Journal of Materials in Civil Engineering

This paper reports on an analysis and experimental verification of the effect of cylindrical anisotropy of softwood loaded in the radial r and tangential t growth plane, i.e., perpendicular to grain. This off-axis loading mode is especially design-relevant for curved and tapered glulam beams. The very low shear stiffness in the r,t on-axis growth plane, together with a complex shear coupling effect in polar coordinates, contributes essentially to extremely variable off-axis strain and stress fields. Thus far, no explicit empiric proof exists for these effects with considerable consequences on damage-relevant stresses. The exemplary off-axis investigations were performed with a cross-sectional slab of a spruce board loaded in tension parallel to the longer edges. True on-axis stiffnesses were determined for the simulations from two on-axis specimens cut from the test volume. Throughout a good agreement between the finite-element predictions and the measured results was obtained. The experiments, for example, proved that off-axis strains parallel and normal to load axes may vary along specimen length by factors of about 4.5 and 9, respectively.

Corrosion rate studies were carried out on carbon steel rebar samples under different pH conditions and in the presence and absence of chloride ions in solution. A known amount of calcium nitrite was added as an inhibitor and the mechanism of inhibition was studied by tracking both the thermodynamic and kinetic properties of the system. The studies indicate that there is a competition between the corrosion and passivation reactions, and the resulting open-circuit potential depends on the relative strength of the corroding and passivating environments. The corrosion rate depends to a great extent on the pH of the solution. Nitrite ions act as anodic inhibitors by increasing the rate of formation of a barrier oxide film. The protective action of the nitrite ions seems to be more pronounced in highly corroding environment. This is due to the mechanism of inhibition, which uses the product of the unwanted corrosion reaction and converts it into a favorable passivating one. For a given amount of chloride, a minimum threshold concentration of nitrite is essential for protecting the steel.

Various forms of heat straightening have been used by fabricators and erectors for many years. However, it has only been in the last 15 years that heat-straightening procedures have begun to be quantified in engineering terms. There has been a general lack of information on the type and distribution of residual stresses that occur in heat-straightened rolled shapes. The purpose of this paper is to report on an experimental study in which residual stresses were measured in heat-straightened plates and rolled shapes. Residual stress patterns were measured for members in which various heat-straightening parameters were varied. It was found that the most reliable data should be obtained from tests on damaged members that have been completely heat straightened. The residual stresses in heat-straightened plates were fairly consistent, having maximum compression stresses of about 159 MPa (20 ksi) at the edges and tension stresses of about one-half that value at the center of the plate. Residual stresses in heat-straightened angles tended to have maximum values approaching yield in compression at the toes and heel. Relatively high tension stresses were found near the middle of each leg. Maximum residual stresses in wide flange beams tended toward the yield stress for both major and minor axes heating patterns. High residual stresses may help or hinder heat straightening. Since jacking forces are usually applied as part of the heat-straightening process, movement could be hindered by the distribution of residual stresses. In addition, overstress may occur due to the combination of residual stresses and jacking stresses. Finally, large compression residual stresses may produce bulges in the compression elements of a cross section. Special heating patterns and sequences may be required to prevent this effect.

An efficient method of fitting Prony-series models to viscoelastic experimental data with power-law presmoothing is presented. A direct fitting of a Prony-series function to experimental data without appropriate presmoothing is difficult when the data have significant variance. A power-law series comprising multiple power-law terms is found capable of portraying a globally smooth, broadband viscoelastic behavior with minimal impact from local variance in the data. However, from a computational point of view, a Prony series representation is preferred to a power-law series representation because of the computational efficiency associated with the exponential basis functions of a Prony series. In this paper, a procedure involving presmoothing of experimental data via power-law-series representation followed by fitting of a Prony-series model to the presmoothed data is discussed and illustrated. Discussion focuses on how experimental data with significant variance can be effectively presmoothed through power-law series representation and how presmoothing improves the quality of the subsequent Prony series fit. The feasibility and effectiveness of the approach are demonstrated through numerical examples in which Prony-series representations of the creep compliance and relaxation modulus of asphalt concrete are determined from available experimental data.

Ideal and optimum fineness moduli are used in combination with the geometrical gradation of aggregates to establish the mix proportioning of mass concrete. The ideal and optimum fineness moduli are experimentally determined for each maximum size of aggregate (MSA) by making concrete mixtures with aggregates of different fineness modulus and determining which of the mixtures give the highest compressive strength for a given cement-to-aggregate ratio. For each MSA, an adequate number of cement-to-aggregate ratios is selected and at least three different water-to-cement ratios are used in order to cover a broad range of strength. Also, for each cement-to-aggregate ratio, five mix proportions with different amounts of sand are studied. Geometrical gradations are used for comparison and for smoothening-out proposed gradations. Geometrical gradation is also utilized as a criterion for the concrete mix proportioning when large aggregates (152 mm) are used. Experimental results are used to exemplify the method.

