Abstract not available

With increasing number of structures reaching their designed life or capacities everyday, retrofitting has become an important area in civil engineering. A popular method of strengthening and stiffening reinforced concrete ( RC ) beams is by adhesively bonding steel or FRP plates to the external surfaces. This technique has been proven to be efficient, inexpensive, unobtrusive and can be applied while the structure is in use. However, it has been found that adhesively bonded plates are prone to premature debonding prior to reaching their designed capacities, which restricts the use of existing design rules and guidelines for retrofitting RC beams using this relatively new form of structure. There are various forms of debonding including : plate end ( PE ) debonding ; critical diagonal crack ( CDC ) debonding ; and intermediate crack ( IC ) debonding. IC debonding is an especially important mechanism as it will occur at plated hinges of continuous members, and unlike other premature debonding mechanisms, IC debonding is very difficult to prevent. This debonding mechanism is associated with the formation of flexural or flexural - shear cracks in the vicinity of the plates, which causes slip to occur at the plate / concrete as well as the bar / concrete interfaces. Most research to date has been focusing on the bond - slip relationship at the plate / concrete interface, while little attention has been given to the IC debonding behaviour of flexural members. To allow safe and effective use of plated structures, it is necessary to model the debonding behaviours at the plate / concrete interface as premature debonding will affect both the strength and ductility of the members, and hence the ability of continuous structures to redistribute moment. Despite the importance of moment redistribution, very limited research has been carried out on the moment redistribution of continuous plated members. Since IC debonding is likely to occur at plated hinges of continuous members hence affecting the ductility of the hinges, the existing approaches for determining moment redistribution of reinforced concrete beams cannot be applied to plated members. In this research a numerical model based on discrete cracking and partial interaction theory has been developed which models the IC debonding of plated beams, taking into account the slips at all interfaces. This model will allow a better understanding of the IC debonding behaviour of plated members, and also from the model, the rotation capacity of both plated and unplated hinges in continuous reinforced concrete beams can be determined. Mathematical models and design rules have been developed for analysing critical diagonal crack debonding, which is dependent on the IC debonding behaviour of the plated members. Moment redistribution of beams with externally bonded and near surface mounted plates is studied through a series of tests and a mathematical model based on variation in flexural rigidity is proposed. Through the tests carried out on continuous plated beams, much moment redistribution is evident as oppose to that suggested by the existing design guidelines for plated members, where no moment redistribution is allowed for members plated with FRP. From the models proposed for IC and CDC debonding in this research, together with the existing PE debonding models available, all debonding mechanisms can now be modelled. Furthermore from the research on continuous plated beams, moment redistribution of plated beams can be analysed, allowing safe, effective and economic use of this retrofitting technique. This thesis is presented in the form of a collection of journal papers published or submitted for publication as a result of the research performed by the author. A selection of ten publications have been included in the following context, together with literature reviews performed on the related areas of studies, as well as further discussions on the papers, which consist of any additional information or work that was carried out in this research but not presented in the papers. / Thesis (Ph.D.)--School of Civil and Environmental Engineering, 2006.

Surface-breaking cracks pose a serious threat to the service life of concrete structures and health monitoring is presently conducted by a visual inspection method, yielding a potential risk to safety. Diffuse ultrasonic techniques have shown their potential as an ultrasonic technique for measuring crack depth in concrete and are currently under development. In this research, the finite element method (FEM) is employed to model the ultrasound diffusion in a concrete specimen. The objectives are to use the commercial finite element (FE) tool Ansys to develop the finite element model of a concrete specimen and verify the applicability of the model by comparing with an analytic solution and experiment data. Further, various crack types are analyzed with the FE model in order to gain physical insight into the interpretation of experimental measurements. The results of this research suggest that a preliminary knowledge of the cracking process is required to correctly interpret the measured impulse responses for an unknown crack geometry, as the impulse response expresses the response of the shortest path through a system of cracks between source and receiver. Moreover, the impulse response can carry some ambiguity, as certain crack types are not uniquely determined.

published_or_final_version / Civil Engineering / Master / Master of Philosophy

Concrete is the most widely used construction material in the world, so the assessment of damage in concrete is critical from the point of view of both safety and cost. Of particular interest are macro cracks that extend through the concrete cover of the reinforcement, which can potentially expose the reinforcement to corrosive elements. The high density of scatterers such as aggregate and voids in concrete makes quantitative imaging with coherent ultrasound difficult. As an alternative, this research focuses on diffuse energy based ultrasonic methods rather than coherent ultrasonic methods for crack depth assessment. Two types of ultrasonic measurements were made on real cracks formed under four point bending: one that focuses on time of flight measurements from an impactor; while the other uses the arrival time of maximum energy in a diffuse field excited by an impulsive load from a transducer. Each of these ultrasonic techniques is used to interrogate a macro crack in a concrete beam, and the results are compared to determine their accuracy and robustness. The actual crack depth is determined using direct surface measurements and a destructive dye-injected approach with drilled cores. The results suggest that the diffusion method, using a maximum energy approach, more accurately estimates the crack than visual inspection and impact echo methods, which overestimate the depth.

