The importance of chloride ions in reinforcement corrosion has led to the concept of a chloride threshold level, namely the minimum chloride content at the depth of steel that would result in the passive film breakdown. The reported chloride threshold level values range from 0.2 to more than 2% by weight cement. The physical condition of the steel-concrete interface appears to be the most important factor affecting the threshold level. The importance of the steel concrete interface is investigated in this research. Extensive experimental work was carried out. This involved the development of methods to create entrapped air voids at the interface, characterise the physical condition of the interface and quantify its effect on the chloride threshold level. A large number of specimens were cast and tested with variable interfacial conditions as well as various cement binders. Voids at the interface were best obtained by varying compaction and mix design, and quantified using an image analysis tool. For the first time chloride threshold levels ranging between 0.2% and 2.5% by weight of cement were reproduced in a single laboratory programme of work. The threshold level was observed to be strongly dependent on entrapped air content, confirming the importance of previously un-quantified interfacial conditions. It was that shown the reduction of percentage voids at the interface to less then 0.2% resulted in a sharp increase in chloride threshold level values to above 2% by weight of cement. The chloride threshold level could be raised by improving compaction, using an admixture and applying an inhibitive steel treatment or electrochemical treatment. In addition to quantifying the effect of the steel concrete interface on the chloride threshold level, new methods of characterising concrete were developed. A novel method was used to measure the chloride binding capacity and the acid neutralisation capacity test was refined to measure residual buffering capacity of concrete. A more appropriate index for presenting chloride threshold level than those currently used was suggested. This was based on the experimental data obtained that showed many of the hydrated solid phases of concrete inhibit corrosion initiation by resisting a fall in pH, while bound chloride presents a corrosion risk due to pH dependent dissolution characteristics of solid phases.

This thesis records the results of an investigation designed to obtain a better understanding of the processes involved in the fracture of cement and concrete. To this end two approaches have been taken. Firstly the characteristics of the fracture process have been studied using mechanical testing methods. Secondly, attempts have been made to develop techniques for studying in a realistic way the microstructure of hardened cement paste and its relation to the fracture process. The investigation was initially restricted to hardened cement paste to reduce somewhat the complexity, and the effect of aggregate addition was studied briefly in the later stages. Specimens containing cut in slits were tested to failure in flexure and direct tension, and the results analysed in terms of various models of fracture. A fracture mechanics approach showed the measured fracture toughness to vary with specimen size and this variation together with the relative notch insensitivity of the material suggested the existence of a region in the vicinity of the slit tip, in which the material did not behave in an ideal linear elastic manner and it was hoped that the microstructural investigation would shed further light on the fracture processes. The standard methods employed in an attempt to detect changes taking place in the microstructure under stress, ranged from simple visual observation to high resolution electron microscopy. However, normal optical microscopy shows up no changes in the microstructure prior to complete failure, and the specimen preparation techniques necessary for electron microscopy introduce large drying out stresses which make interpretation of the results difficult. A new diffuse illumination optical microscopy technique was developed which enables a specimen to be studied in a controlled environment and this technique has shown for the first time that stable micro-cracks occur in stressed samples of hardened cement paste prior to complete failure. Finally an attempt is made to explain the mechanical testing and microstructural observations by means of a tied crack model.

Four instrumented sites, with a total of 14 instrumented piles, have been analysed to understand the structural behaviour of the piles and the geotechnical behaviour of the stabilised slopes. Vibrating wire strain gauges are used for the calculation of the bending moment applied to the piles, while inclinometers are used to measure the displacements. A review of the instrumentation has been carried out and a methodology for processin and analysing strain gauges data has been developed. Concrete is a material with a complex behaviour. Shrinkage, creep, cracking, temperature variations, strength and modulus of elasticity of concrete are critically analysed to understand their influence on the concrete pile behaviour and on the function of the strain gauges. The results show that creep and shrinkage effects can be neglected in underground reinforced concrete structures in saturated clay, while cracking effects and temperature variations have to be analysed case by case. A correction method has been developed to consider the difference between the coefficient of thermal expansion of the strain gauges and that of the concrete. Two types of pile have been analysed, standard reinforced concrete piles and circular concrete-filled steel tubular piles. A review of their behaviour in bending that considers the development of cracking is presented. Two methods for the calculation of bending moment in the piles have been developed taking into account realistic stress/strain curves and the effects of concrete cracking. The bending moment results are compared with the inclinometer profiles using an improved version of an existing curve fitting method. The comparison shows good agreement between the two instrument results. A critical analysis of the pile/soil behaviour has been carried out comparing the results from the instrumented sites with theoretical mechanisms for landslide stabilising piles presented elsewhere. The results show a good match with the theoretical mechanisms as well as showing that the slopes have been successfully stabilised. Other observations have been made during analysis of the monitoring data. These include the seasonal effects of climate and vegetation on stabilised slopes and the structural effect of the external grout ring in laterally loaded circular concrete-filled steel tubular piles

