Forensics investigation of hardened concrete
Chew, Zi Hao
Date of Issue2015
School of Civil and Environmental Engineering
Construction of massive structures such as columns, dam, pier or beam which involves large volume of concrete is known as mass concrete. Hydration of cement being a very exothermic process will lead to a rise in temperature at the core usually reaching above 70°C and as a result, deterioration of concrete will occur. When concrete is subjected to high early temperature, many chemical and physical changes of hardened concrete can occur. It is widely acknowledged that the concrete is likely to crack when subjected to high early temperature with the constant exposure to moisture due to delayed ettringite formation (DEF). DEF which sometimes referred to as internal sulphate attack is responsible for the cracking in massive concrete structures. DEF caused internal swelling reaction of the concrete that occurs in the presence of water without any external ingression of sulphate. This report will evaluate the presence of defects in hardened concrete associated with high heat of hydration at early age through a series investigation. In the investigation presented in this report, samples were extracted from a localized area on an actual foundation slab with a volume of 8200m^3 by means of coring. Temperature sensor has recorded a high early temperature of 90°C at the localized area. Field emission scanning electron microscopy (FESEM) was used to analyse how the microstructure of the samples changes due to high early age temperature and also to detect the possible presence of ettringite in the samples. Since DEF mainly comprises of sulphur, aluminium and calcium, Energy-dispersive X-ray (EDX) was use to identify the element composition of the samples which will provide a more accurate identification of DEF. Compression strength test was also carried out to relate how high early age temperature affects the compressive strength of hardened concrete. In the efforts to support and verify the findings from the core samples, 4 cylindrical concrete samples with dimensions of 300ϕ x 150mm were cast based on the exact mix proportion used for the foundation slab in. To simulate the high core temperature during the early age in mass concrete, special measures were taken. In addition, by adjusting the fly ash content, different trial mix proportions were used in efforts to achieve the optimum temperature in minimizing the chance for the occurrence of DEF. There are many factors which affects the early age temperature which will subsequently lead to cracking and other deterioration of concrete. Experimental investigation has revealed that even if the concrete is subjected to a short period of high early temperature, there is a possibility of long term reduction in strength, increase in permeability and delayed ettringite formation. Therefore, it is a basic requirement to specify limiting temperature for any major concrete concretion. Apart from controlling the maximum temperature rise in concrete during early age, having a good composition of cement, fly ash and GGBS is effective in preventing most of the problems associated with high early age temperature.
Final Year Project (FYP)
Nanyang Technological University