The premature deterioration of reinforced concrete structures is a serious problem in hot coastal environments. Most of this deterioration is caused by chloride ingress through the concrete, resulting in the corrosion of the reinforcing steel. Since the mid 1990s the practice of specifying silica fume in concrete has widened in the Middle East, as engineers work to improve the durability of concrete, which is subjected to the extreme exposure conditions in the region. Current practice is to use mixtures with a maximum water/cement ratio of 0.4 and to replace 5% to 10% of the cement with silica fume depending on the degree of exposure. Are these small amounts of silica fume sufficient to maintain an appropriate life span for concrete structures under the varying exposure conditions, and how much cover is required to the reinforcement to protect the steel from corrosion? In this paper, models are proposed to estimate the chloride ion ingress into portland cement concrete and silica-fume-enriched concrete under different exposure conditions. The results show that small amounts of silica fume are sufficient as long as appropriate concrete covers to reinforcement are maintained and chloride exposure to the concrete is correctly assessed.

A combination of gravimetric, internal relative humidity, and electrical measurements were used to study moisture changes and spatial distribution of moisture within the surface 100 mm of concrete. Specimens were subjected to a repeated drying and wetting regime. The testing program highlighted the influence of binder type, water/binder ratio, initial curing regime, and cyclic drying/wetting on surface-zone absorption and desorption characteristics. It is shown that electrical measurements taken at discrete points within the surface 100 mm can be used to estimate the zone of concrete most influenced by evaporative and wetting processes. A relationship was established between the integrated area between conductivity profiles and cumulative water uptake by the surface region. Over the 12-month test period, electrical conductivity measurements also indicated ongoing hydration and pozzolanic reactions.

Masonry is a material that exhibits distinct directional properties because the mortar joints act as planes of weakness. To define failure under biaxial stress, a 3D surface in terms of the two normal stresses and shear stress (or the two principal stresses and their orientation to the bed joints) is required. This paper describes a method to define a general anisotropic (orthotropic) failure surface of masonry under biaxial stress, using a cubic tensor polynomial. The evaluation of strength parameters is performed using existing experimental data through a least-squares approach. The validity of the method is demonstrated by comparing the derived failure surface with classical experimental results. The ability to ensure the closed shape of the failure surface, the unique mathematical form for all possible combinations of plane stress, and the satisfactory approximation with the results of the real masonry behavior under failure conditions are some of the advantages of the proposed method.

Physicochemical and microstructural developments of a low-lime fly ash modified with 6 and 10% lime and 1% gypsum were studied through X-ray diffraction, differential thermal analysis, scanning electron micrograph, and energy-dispersive X-ray microanalysis tests to gain information on the fly ash-lime and fly ash-lime-gypsum interaction. Specimens were cured up to 10 months at 30 ± 1°C, which prevails commonly in the Tropics. X-ray diffraction test results showed the appearance of new peaks, some of which are not so prominent, indicating that the new phases formed may be in small quantity or in an amorphous state. Scanning electron micrographs of specimens modified with both lime and gypsum showed evidence of the development of a compact matrix at 3 months' curing and a densified compact network of pozzolanic reaction products of fly ash-lime-gypsum with the increase in curing period to 10 months. Further, this development was substantiated by improvement in strength and durability and reduction in permeability of the stabilized material. The modified fly ash with improved engineering characteristics has potential for applications in civil engineering construction.

Roller-compacted concrete (RCC) has proven to be a cost-effective material and construction method in the United States and throughout the world for water resources projects. A methodology for RCC mix design based on the so-called soils approach is proposed for small- to medium-sized projects. The methodology consists of selection of a suitable RCC aggregate, moisture contents to prepare RCC specimens, cementitious material contents, and determination of mix proportions. Specific guidelines are provided for using standard highway base courses for RCC aggregates. Recommendations are given for the need of future research. A variation in specimen size and preparation procedure for durability testing is proposed.

The size of an asphalt concrete test specimen has been traditionally determined based on experience and practical purposes. This study discusses theoretical considerations for selecting the minimum dimensions so that the resulting size of the specimen represents global material properties. The ideal size of the specimen was determined using an image analysis approach and was verified with mechanical tests. The image analysis was accomplished by studying the internal structure of asphalt concrete. The internal structure was captured using a nondestructive X-ray computer tomography technique. Mechanical testing was accomplished by using the Superpave Shear Tester on laboratory-prepared samples. The laboratory samples had dimensions that were both smaller and larger than the theoretically determined representative size. Statistical analysis indicates that the variability of the results is a function of the loading mechanism used in the Superpave Shear Tester. In general, the required representative size increased considerably when the nominal aggregate size was larger than 25.0 mm.

The fracture behaviors of normal strength concrete (NSC) are well understood after about 20 years of worldwide research. As the strength of concrete increases, its fracture properties become more important, because on the one hand high-strength concrete (HSC) is more often used in complicated structures that are vulnerable to damage by external loads, and on the other hand concrete becomes more brittle as its strength increases. And as the strength of concrete increases, the internal damage pattern changes greatly, so it is not proper to transfer knowledge of the fracture properties of NSC directly to HSC. The test results of fracture properties of HSC as well as NSC are presented and compared with the empirical formulas of CEF-FIP Model Code 1990 (MC90) in the paper. It is found that (1) tensile strength and elastic modulus of HSC match MC90's formulas, as well as NSC, and (2) fracture energy and characteristic length of HSC are lower than predictions from MC90's formulas, while those of NSC are slightly higher than predictions from MC90's formulas.