Lattice modelling of size effect in concrete strength

This paper uses a recently improved lattice network model to study the size effect in the strength of plain concrete structures. The several improvements made to the lattice network model are: (i) tension softening of the matrix phase is included in the material modelling; (ii) the structural response is modelled by incrementing the deformation rather than the load. This eliminates the need for introducing arbitrary scaling parameters in the beam element failure criteria and; (iii) a square rather than a triangular lattice beam network is found to be adequate for modelling concrete, thus greatly reducing the computational time.The improved square lattice network has been used to simulate the complete load-deformation response of notched three-point bend beams of different sizes with a view to checking the validity of several size effect models available in the literature. Lattice simulation was found to identify microcracking, crack branching, crack tortuosity and bridging, thus allowing the fracture process to be followed until complete failure. The improved lattice model predicted smooth structural response curves in excellent agreement with test results.The simulated nominal strengths also correlated very well with the test results, apart from that for the smallest beams (depth 38.1 mm). However, even in the relatively broad range of sizes (1:8) of the test beams, there was no clear evidence that one size effect model is superior to the other. In fact, rather surprisingly the test data would appear to be equally well described by all the available size effect models. The lattice simulations however indicated a trend which is better predicted by the multifractal scaling model.     

The influence of corrosion on bond properties between concrete and reinforcement in concrete structures

The rebar corrosion in reinforced concrete is one of the most extensive pathologies affecting the performance of concrete structures. Chloride-induced rebar corrosion damage results mainly from the use of de-icing salts in cold climates and/or exposure to marine environments. Carbonation damage is a further important degradation mechanism. The internal consequences of corrosion are the modification of the steel behavior and degradation of the steel–concrete bond. This work is devoted to the influence of the controlled corrosion on the adherence between steel and concrete. A new geometry of specimen has been designed to: (i) avoid the lateral confining stresses that appear during the classical pullout tests and (ii) permit to impose a known confinement for the study of its influence on the behavior of the interface. Five specimens with different levels of corrosion have been tested in this contribution. Reinforcing bars embedded in concrete were submitted to accelerated corrosion using an external current source. The magnitude of corrosion was measured using both Faraday’s law and the weight loss method. The level of corrosion varied from 0% to 0.76%. The geometry of the specimens allowed us to take series of digital pictures during the tests, which were analyzed using a digital image correlation, the procedure named CORRELI^LMT. The results of pullout tests proposed in this contribution indicate that: (i) levels of corrosion that are less than 0.4% of weight loss improves the bond stress and (ii) levels of corrosion resulting in more than 0.4% of weight loss lead to a reduction of the bond stress value.     

Meso-scale numerical investigation on cracking of cover concrete induced by corrosion of reinforcing steel

Concrete cover cracking induced by corrosion of steel reinforcement is a major influencing factor for durability and serviceability of reinforced concrete structures. Here in this study, the influence of concrete meso-structure on the failure pattern of concrete cover is accounted for. The concrete is assumed to be a three-phase composite composed of aggregate, mortar matrix and the interfacial transition zone (ITZ). And a concrete random aggregate structure is established for the study on the mechanical behavior of radial corrosion expansion. In the present simulations, the plasticity damaged model is used to describe the mechanical behavior of the mortar matrix and the ITZ, and it is assumed that the corrosion of steel reinforcement is uniform. The cracking of concrete cover due to steel reinforcement corrosion is numerically simulated. The simulation results have a good agreement with the available test data and they are between the two analytical results. The failure patterns obtained from the macro-scale homogeneous model and the meso-scale heterogeneous model are compared. Furthermore, the influences of ratio of cover thickness and reinforcement diameter (i.e. c/d), the location of the steel reinforcement (i.e., placed at the middle and corner zones) and the concrete tensile strength on the steel corrosion rate when the concrete cover cracks are investigated. Finally, some useful conclusions are drawn.     

