Short-term effect of sulphate attack on concrete

Abstract

The concrete structures subject to aggressive environment under sulphate attack is of key importance to the service life of the structures. Previous studies have considered the full immersion method to evaluate the concrete resistance to sulphate attack. However, the integration of dry-wet cycles under short-term exposure of external sulphate attack as representing environmental factors in the service conditions was not fully explored in literature. The purpose of this study is to evaluate the properties of concrete under external sulphate attack and drying-wetting cycles for short-term exposure. The dry-wet cycle in this study was defined as immersion of concrete specimens in 5% and 10% sodium sulphate solutions for 6 days and placing at oven at 60oC for 21 hours, followed by 3 hours at room temperature for drying purpose as representing environment factors in the service conditions. The drying process was conducted at the end of 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77 and 84 days. Twelve drying-wetting cycles were conducted with regards to the actual weather conditions in Malaysia and each drying-wetting cycle was lasted for 7 days. Changes on properties over time in term of visual observation, change in mass, ultrasonic pulse velocity (UPV) and residual compressive strength of the concrete under different exposure conditions were performed. Concentration of sulphate ions (SO42-) across the depth of specimens was measured by Ion Chromatography instrument. Microstructure study of the concrete degradation were observed under Field-Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive X-ray Spectroscopy (EDX) analysis and X-ray Diffraction (XRD). From all the tests carried out, it was found the sulphate attack in 10% concentration after 84 days of drying-wetting cycles have a noticeable impact on the degradation of concrete. The results reported that exposing concrete to higher concentration of 10% sodium sulphate after 84 days of drying-wetting cycle, leading to higher mass loss at 0.75%, 9.55% reduction in UPV, 5.30% reduction in compressive strength and sulphate ions penetrated into concrete by reaching depth at 20 to 25 mm within 56 days of drying-wetting cycle. Gypsum, ettringite and calcite are the identified phases after 84 days drying-wetting cycles. Under repetitive drying-wetting cycles, such physical process can significantly accelerate the sulphate attack when high concentration of sodium sulphate travel through the concrete and get deposited at the surface, causing it to scale physically. Therefore, concrete would experience more severe deterioration when drying and wetting process existed in the environments with a higher sulphate content.