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Comparative Analysis of Hollow Clay Blocks and Solid Reinforced Concrete Slabs
International Conference on Advances in Engineering and Technology
C O M P A R A T I V E ANALYSIS OF H O L L O W CLAY BLOCKS AND SOLID REINFORCED CONCRETE SLABS M. Kyakula, N. Behangana and B. Pariyo, Department of Civil and Building
Engineering, Kyambogo University, Uganda
ABSTRACT Over 99% of multi storey structures in Uganda are of reinforced concrete framing. Steel and brick structures account for less than 1%. Of the reinforced concrete structures currently under construction, 75% use hollow clay blocks reinforced concrete slabs. This paper looks at the form of the hollow clay blocks that contribute to its ease of use, and enables it to be held in the slab both by mechanical interlock and friction. It explores its limitations and ways in which its form may be improved.
Designs of single slab panel two storey reinforced concrete structures with one side having a constant dimension of 8m while the dimension is varied from 2m, 3m, 4m, 5m, 6m, 7m up to 8m were carried out for both solid and hollow clay blocks slabs construction. The design loads, moments, reinforcement, shear stresses and costs for each case of solid and hollow blocks slabs were compared. It was found that contrary to common beliefs; solid slabs are cheaper than hollow clay blocks slabs. This is because, hollow clay blocks need a minimum topping of 50mm, and are manufactured in standard sizes of 125mm, 150mm, 175mm, 200mm and 225mm. This implies that for spans of about 2m, solid slabs can be 75mm, 100mm thick, while the minimum thickness of hollow blocks is 175mm. Also unlike solid slabs, for hollow clay blocks slab over 6m long, shear reinforcement may be needed. As the length increases to 8m, the topping for hollow blocks increases to an uneconomic value. However for large structures with over two storeys, hollow blocks slab construction might be cheaper as the reduced weight leads to smaller columns and foundations. Furthermore hollow block slabs are easier to detail, construct and are less prone to errors on site. Keywords: Hollow clay blocks and solid RC slab; block shape; Design loads; shear stress, moments; Reinforcement; cost, ease of design/construction
1.0 INTRODUCTION Concrete slabs behave primarily as flexural members and the design is similar to that of beams except that. The breadth of solid slabs is assumed to be one meter wide while hollow block slabs are designed as T beams with effective width equal to the spacing between ribs. Slabs are designed to span smaller distances than beams and consequently have smaller effective depth (50 to 350ram). Also the shears stresses in slabs are usually low and compression reinforcement is rarely used. Concrete slabs may be classified according to the nature and type of support; for example simply supported, direction of support; for example one way spanning, and type of section; for example solid.
84
Kyakula, B e h a n g a n a & Pariyo
Until recently, the practice has been to use hollow blocks for lightly loaded floors such as residential flats. But a survey of 70 buildings currently being constructed in different parts of the country has revealed that hollow clay blocks are used in flats, hotels, student's hostels, offices, schools, libraries and shopping arcades, (Pariyo, 2005). The basis of design justifies this advance in utilization: The design of hollow clay blocks slabs depend on the fact that concrete in tension below the neutral axis has cracked. Whereas this cracked concrete contributes to the rigidity of the floor, the concrete surrounding the tension bars that holds the bars in the structure and provide bond offers its only contribution to strength. Thus any concrete in tension remote from the bars may be eliminated, thus reducing the weight while at the same time maintaining the strength of the slab. In hollow blocks slab construction, the hollow blocks are laid in a line with the hollow side end to end and the last block has its ends sealed to prevent entry of concrete into the holes. The slab is thus constrained to act as one way spanning between supports. The slab acts and is designed as a T beam with the flange width equal to the distance between ribs but is made solid at about 0.5m to 1.0m from the support to increase the shear strength. A weld mesh is laid in the topping to distribute any imposed load. Thus hollow block slabs can take most loadings. Hollow clay blocks slab construction is the most widespread form of slab construction; 60 of the 70 sites surveyed throughout the nation were using hollow clay blocks slab construction, (Pariyo, 2005). The wide spread usage and acceptability of this material necessitates that it should be thoroughly investigated. This paper is an attempt in this direction. 1.1 Hollow Blocks: A Sketch of a typical clay hollow block is shown below in Figure I below, its surface has small grooves which help introduce friction forces and a key for mechanical interlock, these hold the block in the concrete. The dimensions given in Figure 1 were measured from actual hollow clay blocks on the market. The four hollow blocks sizes available on the Uganda market (from catalogues) are shown in Table 1. The limited number of sizes means that the least depth of hollow blocks slabs is 175mm; this is because the least height of hollow blocks is 125ram and the minimum topping allowed is 50ram. This implies that even for small spans such as l m to 2m, which could require a slab thickness of 5 0 r a m - 100mm, one still has to use 175mm. However, as the span increases to 5m the thickness of the solid floor slab and hollow blocks slab are about equal.
