ASSESSMENT OF THE DETERIORATION OF EXISTING REINFORCED CONCRETE STRUCTURES EXPOSED TO SEVERE MARINE ENVIRONMENT

ABSTRACT

Globally, extreme environmental conditions lead to premature degradation of maritime concrete structures, causing them to fail before their projected service life. The key mechanism responsible for premature degradation of reinforced concrete (RC) structures has been established as reinforcement corrosion. This is primarily controlled by chloride ingress, resistivity, and porosity of RC structures. This has been a major concern for RC structures exposed to Escravos river with seawater predominantly known to be saline in nature and houses many RC structures built to facilitate ease of oil and gas operations. Advances in engineering methods are required for reliable structural assessment of existing structures. This work aims to evaluate the extent of deterioration of two existing RC structures, built in this river and determine the constituent compounds present therein with a view of identifying their influence on deterioration of existing structures and to meet future design needs in an economic way and avoid unnecessary reconstructions.
This work adopted two methods in approaching the deterioration challenges in these structures. First, a general condition assessment methodology consisting of in-situ, non-destructive measurement techniques and detailed testing of samples from the existing structures was studied. The structure's current compressive strength was determined using the Schmidt rebound hammer, and corrosion rates measured based on weight loss of corroded steel reinforcement samples. Secondly, an idealized experimental study was set up under controlled site conditions and electrically accelerated corrosion process to measure chloride concentration at different concrete depths. The analysis also used the chloride threshold value of 0.07% by weight-of-concrete per BSEN 206-1 to determine corrosion initiation point. All specimens were submerged in the Escravos seawater’s sample for maximum of 35days. In compliance with standard practice, water samples were analyzed to determine the presence of corrosion-causing agents in this seawater and their effect on the existing structures.
The results revealed that the quay structure suffered a major chloride attack from surrounding seawater due to its long exposure whereas the jetty structure is still in a safe state since it was recently constructed. In-situ corrosion tests on quay reinforcement samples revealed a high corrosion rate of 0.65uA/cm3 over 45years, as shown by a weight reduction of approximately 47% per meter of steel. The rebound hammer test revealed an average concrete strength of 29N/mm2, a decrease of 27% from the as-built value of 40N/mm2. Chloride diffusion was determined using Fick's second law of diffusion, while weight loss due to steel corrosion was calculated using Faraday's law. Predicted chloride ingress trends showed that the critical chloride level occurred at the reinforcement depth after 16years. With this 45years old structure, the level was exceeded in the quay structure 29years ago with a remaining service life of 15years. This conforms with the recommended 50years intended working life of RC structures in seawater environment per BSEN 206-1. The results showed that the rate of corrosion and strength deterioration depended on the environmental nature and exposure time. The experimental test results will also serve as guidance information for future engineering design of new structures in this region.