Journal of Materials in Civil Engineering

Corrosion of reinforcement is the major cause of deterioration of concrete structures exposed to chloride in the Persian Gulf region. In this hot marine environment, curing is one of the crucial factors that can affect the chloride penetration into concrete and consequently the service life of reinforced concrete structures. In this paper, the values of diffusion coefficient and surface chloride content were determined in concrete specimens made of two mixture proportions, subjected to six initial alternate curing regimes and exposed to seawater in the Persian Gulf for three months. Finally, the effect of various curing regimes was investigated on the estimation of time-to-corrosion initiation of reinforced concrete structures using the DuraPGulf model.

A complex interdisciplinary approach to the reliability assessment of reinforced concrete structures is introduced and demonstrated on a real existing bridge structure. The reliability index of the structure decreases during its life cycle due to material degradation. A retrofitting to the desired reliability level should be performed. This procedure is modeled by advanced life-cycle computer simulation. The main feature of the presented approach is the nonlinear finite-element analysis of the structure employed for the realistic assessment of structural behavior. A suitable technique of statistical sampling, which allows a relatively small number of simulations, is used in this context. Analytical deterioration models combined with in situ monitoring are used to launch the degradation process. The presented methodology for advanced life-cycle analysis is integrated in the innovative decision-making tool that supports maintenance planning of engineering structures.

The performance of nine commercially available corrosion-inhibiting systems for use in reinforced concrete structures exposed to corrosive environments was assessed in the field on bridge barrier walls and in accelerated electrochemical cells in the laboratory. The corrosion-inhibiting systems included concrete admixtures, reinforcing steel coatings, and/or concrete surface coatings/sealers. The results of this study are presented in two companion papers, in which the field evaluation and laboratory investigation are reported. The field evaluation consisted of annual corrosion surveys of half-cell potential and corrosion rate, as well as remote monitoring with embedded instrumentation for the measurement of the environmental conditions. After five years of investigation, the system containing an inorganic-based admixture performed better than others in reducing the risk of reinforcement corrosion in the barrier wall. It was also found that the system using epoxy-coated reinforcement showed good early performance during the first year, but after, the risk of corrosion increased relatively faster than in other systems, possibly due to localized pitting corrosion developing in small defects or pores of the epoxy coating.

Nine commercially available corrosion-inhibiting systems for use in concrete structures exposed to corrosive environments were evaluated on bridge barrier walls and in electrochemical cells. The results of this study are presented in two companion papers reporting on a five-year field evaluation and an accelerated laboratory investigation. The latter, presented in this paper, included the assessment of the effect of the corrosion inhibitors on the oxidation and reduction reactions by the cyclic voltammetry method, and their effectiveness in delaying or reducing corrosion by measurements of chloride thresholds and corrosion rates. The results indicated that the inorganic admixture and some organic admixtures performed very well in the saturated calcium hydroxide solution (pH of 12.6); however, their performance improvement over the control was not observed in a simulated concrete pore solution (pH of 13.5). Although tests in simulated concrete pore solutions cannot adequately simulate all the complex conditions usually found in the field, they were very useful to rapidly identify the effects of the corrosion inhibitors on the corrosion reactions, and to provide supporting information to the corresponding field evaluation.

This study attempted to develop a model for the stress-strain relationship in compression of frost-damaged concrete subjected to fatigue loading. Concrete specimens were prepared and exposed to freeze-thaw cycles followed by application of static and fatigue loading. The strains induced during the freeze-thaw test were carefully measured as well as during a mechanical loading test. It was found that the static strength and the fatigue life of concrete decreases as increasing irreversible tensile strain was induced by frost action. A stress-strain model for frost-damaged concrete under application of static and fatigue loading based on the degradation of initial stiffness caused by frost damage was presented. The degradation of initial stiffness for damaged concrete was empirically formulated as a function of remaining expansion caused by freeze-thaw cycles. The plastic strain under the application of mechanical static and fatigue loading for frost-damaged concrete is higher than that for original concrete. Therefore, plastic strain for damaged concrete was formulated as not only the function of strain level under mechanical loading, but also the function of irreversible strain caused by frost action. The unloading and reloading stiffness factors were introduced to explain the change of stiffness as increasing the number of loading cycles by considering the effect of the degree of frost damage.

On the basis of micromechanics consideration of frost damage, a theoretical model to describe the freeze-induced strain behavior of porous materials such as concrete is presented. The model is derived from the thermodynamic equilibrium condition. Fundamental equations are derived on the microscopic level where temperature-induced phase transitions and the resulting mass transfer within the pore structure are taken into consideration. The local deformation obtained from the microscopic considerations around pores is then averaged to evaluate the nominal strain on a macroscopic level. Components of the macroscopic deformation in the presented model are the thermal strain, the expansion due to the internal pressure caused by the phase transition, the smeared cracking strain due to accumulation of microcracks, and the shrinkage due to the mass transfer caused by the microice-lens mechanism. Experimental studies are carried out with mortar specimens of three different mix proportions. The theoretical prediction is then compared with the experimental results and a good agreement is obtained.

