Journal of Materials in Civil Engineering

Mechanics and materials engineering are essential elements in all of the transcendent technologies of the twenty-first century and the new economy. The transcendent technologies include nanotechnology, microelectronics, information technology, and biotechnology. Research opportunities and challenges in theoretical and applied mechanics combined with materials engineering in the exciting information age are presented and discussed herein.

Understanding the chemomechanical behavior of shotcrete (sprayed concrete) is a prerequisite for better design or failure risk assessment of civil engineering structures. Macroscopic material laws for shotcrete are preferably based on hydration degree–dependent material functions. For a given shotcrete mixture—that is, for a fixed water-cement ratio $(\mathrm{w}∕\mathrm{c})$ and aggregate-cement ratio $(\mathrm{a}∕\mathrm{c})$ —these functions are determined directly from macroscopic experiments. In a purely macroscopic context, considering changes in the $\mathrm{w}∕\mathrm{c}$ ratio (as often encountered in large tunnel projects), or in the $\mathrm{a}∕\mathrm{c}$ ratio would require additional experiments. As an alternative, such hydration degree–dependent material functions can be predicted by considering quantitative information at the microlevel of the cementitious material, in the framework of continuum micromechanics. In this contribution, we focus on the dependence of the elastic stiffness on the hydration degree. We employ a two-step homogenization procedure: within a representative volume element of 50 to $100\mu \mathrm{m}$ , cement grains (clinker), hydrates, water, and air build up a polycrystal. At a larger scale (5 to 10 cm), the aggregates are represented as spherical inclusions in a matrix built up by the aforementioned polycrystal. The input parameters for the model are $\mathrm{w}∕\mathrm{c}$ and $\mathrm{a}∕\mathrm{c}$ . On the basis of seven microelastic properties that are the same for different shotcretes, the chosen approach allows for the prediction of hydration degree–stiffness relationships for different shotcrete mixtures. The high precision of these predictions is shown by comparing them with experimental values obtained from shotcretes with corresponding $\mathrm{w}∕\mathrm{c}$ and $\mathrm{a}∕\mathrm{c}$ values. Changes of $\mathrm{w}∕\mathrm{c}$ between 0.4 and 0.6, which may be encountered on the tunnel site, result in changes of stiffness differing by a factor of up to two. On a structural level, this may change the estimated (high) level of loading of a shotcrete tunnel shell during the first day after installation by more than 40%.

Accelerated durability assessment is one of the promising ways to provide a sound basis for service life prediction of concrete. To follow the rapid pace of research in modern concrete technology, an effective accelerated assessment system for concrete durability with automatic control and consistent working performance is highly desired. In this paper, the general concepts and ways of implementing computerized feedback control of a multipurpose accelerated concrete durability test system are first presented and applied to frost-resistant (one of the main durability factors of concrete structures in cold regions) assessment of different concretes. The experimental investigations showed that the newly developed automatic feedback controlled system was reliable and had a promising future for the accelerated durability assessment of concretes. Second, based on the premise that the deterioration mechanisms and the surrounding aggressive environment of the concrete under consideration cannot be precisely described by classical logic, it is believed that durability assessment of concrete is indeed a decision-making process and belongs to the fuzzy logic field. Following this line of thought, a general framework of synthetic concrete durability evaluation was developed by combining the principles of various individual accelerated durability tests, such as frost resistance, water permeability, crack width, and carbonation penetration and concepts of fuzzy logic. The working examples showed that the fuzzy synthetic evaluation developed agreed well with the experimental results for concrete durability investigated.

External reinforced concrete elements exposed to chloride and/or $\mathrm{C}{\mathrm{O}}_2$ will eventually have a lower pH, which in turn will depassivate the reinforcement and initiate corrosion, thus causing spalling. This paper seeks to address the complex multiphysics nature of concrete environmental damage, which is governed by coupled nonlinear partial differential equations. Heat, relative pore humidity, chloride, and carbonation are all implemented in a two-dimensional coupled nonlinear finite-difference code. Coupling between carbonation and chloride diffusion is explored in the context of both homogeneous and heterogeneous concrete models. Numerical simulations results are presented.

In this paper, we examine the interaction of thermal and mechanical damage processes in heterogeneous materials such as concrete. After a brief introduction of high temperature effects in concrete, we address two topics: (1) the interaction of thermal and mechanical damage at the mesomechanical level of observations, when volumetric and deviatoric degradation take place simultaneously; and (2) the effect of thermal expansion and shrinkage in the two-phase concrete material when thermal softening of the elastic properties leads to massive degradation of the load resistance.