Laboratory studies on influence of transverse cracking on chloride-induced corrosion rate in concrete

To clarify the corrosion mechanism of steel induced by transverse crack, a study on the influence of crack widths and epoxy coating on corrosion of steel bars in cracked concrete is presented here. Microcell and macrocell corrosions of bars were investigated on single crack specimens with crack widths of 0.08, 0.26, 0.38 and 0.94 mm. The entire study was carried out in an artificially created chloride ion-induced corrosion environment. The results show that the steel in cracks was activated once the transverse crack occurred on concrete element, and the macrocell corrosion must co-exist with microcell corrosion of reinforcements in test specimens with transverse crack. The macrocell current of steel elements were separated from the crack width, and the wider the transverse crack is, the higher corroded area and the greater microcell current of the rebar is. Oxygen and water go into concrete through crack instead of through concrete cover. The epoxy coating cannot prevent the occurrence and propagation of crack, so it was not effective to prevent corrosion of steel bars in cracked concrete.     

Lattice modeling of chloride diffusion in sound and cracked concrete

Reinforced concrete structures are frequently exposed to aggressive environmental conditions. Most notably, chloride ions from sea water or de-icing salts are potentially harmful since they promote corrosion of steel reinforcement. Concrete cover of sufficient quality and depth can ensure protection of the steel reinforcement. However, it is necessary to study the effects of material heterogeneity and cracking on chloride ingress in concrete. This is done herein by proposing a three-dimensional lattice model capable of simulating chloride transport in saturated sound and cracked concrete. Means of computationally determining transport properties of individual phases in heterogeneous concrete (aggregate, mortar, and interface), knowing the concrete composition and its averaged transport properties, are presented and discussed. Based on numerical experimentation and available literature, a relation between the effective diffusion coefficient of cracked lattice elements and the crack width was adopted. The proposed model is coupled with a lattice fracture model to enable simulation of chloride ingress in cracked concrete. The model was validated on data from the literature, showing good agreement with experimental results.     

Lattice Discrete Particle Modeling (LDPM) of Alkali Silica Reaction (ASR) deterioration of concrete structures

A large number of structures especially in high humidity environments are endangered by Alkali–Silica Reaction (ASR). ASR leads to the formation of an expansive gel that imbibes water over time. The gel expansion causes cracking and consequent deterioration of concrete mechanical behavior in the form of strength and stiffness reduction. In the recent past, many research efforts were directed towards evaluation, modeling and treatment of ASR effects on structures but a comprehensive computational model is still lacking. In this paper, the ASR effect is implemented within the framework of the Lattice Discrete Particle Model (LDPM), which simulates concrete heterogeneous character at the scale of coarse aggregate pieces. The proposed formulation, entitled ASR-LDPM, allows precise and unique modeling of volumetric expansion; expansion anisotropy under applied load; non-uniform cracking distribution; concrete strength and stiffness degradation; alkali ion concentration effect; and temperature effects of concrete subjected to ASR. In addition, a unique advantage of this formulation is its ability to distinguish between the expansion directly related to ASR gel expansion and the one associated with cracking. Simulation of experimental data gathered from the literature demonstrates the ability of ASR-LDPM to predict accurately ASR-induced concrete deterioration.     

Effect of the water saturation of aggregates on the shrinkage induced cracking risk of concrete at early age

This work consists in studying the effect of the water saturation of aggregates on the development of shrinkage and the potential cracking risk of early age ordinary concrete. Different concretes were obtained from a given concrete mixture by changing only the initial degree of saturation of limestone aggregates. Three degrees of saturation were studied, namely: 0% (dry aggregates), 50% (partially saturated aggregates) and 100% (saturated aggregates). From the experimental results, the early age behaviour and the mechanical properties of the concrete strongly depend on the water saturation of aggregates. A relative cracking risk was estimated from a stress-based approach and experimentally assessed parameters. The potential risk of cracking of these different concretes was shown to be different. Even if the total water content is kept constant, the water remaining in the cement paste actually depends on the initial water saturation of aggregates. The early age behaviour of concrete and the development of its early age properties depend on the amount of added water during the mixing.     

Effect of particle structure on mode I fracture process in concrete

Mode I fracture of concrete is normally regarded at the macro-level, and the effect of micro-structure is reduced to the use of several empirical constants in the fracture law. A key parameter is the softening diagram in tension. In a series of numerical analyses with a simple beam lattice model, the effect of material structure both on the pre-peak and softening regimes of the stress-deformation diagram in uniaxial tension has been established. The simulations are in good agreement with experimental observations that show the propagation of a large ‘crack-like’ feature in the steep part of the softening curve and substantial crack face bridging in the tail of the diagram. The simulations indicate that the density of stiff particles, which ranged between 35% and 83% in the analyses, has a significant effect on pre-peak micro-cracking. This part of the tensile stress-strain behaviour is usually ignored in macroscopic fracture models, but here it is shown that the material composition may have a significant effect in the pre-peak regime, but also on the peak stress. Furthermore the model analyses give output in terms of crack length distributions during the entire fracture process, which may be falsified in new experiments.     

