Literature review of the use of common protective coatings for concrete structures with experiences in the South African context

Jappie, Luqmaan

Literature review of the use of common protective coatings for concrete structures with experiences in the South African context

Master Thesis

2019

Abstract

INTRODUCTION The main objective of this dissertation is to compile a comprehensive literature review of surface protection systems, including their historical development, specification and use, and to conduct an industry review from the South African market. With a vast amount of deteriorating reinforced concrete structures and fast developing technology of surface treatments, it is important that engineers have a good working understanding of concrete deterioration, repair and the use of surface protection systems. Additionally, engineers need to have a good understanding of the pore structure of concrete and its transport properties as this is important to understanding the applicability and use of surface treatments (Breysse and Gérard, 1997; Leeming et al., 1997; Ballim, Alexander and Beushausen, 2009). TRANSPORT PROPERTIES The movement of ions or fluids in concrete occurs due to four basic mechanism; diffusion, permeation, absorption and migration, as briefly outlined below. The kinetics of movement is broadly governed by the size and nature of the concrete pore structure and its exposure environment (Ballim, Alexander and Beushausen, 2009; Basheer and Barbhuiya, 2010). Process Description Diffusion: Movement of ions, gas or liquid under a concentration gradient Permeation: Movement of a fluid through a concrete matrix under an externally applied pressure gradient when saturated with that fluid Absorption: Where a fluid is drawn into the pores of concrete through capillary suction Migration: Movement of ions due to an electrical potential gradient Combined transport mechanisms and long-term changes in transport behaviour may need to be considered, Additionally, the size, nature and degree of cracking is an especially important consideration (Ballim, Alexander and Beushausen, 2009). DETERIORATION OF CONCRETE Rebar corrosion is the biggest threat to the durability of reinforced concrete structures, and is influenced mainly by the quality of the concrete, its cover depth to the reinforcement and the environmental exposure conditions. The primary causes of rebar corrosion are carbonation and chloride ingress. Chemical and acid attack may be of concern in certain environments. When using reactive aggregates, alkali-aggregate reaction may also be of concern to unreinforced and reinforced concrete. Surface protection systems can assist in reducing the effects due to the influence of the above penetration processes. Additionally,in harsh environmental conditions, such as in marine areas, additional protection measures are oftenrequired to ensure that concrete (existing or new) will not prematurely deteriorate during its service life (Beckett et al., 1987; Technical Committee 60-CSC RILEM, 1988; ACI Committee 201, 2008; Larsen, 2008; Ballim, Alexander and Beushausen, 2009; Gjørv, 2011). Typically, the following repair techniques may be considered in the repair and service life extension of concrete structures (Mackechnie and Alexander, 2001): • Crack Repairs • Patch Repairs • Surface Coatings • Migrating Corrosion Inhibitors (MCI’s) • Electrochemical Techniques • Cathodic Protection • Demolition and Reconstruction REPAIR STANDARDS To ensure that a concrete repair project is successful, a systematic approach to the inspection and repair strategy development needs to be followed (Building Research Establishment, 2000b). The European standard EN 1504 and the Concrete Repair Manual from the American Concrete Institute provides such an approach. South Africa does not have such a standard, but is in the process of adopting the European standards for concrete (South African Bureau of Standards, 2017), EN 206 and EN 1992 - it would thus be highly beneficial for South Africa to move towards the adoption of the EN 1504 code as well. EN 1504-9 is a very important part of the code, as it provides a structured approach to the investigation of the cause of deterioration and outlines the “Principles of protection and repair.” EN 1504-2 covers the use of surface treatment systems, and its provisions are intended to be used as “Methods” in order to cover the “Principles” outlined in EN 1504-9 (Atkins et al., 2009; Raupach and Büttner, 2014). SURFACE PROTECTION SYSTEMS Modern technological advances have given rise to numerous surface treatment systems available on the market with various sub-categories that can assist in achieving the durability requirements of a concrete structure, from silanes, siloxanes, many types of polyurethanes and modified cementitious coatings as well as hybrid systems. Each product and system has its use, advantages and disadvantages and the selection of a system and decision on whether to apply a surface treatment or not can be a complicated matter. This decision is often left to the discretion of the engineer, and therefore engineers need to have a good working understanding of surface treatment systems (Leeming et al., 1997; Beushausen and Alexander, 2011; ACI Committee 546, 2014a). Surface protection systems can be classified according to the way in which the protective action is provided. This is the classification system used by EN 1504 and is divided as follows: HYDROPHOBIC IMPREGNATION These are low viscosity fluids which penetrate several millimetres into the concrete and considerably increase the water penetration resistance of concrete, whilst still allowing the passage of water vapour and gases. Typical examples are silanes and siloxanes. They are also referred to as