Journal of Materials in Civil Engineering

A fatigue model based on cumulative damage is developed for predicting the fatigue life of fiber-reinforced polymeric composites. This model incorporates applied maximum stress, stress amplitude, loading frequency, residual tensile modulus, and material constants as parameters. The model is verified with experimental fatigue data on a glass fiber/vinyl ester composite. While the specimens are exposed to air, freshwater, or seawater at 30°C, they are subjected to tension-tension stress at four levels of applied maximum tensile stress in each of two frequencies. Both the residual mechanical properties at specified loading cycles and the number of cycles at which the specimens fail are measured. The results show that, for the material used in this study, the loss in residual tensile strength and modulus in saltwater is approximately the same as that in freshwater and that the fatigue life in these aqueous environments is shorter than that in air. Numerical analysis is carried out to determine the material constants of the composite. The fatigue model agrees well with the experimental data. The model can be used to predict the fatigue life of the polymeric composites subjected to an applied load in different environments and to predict the residual tensile modulus after a number of cycles of service at a given load.

A fatigue performance prediction model of asphalt concrete is developed from a uniaxial constitutive model based on the elastic-viscoelastic correspondence principle and continuum damage mechanics through mathematical simplifications. This fatigue model has a form similar to the phenomenological tensile strain-based fatigue model. Therefore, a comparison between the new model and the phenomenological model yields that the regression coefficients in the phenomenological model are functions of viscoelastic properties of the materials, loading conditions, and damage characteristics. The experimental study on two mixtures with compound loading histories demonstrates that the fatigue model maintains all of the strengths of the constitutive model such as its accuracy and abilities to account for the effects of rate of loading, stress/strain level dependency, rest between loading cycles, and mode-of-loading on fatigue life of asphalt concrete.

Recycled polyethylene (RPE) modified asphalts were prepared and characterized. The RPE was modified with different copolymers before incorporating into the asphalt. A special morphology was found for this system. The RPE particles are highly swollen by asphalt. This morphology is attributed to physical interactions. The presence of copolymers in RPE was found to change the RPE's affinity toward asphalt constituents. Aging of asphalt, oil absorption by polymer particles, and interactions of polymer particles with asphaltenes affect the overall rheological properties of polyethylene modified asphalts. Attempts to approach the rheological behavior of polymer modified asphalt with rheological models for filled polymers were not found to be conclusive. Other specific features of asphalt must be taken into account. These include, for example, changes in the structure that were evidenced by their relaxation time spectra and molecular weight distributions. The polymer modification of asphalt affected the asphalt properties. Well controlled thermomechanical history experienced by the asphalt is needed to differentiate between the chemical changes of the binder due to the mixing process and the beneficial effect of the polymer for high-performance asphalts.

Some states have recommended changing the definition of flat and elongated aggregates from 5:1 to 3:1 and an increase in the maximum allowable percentage of flat and elongated (F&E) particles to 20% or higher in the Superpave aggregate specification. Engineering justification and performance data are required to rationally establish the criteria for this change. The objectives of this research included the statistical analysis of performance indicators of Superpave mixtures containing different levels of 3:1 particles. Three mixes were designed and tested using 0, 15, and 40% of 3:1 particles from a set of Kentucky limestone aggregates. This paper contains the findings, conclusions, and recommendations on the research. The mixtures did not differ significantly in their performance-related properties at lower and higher temperatures. However, at intermediate temperatures, significant differences were observed. A curve shifting analysis confirmed the presence of lower shear stiffness with an increase in the percentage of 3:1 F&E particles. The mix densification trends were inconclusive. Additionally, at low temperatures, the mixes with increasing percentages of 3:1 F&E particles seemed to have higher tensile strengths. This particular finding needs to be further investigated. Overall, increases up to 40% of 3:1 F&E particles did not adversely affect the performance potential of such mixes as measured by the Superpave methodology.

With greater emphasis now being placed on the durability of concrete and the need for on-site characterization of concrete for durability, there is an increasing dependence on the measurement of the permeation properties of concrete. Such properties can be measured in the laboratory under controlled ambient conditions, namely, temperature and relative humidity, and comparisons made between samples not affected by testing conditions. An important factor that influences permeation measurements is the moisture state of the concrete prior to testing. Moisture gradients are known to exist in exposed concretes; therefore, all laboratory tests are generally carried out after preconditioning to a reference moisture state. This is reasonably easy to achieve in the laboratory, but more difficult to carry out on-site. Different methods of surface preconditioning in situ concrete are available; however, there is no general agreement on the suitability of any one method. Therefore, a comprehensive set of experiments was carried out with four different preconditioning methods. Results from these investigations indicated that only superficial drying could be achieved by using any of the preconditioning methods investigated and that significant moisture movement below a depth of 15 mm was not evident.