A non-linear finite element program to simulate the behaviour of infilled frames and plane stress reinforced concrete members under the action of monotonic and cyclic loading has been developed. Steel is modelled as a strain hardening plastic material, and in the concrete model cracking, yielding and crushing are considered. The separation, sliding, and opening and closing of initial gaps at the interfaces between the frame and the infill panels are accounted for by adjusting the properties of interface elements. The non-linear equations of equilibrium are solved using an incremental-iterative technique performed under load or displacement control. The iterative techniques use the standard and modified Newton-Raphson method or the secant Newton method. An automatic load incrementation scheme, line searches, and restart facilities are included. The capabilities of the program have been examined and demonstrated by analysing five reinforced concrete panels, a deep beam, a shear wall, and eight infilled frames. The accuracy of the analytical results was assessed by comparing them with the experimental results and those obtained analytically by other workers and shown to be good. A study of the effects of some material and numerical parameters on the results of analyses of reinforced concrete deep beam has been carried out. Two techniques have been proposed and used to overcome numerical problems associated with local strain concentrations which occur with the displacement control, when path dependent incremental iterative procedures are used for inelastic materials. The displacement control provided with these modifications has been shown to be more efficient than the load control.

Les bétons autoplaçants (BAP), développés depuis plusieurs années sont encore à l'heure actuelle qualifiés de « nouveaux bétons » car leur utilisation reste modeste bien qu'ils possèdent un fort potentiel de développement. Ce frein peut être expliqué par leur coût plus élevé, leur sensibilité à la ségrégation et à la fissuration due à leur fort retrait ainsi que par le manque de connaissances sur leur durabilité. Les BAP se distinguent des bétons ordinaires principalement par un dosage en éléments fins plus important. Ainsi, l'utilisation de fines additions minérales plus économiques et/ou plus écologiques dans ces bétons parait judicieuse sous certaines conditions. La présente thèse s'inscrit dans ce contexte et a pour objectifs de contribuer au développement de BAP plus écologiques et à l'étude de leur durabilité. Notre choix s'est porté sur deux ciments industriels à forte teneur en additions, le ciment CEM Ill à base de laitier de haut fourneau et le ciment CEM V à base de cendres volantes et de laitier de haut fourneau ainsi que sur deux additions minérales encore peu étudiées dans la littéraire : le métakaolin et un sédiment de dragage calciné. Différentes formulations dérivées d'une formulation de BAP de parement de référence à base de CEM 1 ont été mises au point. Les résultats des essais mécaniques et physico-chimiques menés sur les différentes formulations montrent un effet plus ou moins important des additions sur le comportement mécanique au jeune âge et un effet positif certain à plus long terme. Les résultats des mesures de retrait endogène et de dessiccation de l'état frais (plastique) à l'état durci bien hydraté montrent que certaines additions augmentent le retrait de dessiccation des BAP (laitier et sédiment calciné) alors que d'autres le réduisent considérablement (le métakaolin). L'effet indéniable des additions sur l'amélioration de certains indicateurs de durabilité (diffusion des chlorures et perméabilité au gaz) et sur l'augmentation de leur sensibilité à la carbonatation a été quantifié. L'influence assez complexe des différentes additions est étudiée à la lumière des analyses physico-chimiques et microscopique. Enfin, une modélisation mésoscopique est développée et appliquée aux BAP en vue d'une une étude paramétrique. L'effet de la forme des granulats et de leur concentration sur le comportement global en compression et traction d'éprouvette de SAP ainsi que l'effet de l'endommagement mécanique sur l'évolution de la perméabilité sont étudiés. / Self-compacting concretes (SCC), developed several years ago are still considered as “•novel concrete" as their use remains modest although they have a strong development potential. This is mainly due to their higher cos, their sensitivity to segregation and cracking due to their high shrinkage and also due to the lack of information about their durability. SCCs are distinguished from ordinary concrete mainly by their high content of paste. The use of supplementary cementitious materials (SCM), grow increasingly for economic and environmental reasons. This thesis falls within this context and aims to contribute to the development of greener SCCs and to the study of their durability.Two industrial blended cements containing high amounts of SCM, slag cement CEM Ill and CEM V cement containing slag and fly ash as well as two mineral additions still little studied in the literature: metakaolin and calcined dredged sediment are used. Various SCC mixtures derived from a reference SCC based on CEM 1 cement were developed. The results of the mechanical, physical and chemical testing on the different formulations show a greater or lesser effect of SCM on the mechanical behavior at early age and some positive effect over the long term. The results of measurements of the autogenous and drying shrinkage from fresh state (plastic) to well hydrated state show that some SCMs increase the drying shrinkage of SCCs (slag and calcined sediment) while ethers significantly reduce its amplitude (metakaolin).The undeniable effect of SCMs on improving some durability indicators (chloride diffusion and gas permeability) and on increasing the sensitivity to carbonation was quantified. The rather complex influence of the different SCMs is studied in the light of physicochemical and microscopie analyzes. Finally, mesoscopic modeling is developed and applied to SCCs for a parametric investigation. The effect of the shape of aggregates and their concentration on the overall behavior in compression and tensile SCC specimen as well as the effect of the mechanical damage on the evolution of permeability are studied.