pore lining penetrants (Beckett et al., 1987; Leeming et al., 1997; Bijen, 2003; ACI Committee 546, 2014b). IMPREGNATION These are low viscosity solutions that penetrate 1 - 3 mm into the concrete and effectively block pores. They differ from Hydrophobic Impregnations in that they are more restrictive to the passage of water vapour and gases. Typical examples are silicates and silicoflourides as well as certain types of polyurethanes (Beckett et al., 1987; Leeming et al., 1997). COATINGS These are treatments that form a continuous protective layer on the surface of the concrete. They are typically 0.1 mm - 5 mm thick, but may be thicker than 5 mm for certain applications. Typical examples are polymer-modified cementitious systems and polyurethanes (Beckett et al., 1987; BS EN 1504-2, 2004). Surface treatment systems need to meet specified performance criteria. In terms of the concrete durability, these are typically (Beckett et al., 1987): • Ingress protection • Moisture control • Physical resistance / Surface improvement • Resistance to chemicals • Increasing resistivity • Cathodic control Treatment systems may also be required to bridge cracks, be applied to moist concrete or operate in harsh exposure and weather conditions. Therefore, the selection of a system needs to consider these factors and testing may be required for certain critical properties on-site to determine its suitability. Engineers, owners and suppliers need to collaborate in order to achieve a good solution. EN 1504-2 (2004) provides a detailed list of performance criteria for each of the various types of treatments along with the relevant code of practice for testing. In general, suppliers recommended application guidelines should be followed in the application of the system. INDUSTRY REVIEW A review of products and recent applications from Sika and A.B.E. Construction Chemicals is provided in this work. It was found that polymer-modified systems are still the most commonly used system. Siloxanes are often used for hydrophobic impregnation treatments, as the pure silanes are intended for high-performance usage and are only used in special circumstances. Each of the suppliers provides detailed application guidelines for each of their products as well as generalised expected performance criteria. Many of the products available have been formulated such that they are applicable to a wide range of applications, and it appears that some known problems with certain products have been improved on. DISCUSSION & CONCLUSIONS Findings of this report are listed below. • The selection of a treatment system is a complex issue and therefore requires engineers to have a good appreciation of various surface treatments systems. A proper framework and set of guidelines is needed. Notwithstanding the above, collaboration between the owner, engineer and supplier will go a long way to a successful application. • South Africa is lacking in the availability of detailed information for engineers to assess, design, specify and monitor treatment systems. South Africa is also in the process of adopting the Eurocodes for concrete design. The EN 1504 repair code contains specific provisions for surface treatments systems and should now be considered for implementation in South Africa. In the interim, EN 1504 can be used to assist in ensuring a standardised approach has been followed in a repair project. • The current EN 1504 surface treatment classification divides systems into Hydrophobic Impregnation, Impregnation and Coatings. Whilst these are generally acceptable it may be worthwhile to reconsider Coatings as two types i.e. 'coatings’ for thinner coatings and 'overlays’ for thicker coatings, as these treatments may function in a very different manner - overlays function by their thickness providing protection and may not be especially complex treatments. They may also simply be applied for levelling and have a treatment or coating applied over them. • Hydrophobic Impregnation is commonly achieved by the use of silanes or siloxanes or silanesiloxane blends. In particular, silane-siloxane blends are most commonly used. This is due to cost, difficulties in application of silanes, and environmental concerns with the volatile organic compound content of silanes. • There are conflicting reports on the effectiveness in the use of silicate systems for improving concrete durability. This appears to be in-part due to the lack of agreement on the exact nature of the protection mechanism. Further research is required to reconcile differences in reporting. • Polyurethanes are very versatile and are available in various forms. Many differences were found in literature on the effectiveness of polyurethanes for improving durability, and sometimes within the same generic type. One of the problems appears to be that researchers often don’t describe precisely enough the exact nature of the polyurethane that was used in their works. A standardised reporting approach is needed. However, polyurethanes have been shown to positively effect many durability issues in concrete such as water absorption, chloride diffusion and carbonation - depending on the specific polyurethane used. • Polymer-modified cementitious coatings are the most commonly available and used surface treatments, and can be used for a wide range of applications. Their exact properties depend on their formulation, type of polymer and polymer-cement ratio. However, they are generally very versatile and most commercially available products can be used to achieve a wide range of properties, including improving the durability of existing concrete surfaces. They are sensitive to weather conditions during curing and special precautions may be required.

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