maintaining serviceability of concrete structures. It is in this regard that the present paper attempts to develop a quasi-brittle mechanical model to predict crack width of chloride contaminated concrete structures. Assuming that cracks be smeared uniformly in all directions and concrete be a quasi-brittle material, the displacement and stress in a concrete cover, before and after surface cracking, were derived respectively in an analytical manner. Crack width, as a function of the cover depth, steel bar diameter, corrosion rate and time, was then determined. Finally, the analysis results were verified by comparing the solution with the experimental results. The effects of the cover depth, steel bar diameter and corrosion rate on the service life were discussed in detail.