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The patch repair method is one of the most common approaches used by engineers for repairing reinforced concrete structures that have been damaged. Despite its popularity, the knowledge on material properties pertinent to a durable structural repair is still lacking. Most standards and guidelines on concrete repair currently specify arbitrary limits when it comes to the properties of repair materials. In the case of structural patch repairs, these arbitrary limits have led to the development of high-strength cementitious repair mortars, often marketed as “high-performance”, that are tailored to meet the specifications and not the needs on-site. The paper discussed here forms part of a greater study that aims to increase the understanding of patch repairs and inform existing guidelines on designing structural patch repairs. Three commercially available high strength cement-based repair materials were tested for their strength, elastic modulus, shrinkage, and creep properties. These properties were then used as inputs to an analytical model that was developed to determine the distribution of stress in a repaired element over-time. The results from the analytical model suggest that high strength materials do not structurally contribute in the long term, for patch repaired concrete elements under axial compression.

Electrical resistivity measurements are used to assess the potential durability of concrete. The surface resistivity method is a popular method because of the ease of testing, non-destructive nature, and time efficiency. The moisture state of concrete has a significant influence on the resistivity results. Laboratory specimens cured using practical curing methods should be saturated before testing to eliminate this effect and avoid incorrect interpretation of the results. However, the available standard test methods for surface resistivity do not deal with this issue. Consequently, an experimental study has been conducted on the effect of drying on concrete resistivity values, as reported in this paper. The results show that a reduction in the specimen thickness can improve the ability of the surface resistivity method to be used for assessing curing effectiveness.

Unlike natural aggregate particles, recycled concrete aggregates contain hardened cement paste phases and multiple interfacial transition zones. These differences in properties may be linked to a decrease in durability performance in recycled aggregate concrete. This paper presents a review of the literature published on the durability performance of concrete produced using untreated fine recycled concrete aggregate (fRCA). From the available literature, the durability performance of concrete made using fRCA is strongly linked to the w/b used and the porosity of the resulting concrete matrix. Water absorption, gas permeability, and chloride penetration were found to increase with fRCA replacement. A possible maximum replacement level of 30% was observed in this regard. Carbonation resistance was found to increase with fRCA replacement due to the presence of calcium hydroxide in fRCA samples.

Reinforcement corrosion causes destruction to the steel by a loss in cross section and rib height, which can affect the bond between steel and concrete. During repair of corrosion-damaged reinforced concrete members, a protective coating is usually applied to the cleaned corroded steel. There is, however, little information available on the bond performance of repair coatings. Furthermore, the effect of their coating thickness and other parameters like rebar diameter, corrosion degree and repair material properties are not well understood when considered in parallel with repair coatings. This paper seeks to contribute towards understanding the use of anti-corrosive coatings and the influence of their thickness on the bond between cleaned steel reinforcement and repair materials, when used in combination with different bar geometries at varying degrees of corrosion damage. In this investigation, two high tensile steel bars, Y12 and Y16, were considered. Rebar corrosion damage levels of 0%, 10% and 20% (of steel mass loss) were simulated by mechanical grinding of sound rebar samples to represent the condition of cleaned corroded rebar. A commercially available epoxy-modified, cementitious coating material was applied to the steel in one or two coats, with each coat being 0.6 mm thick. Three repair materials with CEM I 52.5N were considered in this study, which included one concrete with a w/b ratio of 0.45 and two mortars with w/b ratios of 0.47 and 0.65. Pull-out testing was conducted on 108 specimens, and the results analysed by means of the one-way ANOVA test to assess the effect of mainly coatings and other parameters on the bond. The results indicate applying coatings to 10% and 20% corroded steel does not significantly reduce the bond for the M65 and C45 mixes. Furthermore, the difference between one and two coats had little influence on the bond of corroded steel for all rebar geometries and repair mixes.