An experimental research program was carried out in order to validate a methodology for the mechanical assessment of structures damaged by alkali-silica reaction (ASR). Long-term deformations of specimens kept under varied environments were measured. Results were collected according to the French methodology for measuring residual expansion, on cores drilled from cylinders and prisms made of the same concrete materials. Numerous testing conditions were intended to check the experimental robustness of the most significant input data for structural assessment. It appears that omitting the measured expansion of the specimens during a rather long period $\left(60\text{days}\right)$ , as presently recommended, can lead to an unsafe estimation of ASR-residual expansions, especially when the core is drilled during the active phase of ASR development. The strains of the specimens during the first days of the test are caused by the superimposition of concrete expansion due to water sorption and additional ASR expansions. Evolutions and improved conditions of application relative to the French standard method for estimating residual expansion of ASR-affected concrete are proposed.

This paper describes the resistance of mortar specimens with two different recycled fine aggregates when they were exposed to sodium and magnesium sulfate solutions up to $15\text{months}$ . The important difference between the recycled fine aggregates used in this study was the water absorption. Control mortar specimen made with only river sand was also tested to compare the degree of deterioration by sulfate attack. The tests include visual examination, compressive strength ratio, expansion and mass loss, as well as microstructural observations. The experimental data indicated that the use of 50% replacement level of recycled fine aggregates led to the increased or similar resistance to both sodium and magnesium sulfate attacks. At the 100% replacement level, however, mortar specimens containing recycled fine aggregate with higher water absorption suffered serious damage in mechanical properties, and the deterioration degree is likely to be more pronounced with further exposure. This work suggests that the absorption characteristics and replacement levels of recycled aggregates have a decisive influence on the sulfate resistance of mortar specimens.

The fireproofing materials used to make firebreak systems are usually hydrated-calcium sulfate, i.e., gypsum or hydrated-calcium silicate panels. Gypsum presents a large latent heat due to its dehydration into hemihydrate and anhydrite. During a fire exposure, the gypsum panels are characterized by an isothermal stage on the unexposed side because of the latent heat effect. This dehydration induces cracks inside the panels, furthermore, the heat flow goes through these cracks and increases the temperature of the unexposed side. The dehydration leads to a large thermal shrinkage, due to the loss of bound-water molecules. In this study, a silica filler was introduced into the plaster. This filler modifies the thermal, physical, and mechanical properties of the gypsum panels and reduces the cracks’ formation. An optimum filler amount is identified. The thermal behavior of gypsum panels and commercially available calcium silicate products exposed to fire are then compared under a standard fire. Above a critical temperature (greater than $\mathrm{1,000}°\mathrm{C}$ ), a melting front diffuses through the material and affects its permeability, as well as its mechanical and geometric properties (thickness). Summing up, the development of new fireproofing materials (exhibiting a real high-temperature resistance) requires mastering high-temperature shrinkage, as shown in the example developed at the end of the paper.

In this study, freezing and thawing tests were conducted on lightweight concrete and aggregate at different freezing rates (one cycle per day and six cycles per day) in order to investigate their freeze–thaw resistance and clarify the effects of various lightweight aggregate properties (density, water content, crushing strength, and pore structure) and freezing rates on their freeze–thaw resistance. The results indicate a high and direct correlation between the freeze–thaw resistance of lightweight concrete and aggregate. The higher density lightweight aggregate concrete showed superior freeze–thaw resistance at each freezing rate, while concrete made with the lower density aggregate achieved freeze–thaw resistance only at the low freezing rate. The aggregate tests showed similar results, too. The freeze–thaw resistance can be influenced by the properties of lightweight aggregate and the freezing rate, but the most fundamental factor is the pore structure of the aggregate. Lightweight aggregate, which has lower pore volume and more small pores, can have good resistance to freezing and thawing.

An experimental program was carried out to clarify the effect of chloride-induced corrosion on posttensioned concrete (PC) beams under different grout conditions using the electrically accelerated corrosion testing method. A series of accelerated corrosion tests were performed to evaluate the influence of insufficient grout condition on corrosion of sheath and prestressing tendon. The radial pressure surrounding the sheath during the corrosion process was also investigated. After a period of accelerated corrosion, the mechanical behavior of the deteriorated PC beams was clarified through loading tests. Prestressing tendon corrosion and its correlation with beam load-carrying capacity was also investigated. The results of the experiments demonstrate a significant influence of corrosion of sheath and prestressing tendon on load-carrying capacity of PC beams. The width of the corrosion crack in PC beams increases with grout filling level inside the sheath. The presence of water inside the sheath leads to increased rates of corrosion of the sheath and prestressing tendon, resulting in an earlier deterioration in the load-carrying capacity of PC beams.