Journal of Materials in Civil Engineering

For many wood species, the grading of timbers [>76 mm (>4-in.) thick] has not changed in decades. Most timbers are still visually graded by methods that originated in the 1930s. Mechanical grading procedures used to accurately grade 38-mm (2-in. nominal) dimension lumber have not been adapted for use with timbers. Furthermore, the only reliable timber test data available for most softwood species were obtained near the turn of the 20th century, and no test data are available for hardwood timber. The lack of better grading procedures and test data makes it likely that current property assignments for timbers are wasteful. Given that the reduced availability of lumber is expected to continue well into the 21st century, the structural engineer must have the widest possible options for structural timbers. Therefore, a study was conducted by the USDA Forest Service, Forest Products Laboratory, to examine use of a pulse echo stress wave technique along the length of timbers to estimate modulus of elasticity (MOE) of green timbers and the development of a machine stress rated grade for oak timbers. Two hundred and ninety-five untreated 178- by 229-mm (7- by 9-in.) mixed oak timbers, 4.9 m (16 ft) in length, were measured, weighted, and nondestructively evaluated to determine their pulse echo MOE. Two hundred twenty were destructively tested in bending; 75 were tested to failure in compression parallel to the grain. The results of these tests determined relationships between pulse echo MOE and static bending MOE and between bending and compressive strength. These tests suggests a clear potential of establishing a machine stress rated grade for oak timbers. This article discusses the implication of these results, potential commercial grades, and a follow-up study to qualify the suggested grades.

The durability of coarse aggregate has a major impact on the durability of Portland cement concrete. Thermogravimetric (TG) analysis is a relatively rapid technique that has shown good potential in previous work for its usefulness in the analysis of carbonate aggregates. In this work, the TG technique is used to analyze 31 limestone aggregates and two dolomitic aggregates through heating the specimens to a temperature above 1,000°C and recording the weight loss and the change of weight loss as a function of temperature. Results show that limestone aggregates yield typical TG curves and a major weight loss takes place around 900°C. Dolomitic aggregates yield typical TG curves with a two-stage weight loss. The first stage is due to dolomitic loss, which occurs around 700°C while the second is a calcite loss similar to limestone aggregates. Results also indicate that a correlation exists between the slope prior to calcite transition in TG curves and some durability aspects of limestone. The TG data can be used to calculate the percent acid insoluble and the pavement vulnerability factor. These values are equivalent to conjugate values determined using the conventional methods. Therefore, TG can be considered a relatively rapid method for acid insoluble and pavement vulnerability factor determination.

The primary objective of this study was to investigate the behavior of concrete subjected to repeated biaxial compressive loading. Square plate specimens of concrete were tested under in-plane repeated loading that included two different unloading schemes (complete unloading to zero and partial unloading to 0.2f_c stress level) and five stress ratios (α = 0, 0.25, 0.5, 0.75, and 1.0). The stress-strain curves of the specimens in each test series were measured and presented here. The regular patterns of strength, strains at the peak stress, residual strains, envelope of stress-strain curves, locus of common points, and unloading and reloading curves are analyzed. An analytical model that predicts the loading, unloading, and reloading curves of concrete subjected to repeated biaxial compressive loading with complete and partial unloading is proposed. The deformation characteristics of concrete under repeated biaxial compressive loading are compared with those under monotonic biaxial compressive loading and repeated uniaxial compressive loading.

Steel fiber is the most common type of fiber reinforcement in cementitious materials. The reinforcement efficiency, in terms of maximum crack bridging force and total energy absorption during fiber pullout, is a function of many parameters including the properties of fiber, matrix, and interface as well as fiber size, volume fraction, geometry, and distribution. In this investigation, we focus on the effect of fiber yield strength on reinforcement efficiency. Fiber pullout specimens are fabricated with fibers of different yield strengths. Because cracks in a concrete member may intersect fibers at different angles, pullout tests are carried out with the fiber inclined at 0, 30, and 60°. The experimental results indicate the existence of an optimal fiber yield strength for the maximization of pullout load and pullout work. Inspection of the pullout curves as well as microscopic studies show that a higher fiber yield strength will result in more severe matrix spalling around the fiber exit point, thus limiting the further improvement in reinforcement efficiency. The results confirm the qualitative trend predicted by an existing theoretical model and indicate that the fiber yield strength is an important parameter that should not be overlooked in the design of fiber reinforced cementitious composites.

An experimental investigation was undertaken to study the flexural fatigue behavior of a stabilized fiber-reinforced pavement base course material composed largely of recycled concrete aggregate with small amounts of portland cement and fly ash. The primary objectives of this endeavor were (1) to evaluate the fatigue resistance of this material; and (2) to determine the extent to which a modest amount of reinforcing fibers (4% by dry weight) can improve the flexural fatigue behavior of this lean cementitious composite. In addition to the repeated load tests, a separate series of static flexural tests and a series of compressive and flexural tests (using notched specimens) were conducted on pieces of failed beams to establish strength correlations to better estimate the static strengths of the specimens; this strength was used to determine the final stress ratio for each beam. The data obtained from this test program showed that a stabilized base course material consisting primarily of recycled aggregate with only 4% cement and 4% fly ash (by weight) has a fatigue strength and endurance limit comparable to virtually all typical stabilized highway materials. The inclusion of 4% (by weight) hooked-end steel fibers significantly improves this material's resistance to fatigue failure. In general, the results of this investigation suggest that a recycled aggregate composite consisting primarily of waste materials has significant promise as a base course for highway pavements.