Table 1. Hollow block types on the Ugandan market S/No Length (mm) Width (mm) Height (mm)
Weight (Kg)
1
400
300
125
7.3
2
400
300
150
8.4
3
400
300
175
11.73
4
400
300
225
13.58
85
International Conference on Advances in Engineering and Technology
1.2 Implications of the Shape: A reasonable arrangement of blocks leaves a minimum width of 75mm, which allows for a 50mm diameter porker vibrator and 12.5mm clearance on either side. Thus a minimum rib width at the bottom is given as 75 + 2x40 = 155ram. This greater than 125mm; the minimum rib width required for fire resistance as given in Figure 3.2, of BS8110. I
t
\
The applied shear stress v, for a ribbed beam is given by; v - [ V / b v d ) , where: V is the applied shear force, d is the effective depth and by is the average rib width. Ribs created between the hollow blocks are 75mm wide at the top and 155mm at the bottom as shown in Figure 2. For a case of a 175mm thick slab, with hollow blocks of 125mm depth, topping of 50mm and 25mm cover to tension bars. It would be more conservative to use the smaller value of b~ = 75mm in shear design calculations, however in practice the larger value of b~ = 155mm is used. Moreover it may be difficult to justify using the average rib width if the rib width is not tapering. One alternative is to modify the hollow blocks such that the key is recessed into the blocks rather than out, as illustrated in Figure 3 This could reduce on the required rib width from 155mm to the minimum allowed of 125mm, thus saving on the concrete, making the calculation of concrete shear stress easier, while at the same time providing the key for holding the hollow blocks safely in the slab. 2.0 C O M P A R A T I V E ANALYSIS Two sets of slabs were designed; one set using hollow blocks while the other used solid blocks. For each set, one side of the slab was kept at 8m while the other was varied from 2m, 3m, 4m, 5m, 6m, 7m, up to 8m. The imposed and partition loads were assumed to be 2 2 2.5N / mm and 1.0N / mm respectively. The floor finish and underside plaster was 3 each assumed to be 25ram and of unit weight 24.0kN / m ; giving a dead load from partitions and finishes of "DL(paF ) = 1.0 + 0.05x24 = 2 . 2 k N / m 2 . The dead load for the hollow block slab is given by: DL(stab~ = 24(h - N b V ~ !+ N b W b , where: h 3is the overall slab depth in meters, N b is number of blocks per m Vb is volume in m of a hollow block and Wb is Weight of a block in k N . The slab was assumed to be an interior panel in a building with over 3 panels in either direction. The corresponding beams, columns, and pad footings were designed. Comparative analyses of the design loading, Moments, reinforcement, shear forces and costs of construction were carried and a few of these are given below.
2.1 Design Loads per Square Meter
2. As the span and thus loading increases, design loads in k N / m increases for both solid and hollow blocks slabs. Figure 4 shows a comparison of design loads for hollow blocks and solid slabs. For hollow blocks slabs less than 4m span, the design load is constant because the slab thickness used is dictated by topping requirements and depth of available blocks. For this depth and span (175ram &<_ 4m ), deflection is not critical. On the other hand the design depth increases with span in solid slabs because slab thickness varies as per allowable deflection requirements.
86
Kyakula, Behangana & Pariyo
Figure 4: Variation of design loads for Solid and hollow blocks Floor slabs 20 A
--B--- Hollow blocks slab
i.