Investigation on the cracking behavior of concrete cover induced by corner located rebar corrosion

Non-uniform corrosion of reinforcement causes concrete cracking in chloride contaminated RC structures. Due to the special boundary conditions, the concrete cover with corner located rebar is often subjected to the attack of chloride ions in a marine environment from two directions, and thus the corresponding non-uniform corrosion distribution should be different from the one for side-located rebar. The aim of the work is to explore the effect of corner located rebar corrosion on the cracking of cover concrete. For corner located rebar, an improved non-uniform corrosion distribution model was established based on the analysis results of two-dimensional chloride diffusion in concrete. Considering the heterogeneities of concrete, a meso-scale mechanical model and method for the study on the failure behavior of concrete cover was built. In the analysis model and method, the non-uniform radial displacement distribution was adopted to simulate the corrosion expansion behavior of the rebar. The cracking of concrete cover with corner located rebar was simulated and studied. The present approach was verified by the available experimental observations. The influences of concrete heterogeneity, corrosion distribution types, rebar diameter and concrete cover thickness on the failure patterns of concrete cover and the expansive pressure were investigated. The simulation results indicate that the developed approach can well describe the cracking behavior of cover concrete and the corrosion-expansion behavior of steel rebar.     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Stochastic service-life modeling of chloride-induced corrosion in recycled-aggregate concrete

The development and implementation of a stochastic service-life model for chloride-induced corrosion in reinforced recycled-aggregate concrete is presented in this work. The 1D model accounts for recycled aggregates that have been initially contaminated with chlorides from previous in-service exposure. Using a probabilistic approach, the model is employed to predict the service life of normal- and recycled-aggregate reinforced concrete with and without aggregate pre-contamination. Specifically, the effect of (a) type and replacement ratio of reclaimed aggregate, (b) chloride boundary conditions, (c) initial aggregate chloride concentration, and (d) thickness of contaminated aggregate shells on time to corrosion cracking was investigated herein. Results suggest that certain levels of contamination may be permissible in the design of reclaimed-aggregate reinforced concrete structures. Furthermore, quality control standards that limit thresholds of recycled aggregate replacement ratios and aggregate purity should be based on anticipated exposure conditions and old mortar thicknesses rather than initial degrees of aggregate contamination.     

Modelling of fracture process in concrete using a novel lattice model

Papers deals with simulations of fracture process in quasi-brittle materials like concrete with a novel lattice model. Concrete was described mainly as a three-phase material composed of aggregate, cement matrix and interfacial transition zones. The calculations were carried out for concrete specimens subject to uniaxial extension, shear and extension and three-point bending. Two-dimensional and three-dimensional simulations were performed. The advantages and disadvantages of the proposed model were outlined.     

Theoretical analysis of the measurement of polarisation resistance in reinforced concrete

Polarisation resistance (R_p) technique based on Stern–Geary equation is one of the most widely used methods of measuring corrosion rate of reinforcement in the field. With the aid of a “sensorised guard ring”, this electrochemical technique is claimed to be able to determine corrosion rate (I_corr) within a given measuring area. However, there are three theoretical problems in the application of this technique: (1) the original Stern–Geary equation is applicable in a uniform corrosion system at its corrosion potential, whereas the reinforced concrete structure may be subjected to non-uniform corrosion or strong polarisation by macro-cell galvanic effects or imposed currents; (2) the value of the parameter B in the original Stern–Geary equation has been estimated to fall within the range 25–52 mV. This may not be suitable for all the corrosion cases of reinforced concrete structures; (3) The polarised surface area of steel may theoretically not always be fully confined by the sensorised guard ring when the cover concrete is too thick.

This paper aims at discussing the theoretical problems. A general relationship between the dissolution rate of steel reinforcement and the measured polarisation resistance is deduced under general conditions. The range of B value is also analysed based on all the possible corrosion situations of reinforced concrete. Furthermore, unsatisfactory confinement by sensorised guard ring on a thick cover concrete is demonstrated.     

