Andrew J. Boyd

Carbonation Curing versus Steam Curing for Precast Concrete Production

AbstractAn investigation was conducted into the beneficial utilization of captured CO2 for early-age curing of precast concrete products. The performance of the carbonation-cured concrete was compared to that of steam curing to investigate the possibility of replacing steam curing with carbonation. The early-age carbonation curing was performed for a short period after an initial curing in a controlled environment. The effect of the carbonation curing was studied in terms of carbon uptake, accelerated strength, and durability. It was found that the short-term carbonation promoted early strength development, while subsequent hydration was essential to obtain later age properties. Durability performance of the carbonation-cured concrete was compared with steamed and normally hydrated references. The carbonation-cured concrete exhibited more resistance to chloride permeability, ion migration, sulfate attack, and freeze-thaw damage. The improved durability by carbonation is attributed to the significantly red...

Geometric Effect of Asperities on Shear Mechanism of Rock Joints

Three-dimensional tracking of changes of asperities is one of the most important ways to illustrate shear mechanism of rock joints during testing. In this paper, the changes of the role of asperities during different stages of shearing are described by using a new methodology for the characterization of the asperities. The basis of the proposed method is the examination of the three-dimensional roughness of joint surfaces scanned before and after shear testing. By defining a concept named ‘tiny window’, the geometric model of the joint surfaces is reconstructed. Tiny windows are expressed as a function of the x and y coordinates, the height (z coordinate), and the angle of a small area of the surface. Constant normal load (CNL) direct shear tests were conducted on replica joints and, by using the proposed method, the distribution and size of contact and damaged areas were identified. Image analysis of the surfaces was used to verify the results of the proposed method. The results indicated that the proposed method is suitable for determining the size and distribution of the contact and damaged areas at any shearing stage. The geometric properties of the tiny windows in the pre-peak, peak, post-peak softening, and residual shearing stages were investigated based on their angle and height. It was found that tiny windows that face the shear direction, especially the steepest ones, have a primary role in shearing. However, due to degradation of asperities at higher normal stresses and shear displacements, some of the tiny windows that do not initially face the shear direction also come in contact. It was also observed that tiny windows with different heights participate in the shearing process, not just the highest ones. Total contact area of the joint surfaces was considered as summation of just-in-contact areas and damaged areas. The results of the proposed method indicated that considering differences between just-in-contact areas and damaged areas provide useful insights into understanding the shear mechanism of rock joints.

Sulfate Attack on Concrete: Effect of Partial Immersion

Traditionally, the extent of sulfate attack is qualified through visual rating or quantified by the percent expansion of slender bars completely submerged in sulfate solution. There are currently no standardized test methods that take into account the change in engineering properties because of deleterious mechanisms. Moreover, the exposure regime used to evaluate sulfate attack, complete immersion, is not typically representative of that encountered in the field. For these reasons, the objective of the research presented herein is to quantify the degree of sodium sulfate attack through the degradation of mechanical properties, specifically the compressive and splitting tensile load capacities of standard cylindrical specimens. A novel exposure regime is utilized wherein the specimens are only partially submerged in 5% sodium sulfate solution, creating an evaporation front similar to that of field exposure. It was found that the portion submerged in sulfate solution, although visually pristine, was the weaker portion of the cylinder for both mechanical tests, even though the other half showed extensive signs of surface disintegration caused by salt crystallization.

Detection and Assessment of Structural Flaws in Concrete Bridges with NDT Methods

Researchers investigated the applicability of various nondestructive testing (NDT) methods on full-scale concrete bridge structures, including ultrasonic pulse velocity tomography and the impact-echo method. The combination of the two methods enabled researchers to three-dimensionally evaluate material consistency and locate flaws or regions of deterioration within structural members with a new level of efficiency. Although research involving NDT methods has been conducted for many years, the use of NDT methods for primary investigation of full-scale structural elements is not typically performed. In this study, the investigation of bridge structures for flaws and damage resulted in the discovery of a previously undetected yet recurring form of damage, which led investigators to suspect a design flaw. The results of the ultrasonic pulse velocity tomography enabled researchers to assess the actual cause of the damage, which allowed the formulation of recommendations for the repair and redesign of the defective structural elements.

