Optimisation of Cavity Size in Hollow Sandcrete Blocks

Transcription

1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3 (1): Scholarlink Research Institute Journals, 2012 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(1):86-90(ISSN: ) Optimisation of Cavity Size in Hollow Sandcrete Blocks J. C Ezeokonkwo Department of Civil Engineering, University of Nigeria, Nsukka Abstract Two-cell hollow sandcrete blocks constitute the dominant wall construction material for modern shelter provision in many African countries. Obviously, the size of the cavity in a block unit would affect its quality, especially its compressive strength. Therefore, the thrust of this paper is the optimisation of cavity sizes in hollow sandcrete blocks. Consequently four different rectangular cavity sizes (117mmx129.5mm; 137.5mm x 150mm; 153mm x 165mm and 168mm x 180.5mm) were introduced into sandcrete block with standard overall dimensions (450mm x 225mm x 225mm), to produce two-cell hollow sandcrete blocks with centre-web to endweb ratios of up to used to study the strength pattern of two-cell hollow sandcrete blocks. Sand/cement ratio of 6 and water/cement ratio of 0.7 were used. The blocks were hand compacted and curing was according to Nigerian Industrial Specifications for sandcrete blocks. Analysis of the results showed that there is an optimum combination of cavity volume with centre-web to end-web ratio. This optimum combination is best defined when the as-cast side is the bearing area at test, and the strength evaluated on the basis of effective area Keywords: hollow sandcrete blocks, apparent strength, effective strength, end-web, centre-web Notations employed in the production and the properties of the The following symbols are used in this paper: constituent materials (Baiden and Tuuli, 2004 and A e effective area of the web Abdullahi, 2005). Surveys of block making industries a length of cavity B width of the hollow sandcrete in Nigeria by researchers (Nwoke, 1975; Ibeh, 1975; b width of cavity Ebuka, 1980; Florek, 1985; Momah, 1987; Adeshola, F xa apparent strength based on apparent bearing area of as-cast 1992 and Mohammed, 1992) have shown that the side geometry of hollow sandcrete blocks vary, as a result F xe effective strength based on effective bearing area of as-cast side of the variation in cavity sizes and centre-web to endweb ratios. Moreover, the variation in the centre-web F za apparent strength based on apparent bearing area of as-cast face to end-web results in different sizes of the solid F ze effective strength based on effective bearing area of as-cast bearing surface of hollow sandcrete blocks as well as face H height of the hollow sandcrete block in the face shell thickness. These variations L length of the hollow sandcrete block ultimately produce a variation in the axial P x failure load on as-cast side compressive strength of hollow sandcrete blocks. P z failure load on as-cast face Therefore, the overall effect of cavity size on the R sg ratio strength of two blocks with different geometry R t ratio of average solid thickness of two blocks with different strength of sandcrete hollow blocks is required in the geometry production of quality blocks, as it has also been t c centre-web thickness shown that the strength of a block is a function of its t e end-web thickness average solid thickness (Eze-Uzomaka, 1977). V volume of hollow block V c volume of cavity t a average solid thickness Rs ratio of strength of hollow block to strength of control sandcrete cube R g ratio of average solid thickness of hollow block to thickness of control sandcrete cube INTRODUCTION The growing demand for hollow sandcrete blocks has mounted pressure on the supply of the raw materials and also raised the issues of sustainability of the raw materials and affordability of the built shelter. In order to minimize the quantity of the materials used in producing a block unit, blocks are made to incorporate different sizes of rectangular cavities. The quality of blocks produced, however, differs from each manufacturer due to the different methods 86 In Nigeria, it is required that the strength of sandcrete blocks for building construction should comply with the Nigerian Industrial Standards Specification (NIS- 75 and (NIS 87: 2000) or with the Federal Ministry of Works (RCRDMT-1979) minimum strength specification. However, the Nigerian Industrial Standard definition of the compressive strength needs to be reviewed in order to effectively simulate the functional mode of the hollow sandcrete block in a structure. This is necessary for the full load carrying capacity of a block to be utilized and thereby optimise the use of the scarce block materials in order to minimize costs. Therefore, the thrust of this paper is the determination of the optimum size and configuration of block cavities with respect to the

