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.
