Assessment of usually encountered damage on concrete bridges and its influence on the load carrying capacity capacity and safety of users

Thesis (MEng)--Stellenbosch University, 2018.ENGLISH ABSTRACT: The assessment of existing bridges is becoming increasingly important since it is necessary to ensure that all bridges maintain structural integrity in terms of the ultimate limit state for which they were designed. South Africa uses a Bridge Management System (BMS) to assess and manage existing structures. The assessment is based on visual inspection, and ratings form 0-4 are assigned to the extent of damage (DERU-rating system) which occurs on the bridge. Using the condition analysis, a prioritised list is compiled where ranking all inspected bridges from worst to least degraded. This list is then used to determine what resources should be allocated and which bridges need to be repaired first. A more accurate way of assessing existing bridges is however required to determine which bridge will fail first, if not repaired. This study developed a method for assessing existing bridges based on their reduction in structural integrity. It was done by relating compiled damage distributions to the DERU-ratings, then using the assigned DERU-ratings in accordance with a reliability-based model, to determine reductions in structural strength. The critical damages identified were reinforcement corrosion, cracking, and spalling of concrete. These damages were compared and it was assumed that a natural sequence will follow; the reinforcement will start to corrode, induce cracking and lead to concrete spalling. This natural sequence was divided into categories, each representing a state of degradation. Distribution functions were compiled to represent the critical damage and were related to the established categories. To determine the effect of these categories, a representative beam was used and degradation applied. The reduction in structural strength was determined according to a reliability assessment, and these results were compared to the results if the beam was analysed according to the BMS. It was concluded that the reliability assessment gave a better representation of structural degradation since, in this case, most of the degradation was not visible from the outside, and thus could not be correctly assessed by visual inspection.AFRIKAANSE OPSOMMING: Die strukturele assessering van bestaande brûe is ‘n kritiese aspek van infrastruktuur bestuur, aangesien die integriteit van ‘n struktuur in terme van sy uiterste ontwerps-limiet verseker moet wees oor sy lewensduurte. Die brug bestuur sisteem genaamd “BMS” word in Suid-Afrika gebruik vir die bepaling van bestaande strukture se kondisie, en is meestal gebaseer op visuele inspeksie en ander niedestruktiewe toetsmetodes. ‘n Objektiewe asseseringsmodel genaamd “DERU” word gebruik om verskeie tipes skade teenwoordig op ‘n brug te dokumenteer. Deur gebruik te maak van kondisie analise kan ‘n geprioritiseerde lys opgestel word vanaf hierdie data, waar alle brûe gerangeer word in terme van skade en herstelwerk benodig. Die instandhoudings outoriteit kan dan objektief besluit of die bestaande brûe herstel of vervang moet word. Die studie het ‘n metode ontwikkel om bestaande, beskadigde brûe te assesseer in terme van hul strukturele verrigting. Dit was bereik deur die versamelde skade distribusies volgens die DERU-model te vergelyk deur gebruik te maak van ‘n betroubaarheid-gebaseerde model. Die resultate verskaf dan ‘n objektiewe uitslag oor die strukturele integriteit van die struktuur. Kritiese skade kriteria is geïdentifiseer as die drievoud van bewapening korrosie, kraak van omhullende beton en die dienooreenkomstige aftrek van die buitenste betonlaag. Hierdie skade kondisies is vergelyk en aangeneem as die normale verloop van degradasie op ‘n strukturele bewapende beton element. Hierdie natuurlike gevolg is ingedeel in kategorieë, waar elke afsonderlike kategorie ‘n element van degradasie verteenwoordig het. Distribusie funksies is saamgestel om die kritieke skades te vergelyk met die bestaande kategorieë. Laastens is die effekte vergelyk met die bestaande BMS deur gebruik te maak van ‘n verteenwoordigende strukturele balk met verswakking a.g.v. die kritiese skade kriteria. Die studie het bevind dat die betroubaarheids assesserings model ‘n beter verteenwoordiging van die struktuur se verswakking toon, veral in die spesifieke gevallestudie aangesien die verswakking nie visueel sigbaar was nie