2 compressive strength of the block using two-cell hollow sandcrete blocks. TEST DETAILS Specimen Preparation Fig. 1 show different dimensions and configurations of the cavity created in sandcrete blocks with the standard overall dimensions of 450mm x 225mm x 225mm. There are four groups of cavities: 30%, 40.7%, 50% and 60%. The percentage of cavity volume and end-web thickness is used to identify the block types, e.g. 30H12.5 refers to a block with 30% cavity volume and 12.5mm end-web thickness. It has been shown that for a sandcrete mix with sand/cement ratio of 6, the optimum water/cement ratio is 0.7 (Uzomaka, 1977). Opi River sand was used in this study. It has high specific surface area and belongs to zone 3 sand (Uzomaka, 1977; Ebuka, 1980 and Ezeokonkwo, 1986). The block specimens were prepared with wooden moulds. Compaction was done in three layers, each being tamped 25 times with a standard wooden tamping rod. Standard 100mm x 100mm x 100mm cube specimens were prepared for control. The procedure of rubbing down the test face with carboradum stone was used to achieve the desired even surface for the as-cast face. TEST PROCEDURE Each block was tested in compression in a 2000 KN capacity Dension Testing Machine. No effort was made to produce frictionless ends in order to simulate as closely as possible the actual procedure in practice. Computation of Compressive Strength Compressive strength was calculated on the basis of the apparent and effective bearing areas of the two perpendicular surfaces as shown in figure 2. (a) Isometric View of Hollow Block 87

3 L a a b b (b) Plan View of Hollow Block te tc B centre-web to end-web ratio as seen from figures 3 and 4. From figures 3 and 4 it is apparent that compressive strength ratio (compressive strength of block/compressive strength of cube) is a non-linear function of the centre-web to end-web ratio of hollow sandcrete blocks. The function depicts a minimum centre-web to end- ratio needed to achieve minimum compressive strength for each of the four groups of hollow sandcrete blocks investigated. This value is not constant but dependent on the cavity volume. The fact that high centre-web to end-web ratio gives high compressive strength is also highlighted. But in all it can be argued that a minimum and a maximum endweb to centre-web ratios exist for hollow sandcrete blocks. te L (c) Section A-A showing the Effective Bearing Area of As Cast Side Fig. 2: Orientations and Dimensions of Hollow The compressive strength of hollow sandcrete blocks, when as-cast face was the bearing surface at test, is calculated as follows: F za = Pz [1] L x B F ze = Pz [2] LB-2ab The compressive strength when as-cast side was the bearing surface at test is calculated as follows: F xa = Px [3] L H F xe = Px [4] A e Where A e = H (2t e + t c ) [5] The standard 100mm x 100mm x 100mm sandcrete cubes have a mean strength of 9.640N/mm 2. ANALYSIS OF TEST RESULTS Effects of Centre-web to End-web Ratio (t c /t e ) For each of the four different hollow blocks studied, the strength based on the as-cast face and on the ascast side surface strength showed that the compressive strength increases with increasing The value of 2 for 30% cavity and 1 for 40.7%, 50% and 60% cavities should be the lower limit while 6 should be the upper limit value for 30%, 40.7%, 50% cavities and 4 for 60% cavity. This fact that it is advantageous to have a thicker centre-web than endweb is important and should be exploited in practice. It was observed that hollow blocks with end-web thickness less than 25mm are difficult to mould, although they possess high centre-web to end-web 88

4 ratio. Such blocks developed cracks while still plastic. This observation confirms that the 25mm minimum thickness for webs and face shells specified in the Nigeria Industrial Standard for sandcrete blocks is reasonable. Such blocks are easy to work on, and are free from cracks. They also showed good strength development. Effects of Average Solid Thickness on Block Strength Researches Eze-Uzomaka (1977) and Ezeokonkwo (1988) have shown that the solid part of hollow block is highly significant and it influences the strength of hollow blocks. In this study, each hollow block type has the same bearing area, but different average solid thickness. The average solid thickness is defined as the solid area divided by the median perimeter of the solid area and the value, with respect to fig. 2, is given below: t a = (LB 2ab) [6] {2(L-t e ) + 3(B-t s )} Both the strength and average solid thickness of hollow blocks were expressed as ratios (R s and R g ) of the corresponding values or sandcrete cube specimen. Fig.5 shows that the solid part of hollow block affects the strength of block in an exponential manner the strength increases exponentially with increase in the average solid thickness of the blocks. Also, correlation between R s and R g is improved when grouped together for the same end-web thickness. The exponential curves shown in fig. 5 have coefficient of correlation ranging from to lead to premature failure. Hollow block is composed of webs and face shells that have different dimensions. As the cavity volume increases, these webs and face shells decrease accordingly. The measured strength of hollow blocks decreases as the solid area decreases. Fig. 6 shows that cavity volume expressed as a percentage of solid volume affects the strength, the strength being inversely proportional to the cavity volume. The inverse proportion between apparent strength, effective strength and cavity volume is depicted when the as-cast face was the bearing surface at test. This relation also exists between the apparent strength and cavity volume when the as-cast side was the bearing surface at test. This inverse proportion is better described by a non-linear curve through sets of blocks strength with the same end-web thickness. Fig. 7 shows the trend of change in compressive strength as the volume of cavity expressed as a ratio of the volume of solid part is increased. This trend is rather interesting in that as the cavity volume increases the strength decreases to a minimum and then begins to increase. This would appear to be anomalous, as one would generally expect the strength to continue to decrease with increasing cavity volume. The observed trend may be ascribed to some sort of size effect. Although, the failure load of the block is less after the trough than at the trough, the strength that is expressed as stress is higher after the trough than at the trough because of the smaller bearing area after the trough. Effects of Cavity Volume on Strength It has been observed that greater volume of specimen (solid volume) leads to a more uniform stress distribution and therefore a lesser likelihood of premature failure (Neville, 1966). The introduction of cavity in a specimen reduces the available solid volume for uniform stress distribution, which may Consider, for example the for 37.5mm end-web block. The trough occurs at a cavity volume ratio of 120%. The corresponding failure load is 94.5KN while the corresponding load bearing area is 23,625mm 2 giving a strength value of 4.0N/mm 2. At cavity volume ratio of 150%, the load bearing area is 22,500mm 2 while the failure load is also 94.5KN thus giving a strength value of 4.2N/mm 2? 89