Solid slab
~6
--'~
J
.a4 Z
~2
~o
U
0
1
m
2
3
m
4 5 Span Length (m
6
7
8
9
2.2 Moments and Reinforcement From Figure 5 it is seen that, despite the fact that the solid slab has a greater load and thus greater applied moment it has a greater reserve capacity, its ratio of applied to ultimate moments is less than that of hollow blocks for all spans greater than 3m. Also its areas of 2 reinforcement in mm per m width of slab are less than that for hollow blocks slab for all spans. This is because for lower than 4m, even where required area of reinforcement is small, one must provide the minimum allowed, the hollow blocks slab is treated as a Tee beam and one is required to provide a minimum of area of steel given by
(100A~/b w h ) - 0.26 fy - 460N / mm
2
for
flanged
beams
with
the
flange
in
compression
and
as per table 3.25 BS8110. On the other hand, Solid slabs are provided
with a minimum of ( 1 0 0 A ~ / b h ) - 0 . 1 3 in both directions. Also for hollow slabs it is preferable to provide one bar per rib, thus the next bar size has to be provided even where required area of steel has exceeded the previous bar size by a small value.
Figure 5: Variation of ratio of applied moment to Moment of resistance for solid and Hollow blocks Slabs 0.25
E 0
0.20
=E o r
C .~
Hollow blocks slab J
- - A - - Solid
~
0.15
0 =En~
0.10
9-r r
0.05
<
0.00 0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
Span Length (m)
87
International Conference on Advances in Engineering and Technology
2.3 Applied and Concrete Shear Stresses The value of applied shear stress v and concrete shear stress vCobtained depends on the value of b v used. The usual practice is to stop hollow blocks at about 500mm to 1000mm from the support and for this length the slab is made solid. This serves to increase the shear resistance of slab close to the support It is also the practice to ignore the keys, then. b v -155mm and . ( v c > v ) . However if the keys are not ignored and b v - 7 5 r a m , then as shown in Figure 6, for span greater than 3m (vc < v), thus necessitating shear reinforcement or using a solid slab up to .a length when the applied shear stress is no longer critical. On the other hand, the design concrete shear stress for the solid slab was greater than the applied shear stress for all length of span.
Figure 6: Comparision of applied shear and Concrete shear stresses for hollow blocks slab (bv =75mm) 1.2 A
t~ :3
O"
r 0.8 E E 0.6 #
0.4
Applied Shear stress v
------Zk--Concrete Shear stress Vc
L
I o.2
0
1
2
3
I
l
i
I
I
l
4
5
Span Length (m)
I
,
6
7
8
9
2.4 Cost Comparisons The cost of various Structural Elements were derived and compared for both solid and hollow blocks slabs. The cost of each element designed using solid slab was divided by that of the hollow blocks slab and this ratio was plotted against span. Figure 7 shows the variation of the cost of solid and hollow blocks slabs with span length. It is seen that for slabs less than 4m and greater than 5m, the cost of hollow slabs is higher than that of solid slabs. This is due to the fact that for spans less than 4m, solid slabs allow smaller depth, as per deflection requirements and hollow blocks slabs dictates slab thickness based on the depth of available blocks and topping. Thus for spans of 2m and 3m, hollow blocks have bigger slab depth than solid slabs with corresponding material requirements. At 4 and 5m, the hollow blocks slab becomes cheaper. Above 5m, the minimum topping (50mm) cannot be use because the available hollow blocks offer few standard depths and in order to meet deflection requirements as the span increases, the only option is to increase the topping. Thus for 8m span, deflection requirements dictate the overall depth of
88
Kyakula, Behangana & Pariyo
340mm, yet maximum depth of available hollow blocks is 225mm, giving uneconomical a topping is 115mm. The comparison of the cost for beams revealed that for spans less than 4m, and greater than 5m beams supporting solid slabs were slightly cheaper. This is because. The current practice of using the beams of the same size even when the hollow blocks have constrained the slab to act as one way spanning maintains rigidity of the structure and reduces effective height for the columns, but offers no reduction in the materials used in beams. The cost of columns were found to be the same for both cases because, the case considered carried little weight and the reinforcement areas were dictated by the minimum requirements rather that loading conditions. This implies that for structures supporting many floors, the columns of one for hollow blocks slabs will be cheaper because it will carry less loads and the bending may be assumed to act about only one axis for all the columns. On the other hand the foundation for a structure supporting hollow blocks slab were found to be cheaper by an average of 10%. This is because the hollow blocks slabs ensured a reduced weight.
2.5 Design and Construction Use of hollow blocks constrains the slab to act as one-way spanning. These are simple to analyse, and design. The Structural drawings are easy to detail and understand. During construction it is easier to lay the reinforcement, thus minimizing mistakes on site. The weld mesh included in the topping ensures distribution of imposed loading to the whole slab. Its ease of construction has contributed to its growing popularity such that it now occupies 75% of the market share.