A distressed material can be considered as a two-phase composite material with the distressed areas (volumes) as one phase and the original material as the other phase. A parallel diffusion model was developed in which the damaged phase and the original phase are arranged parallel to the direction of diffusion. The parallel model is associated with the isogradient principle in that the concentration gradients in the damaged phase and in the original phase are the same. A serial diffusion model was developed in which the two phases are arranged perpendicular to the direction of diffusion. The serial model is associated with the isoflux principle in that the fluxes in the two phases are the same. The two-phase composite models were further extended into general multiphase composite models to evaluate the effect of multilevel damage on the effective diffusivity of distressed materials. A principle of minimum complementary “chemical-flux energy” and a principle of minimum “potential chemical energy” were established similar to the commonly used principles of minimum complementary energy and minimum potential energy. Based on the two new minimum energy principles, it was shown that the parallel model is the lower bound and the serial model is the upper bound for effective diffusivity of distressed materials. Diffusivity of distressed concrete was used as an example. The experimental data agreed quite well with predictions of the theoretical models.

The time-varying performance of deteriorating materials and structures can be simulated using probabilistic methods. The propagation of performance uncertainties is affected by the characteristics of input random variables. In general, the number of samples and/or their quality do not satisfy the rigorous statistical constraints required to obtain reliable probability distributions. This deficiency encountered at the beginning of assessment process can be alleviated later by investigating the importance of variables through variability and sensitivity analyses. These analyses help the identification of significant variables in a quantitative way. In this paper, evaluation of the time-varying performance of deteriorating materials and its effect on structures measured by the reliability index profile is addressed based on three aspects: (1) acquisition of statistical characteristics of input random variables from imperfect samples; (2) model-based simulation; and (3) evaluation of variability and sensitivity measures of the nondeterministic performance model. Examples are presented to illustrate these aspects.

There is an ongoing debate in the chemomechanics community over whether calcium leaching leads to an increase in brittleness or ductility of cementitious materials. While other research teams have addressed this question by size-effect studies of the fracture process of partially degraded mortar beams, we address this question, in this paper, through original results from direct tensile tests on unleached and asymptotically leached cement pastes and mortars. The test device is described first, followed by the presentation and a discussion of deformation and strength results. It is shown that: (1) the tensile strength of pastes degrades much less than that of mortars; (2) the strength degradation in tension is much lower than the compressive strength degradation; and (3) the deformation at failure increases substantially due to chemical softening for both paste and mortar. Finally, by employing different ductility definitions, ranging from deformation to fracture properties, it is concluded that cement-based materials become more ductile in the course of chemical softening by calcium leaching. This finding is substantiated by an analysis of the micromechanical sources of the material failure mechanisms in cement-based materials.

The deterioration process of reinforced concrete structures due to chemical–physical phenomena can be profitably analyzed by means of model-based simulations, performed by using a mathematical/numerical approach. In this work both the model of humidity, pollutant and temperature diffusion, and the mechanical damage approach have been applied to some real cases with the aim of establishing a possible develop-line for the analysis of degradation that can be applied to real engineering structures. The intend is the evaluation of the structural safety and the prediction of the expected service life of reinforced concrete structures.

Hexcel AS-4 12K carbon fibers were sized with two different thermoplastic sizing agents. The sizing agents tested were K-90 poly(vinylpyrrolidone) and modified polyhydroxyether of bisphenol A (Phenoxy™). Hexcel AS-4 36K carbon fibers sized commercially with the standard $G^{\prime }$ was also examined as reference. The sized carbon fibers were processed into composites utilizing a pilot scale pultruder and a pultrudable vinyl–ester resin. Both the thermoplastic sizing materials especially the Phenoxy™ sizing had improved processability over the traditionally utilized $G^{\prime }$ sizing material. These improvements in processability translated into improvements in final composite surface finish and quality. In addition, the thermoplastic sized fibers entered the dip bath stiff, thus minimizing fraying and damage thereby ensuring a better product. The thermoplastic sized composites outperformed the standard sizing $G^{\prime }$ by at least 25% in static tensile strength, by at least 11% in longitudinal flexure strength, by at least 30% in short beam shear strength, and 31% in static compressive strength. The tensile, longitudinal flexure, and compressive modulus appeared to be unaffected by variations in the sizing. Therefore, the mechanical performance of pultruded, carbon fiber/vinyl–ester composites is greatly affected by the nature of the sizing agent present and is believed to be the result of both improved fiber processability and altered interfacial properties. The affect of improved fiber/matrix adhesion is also believed to be a contributing factor but is secondary to the affects of processability and altered interphase properties.

