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Thirty full scale prestressed concrete struts all having a slenderness ratio of L/D = 40 were tested with three different eccentricities. Stress-strain curves of the corresponding test cylinders were established and the Young's Moduli evaluated. The specimens were fabricated with different percentages of prestressing steel ranging from 0.23 to 1.20%. The initial prestressing forces were set to obtain a final prestress of 140,000 psi in all specimens. The dimensions of the struts, and the quality and manufacturing process of the concrete, were kept constant. Special adjustable supporting devices simulated ideal hinged conditions. The struts were tested to their ultimate capacity and their critical buckling loads were evaluated. The tests proved that, provided the column is made of high quality concrete, the critical load values can be obtained by using Euler's fundamental formula, and these values are independent of the amount of prestressing. For eccentrically loaded struts maximum stresses from a modified secant formula, incorporating effect of prestressing, were compared with those calculated on basis of observed deflections. According to the test results the secant modulus corresponding to the average compressive stress should be used in Euler's formula. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

During its service life, a structure may be subjected to various environmental and loading conditions. However, in general, the properties determined under one set of conditions may not be used to determine the behaviour of the material under a different set of conditions. For example, it is well known that concrete is a strain rate sensitive material; therefore, its properties determined under conventional static loading cannot be used to predict the performance of concrete subjected to high strain rates. The problem is serious because these high strain rate loadings are associated with large amounts of energy imparted to the structure in a very short period of time, and concrete is a brittle material. Since the strain rate sensitivity of concrete prohibits the use of its statically determined properties in assessing its behaviour under dynamic conditions, high strain rate tests are required. Impact tests were carried out on about 500 concrete beams. An instrumented drop weight impact machine was used. The instrumentation included strain gauges mounted in the striking end of the hammer (called 'the tup'), and also in one of the support anvils. In addition, three accelerometers were mounted along the length of the beam in order to obtain the beam response, and also to enable the inertial correction to the observed tup load to be made. Two different concrete mixes, normal strength with a compressive strength of 42 MPa, and high strength with a compressive strength of 82 MPa, were tested. The effect of two types of fibres, high modulus steel, and low modulus fibrillated polypropylene, in enhancing concrete properties was investigated. In addition, tests were also conducted on beams with conventional reinforcement. Hammer drop heights ranging from 0.15m to 2.30m were used. Static tests were conducted on companion specimens for a direct comparison with the dynamic results. In general, it was found that concrete is a very stain rate sensitive material. Both the peak bending loads and the fracture energies were higher under dynamic conditions than under static conditions. Fibres, particularly the steel fibres, were found to significantly increase the ductility and the impact resistance of the composite. High strength concrete made with microsilica, in certain circumstances, was found to behave in a far more brittle manner than normal strength concrete. High speed photography (at 10,000 frames per second) was used to study the propagation of cracks under impact loading. In general, the crack velocities were found to be far lower than the theoretical crack velocities. The presence of reinforcement, either in the form of fibres, or of continuous bars was found to reduce the crack velocity. A model was proposed based on a time step integration technique to evaluate the response of a beam subjected to an external impact pulse. The model was capable of predicting not only the experimentally observed non-linear behaviour of concrete under impact loading, but also the more pronounced brittle behaviour of high strength concrete. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

The extensively used AASHTO structural design procedures for flexible pavement indicate the required pavement design in terms of a structural number. For a particular pavement thickness design, this structural number can be computed from the sum of each pavement layer's thickness multiplied by its strength parameter, called the structural layer coefficient. The research work reported herein presents methods for determining the structural layer coefficients for asphalt emulsion-aggregate mixtures. A hot plant-mixed asphaltic concrete was evaluated for structural layer coefficient, and the radial stress vs. fatigue failure relationship was developed using circular specimens and the Jimenez deflectometer. Relationships between structural number and load repetitions to failure for different loading conditions were developed. These relationships were used to evaluate the structural numbers of other specimens when tested to failure in flexural fatigue. Three asphalt emulsion-aggregate mixtures were designed using CSS-lh asphalt emulsion. The aggregates used for the three mixtures were: (1) Type I aggregate using dense-graded, crushed, river gravel; (2) Type II aggregate using pit-run, coarse sand; and (3) Type III aggregate using a silty sand. These mixtures were evaluated for Marshall stability, Hveem stability and cohesiometer value, unconfined compressive strength, double punch tensile strength and dynamic modulus of elasticity at various ages from 3 to 28 days. Flexural fatigue life, when tested in the deflectometer, was determined for all mixtures at 7 and 28 days. Structural numbers for the specimens and structural layer coefficients for the mixtures were determined. Relationships were developed between the evaluation tests performed and the structural layer coefficients at various mixture ages by using test results from the three mixtures and a regression analysis procedure. A fourth asphalt emulsion-aggregate mixture using CSS-lh asphalt emulsion and a Type II crusher-run aggregate was designed. Evaluation tests were performed at 3 and 7 days and layer coefficients for the mixture were predicted for 7 and 28 days using the regression equations developed. Layer coefficients at 7 and 28 days were also determined by testing specimens in fatigue in the deflectometer and computing their structural numbers and layer coefficients. Layer coefficients determined in these two manners indicated favorable comparisons. The results of this research provides information about the structural layer coefficients for asphalt emulsion-aggregate mixtures. The relationships between the evaluation tests and structural layer coefficient can be used for determining layer coefficients for other asphalt emulsion-aggregate mixtures. Because the evaluation tests used were tests commonly performed in most asphalt laboratories, these determinations can be made without the necessity of additional equipment or procedures in most cases.

Typescript. Bibliography: leaves 318-338.

published_or_final_version / Civil Engineering / Master / Master of Philosophy

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