Performance of High Strength Concrete Containing Rice Husk Ash and Bagasse Ash under Elevated Temperatures

Abstract

Use of high strength concrete (HSC) in the columns of a high-rise building has become a complementary practice as it results in economy in the form of space-savings due to reduced cross-sections in addition to offering higher strength and durability. Fire is one of the most deleterious situation to which a concrete structure can be exposed in its service life. Columns are the most affected structural elements during any instance of a building fire and therefore a lot of study is being conducted to characterize the performance of HSC at elevated temperatures. It has been observed that, in contrast to room temperature performance, HSC performs poorly at elevated temperatures as compared with normal strength concrete (NSC). Also, a severe kind of failure mechanism has been reported to exist when HSC is exposed to elevated temperatures known as fire explosive spalling. Rice husk ash (RHA) and bagasse ash (BA) are agro-industrial wastes produced in large quantities. Landfilling of RHA and BA causes severe environmental burden. A lot of research conducted on RHA and BA claims that, under optimum production and processing conditions, they can contain a fair amount of amorphous silica and hence can be used as mineral admixtures (MAs) in concrete. Studies confirm that, if processed wisely, inclusion of RHA and BA into pastes, mortars or concrete, results in improved rheological behavior, enhanced mechanical performance at room temperatures and highly advanced durability aspects. Thus, using RHA and BA as concrete mix ingredients will not only yield a concrete with superior quality but also will reduce the carbon footprints on environment associated with the cement production. Adopting them as MAs will also mitigate the environmental burdens associated with their landfilling. It has been observed that the constituent materials of HSC highly influence its performance under elevated temperatures. Inclusions of fly ash (FA) and ground granulated blast furnace slag vii (GGBFS) in HSC have been reported to result in its improved performance, whereas, inclusion of silica fume (SF) and metakaolin (MK) have been reported to result in its reduced performance under elevated temperatures. Human safety is the foremost priority during the design of any concrete structure which, during its service life, might be exposed to any emergency condition like fire. So, the performance of concrete containing any new material under fire must be fully understood prior to its use. To author’s knowledge, there is no data available on the elevated-temperature performance of HSC containing BA or RHA. Hence this study is opted to evaluate the performance of HSC mixes containing RHA and BA under elevated temperatures. An experimental program was designed to access the mechanical properties including compressive strength, compressive stress-strain behavior, static elastic modulus and splitting tensile strength of three HSC mixes made with 10% RHA as cement replacement (RHSC), 10% BA as cement replacement (BHSC) and a control mix with pure cement (CHSC) at elevated temperatures of 100, 200, 400, 600 and 800°C. A relatively slow heating rate of 3°C/min was adopted. Changes in the macrostructure at elevated temperatures were accessed by visually inspecting the crack patterns and color changes. Microstructural changes caused by elevated temperature exposures were studied using scanning electron microscopy (SEM). Mass losses resulting from elevated temperatures’ exposures the exposures were recorded. Results of mechanical property tests reveal that both unstressed and residual mechanical properties of all HSC mixes deteriorated under elevated temperatures. At elevated temperatures, the mechanical performances displayed by RHSC and BHSC mixes were inferior than those of control mix. Moreover, at temperatures closer to 672°C, the specimens made with RHSC mix underwent explosive spalling. Linear regression analysis of the experimental data was carried out to propose temperature-dependent relationships for compressive strength, splitting tensile strength and elastic modulus. Visual observations of the viii crack patterns of specimens’ surfaces reveal that the cracking mostly initiates during cooling phase which follows the heating phase. No appreciable cracking was observed in any of the mixes up to 200°C. At 400°C, however, significant but narrow cracking was just present in RHSC. At temperatures beyond 400°C, each mix exhibited extensive cracking. SEM studies reveal that the microstructures pertaining to different mixes became coarser and more crystalline with the increase in exposure-temperature. RHSC suffered highest coarseness of its microstructure out of all mixes. This study concludes that the inclusion of RHA and BA in HSC results in its poor performance under elevated temperatures.