Durability is a major concern for reinforced concrete (RC) structures. RC structures both in service and new, are subject to cracking. Irrespective of the cause of the cracking, cracks can increase the rate of penetration of aggressive species into concrete and modify the transport properties. Consequently, the service life of corrosionaffected RC structures may be drastically reduced in the presence of cracks. However, no modifications are made for the influence of cracking on the penetration of aggressive species into concrete when analysing durability test results or making service life predictions, even through concrete is very often in a cracked state. This study focused on the influence of cracks on the ingress of aggressive species (carbon dioxide and chlorides) into cracked concrete in comparison to uncracked concrete. The aim was to establish any correlations between the transport properties in uncracked and cracked concrete. Furthermore, in a broader context, the aim was to assess to what extent the modified cracked concrete parameters used in service life predictions affect the service life outputs, when compared with service life outputs obtained using the uncracked concrete parameters. Six concretes mixes were investigated comprising two water/binder (w/b) ratios (0.40 and 0.55) and three binder types (100% CEM I 52.5N (PC), 70/30 PC/FA and 50/50 PC/GGBS). 100 x 100 x 500 mm beams were cast and cracks were induced after seven days in the mid-span of each beam using three-point loading. Two crack width ranges were investigated; 0.1-0.4 mm (wcr1) and 0.5-0.8 mm (wcr2). The central section of the beam that contained the crack was sawn from the rest of the beam and used for either accelerated carbonation or bulk chloride diffusion testing. Cores were drilled from the outer sections of the beam and used as specimens for the Durability Index tests. The cracked specimens were monitored for carbonation (accelerated carbonation) and chloride ingress (bulk diffusion), while the uncracked ones were monitored for durability parameters (OPI, WSI & CCI) after 8 and 16 weeks of exposure. Firstly, it can be concluded that the presence of cracks modifies the transport properties of concrete by promoting rapid increase of ingress of aggressive species (CO₂ & Cl-) into the concrete matrix. It was found that the degree to which the transport properties were modified increased as the crack width increased. This was primarily attributed to the increase in surface area created by the crack, which allowed increased amounts of species (CO₂ & Cl-) to penetrate into the concrete matrix. In the case of carbon dioxide ingress, the presence of cracks significantly increased the rate of carbonation (up to 50 %) in the concrete specimens that contained blended cements PC/FA and PC/GGBS when compared to the PC concrete specimens. However, in the case of chloride ingress the effects of cracks in the PC mix resulted in the highest presence of chlorides (up to 78 %) in the concrete specimens in comparison to the chlorides present in the PC/FA and PC/GGBS concrete specimens. Secondly, when the sound (DI) and cracked durability parameters (carbonation and diffusion coefficient) where used in carbonation and chloride ingress service predictions, it was found that the DI service life prediction outputs were more conservative in relation to service life outputs from the durability parameters obtained from cracked concrete specimens. These results highlighted the degree of influence which the presence of cracks had on modifying transport properties in concrete. Furthermore, it also highlights the impact of the presence of cracks on the service life of RC structures and the prediction of long-term carbonation- and chloride- induced corrosion. Due to the significant influence that cracks have on modifying the transport properties of concrete, the results show that some reduction factors need to be applied to the results from the DI approach to reflect a more realistic durability potential of the concrete. Further research into understanding how other crack parameters (crack depth, frequency etc.) modify transport properties in concrete will lead to a more accurate insight into dealing with and accounting for the presence of cracks in RC structures.

The aim of this thesis is to introduce the concept of recycling demolished concrete as aggregate which is then used in fresh concrete - to be known as "recycled concrete". Various aspects of concrete technology are covered and in this way recycled concrete is compared to conventional concrete. The work was performed in three phases, and it should serve as a guide to prospective users. Phase 1: Various recycled aggregates were tested according to standard specifications and were found to be satisfactory in most aspects. Recycled fine aggregate is very coarse though, and should be used with caution. The absorption and porosity of recycled aggregates should always be determined to enable their use in concrete. The specific gravity of such an aggregate should also be found to enable more accurate mix calculations. The highest compressive strengths normally possible for recycled concretes are between 56 and 71 MPa, but an average strength of 50 MPa should not be exceeded without thorough investigation, even though it is easily attainable. Phase 2: A wet-batching method of mix design was investigated and satisfactory recycled concretes were produced. Strength charts for such concretes are given. Methods of dry-batching are also presented, but are more complex than the wet-batch method. The water demand of recycled· fine aggregates was found to be considerably higher than for natural sands, and again the use of fine recycled aggregate should be carefully considered. Phase 3: The mechanical properties of recycled concretes were tested and Little difference found between recycled and conventional concretes. The compressive strengths were satisfactory and the elastic moduli sufficiently high, even though they were 15 to 20 percent Lower than those of corresponding dense concretes. The shrinkage of recycled concrete is comparable to that of conventional concrete, and the creep potential somewhat greater, although not excessivly so. The use of recycled coarse aggregate in both plain and structural concrete is then recommended as an alternative to the dwindling supply of natural aggregates. The use of recycled fine aggregate, however, is not recommended, although its use in Low-grade or mass concrete is condoned.

