Concrete is the most widely used materials apart from steel for construction of infra structural structures like bridges, cooling tower, jetties and silos. In the coming future there will be more focus on developing infra structural facilities in India and cement concrete consumption is expected to grow. Although normally RCC structures are preferred as it involves less maintenance cost and it’s over all life cycle cost is low. But if proper quality checks are not observed during construction one may have to repair the concrete structure at the early age increasing life cycle cost of the structure and causing inconvenience to public using that particular infra structural facility. This paper present one such a situation where RCC deck slab of the National Highway Bridge has to undergo major repairs within 2 years of commissioning because of accelerated corrosion of the reinforcement. Papers discusses approach taken for analyzing the cause of the problem and solution offered and executed for fixing the problem.
The bridge under consideration is along the west cost of India and near the sea. This was a major flyover and was open to traffic two year back. Within one year of commissioning there were appearance of crack on the surface of the road. Concessioner carried out some minor repairs considering it as local cracking of concrete. However, cracks keep appearing on concrete surface and that’s when concessioner thought of engaging specialized retrofit contractor Sanrachana Structural Strengthening Pvt. Ltd. (SSSPL) to offer the long term solution for the problem.
On inspection cracks were appeared on wearing coat of the bridge and local pot holes type formations were observed on deck because of popping up of wearing coat. We checked the old concrete records and concrete grade M40 was found to be ok. Also designs were rechecked and overall design found to be correct. It is decided to carry out non destructing testing of the concrete and chemical and electro chemical analysis of concrete. Following tests were conducted on concrete,
Core compression testing of deck concrete
Progressive chloride profile of concrete by IS 14950 (Part 2): 2001
Half-cell potential test of RCC Deck
Concrete resistivity, I Cor – value to find the rate of corrosion of reinforcement.
Results and Discussion
Core compression test:
As discussed previously total 12 nos. of concrete core extracted from deck slab. The average equivalent cube compressive strength of the concrete is M45 which is much above prescribed concrete grade of M40.
Progressive chloride profile of concrete by IS 14950 (Part 2): 2001:
It can be seen from the table above that chloride content is higher than permissible limit in the upper 25 mm layer of concrete and is in normal range as it goes down.
It can be concluded from the results that there is early age corrosion in the reinforcement. Main cause of this early age corrosion in high chloride concentration in the concrete. It looks that although concrete quality was maintained while casting because of water scarcity water used for curing is from local tanker vendors who used the water from well nearby sea having high chloride content. These chloride had penetrated in upper cover zone of concrete leading to accelerated corrosion of the reinforcement. So not only water used for concrete but also water used for curing has to be checked thoroughly.
As it is clear from results and discussions that main caused of cracks in the reinforcement is accelerated corrosion because of high chloride content. It is also observed that chloride concentration is higher in top 25 mm layer of concrete. So it is decided to remove top 50 mm layer of chloride infected concrete and recast the whole section by new concrete. Following steps were followed while rehablating the concrete top layer,
Completely remove the chloride infected concrete in the vicinity of top steel, initially it was decided to remove up to 50 mm top concrete layer but practically total dept of 75 mm was removed from top surface of concrete.
Expose reinforcement was cleaned thoroughly wherever required extra reinforcement was provide. Reinforcement treated with alkaline rust converter Feovert, followed by IPN coating on rebars.
Entire concrete surface was treated with migratory corrosion inhibitor EPCO KP 100.
12 mm dia shear keys were provided at the staggered spacing of 300 mm
Gravity Flooding the deck with low viscosity monomer –Monopol 85 followed by sand broadcasting
Bond coat was provided to ensure good bond between old and new concrete.
Entire top surface was re-casted with M50 grade micro-concrete
Surface is cured for 4 days and after it wearing coat was applied before opening the bridge for traffic.
RCC structure can show accelerated aging if proper precaution is not taken on durability issues while construction. Day by day natural sources like river sand and water are becoming limited. Proper checks and quality controls need to be set for testing the materials when casting and curing of concrete.
Modern tools like progressive corrosion profiling, concrete resistivity and ICor are available for the investigation. These tools can be used effectively to optimize the cost of repair and retrofit of the structure suffering from accelerated corrosion and early aging. Modern materials like migratory corrosion inhibitors, low viscosity monomers and micro concrete can be effectively used for the repair and retrofit of the structure.
The structure under consideration was effectively restored in 30 days period by following state of the art investigation and repair methods.
Consultants: IIT Bombay Civil engineering department
Electro Chemical Investigation: Vijna Consulting Engineers Pvt. Ltd. Thane