This research presents the results on an experimental investigation to identify the significant factors influencing horizontal cracking in continuously reinforced concrete pavements (CRCP). An in-depth analysis of the microstructure, morphological characteristics of the interfacial transition zone (ITZ) and the observation of cracking using the environmental scanning electron microscope (ESEM) was done. Characterization of oxides using Fourier transform infrared spectroscopy (FTIR) and electron dispersive x-ray spectroscopy (EDS) was also performed. Water to cement ratio (w/c) and rebar temperature had a significant influence on the rebar-concrete bond strength. The 28-day shear strength measurements showed an increase in rebar-concrete bond strength as the water to cement ratio (w/c) was reduced from 0.50 to 0.40. There was a reduction in the peak pullout load as the temperature increased from 14oF to 252oF for the corroded and non-corroded rebar experiments. The corroded rebar pullout test results showed a 20-50 % reduction in bond strength compared to the non-corroded rebars. FTIR measurements indicated a presence of lepidocrocrite (γ -FeOOH) and maghemite (γ -Fe2O3) on the ITZ. ESEM images showed the existence of microcracks as early as three days after casting with the bridging of these cracks between coarse aggregate locations in the interfacial zone propagating through the mortar.

In addressing the need for a site specific corrosion model for concrete structures, Bazant’s time to first cracking model was used as the basis for an experimental research study. The theoretical model, which expresses the time to first cracking as a function of the corrosion rate and physical properties of the reinforced concrete, was never experimentally validated. This research study focuses on examining Bazant’s model and its validity using commercially available corrosion rate measuring devices. With validation of the model, the present condition of the structure could be assessed, damage rate predicted, and repair or rehabilitation methods selected. The experimental program examined 18 test blocks and 52 test slabs constructed to simulate sections of a reinforced concrete bridge deck. The test blocks and slabs contained a single layer of electronically isolated steel reinforcement embedded below the surface of the specimens. The design variables included cover depth, bar spacing, bar diameter, and corrosion rate. Six chloride concentrations 0.0 to 9.6 lb/yd³ (0.0 to 5.7 kg/m³) added directly to the fresh concrete were used to vary the corrosion rate. Two environmental conditions were also considered: outdoors in Blacksburg, Virginia and indoors with near constant moisture and temperature. The slabs were designed with two reinforcing mats. The lower mat of fiberglass reinforcement was placed in both directions and the upper mat was one layer of fiberglass and one layer of steel. Thus, the corrosion rate a single layer of electrically isolated steel bars was monitored. The corrosion rates were monitored from the date of casting for approximately one year using the 3LP and Gecor corrosion rate devices. The amount of metal loss of the bars was determined both destructively and non-destructively to allow a comparison between the two. The metal losses were compared to the predictions made according to Bazant’s equations as well as to other existing time to first cracking criteria to determine their validity. Tests were performed to verify the chloride contents of the slabs. The effect of temperature on the corrosion rate was also investigated. Preliminary results indicate that temperature strongly influences the corrosion rate. To date no cracks have formed in the surface of the test specimens. The corrosion rate of the slabs has been progressing at a maximum rate of 1.24 mils/yr (31.5 microns/yr) for over one year. The test blocks and slabs are to be monitored until cracking occurs at which time Bazant’s model will be either validated or adjusted to reflect the predictions derived from the commercially available corrosion rate devices used in this experimental program. Additional tests must also be performed to further quantify the effects of temperature and other variables. / Master of Science

Thesis (MScEng)--Stellenbosch University, 2013. / Durability and impermeability in a water-retaining structure are of prime importance if the structure is to fulfill its function over its design life. In addition, serviceability cracking tends to govern the design of water retaining structures. This research concentrates on load-induced cracking specifically that due to pure bending and to direct tension in South African reinforced concrete water retaining structures (WRS). As a South African design code for WRS does not exist at present, South African designers tend to use the British codes in the design of reinforced concrete water-retaining structures. However, with the release of the Eurocodes, the British codes have been withdrawn, creating the need for a South African code of practice for water-retaining structures. In updating the South African structural design codes, there is a move towards adopting the Eurocodes so that the South African design codes are compatible with their Eurocode counterparts. The Eurocode crack model to EN1992 (2004) was examined and compared to the corresponding British standard, BS8007 (1989). A reliability study was undertaken as the performance of the EN1992 crack model applied to South African conditions is not known. The issues of the influence of the crack width limit and model uncertainty were identified as being of importance in the reliability crack model.