Figure 7" Variation of the cost of solid and hollow blocks slab with span 1.1
1 A
E -c
09
c ,_1
c
0.8
0.. t~
#
V a r i a t i o n of c o s t y ratio
0.7
0.6 2
3
4
5
6
7
8
Cost of Solid/hollow blocks slab
89
International Conference on Advances in Engineering and Technology
3.0 CONCLUSION The current shape of the hollow clay blocks has keys and groves that provide mechanical interlock and friction resistance to hold the blocks firmly in concrete. However, his shape could also decrease the shear resistance of the slab. A shape has been proposed that has all the advantages of the one currently used, while at the same time increases shear resistance of the slab and a saving in the concrete used. The limited range of hollow blocks available on the market makes hollow blocks slabs more expensive than solid slabs for spans less than 4m or greater than 5m. For spans less than 4m the minimum slab depth is 175mm, because the minimum available block depth is 125mm and minimum topping required is 50mm. Yet for solid slabs the depths can vary from 50mm to 150mm for spans varying from lm to 3m depending on loading and deflection requirements. For spans greater than 5m, deflection requirements dictate increasing depth with spans, yet the maximum depth of available blocks available is 225mm, leading to uneconomical depth of the topping. Using the beams of the same size even when the hollow blocks have constrained the slab to act as one way spanning maintains rigidity of the structure and reduces effective height for the columns, but offers no reduction in the materials used in beams. The reduced weight due to use of hollow blocks slabs results in reduced cost of columns and foundations. Moreover since use of hollow blocks constrains the slab to be designed and act as one way spanning, the loading and thus moments from one set of beams framing into the column is negligible compared to the other. Thus the columns experience uniaxially moments, which causes a saving in reinforcement. REFERENCES Balu Tabaaro. W. (2004), "Sustainable development and application of indigenous building materials in Uganda" Journal of Construction exhibition. Issue 1 Page 4-5 BS8110-1 (1985, 1997) Structural use of Concrete- Part 1, code of practice for design and construction MosleyW.H. and Buney J. H. (1989) "Reinforced Concrete Design" 5th Edition Macmillan, London Pariyo Bernard. (2005) "Comparative cost analysis of solid reinforced concrete slab and hollow clay blocks slab construction", Final year undergraduate project, Kyambogo University" Seeley I. H. (1993)"Civil Engineering quantities" 5th edition Macmillan London
Uganda Clays, Kajjansi catalogue (2004) Price lists and weights of suspended floor units.
90
C O M P A R A T I V E ANALYSIS OF H O L L O W CLAY BLOCKS AND SOLID REINFORCED CONCRETE SLABS M. Kyakula, N. Behangana and B. Pariyo, Department of Civil and Building
Engineering, Kyambogo University, Uganda
ABSTRACT Over 99% of multi storey structures in Uganda are of reinforced concrete framing. Steel and brick structures account for less than 1%. Of the reinforced concrete structures currently under construction, 75% use hollow clay blocks reinforced concrete slabs. This paper looks at the form of the hollow clay blocks that contribute to its ease of use, and enables it to be held in the slab both by mechanical interlock and friction. It explores its limitations and ways in which its form may be improved.
Designs of single slab panel two storey reinforced concrete structures with one side having a constant dimension of 8m while the dimension is varied from 2m, 3m, 4m, 5m, 6m, 7m up to 8m were carried out for both solid and hollow clay blocks slabs construction. The design loads, moments, reinforcement, shear stresses and costs for each case of solid and hollow blocks slabs were compared. It was found that contrary to common beliefs; solid slabs are cheaper than hollow clay blocks slabs. This is because, hollow clay blocks need a minimum topping of 50mm, and are manufactured in standard sizes of 125mm, 150mm, 175mm, 200mm and 225mm. This implies that for spans of about 2m, solid slabs can be 75mm, 100mm thick, while the minimum thickness of hollow blocks is 175mm. Also unlike solid slabs, for hollow clay blocks slab over 6m long, shear reinforcement may be needed. As the length increases to 8m, the topping for hollow blocks increases to an uneconomic value. However for large structures with over two storeys, hollow blocks slab construction might be cheaper as the reduced weight leads to smaller columns and foundations. Furthermore hollow block slabs are easier to detail, construct and are less prone to errors on site. Keywords: Hollow clay blocks and solid RC slab; block shape; Design loads; shear stress, moments; Reinforcement; cost, ease of design/construction
1.0 INTRODUCTION Concrete slabs behave primarily as flexural members and the design is similar to that of beams except that. The breadth of solid slabs is assumed to be one meter wide while hollow block slabs are designed as T beams with effective width equal to the spacing between ribs. Slabs are designed to span smaller distances than beams and consequently have smaller effective depth (50 to 350ram). Also the shears stresses in slabs are usually low and compression reinforcement is rarely used. Concrete slabs may be classified according to the nature and type of support; for example simply supported, direction of support; for example one way spanning, and type of section; for example solid.
