This paper presents an experimental and numerical investigation on concrete members strengthened by embedded through-section (ETS) steel and glass fiber-reinforced polymer (GFRP) bars attached preferably with mechanical anchorage at the tension ends. The pullout tests to analyze the bond performance between ETS bars and concrete under various influences such as anchorage presence, embedment length, ETS bar diameter, ETS-material types, and anchorage length are carried out. An analytical method for deriving the local bond stress-slip relationship of GFRP bars-concrete interfaces is developed. The overall responses of the pullout test specimens in terms of pullout force-slip curves, failure modes and strain profiles along the embedment length are discussed. Based on a careful interpretation, the analytical results demonstrated the effectiveness of the local bond stress-slip model developed in this study. Additionally, the finite element (FE) simulation of the beams intervened with ETS bars, which were tested in a previous study by the authors, incorporating with the proposed interfacial model is conducted. Comparison between the results achieved from the FE modelling and the experiment implies that the FE method was an accurately applicable tool to assess the behaviors of the beams strengthened in shear by ETS bars.
As a kind of concrete material, the self-compacting concrete (SCC) is usually subjected to external cyclic load after cracking. In this paper, three-point bending tests were carried out for SCC specimens with different notch-to-depth ratios. The whole loading process was monitored by acoustic emission (AE) technology and digital image correlation (DIC) method. The post-peak cyclic characteristics of SCC were analyzed, and the effects of notch-to-depth ratios on mechanical performance of SCC were explored. The change rules of acoustic emission energy, b-value and Ib-value were obtained by processing the collected acoustic emission signal data. Besides, the development law of internal damage for SCC specimens was elaborated and the active cracks were classified according to the ratio of RA value to AF. In addition, the strain field diagrams of specimens with notch-to-depth ratio were displayed and analyzed, and the propagation laws of effective crack length and crack tip opening displacement (CTOD) of SCC concrete with different notch-to-depth ratios were obtained based on DIC technology.
The prediction of delayed strains is very important in prestressed concrete structures. The temperature and relative humidity of concrete structures are two important parameters for this prediction, and they vary over time on site. Based on model code 2010 and the superposition principle, the authors propose an analytical method that takes account of such variations in temperature and relative humidity. The coupling between delayed strains in concrete and stress relaxation in prestressing bars is also considered. The proposed method is validated with respect to in-situ measurements on the VeRCoRs mock-up, which is a 1/3 scale mock-up of a biaxially prestressed confining structure. At the end of the paper, the authors discuss the importance of taking into account the coupling between delayed strains and stress relaxation, the influence of variations in temperature on the delayed strains of concrete and the value of Poisson’s ratio for drying creep.
In this study, creep characteristics of concrete under multiaxial stress caused by uniaxial loading in confined conditions are experimentally investigated. Focus is hereby given to the creep Poisson’s ratio with regard to different degrees of lateral confinement as well as different drying conditions. The unconfined and confined specimens with the steel tube are used in the creep test. As results of the test and reproducing calculation based on the theory of elasticity, Poisson’s ratio of concrete obtained by instantaneous loading test could be also used as creep Poisson’s ratio regardless of conditions of lateral stress and drying.
The present research aimed at evaluating early age thermal cracking risk of durable RC slabs incorporating slag cement and expansive additive on multiple span steel box girder bridges utilizing full-scale 3D FEM simulation. First, laboratory investigations were conducted to calibrate the material models of durable concrete. Second, the material models were utilized in several member level FEM models and the simulation procedure was verified regarding early age volume changes calibrating parameters for expansion energy and reduction factors for creep. Third, thermal and volumetric changes in RC slab were monitored and the simulation procedure was further validated in structural level utilizing full-scale FEM model of the real bridge. The simulated maximum tensile stress along bridge axis in RC slab signify the risk of early age transverse cracking where the accumulated stepping construction stress is comparatively large. The effectiveness of expansive additive in reducing the risk of transverse cracking is revealed from the simulation. However, parametric studies of the validated model indicate that the RC slab on the permanent form of seven span steel box girder bridge is vulnerable to early age thermal cracking regardless of ambient conditions and placing temperatures when coefficient of thermal expansion of concrete is larger than 6×10-6/℃.
The electrical properties of porous systems are intimately linked to mass transport and flow processes such as diffusion and permeability and offer a simple testing methodology for assessing those properties which are responsible for the durability and long-term performance of construction materials. In the current study, electrical impedance spectra for concretes containing both plain and blended Portland cement binders were obtained over a period of 360 days. In-situ impedance measurements were used to accurately identify the bulk resistance (hence evaluation of resistivity) of the concretes and the optimum frequency range for bulk resistance measurements. The bulk resistivity was normalised by that of the pore-fluid resistivity obtained from computer simulations and the results indicated that the pore-fluid resistivity decreased only marginally with time once the hydration process had advanced beyond 28 days. It is shown that the normalised resistivity – termed the Formation Factor – displayed a continual increase with time, highlighting on-going hydration/pozzolanic reaction and pore structure refinement over the entire test period. This was particularly evident for the slag concretes. Using the normalisation process, a simple approach is presented to evaluate the effective diffusion coefficient of the concretes and a durability/performance classification system, based on the Formation Factor, is presented.