5 Ebika, J.C Survey of block industry in some States of Nigeria. B. Sc thesis. University of Nigeria, Nsukka, pp Ezeokonkwo, J.C Uniaxial compressive strength of sandcrete hollow blocks and its dependence on geometry. M Eng. Thesis, University of Nigeria, Nsukka, pp Eze-Uzomaka O. J Some factors which affects the crushing strength of sandcrete blocks. Materiaux et Construction, 10, No. 55, However, for cavity volumes greater than the trough value, the failure of the block is sudden and rather catastrophic making this regime after the trough very undesirable. Therefore, the maximum volume of cavity that should be incorporated in the blocks is that corresponding to the trough value. CONCLUSIONS The following conclusions are drawn from this study: 1. A non-linear relationship exists between strength and centre-web to end-web ratio. This indicates a minimum centre-web to end-web ratio that is dependent on cavity volume. It also shows that high centre-web to end-web ratio gives high strength. 2. The strength increases exponentially as the average solid thickness of block increases. 3. The strength decreases geometrically as the cavity volume increases. This is the case when as-cast face was the bearing surface at test and also for the apparent strength of ascast side. 4. Parabolic relationship exists between the effective strength on as-cast side and cavity volume. It shows that there is a minimum volume of cavity for a given centre-web to end-web ratio. REFERENCES Abdullahi, M Compressive Strength of Sandcrete Blocks in Bosso and Shiroro Areas of Minna, Nigeria. AU Journal of Technology. 9(2). Pp Adeshola, F. A The Quality of Sandcrete Hollow Blocks Manufactured in Kwara State (i.e. Ilorin and its Environs). M Sc Thesis. Ahmadu Bello University, Zaria. Baiden, B. K and Tuul, M. M Impact of Quality Control Practices in Sandcrete Blocks Production. J. Archit. Eng. 10, 53 (2004); doi: / (ASCE) (2004)10:2(53). Florek, A Quality of sandcrete hollow block manufactured in Northern Nigeria. Material Testing, Research and Control. Federal Ministry of Works and Housing, Lagos, pp Ibeh, L.C A survey of block-making industries in some urban centres of East Central State. B. Sc Thesis University of Nigeria, Nsukka, pp Mohammed, M The Quality of Sandcrete Hollow Blocks Produced in Kano State. M. Sc. Thesis, Ahmadu Bello University, Zaria. Momah, C.C Survey of block industries in some Town in Anambra and Imo States. B. Sc Thesis, University of Nigeria, Nsukka, pp Neville, A.M A General Relation for Strengths of Concrete Specimens of Different Shapes and Sizes. Journal American Concrete Institute, Vol.63 No. 10, October, pp Nigerian Industrial Standard (NIS 87: 2000). Specification for sandcrete Blocks. Standard Organization of Nigeria, Lagos, Nigeria. Nigerian Industrial Standard (NIS-75). Specification for sandcrete blocks. Standard Organization of Nigeria, Lagos, Nigeria Nwoke, O. I An investigation into the strength characteristics of sandcrete blocks. Proceeding of Symposium on Sandcrete Blocks and Construction Industry. Department of Civil Engineering, University of Nigeria, pp Recommendation of Committee on Review of Decision (RCRD-1979). Material Testing, Control and Research. Federal Ministry of Works and Housing, Lagos, p 102. Uzomaka, O.J An Appraisal of Method of Testing Some Physical Properties of Sandcrete Blocks. Proceedings Institution of Civil Engineers, 63, Part 2, September,