Journal of Materials in Civil Engineering

A two-dimensional algorithm has been developed to determine the locations and sizes of steel bars in reinforced concrete members. This algorithm uses magnetostatic relationships to relate measured distortions in the magnetic field outside of a concrete member to the distribution of steel within the member. An optimal solution for the locations and sizes of the steel bars is determined by minimizing the difference between measured and computed distortions of the magnetic field. Finite-element analyses were conducted to simulate the interaction between steel bars and an imposed magnetic field. On the basis of the simulated magnetic distortion, the algorithm accurately identified the location and size of a single bar even when 20% error was induced in the magnetic distortions. The algorithm also identified two bars spaced horizontally or vertically. However, if one bar was much closer to the receivers than the other, only the closest bar was identified accurately. When 10% error was induced in the simulated magnetic distortions, the algorithm accurately located two bars only when the areas were already known.

This paper presents an experimental observation of the relation between the plastic damage and the acoustic emission (AE) events in concrete subjected to uniaxial compressive loading. To examine the effect of the degree of the initial deterioration on the plastic damage, both undamaged and damaged concrete specimens were tested. In the damaged specimens, the damage was induced using different numbers of freezing-thawing cycles to achieve different initial deterioration before loading. It is found that as the plastic damage increases the activity of AE signals increases. Moreover, it is noticed that the degree of the nonlinearity of plastic damage–AE event relationship depends significantly on the initial deterioration. The current investigation may provide a new procedure for core testing to evaluate concrete deterioration because the conventional strength and Young's modulus are less sensitive to the damage.

The Kuwaiti environment was contaminated with oil residue, gaseous pollutants, soot, and smoke resulting from the destruction of oil wells by Iraqi forces. This study investigated the effects of oil-based pollutants on the strength and integrity of building materials. The investigation was initiated as soon as the oil-well fires were put out and the visible pollutants had settled to the ground. Samples were collected from all over Kuwait and were subjected to physical, chemical and mineralogical [x-ray diffraction (XRD) and x-ray fluorescence (XRF)] testing. The physical testing program indicated a positive effect with regard to compressive strength and slump of concrete made from contaminated aggregate. The chemical and mineralogical testing program indicated that the hydrocarbon concentration and penetration in building materials depends on the type of building material, and showed that the pollutants reduce the integrity of the building materials when comparing the exposed surface with the interior. Overall, the dry environment reduced the extent of deterioration, because harmful chemical compounds were not developed.

This investigation deals with the effect of curing age on the strength development of high-strength concrete containing silica fume and fly ash and exposed to cold temperatures. Tests were performed on three sets of specimens, having initial curing ages of 1, 14, and 28 days at room temperature. The test specimens were exposed to five temperatures varying from −10°C to 20°C for periods over 3 months in cold ocean water. The gains in compressive and tensile strength with time were found to be directly proportional to the temperature increase. The highest gain in strength with time was recorded at 20°C and the lowest at −10°C for all three sets of specimens. For the set initially cured for 1 day, the reduction in the rate of strength gain was profound. However, it was minor for the set cured initially for 14 days, and negligible for the set cured initially for 28 days. A maturity analysis using a hyperbolic function was performed. The maturity functions gave reasonable agreement with the three sets of specimens at different temperatures except for the specimens cured for 1 day and exposed up to 7 days.

A simple method is proposed for determination of fracture energy of materials exhibiting quasi-brittle behavior during mode I cracking. A series of graphs showing the relationship between the nondimensional peak load and the nondimensional beam size are generated for a three-point beam configuration using a new and computationally efficient finite-element program. The method only requires the measurement of the peak load of a single-size beam specimen. It is preferred that the beam specimens have a small preexisting notch to ensure mode I cracking in a three-point bending test. The other required material properties for the use of nondimensionalized graphs include Young's modulus and tensile strength. Since the three-point bending test can be easily carried out in the laboratory, the proposed method offers a practical, yet expedient, way of characterizing the nonlinear fracture properties of materials that follow the linear stress-separation behavior in the process zone.

Pore-size distributions determined experimentally and durability factors (DFs) assigned on the basis of the observed state of weathering of certain Sphinx strata were used as a pattern of input and output, respectively, to train artificial neural networks. The weights obtained were then used to calculate the DF of the remaining Sphinx strata. The calculated values excellently matched the observed relative condition of these strata as well as the durability indicated by their lithological and petrofabric makeup.

Mixtures of geomaterials and by-products have potential as barriers for containing contaminants. In this investigation, the permeability of fly ash to distilled water was found to decrease due to the addition of pure bentonite. On subjection of the compacted bentonite-modified fly ash to cycles of freeze-thaw action, using a specially designed freeze-thaw permeameter, the permeability of the composite material decreased to a constant value after one cycle. It is postulated here that the observed permeability decrease stems from the dispersion of flocculated clay material that occupies the intergranular regions among the fly-ash particles. The specific results of this study are fitted into a general framework for describing textural changes and deterioration of composite materials under cyclic thermal gradients. In this framework, a comparison of the response of clay-modified fly ash and ash-modified clay to the stresses induced by freeze-thaw action is made. Both class-C and class-F fly ash are examined.

In this study the physical and engineering characteristics of various no-fines concrete mixtures are investigated. No-fines concrete mixtures subjected to impact compaction are studied under unconfined compression, indirect tension, and static modulus of elasticity; and the results are interpreted as functions of mixture proportions. The effect of impact-compaction energies, consolidation techniques, mixture proportions, curing types, and testing conditions on physical and engineering properties are presented. The abrasion characteristics and resistance to freezing and thawing of no-fines concrete are also discussed. It was found that the strength of no-fines concrete is strongly related to its mixture proportion and compaction energy. A sealed compressive strength of 20.7 MPa (3,000 psi) can readily be achieved with an aggregate cement ratio of 4.5:1 or less and a minimum compaction energy of 165 J/m ³ (4,303 ft-lb/cu ft). The splitting tensile-compressive relationship followed a pattern similar to that of conventional concrete. No-fines concrete had a lower modulus of elasticity than conventional concrete. The ultimate drying shrinkage of compacted no-fines concrete was found to be approximately 280 × 10 ⁻⁶ , about half that typically expected in conventional concrete. Air-entrained no-fines concrete exhibited a higher resistance to freezing and thawing than non-air-entrained mixtures.

This paper presents a nondestructive evaluation method to characterize portland cement concrete (PCC) based on its dielectric properties. The work reported is part of an ongoing study to develop a nondestructive evaluation method for PCC structures using electromagnetic waves. A custom-designed parallel plate setup with an HP 4195A Network/Spectrum Analyzer was used to form the measurement system. A model was established for the measurement system and a set of calibration standards was developed for the system calibration. Six different PCC batches were cast and their complex permittivity was measured over 28 days of moist curing, with an average of four specimens per batch. The selected batches covered a representative variation of water to cement ratio and two aggregate types using Type I portland cement. The complex permittivity of PCC was evaluated over a frequency range of 0.1–40 MHz. Preliminary measurements showed significant changes in the real part of the dielectric constant versus the curing time. However, the change in the loss tangent was less pronounced. Significant differences were also observed in the dielectric properties of PCC specimens due to mix-design parameters' variations and the curing state.