Summary in English. / Includes bibliographical references. / This dissertation outlines a model developed to allow for the comparison of various design options on a life-cycle cost basis for reinforced concrete structures. The model consists of two interlinked components: the first part of the model is technical, and can be used to estimate the service life of a structure within a specified environment based on a set of prediction models; the second part of the model is used to determine the economic implications of the various design options over a specified evaluation period. The use of a particular predictive model is subject to the environment in which it was calibrated. In many cases the models which are presented were developed overseas and as such their values may not be directly applicable to South African environments. The approaches and development of the models are however useful and if calibrated to particular South African environments could be of considerable benefit. A survey of consulting engineers was also conducted to determine common perceptions of various durability related issues. The results of the survey are presented in this project. The size of the sample was relatively small and as such it would be inappropriate to apply the results categorically to all engineers or organisations. The survey is of value however in that it identifies some areas of potential opposition to the concept of designing for durability and highlights other areas where its acceptance and implementation may be more favourable.

Wastewater conveyance and treatment facilities, which include outfall sewers, manholes, and treatment works, are among the key constituents of a country's infrastructure. Most of these facilities are made of concrete due to its low production costs, versatility, inherent strength and durability under most conditions. However, under certain conditions, sewage that is conveyed through outfall sewers becomes septic and hydrogen sulphide (H2S) gas is generated. When this gas is released from the sewage and absorbed onto the moist concrete sewer pipe walls, it is microbially converted by sulphide-oxidising bacteria to sulphuric acid (biogenic H2SO4) which reacts with the acid-soluble components of concrete causing it to corrode. In principle, the biogenic H2SO4 concrete corrosion mechanism entails simultaneous destruction of the calcium hydroxide (CH) in the hydrated cement paste (HCP) and substituting a larger molecule of calcium sulphate into the concrete matrix thus causing pressure and spalling of the adjacent concrete and aggregate particles. In addition, the calcium sulphate precipitates as gypsum which reacts with various aluminates to form secondary ettringite. These mechanisms lead to the loss of stiffness and strength, accompanied by expansion and cracking, and eventually transformation of the affected concrete matrix into a soft and pulpy non-cohesive layer. The biogenic concrete corrosion rate depends, inter alia, on the chemical composition of binders (cement and supplementary cementitious materials (SCMs)) and microstructural characteristics of concrete mixtures used in the manufacture of sewer pipes. The needed properties of concrete mixtures for sewer pipe applications can be determined by biogenic corrosion prediction models, such as the widely used deterministic (mechanistic) Life Factor Method (LFM). The service life of wastewater treatment facilities made of concrete depends on the input parameters in corrosion prediction models. The motivation behind the current study was based on the need to improve the ability to predict the design life of concrete sewers by improving the input parameters in the LFM, which is used in South Africa. The design life of concrete sewers in South Africa has traditionally been 40 years. The main objective of the current study was to characterise the microstructure of both Portland cement (PC) and calcium aluminate cement (CAC) based concrete mixtures that had been subjected to biogenic corrosion mechanisms in an operational sewer environment for approximately 127 months (10½ years); further, based on the understanding of the underlying mechanisms of attack, proposals were made to improve the LFM, for which the corrosion rate-controlling input parameter, referred to as alkalinity (or equivalent CaCO3, as a summation for both binder and aggregate) is based on the characteristics of plain PC-based binder systems. In addition to the main objective above, a parallel study was undertaken to characterise parameters that influence biogenic concrete corrosion rates based on measurements taken in two sewer environments/sites in different geographical locations in South Africa. One of the study sites was the Virginia Experimental Sewer (VES) in Virginia, Free State Province, while the other site was a manhole within the Langa Pump Station in Cape Town, Western Cape Province. The VES consists of 900 mm diameter by 300 mm long concrete pipe samples made from both PC- and CAC-based (plain and blended) binder systems, the top 120° being cut to form 'lids', so that they are removable. The removable 'lids' enable scheduled observations and sample recovery to be undertaken. Moreover, the 'lids' also act as windows through which core samples can be placed in plastic baskets that are hung at certain sections in the sewer headspace, so that they can be accessed for monitoring.

Traditional metallic materials lead steel-reinforced concrete structures to a durability problem due to its low value of resistance to corrosion. The superior performances of FRP, including the high resistance to corrosion, the flexible and complex shapes… give it a big advantage. However, premature failure due to debonding of adhesives between concrete and reinforcing materials is the major concern for all types of reinforcement containing FRP plate reinforcement. This thesis gradually develops three elastic foundation models, which are mainly derived from the solution of superficial stress in the foundations-soil system. The one-parameter Winkler’s elastic foundation model is simple and easy. The two-parameter elastic foundation model thinks over the interfacial shear force of the joint bond. And the three-parameter foundation model additionally considers the adhesive layer’s transverse displacement to meet the boundary condition of zero shear stress. Finite element analysis (FEA) is used to compare with the proposed three foundation methods.

This thesis contains the results of various green concrete samples subjected to different vibration intensities to determine how green concrete withstands against these vibration intensities. The green concrete was exposed to these vibrations at times before, during, and after the concrete had set. The concrete was also exposed to different timed durations while being subjected to the different vibration levels. Every batch of concrete mixed included a controlled (un-vibrated) set of cylinders and a vibrated set of cylinders. The compressive strength and the resistivity of these concrete cylinders were measured and compared to determine if there was any significant difference between the two sets. It was found that the vibrations subjected to the cylinders did not create a significant effect; given the vibrations levels and timed durations stay within the limits of this study.

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