Civil Engineering March 2021 | Vol 29 No 2

36 March 2021 Civil Engineering INTRODUCTION TO DURABLE CONCRETE Durable concrete has the ability to with- stand specific environmental and loading conditions throughout its service life, without significant deterioration. Concrete durability is not a single property that con- crete possesses, but rather a combination of properties which allow certain concrete mixes to withstand exposure conditions more effectively. Standard test methods and acceptability parameters are well documented and researched to measure these properties. Properties such as porosity, perme- ability, capillarity, and diffusivity of the concrete are typical indicators of the durability performance of concrete used for new construction; measuring these properties on existing concrete structures, along with the extent of deterioration, provides insight into the defect causing mechanism and the rate of deteriora- tion. Understanding the deterioration mechanism of the concrete and the rate thereof is the only way to appropriately specify the repair and maintenance of concrete, particularly concrete in steel reinforced structures. LOSS OF CONCRETE DURABILITY Concrete deteriorates with time through various actions which can be principally characterised in three groups: chemical, physical, and electrochemical. Both chemical and physical attack can result in electrochemical attack, which is the pro- cess by which steel corrodes. The leading defect caused by the deterioration or loss of durability in reinforced concrete is the corrosion of the steel reinforcement. Concrete is highly alkaline because of the alkalis and the hydration products from the cementitious materials. This alkalinity provides a favourable environ- ment for steel as it forms a ‘passivating’ protective layer around the embedded steel reinforcing. This layer protects the steel from corrosion, even in the presence of water and oxygen. The loss or disruption of the protec- tive passivating layer can be attributed to carbonation (reaction with carbon dioxide) and/or chloride attack (primarily to chloride ingress). CARBONATION Carbonation occurs due to the reaction of carbon dioxide with the calcium and hydration products in concrete. As carbon dioxide from the atmo- sphere reacts with the water in the pore system of the concrete and passes through the capillary tracts, it lowers the pH of the concrete and the concrete becomes “carbonated”. When this carbonation front reaches the depth of the embedded steel reinforcement, the passivation layer breaks down, and corrosion is initiated. Carbonation-induced corrosion is a micro cell electrochemical reaction with adja- cent anodic and cathodic areas. CHLORIDE ATTACK Chlorides may be introduced to concrete either at mixing and placing stage, or by ingress into the finished concrete. When chlorides enter through the surface of the concrete, they migrate into the concrete, increasing its chloride concentration. When the critical chloride concentration is reached at the depth of the embedded reinforcement, the breakdown of the pas- sivation layer commences, and corrosion is initiated. Chloride-induced corrosion is a macro cell electrochemical reaction with distinct anodes and cathodes. This is a more aggressive, localised form of corrosion; as opposed to carbonation-induced corro- sion which occurs at a moderate rate, at various parts along the reinforcing bars. CORROSION TREATMENT It is essential to understand the corrosion causing mechanism in concrete so that an appropriate treatment of the concrete can be carried out thereafter. Corrosion of the steel reinforcement reduces its cross-sectional area, and the corrosion products fill a larger volume than the uncorroded bar. This causes the concrete cover to crack and spall which is commonly repaired by patching, an ac- ceptable solution for carbonation-induced corrosion. Localised patch repairs of spalled concrete occurring as a result of chloride-induced corrosion may, however, still leave the reinforcement susceptible to corrosion should all the chloride-affected concrete not be treated. The new patch of concrete often leads to incipient anode formation and new areas of corrosion which in turn leads to patch failure. The accurate diagnosis of corrosion therefore becomes essential so that expen- sive concrete repairs are not undertaken with an adverse result. Testing is necessary to ensure the appropriate repair method is specified and applied to the defected concrete. Ultimately, the repair undertaken should prolong the structure’s service life. Concrete testing , an imperative for accurate assessment and maintenance of concrete structures Kerusha Ayer Graduate Engineer Naidu Consulting kerusha.ayer@naiduconsulting.com Kevin Volmink Pr Eng Sector Manager: Construction Naidu Consulting kevin.volmink@naiduconsulting.com Andries van der Merwe Pr Eng Senior Engineer Naidu Consulting andries.vandermerwe@naiduconsulting.com