Hexcel AS-4 12K carbon fibers were sized with two different thermoplastic sizing agents. The sizing agents tested were K-90 poly(vinylpyrrolidone) (PVP) and modified polyhydroxyether of bisphenol A (Phenoxy™). In addition, Hexcel AS-4 36K carbon fibers sized commercially with the standard $G^{\prime }$ were tested. The sized carbon fibers were processed into composites utilizing a pilot scale pultruder and a pultrudable vinyl–ester resin. Tension-tension fatigue tests were performed on the composites at a frequency of $10\mathrm{Hz}$ . Residual stiffness and $S–N$ curves were generated. The slopes of the normalized $S–N$ were different for the $G^{\prime }$ sizing and the thermoplastic sizings albeit the slopes of the two thermoplastic sizings were similar. Moisture absorption studies were performed on the composites by immersing the samples in tap water at $65°\mathrm{C}$ . The diffusivity as well as maximum moisture content was observed to be sensitive to the type of sizing. In addition to the fiber-reinforced composites, blends of the sizing (PVP and phenoxy) and vinyl–ester resin were tested to evaluate the influence of sizing concentration on moisture uptake. These immersion tests were performed in a $65°\mathrm{C}$ water bath. Preferential hydrogen bonding between the PVP and vinyl–ester was seen to affect the uptake behavior of the blends as opposed to the pure components. Finally, the residual tensile properties of the saturated composites were obtained and again the influence of the different sizings was seen to be prominent. The residual strengths for the Phenoxy™, PVP, and $G^{\prime }$ composites were 0.85, 0.87, and 0.82, respectively.

This paper addresses proposed improvements to the carbon/vinyl ester interface via the use of engineering thermoplastic polymers as sizing agents. Two thermoplastic polymers, K-30 poly(vinylpyrrolidone) (PVP) and a modified polyhydroxyether of Bisphenol A (Phenoxy™) were used as sizings in a controlled study. These sizings were applied to surface treated but unsized AS-4 carbon fibers using a lab-scale sizing line. The standard and commercially available $G^{\prime }$ sized AS-4 fiber, was also used for comparison. Predictions of observed tensile and compression strengths of unidirectional, sized carbon fiber–vinyl ester composites fabricated using the pultrusion process were assessed using micromechanical models that take into consideration the influence of the different interfaces. The micromechanical models require inputs on either the interfacial shear strength or the global laminate shear response. The influence of both processing induced differences (misalignment of fibers and fiber damage) as well as the differences in interphase properties (shear response) have been shown to play an important role in describing the observed differences in the interphase varied composites.

The compressive strength of concrete is a commonly used criterion in producing concrete. However, the tests on the compressive strength are complicated and time consuming. More importantly, it is too late to make improvements even if the test result does not satisfy the required strength, since the test is usually performed on the 28th day after the placement of concrete at the construction site. Therefore, accurate and realistic strength estimation before the placement of concrete is very important. This study presents the probabilistic technique for predicting the compressive strength of concrete on the basis of concrete mix proportions. The estimation of the strength is performed using the probabilistic neural network which is an effective tool for the pattern classification problem and provides a probabilistic viewpoint as well as a deterministic classification result. Application of probabilistic neural networks in the compressive strength estimation of concrete is performed using the mix proportion data and test results of two concrete companies. It has been found that the present methods are very efficient and reasonable in predicting the compressive strength of concrete probabilistically.

When conventional concrete is used in congested drilled shafts, coarse aggregates may bridge between reinforcing bars, which may lead to segregation of the concrete between the inside and outside of the reinforcing cage. The use of self-consolidating concrete (SCC) was evaluated to overcome this problem. The purpose of this study was to compare conventional drilled shaft concrete and SCC in a drilled shaft application. This paper reports on the data collected during the construction and evaluation of five drilled shafts that were 1.0 m in diameter and 7.3 m deep. Three shafts were constructed with conventional drilled shaft concrete and two shafts were constructed with SCC. The five shafts were exhumed, cleaned, and visually inspected to evaluate the quality of the as-built concrete. Cross sections of the exhumed shafts were cut in order to observe the uniformity and the amount of segregation between the inside and outside of the cage. Based on the results of this project, it is concluded that SCC may be feasible for use in congested drilled shaft applications.