Durability of precast prestressed concrete piles in marine environments.

摘要

The purpose of the research was to (1) characterize the durability requirements and field performance of prestressed concrete piles in marine environment, and (2) develop potential high performance marine concretes (HPMC) that would be capable of 100+ year service lives in brackish and seawater environments.;The characterization of pile durability was accomplished through a forensic investigation of bridge piles from a decommissioned bridge, interviews of Georgia Department of Transportation engineers and inspectors, and site inspections of bridges showing substructure deterioration. The results suggested that potential HPMC's must be able to provide adequate resistance to chloride ingress, carbonation, and sulfate attack.;Nine potential HPMC mixture designs were developed utilizing Class F fly ash, slag, metakaolin, and silica fume in binary and ternary mixes. Extensive testing was performed to quantify chloride ingress, carbonation, sulfate attack, and the influence of cracks and self-healing on chloride ingress.;Chloride ingress tests found that ternary mixture designs provided superior resistance compared to binary mixture designs; including of 5 to 10% silica fume to a Type II Portland cement plus Class F fly ash mix increased the service life approximately 25%. Bulk diffusion tests and service life modeling considering only chloride ingress showed that ternary mixture designs with Type II cement, slag and metakaolin provided service lives over 100 years before corrosion initiation would occur.;Sulfate attack resistance was characterized using both expansion and strength degradation test methods. Compressive strength degradation testing demonstrated that mixture designs with a high initial CaO content, determined through oxide analysis of the cement and SCMs, performed well, presumably due to the formation of calcium hydroxide (CH) which served as a buffer to the decalcification of calcium-silicatehydrate (C-S-H) in the formation of gypsum. However, high CaO contents led to poor performance on expansion testing due to the availability of large amounts of calcium hydroxide to react with sulfate ions to form expansive ettringite. Slag mixture designs containing metakaolin performed well with both test methods. Slag mixture designs containing metakaolin performed well on both criteria, and they had moderate CaO contents (44 to 50%) and utilized sulfate resistant ASTM C 150 (2009) Type II cements with low C3A contents.;Carbonation resistance was determined by exposing concrete prisms to a high CO2 environment and by measuring the depth to the carbonation front. The results were used to perform service life modeling of times to carbonation induced corrosion initiation and to determine necessary cover dimension. Carbonation performance of ternary mixture designs showed that they were capable of providing service lives in excess of 200 years using a 5 mm (0.2 in.) cover. Mixture designs with 5% to 10% silica fume and 25% fly ash performed the best, resulting in a service life over 95% longer than 25% fly ash alone.;The investigation into the influence of self-healing on chloride ingress into cracked sections demonstrated that prestressed concrete piles in marine environments can undergo self-healing of tension and flexure induced cracks where the crack widths were less than 186 mum (0.007 in.). Service life modeling considering initiation of reinforcement corrosion was performed, and the model demonstrated that the presence of cracks decreases the service life of a structure by over 40% compared to uncracked concrete whether self-healing occurs or not. Mixture designs containing slag and Type II cement or cement-only show the greatest propensity for self-healing to occur according to crack width measurements and chloride ingress resistance measurements. If cracks cannot be prevented, corrosion resistant reinforcing steel should be used to achieve a 100+ year life span.;The results of the durability assessments for chloride ingress, carbonation, and sulfate attack led to the development of mixture design capable of achieving 75 and 100 year service lives for prestressed concrete piles. The research concluded that uncracked prestressed concrete piles made with non-corrosion resistant steel required protection from sulfate attack, carbonation, and chloride ingress to achieve 75 to 100 year service lives. For cracked sections, corrosion resistant prestressing steel should be used with concrete resistant to sulfate attack; such mixes include Type II cement, at least 25% cementitious content of Class F fly ash or slag, and 5% or more silica fume or metakaolin.