Polymers are being increasingly used for modification of bitumen to enhance the properties of bituminous mixes. This paper reports the procedure for modification of 80/100 penetration-grade bitumen using ethylene vinyl acetate copolymer. Some of the basic physical properties of the bitumen and the modified binder are presented. The mechanical properties of the mixes containing such binder have been studied and compared with neat bitumen. It was found that 5% EVA concentration in modified binder by weight is adequate in terms of the enhanced properties studied.

Four tests for measuring the surface properties of concrete have been studied: (1) The initial surface absorption test; (2) Figg's air permeation index (Figg) test; (3) the covercrete absorption test (CAT); and (4) the air permeability of the near surface test. Analytical models are presented for each of the tests to relate the results to fundamental properties of the concrete. Experimental results are presented for the application of a vacuum technique for preconditioning concrete in situ prior to the Figg test and CAT. The application of vacuum using the initial surface absorption test cap did not lead to satisfactory results with these tests. However, direct application of vacuum to the Figg and CAT heads prior to testing resulted in improvement of the reproducibility of the permeation indices obtained from these tests. The analytical models were then used in combination with the experimental results to draw conclusions about the choice of test method for practical applications.

This article focuses on a laboratory materials-testing program that was aimed at: (1) identifying candidate shrinkage-compensating concrete (SCC) mixture designs for possible later field evaluations; and (2) performing mechanical property evaluations of the candidate mixtures under various simulated weather and curing conditions. The SCC mixtures were produced by using a Type K portland cement. Results of the laboratory concrete mixture design, property evaluation, and sensitivity testing verified that SCC mixtures should be viable mixtures for bridge decks to reduce shrinkage cracking. The testing revealed that: (1) time-of-set of SCC was faster than for ordinary portland cement concrete by about 100 min, as was the rate of slump loss; (2) the compressive and splitting tensile strengths of the SCC mixtures were higher than those of conventional concrete mixtures using Type I portland cement; (3) the compressive strengths of concrete mixtures with both the Type K and Type I cements were enhanced by the addition of silica fume; (4) the SCC concrete restrained length bar specimens exhibited significant early (first 7 days) expansion; (5) type K cement restrained mortar bars exhibited ∼4 times more expansion than mortar bars made with Type I cement; and (6) type K cement mixtures are quite sensitive to proper moist curing if they are to achieve proper shrinkage compensation.

This paper outlines the theoretical foundations governing the rheological properties of concrete. The rheological equation for fresh concrete is given as a modification of Bingham's equation. The shear resistance of the concrete mix is explained in terms of two constants—yield value and plastic viscosity. The complex nature of the yield value is described. It is pointed out that there are three different zones of rheological behavior of fresh concrete, which arise as a result of the relation between shear stress and yield value of the mix. This paper also describes an experimental study that measures the effects of superplasticizer, water-to-cement (w/c) ratio, condensed silica fume (CSF) and elapsed time on the rheological parameters for fresh concrete. The rheological tests were carried out on a ROD1E rotational rheometer. In total, 102 flow curves were obtained from which the yield values g and the plastic viscosities h for a wide range of concrete mixes were determined. The results showed that superplasticizers became less effective with increase in w/c ratio and that different superplasticizers have widely different effects on changes in g and h. It was also established that CSF additions generally reduce workability, although above a certain CSF content a sharp reduction in plastic viscosity occurs. In addition it was observed that superplasticizers generally extend the period over which a mix remains workable. The results are discussed in terms of the theoretical foundations.

TEMPCORE is a patented mill process that produces high-strength reinforcing steel bars via in-line quenching and self-tempering. This paper reports on the fatigue performance of 400 MPa-grade reinforcing steel produced by TEMPCORE process, called TEMPCORE steel in this paper. Seven bar diameters were axially fatigue-tested in air, and the results in terms of S-N curves and failure features are presented, and compared with test data for other types of reinforcing steels from the literature. Comparisons indicate that this type of reinforcing steel has higher fatigue resistance. Although the general geometrical features are similar to those of other types of reinforcing steels, existing models fail to predict its fatigue properties. It is suggested that the metallurgical microstructure and residual stresses developed during the TEMPCORE process contribute to the superior fatigue properties and must therefore be considered in the models.

This paper presents the results of a study on the freeze-thaw durability and deicer salt scaling resistance of particular non-air-entrained very high strength cement-based materials. Variables investigated were the microsilica content (0, 10, and 20% by weight), as replacement of an equal amount of cement, and a different quantity of randomly dispersed steel microfibers (0, 2, 4% by volume). Very little research has been undertaken regarding this subject, and these materials are normally considered to be frost resistant because of their intrinsically low porosity. In addition to standard durability tests, compression and bending experiments have been conducted in order to compare the strength and the global response of both reference and freeze-thaw specimens. Tests confirmed the very good behavior of all compositions considered, even those containing high percentages of microsilica and high volume contents of steel microfibers.

This paper presents a continuous, multilinear expression that can be used as a general expansion-temperature equation. The equation can accommodate an unlimited number of connected linear portions. Therefore, it can be easily adapted to represent the expansion behavior of materials—including those of steel, the behavior of which changes abruptly with temperature during phase transformations. Being a continuous curve, it has a major advantage over other comparable expressions, in that all the parameters in the equation can be determined simultaneously in a single curve-fitting operation. Consequently, it can be useful and powerful for characterizing test results. This paper also shows examples where the equation is fitted to experimental data obtained from various grades of structural steel. The examples show that a curve with four linear portions is sufficient to closely fit the test data. Accordingly, a model is constructed for representing the expansion-temperature behavior of structural steel. This model is also presented in this paper.