Civil Engineering March 2021 | Vol 29 No 2

Civil Engineering March 2021 39 reinforcement is regarded to be at risk of chloride-induced corrosion. In the case of all three bridge struc- tures, the chloride content values were established to be well below these limits, even with the actual cementitious material content of the concrete unknown. For the 0.2% chloride limit to be exceeded, the ce- mentitious material content in the concrete mixes reported in Table 2 would have to be less than 250 kg per cubic meter and would not have achieved the compressive strength results reported. With this information, the possibility of any defects occurring due to chloride-induced corrosion were ruled out. CARBONATION TESTING Carbonation results were found to vary across the different elements of Bridge 1. Carbonation depths were not severe on the parapets, abutments, and piers. High carbonation depths were however recorded on the bridge deck. The discrepancy was attributed to ‘micro-climates’ that occur in a structure where different elements exposed to different moisture and thermal conditions cause different rates of carbon- ation in each element of a structure. Cyclic wetting and drying of the bridge deck was also noted as a possible reason for the high carbonation depth recorded. COVER TESTS The recorded cover measurements on certain bridge elements were found to be lower than the acceptable standards and corrosion of the steel reinforcement was observed. Certain elements recorded adequate cover, but concrete spalling was still observed. The spalled concrete was attri buted to localised areas of minimum cover that is a result of poor workmanship at placement stage of construction, as op- posed to significant carbonation. A visual inspection of the concrete par- apets at Bridge 1 showed minimal cover in localised areas, with corrosion occurring as a result. In this case, it was recommended that the spalled areas be repaired, and a carbonation barrier coating be applied to the entire surface of the parapets. Coatings that are compliant with EN 1504-2 and possess a minimum Sd value of 50 m, should result in the carbonation front re- maining at approximately the same depth. The Sd value of the anti-carbonation coating is its diffusion resistance to carbon dioxide permeation. DURABILITY TEST RESULTS Durability results of the concrete at Bridge 1 varied, and are shown in Table 3. The oxygen permeability and water sorptivity test results were poor, indicating that the concrete was susceptible to ingress of moisture and gases. An interesting outcome was the chloride conductivity test that produced an excellent result. This provided supplementary information sup- porting the conclusion that the ingress of chlorides was not the cause of the defects. Despite the presence of chlorides, the evidence indicated that these may have been bound in the mix, possibly due to supplementary cementitious materials in the concrete such as ground granulated blast furnace slag or fly ash. This is im- portant to note as a high chloride content may not always result in corrosion. It is essential for the engineer to make informed decisions on which tests to specify. Chlorides can be ruled out with adequate information on the structure’s location and exposure conditions, however, cognisance must be given to chemically bound chlorides which may be released due to deterioration of the cement hydrates caused by carbon- ation. The chlorides would then become available for the corrosion reaction. REMAINING SERVICE LIFE OF CONCRETE STRUCTURES A remaining or residual service life of a con- crete structure in the context of corrosion is understood as the time from the present condition of a structure to a time where a threshold limit of deterioration is reached, and the structure is no longer serviceable. Figure 8 is a sketch of a Tutti’s model that Table 3: Durability results of Bridge 1 DI value Qualitative descriptor Oxygen permeability index (no unit) 9.45 Poor Water sorptivity (mm/hour0.5) 10.32 Poor Chloride conductivity (mS/cm) 0.65 Excellent Table 2 Summary of Bridge 1 test results Element Compressive strength (MPa) Average chloride content (%weight of sample) Average measured cover (mm) Carbonation depth (mm) Parapets 34.3 0.019 29 5 Abutments 48.1 0.020 57 15 Pier 48.5 0.019 71 15 Deck 36.1 0.017 26 30 Figure 5 Carbonation depth of Bridge 1 core Figure 6 Spalling due to inadequate cover