Reasons for the use of automated methods for the detailing of reinforced concrete members are described and the application of automation to the various parts of the detailing process is discussed. Particular attention is given to the problems of producing working information economically without suffering a loss of quality and clarity compared with existing manual techniques and the development of a suitable system for the production of working information is described. The problems of preventing interference between the reinforcement in different members at intersections are described and techniques for ensuring compatibility are discussed. The use of various types of computer is considered and the results of a survey into the capabilities of a number of types of small computer are set out. Finally the effects on both the design office and the site of the use of automated detailing techniques using the proposed method of producing working information are considered and an estimate made of the costs of implementing such a system, together with its possible savings.

A study is presented of the behaviour of reinforced concrete slab strips under the combined effect of bending and compressive membrane action. The existing methods for allowing for membrane action in predicting the plastic behaviour of reinforced concrete slabs are reviewed and their limitations outlined. A new theory of the plastic behaviour of materials with tension cracks based on 'total strain' and 'strain rate' flow rules is proposed and applied to problems of axially restrained concrete slab strips. The effect of elastic axial strains, flexible restraints and physical gaps at the boundaries are carefully considered. The results of a series of experiments on slab strips designed to test the proposed theory are presented.

The thesis describes a theoretical and experimental investigation into the ultimate flexural strength of uniformly loaded two-way rectangular concrete slabs (reinforced and unreinforced) with membrane action induced by restraint against lateral displacement at the boundaries. The experimental part of the programme involved the making up and testing of 45 concrete slabs. In addition, the results of 22 slabs tested by other investigators are analysed. The investigation is divided into four main parts. Firstly, a yield-line theory incorporating a rigid-plastic strip approximation is developed which defines the loaddeflection curve at and after the ultimate flexural load for slabs with rigid boundaries during the compressive membrane action stage. Cases of boundary restraints which produce compressive membrane action in either one or two directions are considered and the accuracy of approximate yield-line patterns are investigated. On the basis of an empirical value~first for the width of the yield bands and thence for the central deflection at the ultimateload~expressions are obtained for the ultimate flexural strength of slabs which compare well with results obtained from slabs tested under short-term loading in very stiff surrounding frames. Secondly, the theory is generalized to take into account the effects of axial strains in the slab and of small lateral displacements at the boundaries,in order that the etfects of long-term loading and elastic boundary restraints may be included •. The theory is checked against test results obtained :f'rom slabs tested under periods of sustained loading. It is shown that axial strains and small lateral displacements at the boundaries can significantly reduce the compressive membrane action, especially in the case of thin slabs. Thirdly, the behaviour of a slab as a tensile membrane at large deflections beyond that at the ultimate :f'lexural load is examined. A theory is developed to de:f'ine the loaddeflection characteristics ,assuming that the reinforcement acts as a plastic membrane. It is shown that heavily reinforced slabs can carry loads by tensile membrane action which exceed the ultimate :f'lexural load. Finally, the laterally sti:f'fness and strength required 01' surrounding beams and panels of slab and beam floors in order to entorce membrane action in interior panels is investigated. Results obtained from tests on the interior panels of 9 panel floors are analysed and show good agreement with the theory. An appendix containing some results obtained by the conventional Johansents yield-line theory is also included

The thesis expounds the logic behind a new concept of composite construction which exploits fibre reinforced cement composites in the form of channels, placed in the tensile zone of reinforced concrete T-beams. The fibre reinforced cement composites, which act as 'surface reinforcement', incorporate high modulus, asbestos fibres in a predominantly two-dimensional random distribution in a cement matrix. Compared with concrete, these units have a high tensile strength and greater extensibility. The first stage of this investigation is primarily concerned with the design of the fibre reinforced cement composites of suitable mechanical properties for use as surface reinforcement in large structural components. The mechanical properties are affected both by the thickness and the composition of these composites. In the second stage the structural utilisation of the composites is examined in relation to strength, cracking and deflection characteristics; static, fatigue and sustained loading tests are carried out on reinforced concrete T-beams cast in conjunction with asbestos-cement composites. Altogether twenty-four beams are tested, the cross-sections of which, the type and the amount of tensile steel remain constant, whereas the variables investigated are the effect of the upstand length, thickness and composition of the surface reinforcement. It is shown that this type of construction considerably enhances the structural performance of reinforced concrete beams; the degree of improvement being a function of the composition and the upstand length of the surface reinforcement. At the design working load, for example, the deflection is reduced by up to 33% whilst cracks can be completely eliminated. Furthermore, the resulting composite could, in using surface reinforcement that extends a full depth of the web of a T-beam, ameliorate shuttering and consequently reduce the cost of the product.