Influence of corrosion inhibitors on concrete transport properties

The effect on transport properties of admixing corrosion inhibitor in Portland cement based concrete is studied experimentally in this paper. Two commonly available types of corrosion inhibitor have been evaluated: an organic corrosion inhibitor and a nitrite based inhibitor. The gas permeability, water permeability and chloride migration of the resulting concrete were evaluated by means of standard test methods. In some cases, 1% sodium chloride was added to the concrete. It is found that both types of corrosion inhibitor lead to higher transport coefficients. The increment caused by the nitrite based inhibitor is higher than for the amine-ester based inhibitor. The influence of sodium chloride addition during mixing is not prominent. The obtained transport properties are analysed and discussed by means of BSE experiments and image analysis.     

Double shear tests to evaluate the bond strength between GFRP/concrete elements

Externally bonded reinforced systems have been widely used in civil engineering. However, the problems associated with bond between structural elements are not yet fully solved. As a consequence, many researchers have been proposing tests and techniques to standardize procedures and reach better agreement for design purposes. In the present paper, an experimental program is described that was developed to characterize the glass FRP/concrete interface by double shear tests made on 15 cm side cubes with GFRP bonded on two opposite faces. The GFRP wrap had two layers applied by the wet lay-up technique and three classes of concrete were considered. With the support of the experimental program, cohesion and friction angle for GFRP-concrete interfaces were found leading to different envelope failure laws, based on the Mohr-Coulomb failure criterion for each concrete class, capable of predicting GFRP debonding. Results are discussed.     

Evaluation of the stress corrosion cracking behaviour of damage-tolerant Al---Li sheet using the slow strain rate testing technique

Slow strain rate tests were performed on longitudinal tensile specimens of 8090-T81 sheet under permanent immersion conditions in various synthetic environments. Strain rates were in the range 10⁻⁷−10⁻⁴ s⁻¹. Environmentally assisted cracking is observed in aqueous chloride-carbonate-hydrogencarbonate solutions. Near neutral 3.5% NaCl solution and also 3% NaCl solution with hydrogen peroxide added do not promote stress corrosion cracking with 8090-T81 alloy sheet. The degradation of ductility found with tensile specimens immersed in the latter corrosive environments is caused by localized corrosion independent of stress. Fracture energy data obtained from slow strain rate tests in substitute ocean water reveal a large scatter. Again, the deterioration observed is not related to stress corrosion cracking. Slow strain rate tests were also carried out with longitudinal tensile specimens of 2091-T8X and 2091 CPHK-T8X alloy sheet using an aqueous solution of 3% NaCl + 0.3% H₂O₂. For the alloy 2091 CPHK-T8X, similar results were obtained to those with 8090-T81, whereas 2091-T8X sheet is prone to environment-induced cracking in the aqueous chloride-peroxide solution.     

浅谈钢筋混凝土中钢筋锈蚀修补技术

研究钢筋混凝土结构的修补原理、技术与材料对保证钢筋混凝土结构工程的长期、耐久性及使用寿命有着重要的指导意义及实用价值。本文介绍了钢筋混凝土中钢筋锈蚀的有关修补技术。     

Effect of ground granulate blast-furnace slag on corrosion performance of steel embedded in concrete

Corrosion of steel in concrete is one of the major causes of premature deterioration of reinforced concrete structures, leading to structural failure. To prevent the failure of concrete structures because of corrosion, impermeable and high performance concretes should be produced various mineral admixtures. In this study, plain and reinforced concrete members are produced with mineral admixtures replacing cement. Ground granulated blast-furnace slag (GGBFS) has replaced cement as mineral admixture at the ratios of 0%, 25% and 50%. The related tests have been conducted at the ages of 28 and 90, after exposing these produced plain and reinforced concrete members to two different curing conditions. The unit weight, ultrasonic pulse velocity, splitting tensile and compressive strength tests are conducted on plain concrete members. Half-cell potential and accelerated corrosion tests are also conducted on reinforced concrete members. According to the test results, it is concluded that the curing age and type are important and corrosion resistant concrete can be produced by using GGBFS mineral admixture at the ratio of 25%.