Effect of wetting and air curing durations on strength of stabilized sands
- Moatlhodi, Lerato J., Okonta, Felix N.
- Authors: Moatlhodi, Lerato J. , Okonta, Felix N.
- Date: 2014
- Subjects: Sandcrete blocks , Low-cost houses - South Africa , Building materials , High strength concrete , Portland cement , Fly ash
- Type: Article
- Identifier: uj:5076 , ISBN 9781614994657 , ISBN 9781614994664 , http://hdl.handle.net/10210/13647
- Description: Low cost houses in most developing countries are constructed from sandcrete blocks manufactured from sand-cement mortar. In South Africa, failure of many of these blocks has been experienced leading to collapse of the structures. Lack of sufficient compressive strength in the manufactured blocks was attributed as the main cause of the failures. The unconfined compressive strength (UCS) of sandcrete blocks can however be improved by curing the blocks under different conditions. Therefore the objective of this study was to evaluate the effect of different wetting and air drying durations would have on the strength and the stiffness of the cubes produced. Weakly cemented sand cubes, with different cement contents and cement-fly ash binder ratios compacted at the optimum moisture content, were prepared. The 28 days UCS was determined, modulus of elasticity and moisture contents of sample specimen was also determined. The results showed that curing cubes under water for 7 days and then air curing for 21 days yield maximum UCS and modulus of elasticity for sands cemented by plain Portland cement, and that 3 days under water curing produced maximum results for sands stabilized by cement-fly ash binder. It was however found that under water for 28 days results in minimum strength. Therefore the longer the cubes are cured under water minimum strengths were achieved.
- Full Text:
- Authors: Moatlhodi, Lerato J. , Okonta, Felix N.
- Date: 2014
- Subjects: Sandcrete blocks , Low-cost houses - South Africa , Building materials , High strength concrete , Portland cement , Fly ash
- Type: Article
- Identifier: uj:5076 , ISBN 9781614994657 , ISBN 9781614994664 , http://hdl.handle.net/10210/13647
- Description: Low cost houses in most developing countries are constructed from sandcrete blocks manufactured from sand-cement mortar. In South Africa, failure of many of these blocks has been experienced leading to collapse of the structures. Lack of sufficient compressive strength in the manufactured blocks was attributed as the main cause of the failures. The unconfined compressive strength (UCS) of sandcrete blocks can however be improved by curing the blocks under different conditions. Therefore the objective of this study was to evaluate the effect of different wetting and air drying durations would have on the strength and the stiffness of the cubes produced. Weakly cemented sand cubes, with different cement contents and cement-fly ash binder ratios compacted at the optimum moisture content, were prepared. The 28 days UCS was determined, modulus of elasticity and moisture contents of sample specimen was also determined. The results showed that curing cubes under water for 7 days and then air curing for 21 days yield maximum UCS and modulus of elasticity for sands cemented by plain Portland cement, and that 3 days under water curing produced maximum results for sands stabilized by cement-fly ash binder. It was however found that under water for 28 days results in minimum strength. Therefore the longer the cubes are cured under water minimum strengths were achieved.
- Full Text:
Preliminary study of strength of coal composites
- Moatlhodi, Lerato J., Okonta, Felix N.
- Authors: Moatlhodi, Lerato J. , Okonta, Felix N.
- Date: 2014
- Subjects: Columns, Concrete , Composite materials , Pillar failure
- Type: Article
- Identifier: uj:5081 , ISBN 9781614994657 , ISBN 9781614994664 , http://hdl.handle.net/10210/13652
- Description: Large scale floor convergence and sudden failure of pillars in room-and pillar underground mining have been reported in various countries over many years. Failure of these mines has been attributed, amongst other causes, to the stiffness criterion used by the mining operators and most importantly mine design practices, that tend to reduce the width-to-height ratio i.e. slender pillars in pursuit of greater coal recovery yields, in order to maximize coal recovery in the seam. It is recorded that the room-and-pillar method can leave behind about 40% of the total coal available for mining. The current study seeks to determine the appropriate width-height ratio for coal composite and stabilized coal pillars. Pure coal was mixed with granular soil and stabilized with cement to form cylindrical composite columns which were crushed to determine their unconfined compressive strength at different width-to-height ratios. The strength increased with the increase in W/H of the columns. Increase in coal percentage in a composite reduces the strength. The stabilized coal pillars mobilized less strength than the composite pillars. Based on limited data, relationships amongst column strength W/H and mix ratios were formulated and are proposed for the estimation of strength of mine support structures. More data is however required to formulate a general regression equation.
- Full Text:
- Authors: Moatlhodi, Lerato J. , Okonta, Felix N.
- Date: 2014
- Subjects: Columns, Concrete , Composite materials , Pillar failure
- Type: Article
- Identifier: uj:5081 , ISBN 9781614994657 , ISBN 9781614994664 , http://hdl.handle.net/10210/13652
- Description: Large scale floor convergence and sudden failure of pillars in room-and pillar underground mining have been reported in various countries over many years. Failure of these mines has been attributed, amongst other causes, to the stiffness criterion used by the mining operators and most importantly mine design practices, that tend to reduce the width-to-height ratio i.e. slender pillars in pursuit of greater coal recovery yields, in order to maximize coal recovery in the seam. It is recorded that the room-and-pillar method can leave behind about 40% of the total coal available for mining. The current study seeks to determine the appropriate width-height ratio for coal composite and stabilized coal pillars. Pure coal was mixed with granular soil and stabilized with cement to form cylindrical composite columns which were crushed to determine their unconfined compressive strength at different width-to-height ratios. The strength increased with the increase in W/H of the columns. Increase in coal percentage in a composite reduces the strength. The stabilized coal pillars mobilized less strength than the composite pillars. Based on limited data, relationships amongst column strength W/H and mix ratios were formulated and are proposed for the estimation of strength of mine support structures. More data is however required to formulate a general regression equation.
- Full Text:
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