Strengthening of reinforced concrete (RC) beams by external plate bonding (EPB) technique has become very popular for the last two decades, especially in the past few years with the increasing applications of the advanced composites. Unlike the unstrengthenedb eams, the composite structure shows an undesirable failure of plate debonding. It has been well recognized that central to this failure mode is the stress concentration at the plate end. It is prerequisite to well understand the stress characteristics at this zone. This thesis reports a systematic investigation on the stress distribution at the interface between the concrete and the bonded plate and the prediction of the failure loads using the obtained stresses. A Finite Element Analysis (FEA) based on elastic fracture mechanics is carried out first to provide a detailed study on the stress fields near the plate ends. This becomes the benchmark for the analytic solutions presented in the following chapters. A completed analytical solution is developed by the principle of complementary energy and provides consistent results with the FEA with less computational efforts. A closed form rigorous solution is proposed such that a spreadsheet package is sufficient to obtain the numerical results. This rigorous solution provides the basis to further develop a simplified solution of the interfacial stresses that are subsequently used to develop the strength models. To consider the nonlinear properties of concrete a detailed nonlinear FEA simulation is conducted in the thesis and extensive results are computed at various load levels, from the elastic state to the ultimate state. In addition, a Nonlinear Fracture Mechanics (NLFM) method is developed taking into account the pre- and post-cracking behaviors and the interactions between the shear and transverse normal stresses. This solution is able to predict the load level at the onset of plate end cracking (serviceability load) and that at the ultimate failure state (ultimate load). Finally some of the proposed solutions are applied to selected beams whose test results are available for comparison. In combination with existing material failure criteria, the elastic simplified solutions are also used to predict serviceability loads. Two groups of serviceability loads predicted from the elastic solution and NUM solution, respectively, and one group of ultimate load predicted from NLFM solution are all compared with the experimental data. Encouraging correlations are achieved. Other useful results, such as development length, the size of Fracture Process Zone (FPZ) are also calculated. Before concluding the work, a series of parametric analyses are carried out to assess the impact of various parameters on the interfacial stress fields, which provides some fundamental information related to design of the strengthening scheme.

No description available.

Many bridges in the world are classified as deficient and in need of rehabilitation or replacement. Some of them are deficient because their load-carrying capacity is inadequate for today's increased traffic load. To improve their efficiency and increase their load capacity, several methods can be used, one of them is the external prestressing. Also, to avoid corrosion problem that faced this type of strengthening, Fibre Reinforced Plastics (FRP) can be used instead of steel tendons. Within the different types of FRP, Parafil rope was established to be well suited for prestressing system, combining the benefits of light-weight, high strength, easy handling and efficient anchorage system. Thirteen prestressed beams,one with internal prestressing steel only, and the rest strengthened externally using Parafil Ropes Type G were tested up to failure under two third point loading. Six factors were studied to investigate their effect on the behaviour of strengthened beams. These factors are the value of the external prestressing force and its eccentricity, deviator position, previous loading stage before strengthening, concrete strength and (span/depth) ratio. Also, analytical investigations were conducted to propose simple equations could be used in the analysis of this beam type, regarding its deflection and flexural strength with an acceptable accuracy. The study indicated that, external prestressing using Parafil rope is a very powerful system for strengthening or rehabilitation of prestressed concrete structures. Also, providing external prestressing force by a moderate amount improves the stiffness, and both cracking and ultimate flexural strength of prestressed concrete beams without significant reduction in ductility even for cracked beams. The modifications made to the methods used to calculate deflection and ultimate moment of bonded prestressed concrete, generalised these methods so, they can be used with beams have different types of prestressing tendons (bonded, internal unbonded, external unbonded or mixture of them). These methods, after modification, were found to give fairly accurate results. Also, the novel equation described the relation between the increase in the external prestressing force and the deflection makes the analysis of the externally strengthened beams under service loads,less complicated with a reasonable accuracy.