PSI - Issue 26

Petr Konečný et al. / Procedia Structural Integrity 26 (2020) 430 –438 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The authors already published preliminary conclusions from the field measurements of two bridges in Vořechovská et al. (2018), the effect of the core sample locations was considered in Konečný et al. (2020), but the differences between main road and motorway bridges were not taken into account. For the assessment of the corrosion risk on the sampled cores, it is needed to define the threshold value related to the corrosion initiation based on the presence of the aggressive agent. Neville (2012) reported that conventional steel reinforcement in concrete is depassivated if pH drops below 11.8, and usually, the reduction of pH is related to the carbonation effect. However, the threshold values reported by researches vary significantly, e.g. Glass and Buenfeld (1997). The chloride ion concentration is typically expressed as a percentage per mass of cement; therefore, the equation for conversion to the percentage of the mass of concrete may be helpful. Locke et al. (2009) reported the chloride threshold for the corrosion initiation as 0.4 – 0.8 [wt.-%/cement], while ACI 201.2R-01 reports 0.1 – 0.2 [wt.-%/cement]. That might be also given as concentration per mass of concrete. The approximate ratio 5.5 for the recalculation from [wt.-%/cement] to [wt.-%/concrete] might be derived when considering for simplicity the weight of cement 400 kg and the weight of concrete without steel 2200 kg, which would be around 0.078 – 0.145 [wt.- %/concrete] according to Locke et al. (2009) and 0.018 – 0.036 [wt.-%/concrete] according to ACI 201.2R-01. The value of 0.1 [wt.-%/concrete] is considered as a chloride threshold herein based on Huet et al. (2003). The high scatter of the chloride threshold values may be related to the strong correlation to the pH of the concrete, see Raharinaivo et al. (1986), Vořechovská et al. (2018), Vořechovská et al. (2009), Huet et al. (2003). However, the value of the pH related to the carbonation of concrete is widely missing, e.g. Stewart and Rosowsky (1998), Lehner et al. (2014), Tikalsky et al. (2005), Bentz et al. (2013). In this paper, the comparison of chloride concentration and corrosion risk with consideration of the carbonation effect is of high interest. Furthermore, the “critical” parts in the examined bridges that are most affected by the degradation effects are localized and the influence of whether the bridge belongs to the main road or motorway is taken into account. 2. Methodology 2.1. In-situ investigation The bridges under service in the portfolio of Directorate of Road and Motorway of the Czech Republic are regularly inspected. During the inspections, the concrete samples were drilled out of the specific parts of constructions – the representative locations on girders, abutments, sealing of longitudinal joints between the precast girders, bearing seats, or injection grout in precast girders. The samples were taken in three depths from the concrete surface (0–10, 10–20 and 20–30 mm), in the case of injection grout, only one sample was taken per location. In total, 298 cores with 803 samples were analyzed. 2.2. Chemical analysis The value of pH and the chloride concentrations (number of water-soluble chlorides) of the concrete samples drilled out of the construction were analyzed in the laboratory of the University of Technology in Brno. Based on the measured parameters, the risk of the corrosion is expressed by the ratio of the concentrations of Cl – and OH – that indicates the ability of concrete to protect reinforcement. The potential risk of corrosion increases with the ratio of chloride and hydroxide ions; the critical value of c(Cl – )/c(OH – ) was determined as 0.6 as in Raharinaivo and Genin (1986). 2.3. Evaluation of the measured data The results were grouped based on the locations in the construction, so the intensity of the exposure to the aggressive environment could be considered. Therefore, the first group consists of abutments and bearing seats; in the second group are girders, cross bars, and longitudinal joints together; in the third group are concrete patches, and the fourth and fifth group consists of injecting grouts and support columns and intermediate support together.