There is a growing trend to include service life (SL) and durability considerations in the design of reinforced concrete structures. This is partially a consequence of increasing deterioration of ageing concrete infrastructure worldwide and associated financial burdens and structural failures. Against this trend, this contribution critically explores the principles of SL and durability design and highlights the differences between SL modelling, SL prediction, SL design, and performance-based design, focussing on aspects related to reinforcement corrosion in concrete. Based on extensive research and practical experience, the Southern African perspective on concrete durability is then explored in more detail, in particular the South African performance-based design methods, associated SL modelling, and deterioration prevention methods. Based on chloride ingress measurements in-situ, a critique of design exposure classes for marine environments is presented, highlighting the need not only to consider chloride ingress, but also the various factors promoting reinforcement corrosion. [ABSTRACT FROM AUTHOR]

Hydrophobic (silane) impregnation represents a cost-effective way to increase the durability of concrete structures in cases where insufficient design cover quality and depth have been achieved. The water repellent product lines the internal capillary pore structure and provides a water-repellent concrete surface. Thus, the risk of reinforcement corrosion initiation and subsequent deterioration can be reduced as the ingress of water-dissolved aggressive species (chlorides) is minimised or prevented. The purpose of this study was to investigate the effect of silane impregnation on durability indicators, including penetrability tests and chloride ingress (bulk diffusion). The results indicate that silane impregnation reduces capillary absorption and conductivity of chloride ions. Similarly, chloride ingress in the treated concrete mixes was suppressed.

One of the most common methods adopted in the rehabilitation of corrosion-damaged concrete is the patch repair procedure. However, in practice this method has shown to often be unreliable as a consequence of the widespread occurrence of shrinkage induced cracking and poor substrate-patch adhesion leading to debonding of the patch repair. From a practical point of view, such failed repair systems essentially restore the repaired concrete back to a deteriorated state. There is a common belief that repairing concrete with specialised proprietary repair materials would guarantee durability. However, the widespread premature failure of patch repairs conducted using such materials has proven the contrary. This paper presents an understanding of the materials and issues concerning the durability and serviceability of concrete patch repairs, with the aim of identifying alternative non-structural patch repair materials for the effective repair of corrosion-damaged concrete structures. The potential patch repair materials researched were polymer-cement concrete (copolymer of vinyl acetate and ethylene with 5% cement replacement) and 60%, 80% and 100% fly ash (FA) mortar. Patch repairs were conducted on substrate moulds to test application and observe cracking/debonding occurrence. Furthermore, compressive strength, durability index, accelerated drying shrinkage, restrained shrinkage, workability and scanning electron microscopy (SEM) tests were conducted to determine the properties of the materials developed with reference to performance requirements of durable concrete repairs. It was concluded that the 60% FA and polymer-cement concrete repair materials had the best overall performance. This research established that innovative alternative repair materials such as a 60% FA or polymer-cement concrete material, can be developed for non-structural patch repairs with improved long-term performance relative to conventional materials.

Many studies have been realised on polymer-modified mortars (PMMs). Among the polymers used, ethylene vinyl acetate (EVA) has revealed evident interaction between calcium ions and its acetate groups. Most of the studies have shown a positive impact of EVA on mortar performance, which is enhanced by the combination of the EVA with calcium oxide CaO. However, there is still a lack of understanding of the nature of these interactions and no clear link has been established between these interactions and the properties of the cementitious materials at early ages. This article aims to tackle this topic by focusing on the evolution of EVA particles in a pore solution and the properties of a cement modified with EVA and CaO, especially the rheological behaviour. As results, it is observed that the zeta potential of the pore solution decreases when EVA is added. Furthermore, the hydrodynamic radius of this polymer tends to increase over time in the pore solution. On the other hand, the EVA tends to delay the setting time while the CaO accelerates it. For the rheological state, EVA tends to govern the plastic viscosity of the cement paste while CaO governs the yield stress and these parameters are not affected by the mixing time during the first 100 min. Their combination enhances these rheological parameters.

A range of different surface coatings are available for the protection of concrete surfaces, typically aimed at reducing the ingress of deleterious substances into the concrete and providing adequate aesthetic appeal on patch-repaired elements. The coating systems are usually of high material quality and perform well when applied and maintained correctly. The long-term success of protective surface coatings is largely a function of application procedures. In particular, coating durability with regards to bond strength and crack resistance is dependent on the preparation of the concrete substrate prior to coating application. This research gives insight into the effects a sound, clean and profiled concrete substrate that is subjected to different moisture conditions has on coating bond strength. The influence of three different methods to precondition concrete substrates on the tensile pull-off strength of two commercial coatings, applied to substrate concrete with various strengths was investigated. The moisture condition was found to have a significant influence on the pull-off strength of the coatings, higher degrees of substrate saturation typically resulting in higher strengths. Similarly, with increasing substrate moisture content, the failure mode increasingly shifted towards substrate cohesion failure.