Civil Engineering March 2021 | Vol 29 No 2

40 March 2021 Civil Engineering divides the service life of reinforced con- crete structures into two phases, namely: Q Q Initiation phase – the time required for carbon dioxide or chlorides to reach re- inforcement level and initiate corrosion. Q Q Propagation phase – the time between the initiation and failure. The corrosion initiation and propagation time for a concrete structure can be mod- elled by using diffusion laws that govern how carbon dioxide and chlorides move via dif- fusion. An estimate can in turn be made on the remaining service life of the structure. As chloride induced corrosion was ruled out, a carbonation model was used to model the initiation time and remaining service life of the five bridge structures to provide the client with additional informa- tion to ascertain the extent of the repairs. Fick’s Second Law of Diffusion was used for the carbonation model; the equa- tion of the model is as follows: Cd = k √ t Where, Cd = carbonation depth (mm) k = co-efficient of carbonation (constant) t = time (years) To obtain an estimate of the k value, the carbonation depths and ages of the bridges were used. This k value is sensi- tive, and the results calculated using the model were concluded to be conservative. Although conservative results for the initiation time were obtained, the results calculated for certain bridge elements did provide supplementary information to the concrete test results for the selection of appropriate repair methods. For example, the model predicted a time until the carbonation front reaches reinforcement level in the walls of the culvert structure as 28 years. This information was used to decide, considering the age of structure, if the walls of the culvert should be coated with an anti-carbonation coating to halt the movement of the carbonation front and extend the structure’s service life. CODES AND STANDARDS In the new COTO standards, reference is made to the EN 1504 code, the European standard which provides an extensive guide to concrete protection and repair. The standard contains product specifications as well as repair methods that should be ad- hered to for effective treatment of corrosion. For new construction, the current South African guide for the design of durable concrete involves the use of Table 6000/1 specified by SANRAL. This table includes design specifications for both carbonation and chloride induced corrosion which makes use of exposure conditions to allow the selection of cover depths and blends of cement at design stage. Additionally, the table provides recommended values for sorptivity tests conducted on the concrete. Table 6000/1 could further be used to assess cover and durability performance of existing structures by determining what these measurements should be for the structure’s exposure conditions. CONCLUSION In addition to concrete testing, standards and codes were found to be essential for ensuring appropriate solutions are specified for concrete repair. The EN 1504 code is a necessary reference for repair, but it is required to be purchased which may create a gap in the specifications, tender and construction process. The development of a South African standard may resolve this possible gap. Systems such as SANRAL’s BMS provide an essential tool that enables structures requiring urgent attention to be prioritised. This is done through informa- tion stored on the BMS from inspections and repair work carried out over the years. Recorded information is also valuable when tests and repair work are carried out. It is advisable for concrete testing to be done prior to undertaking the repair work to ensure the appropriate repair measures are specified and quantified during the design stage. The test results should also be well documented on the respective cli- ent’s asset management system for future accessibility and reference in establishing the rate of deterioration. Engineers should exercise good judge- ment and ethics when specifying tests and repair of concrete structures. Tests that are not necessary should not be specified; this decision should be made with adequate information, knowledge and references to literature and guidelines. Codes and stan- dards should be adhered to for repair to ensure the safety of users of the structure is not compromised, and extra costs are not incurred as a result of the application of incorrect repair methods.  NOTE Reference details are available from the authors. Figure 7 Cover testing being carried out at Bridge 1 Deterioration Limit state Initiation period Propagation period Service life Carbonation Reinforcement corrosion Penetration of chlorides Figure 8: Service life graph (Raupach, M (2014) after Tutti (1982)) Time