Tensile Strength Prediction in Concrete Using Nondestructive Testing Techniques

Field studies have suggested that wave velocities through concrete samples decrease with increasing damage. However, to date there has been no replication of this effect in a laboratory setting allowing for a controlled experiment to quantify this effect. The primary objective was to see how the exposure of concrete to sulfate solutions related to surface-wave velocity and through-wave velocity. The impact–echo method and the ultrasonic pulse velocity test were used to quantify these relationships, respectively. Laboratory research focused on correlating nondestructive test (NDT) data with destructive test results from field-sized concrete samples exposed to continuous sulfate attack over time. The intent was to evaluate the capabilities of the NDT techniques in identifying and quantifying damage due to sulfate attack. Prior research has shown that tension testing tends to be far more sensitive than compression testing to such damage. As a result, it was expected, and confirmed, that stress wave velocities from the two NDT techniques correlate better with tensile strength than with compressive strength.

Pressure-tension test for assessing fatigue in concrete

In a pressure-tension test, a cylindrical concrete specimen is inserted into a cylindrical steel jacket, with a rubber ‘‘O’’ ring seal at each end to prevent gas leakage. Gas pressure is then applied to the curved surface of the concrete cylinder, leaving the ends free. As the gas pressure is increased, the specimen eventually fractures across a single plane transverse to the axis of the cylinder. The gas pressure at fracture may then be considered as the tensile strength of the concrete. In this study, the pressure-tension test is used to study fatigue in concrete. A total of 22 standard concrete cylinders (100 mm × 200 mm) were tested. Both dry and wet specimens have been studied. Low-cycle loading, which involves the application of a few load cycles at high stress levels – such as a concrete structure under earthquake load – has been used in this study. It was found that the concrete specimens in a low-cycle loading fail after only a few cycles of loading and interestingly at a stress level lower than the maximum value applied in the cyclic loading. In addition, non-destructive testing (NDT) was performed to determine the progressive damage due to tensile load in concrete cylinders using Ultrasonic Pulse Velocity (UPV). It was found that UPV can be used to evaluate the damage in concrete even after the application of a very low-level of tensile stress – as low as 10% of its tensile strength.

Evaluating Freeze-Thaw Deterioration with Tensile Strength

Freeze-thaw damage is one of the leading contributors to infrastructure deterioration in temperate northern climates. Deterioration caused by freeze-thaw cycling is primarily induced by hydraulic pressures within the hydrated cement paste matrix that cause tensile cracking. Such damage should, therefore, be more effectively detected with tensile testing. This work presents the detection and evaluation of ongoing freeze-thaw (F/T) damage in plain concrete cylinders using the pressure tensile strength test, as it compares to compressive strength evaluation. Pressure tension test results exhibited significantly higher levels of deterioration compared to compression testing, with the samples losing up to 90% of their undamaged tensile capacity. Moreover, it was shown that tensile strength testing is far more sensitive to freeze-thaw deterioration, evidenced by a significant drop in the tensile to compressive strength ratio to below 5%.

Retrofit of Shear Strength Deficient RC Beams with Sprayed GFRP

Worldwide, a great deal of research is currently being conducted concerning the use of fibre reinforced plastic wraps or laminates in the repair and strengthening of reinforced concrete members. Such techniques can be both effective and economical when compared to the existing practice of retrofitting with steel plates. A novel technique which further simplifies the application procedure is to apply the fibre using a spraying process. By spraying the fibres onto the member surface concurrently with a suitable matrix resin a two dimensional random distribution of discontinuous fibres is obtained, resulting in an FRP plate that exhibits isotropic in-plane behaviour.