Predicting depth of carbonation of concrete for varying climatic conditions

Conference ProceedingsCorrosion of reinforcing steel is the result of poor durability performance of reinforced concrete structures. Carbonation of concrete caused by the diffusion of carbon dioxide into the concrete is one of the major factors responsible for reinforcing steel corrosion. A carbonation model which predicts the rate and extent of carbonation is useful not only in the design phase by facilitating the right choice of materials, but also in helping to assess the rate and extent of carbonation of existing structures. Most of the currently available carbonation models predict the depth of carbonation based on constant humidity conditions. The influence of varying climatic conditions (i.e., drying and wetting cycles) is not taken into consideration in predicting depth of carbonation. However, current approaches may be conservative or non-conservative, depending on the different climatic conditions which influence the rate of carbonation. The main aim of this study was to develop or modify a carbonation model to as to accommodate varying climatic conditions. The carbonation model developed is validated based on experimental data from specimens prepared with different concrete mixes exposed to natural carbonation

Long-term performance of bonded concrete overlays subjected to differential shrinkage

Includes bibliographical references (leaves 231-243).The performance of bonded concrete overlays relates mainly to the resistance against cracking and debonding. The associated failure mechanisms are largely a result of differential volume changes between substrate and overlay. The objective of this research was to develop analytical tools to facilitate the design of bonded overlays subjected to differential shrinkage. The experimental programme included the identification of fundamental strain characteristics and bond strength development of composite members in relation to different interface textures and overlay materials. Existing analytical models for the prediction of strains and stresses in bonded overlays were evaluated. Results from the experimental work indicated that existing models, which are based on simple beam theory, are deficient in modelling overlay strains in a realistic manner. The degree of overlay restraint was found to depend far less on relative sectional dimensions of substrate and overlay as commonly assumed. Based on fundamental aspects concerning strain characteristrics of bonded overlays identified through experimental tests and numerical simulations, an analytical prediction model was developed based on localised strain conditions at the interface

The effect of hydrophobic (silane) treatment on concrete durability characteristics

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

Performance-based specifications and control of concrete durability: state-of-the-art report RILEM TC 230-PSC

This work gives an overview of significant research from recent years concerning performance-based design and quality control for concrete durability and its implementation. In engineering practice, performance approaches are often still used in combination with prescriptive requirements. This is largely because, for most durability test methods, sufficient practical experience still has to be gained before engineers and owners are prepared to fully rely on them.   This book, compiled by RILEM TC 230-PSC, is intended to assist efforts to successfully build the foundation for the full implementation of performance-based approaches through the exchange of relevant knowledge and experience between researchers and practitioners worldwide.  

The influence of concrete substrate moisture condition on the tensile pull-off strength of protective coatings

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

Bond (chapter 4)

Good bond is a key factor for providing monolithic action in bonded concrete overlays. This chapter starts with three theoretical sections classifying bond in three groups (complete, uncertain, and poor), defining bond strength, and describing the fundamental bond mechanisms. The main part of this chapter covers a thorough description in chronological order of how the 13 most important factors affect bond. All events from removal of deteriorated concrete to concrete placing and curing and long-term exposure are investigated. There exist a number of different methods to determine bond strength and the most frequent ones are briefly described in a separate section. It is shown that there is an evident relationship between the two most frequently used families of test methods, i.e., between methods determining bond strength in tension and bond strength in shear. Furthermore, it is shown that it is possible to provide durable bond in concrete overlays if all operations for concrete removal, surface cleaning, concrete placing, and curing are conducted meticulously. The chapter is ended by two sections devoted to design strength values in various international codes and performance requirements that can be used for quality control

The effect of curing, specimen thickness, and saturation on surface resistivity of concrete

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

Investigating the structural contribution of patch repairs to reinforced concrete elements

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

Durability of untreated fine recycled aggregate concrete: a literature review

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

Alternative patch repair materials for rebar corrosion damage

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