DETERMINATION OF FRACTURE PARAMETERS OF HIGH STRENGTH CONCRETE DERIVED FROM RICE HUSK ASH CEMENT BLENDS

Abstract

Concrete, a conventional building material is prone to fracture crack propagation, due to temperature and shrinkage stresses development, resulting in strength loss. Efforts in recent times have been directed at improving the resistance of concrete to crack propagation using pozzolanic materials such as Rice Husk Ash (RHA). However, information on fracture characteristics of High Strength RHA blended High Strength Concrete (HSC) are limited. This study was designed to investigate fracture characteristics of modified RHA-HSC using Crack Tip Opening Displacement (CTODc) and Stress intensity factor (KSIC). Rice husk obtained from Ire-Ekiti was calcined for six hours at 700°C in a closed furnace and cooled over a 48-hour period. The RHA produced was milled to 5 µm, and the chemical and microstructural properties were determined using ASTM C 618 and Xray Diffraction (XRD), respectively. The BRE/DoE mix design method was used to determine the concrete mix for targeted compressive strength of 60 MPa. Portland limestone cement was replaced with RHA at 0, 10, 20, 30, 40 and 50% by weight of cement. Seventy-six (milled and unmilled each) 150 mm RHA-HSC cubes were cast and tested for compressive strength at 7, 14, 21 and 28 days. Based on the preliminary results 78 beams of milled (0, 10 and 20%) RHA-HSC blends were prepared to obtain CTODc and KSIC using Reunion Internationale des Laboratoires et Experts des Materiaux method. The CTODc and KSIC for 60 MPa were modelled using numerical analysis, while Scikit-learn statistical method was used to model varying RHA-HSC blends. Adequacy of the model was determined using coefficient of Regression (R2). The RHA comprised of SiO2 (87.3%), Al2O3 (3.1%), and Fe2O3 (1.1%). This satisfied the ASTM C 618 70% minimum requirement for oxides. The observed pattern of peak broadening, smaller grain size and distinct peaks in RHA-HSC blends, implied the presence of a periodic crystal lattice structure. The compressive strengths of milled and unmilled RHA concrete blends ranged from 54.5 to 60.2 MPa and 11.3 to 44 MPa, respectively. This implied that RHA concrete did not meet the targeted compressive strength of 60 MPa. The corresponding CTODc at 10% and 20% RHA concrete cement blends were 0.02 and 0.32 mm, respectively while that of KSIC were 1.32 and 1.42 MPa√m, respectively. The corresponding CTODc of 10% milled RHA-HSC increased by 20% crack width, while the 20% milled RHA-HSC increased by 58.5%, when compared with the control mix. The KSIC of 10% RHA-HSC samples yielded 7.9% increase, while the 20% RHA-HSC concrete yielded a 16.2% increase, when compared with the control mix. The CTODc and KSIC from varying RHA-HSC blend fracture models yielded 0.02 and 1.24, respectively, and compared favourably with experimental data (R2=0.873). The incorporation of rice husk ash enhanced the fracture resistance characteristics of blended high strength concrete. The adopted model is suitable for predicting the potential failure of high strength concrete derived from rice husk ash cement blends.