This paper describes axial compression test results of 27 concrete cylinders confined by carbon fiber reinforced polymer composite jackets. The experimental parameters include plain concrete compressive strength and the thickness of the composite jacket. It is found that the carbon fiber composite jacketing can significantly increase the compressive strength and ductility of concrete. The test results indicate that concrete strength and confinement modulus, defined as the ratio of transverse confinement stress and transverse strain, are the most influential factors affecting the stress-strain behavior of confined concrete. The failure of the confined concrete was dominated by the rupture of the jacket at an average strain much smaller than the ultimate strain obtained from tension tests of flat coupons. In order to describe the main mechanical features of the confined concrete, a simple bilinear stress-strain model is suggested based on the theory of elasticity and minimum number of empirical equations determined from the tests. The simple model is shown to compare well with test results from previous studies by other researchers.

This study proposes a new theoretical approach for predicting the tensile stress-strain relation of random short-fiber-reinforced cement composites showing pseudostrain hardening. This approach is grounded on the solid basis of micromechanics, which describes the pseudostrain hardening phenomenon in terms of constitutive properties of the fiber, matrix, and fiber/matrix interface. The proposed modeling requires theoretical treatment of an inelastic strain due to multiple cracking. This modeling is achieved by employing a probabilistic description of initial flaw size distribution, which should be known for predicting the stress-strain relation. This study proposes a practical method for this identification using the tensile test result of a reference composite. A comparison with the test data indicates that the proposed model is capable of reasonably reproducing the stress-strain relation of “similar” composites. Such composites have a configuration similar to the reference configuration but different in fiber volume fraction and fiber length. Finally, the proposed theory is a potentially powerful tool for tailoring composites to satisfy targeted structural performance.

Microstructural, chemical, mineralogical, and thermal analysis was carried out in a study to use Eraring Power Station (New South Wales, Australia) fly ash as pavement base material. The fly ash was stabilized with cement and lime separately. The effects of cement and lime stabilization were examined in terms of chemical composition, crystalline structures, and hydration products. Unconfined compressive strength of samples was also measured to observe the effect of stabilization over time. The results obtained from cement and lime-stabilized samples showed that the hydration products that account for strength gain of the stabilized fly ash are almost the same for both types of stabilizing agents. The discrepancies between the resulting strengths are due to the proportion and density of these products.

A marine environment may be characterized by tidal, submerged, and atmospherically exposed zones. Cathodic protection (CP) of reinforced concrete in the submerged zone may induce reducing conditions associated with immunity to corrosion characterized by negative steel potentials. In the atmospheric zone CP induces an improvement in the local environment at the steel, promoting steel passivity, which may be monitored using a potential decay criterion in relatively dry conditions. Many protective effects will persist following the interruption of the current, providing a basis for intermittent CP in the tidal zone. However, conventional criteria may not be reliable in this case. The importance of the current-induced improvement in the tidal zone environment provides justification for the use of a protection current density criterion. The integrated current densities required to arrest high initial corrosion rates are relatively low. Protection criteria may also be based on a positive trend in measured potentials indicative of passivating steel and the induced voltage drop through the concrete cover that drives the removal of chloride ions and sustains a high hydroxyl concentration gradient between the steel and the external environment.

The influence of steel fibers on the fatigue resistance of concrete in direct compression, based on experiments, and an assessment of it through neural network modeling have been presented. Both straight and hooked fibers were incorporated in the concrete. The volume fraction was 1% for straight fibers, and both 1 and 2% for hooked fibers. The applied stress levels ranged from 0.55 to 0.95 of the (static) compressive strength. The number of cycles was up to 1,000,000. The neural network was based on a feedforward back-propagation algorithm. Steel fibers enhanced the fatigue strength, distinctly, at all stress levels. Hooked fibers improved the fatigue strength more than the straight ones; the larger the fiber content, the greater was the enhancement of fatigue strength. Strains measured just before failure showed that fiber reinforced concrete could undergo larger strains before failure than plain concrete. Based on strains measured throughout the fatigue cycling, stages of dilation and failure have been apparent. The adopted neural network modeled both the S-N (strength–number of cycles) and strain-N behaviors extremely well. It was considered possible to extrapolate the S-N behavior beyond 1,000,000 cycles in order to estimate the fatigue strength in the super-high cycle ranges of 10⁷ and more.

The ultrasonic pulse velocity technique was used to assess the structural integrity of timber elements in monumental buildings. Six species of timber of different strength values were selected to establish the pulse velocity ranges. The effect of moisture content, specific gravity, internal voids, and external voids on the pulse velocity was measured from tests on different samples. The pulse velocity perpendicular to the grain was found to increase with increase in specific gravity or compressive strength parallel to the grain. A correlation between compressive strength parallel to the grain and ultrasonic pulse velocity perpendicular to the grain was established from test results on six species of timber. A condition-rating system was developed to estimate the strength and to assess the extent of deterioration or the reduction in strength in timber beams. Tests on laboratory samples and extensive field studies on large timber joists and beams of a nearly 140-year-old church building suggest that the recommended procedure can be used effectively for in-service assessment (nondestructively) of wood.