84
Kyakula, B e h a n g a n a & Pariyo
Until recently, the practice has been to use hollow blocks for lightly loaded floors such as residential flats. But a survey of 70 buildings currently being constructed in different parts of the country has revealed that hollow clay blocks are used in flats, hotels, student's hostels, offices, schools, libraries and shopping arcades, (Pariyo, 2005). The basis of design justifies this advance in utilization: The design of hollow clay blocks slabs depend on the fact that concrete in tension below the neutral axis has cracked. Whereas this cracked concrete contributes to the rigidity of the floor, the concrete surrounding the tension bars that holds the bars in the structure and provide bond offers its only contribution to strength. Thus any concrete in tension remote from the bars may be eliminated, thus reducing the weight while at the same time maintaining the strength of the slab. In hollow blocks slab construction, the hollow blocks are laid in a line with the hollow side end to end and the last block has its ends sealed to prevent entry of concrete into the holes. The slab is thus constrained to act as one way spanning between supports. The slab acts and is designed as a T beam with the flange width equal to the distance between ribs but is made solid at about 0.5m to 1.0m from the support to increase the shear strength. A weld mesh is laid in the topping to distribute any imposed load. Thus hollow block slabs can take most loadings. Hollow clay blocks slab construction is the most widespread form of slab construction; 60 of the 70 sites surveyed throughout the nation were using hollow clay blocks slab construction, (Pariyo, 2005). The wide spread usage and acceptability of this material necessitates that it should be thoroughly investigated. This paper is an attempt in this direction. 1.1 Hollow Blocks: A Sketch of a typical clay hollow block is shown below in Figure I below, its surface has small grooves which help introduce friction forces and a key for mechanical interlock, these hold the block in the concrete. The dimensions given in Figure 1 were measured from actual hollow clay blocks on the market. The four hollow blocks sizes available on the Uganda market (from catalogues) are shown in Table 1. The limited number of sizes means that the least depth of hollow blocks slabs is 175mm; this is because the least height of hollow blocks is 125ram and the minimum topping allowed is 50ram. This implies that even for small spans such as l m to 2m, which could require a slab thickness of 5 0 r a m - 100mm, one still has to use 175mm. However, as the span increases to 5m the thickness of the solid floor slab and hollow blocks slab are about equal.
Table 1. Hollow block types on the Ugandan market S/No Length (mm) Width (mm) Height (mm)
Weight (Kg)
1
400
300
125
7.3
2
400
300
150
8.4
3
400
300
175
11.73
4
400
300
225
13.58
85
International Conference on Advances in Engineering and Technology
1.2 Implications of the Shape: A reasonable arrangement of blocks leaves a minimum width of 75mm, which allows for a 50mm diameter porker vibrator and 12.5mm clearance on either side. Thus a minimum rib width at the bottom is given as 75 + 2x40 = 155ram. This greater than 125mm; the minimum rib width required for fire resistance as given in Figure 3.2, of BS8110. I
t
\
The applied shear stress v, for a ribbed beam is given by; v - [ V / b v d ) , where: V is the applied shear force, d is the effective depth and by is the average rib width. Ribs created between the hollow blocks are 75mm wide at the top and 155mm at the bottom as shown in Figure 2. For a case of a 175mm thick slab, with hollow blocks of 125mm depth, topping of 50mm and 25mm cover to tension bars. It would be more conservative to use the smaller value of b~ = 75mm in shear design calculations, however in practice the larger value of b~ = 155mm is used. Moreover it may be difficult to justify using the average rib width if the rib width is not tapering. One alternative is to modify the hollow blocks such that the key is recessed into the blocks rather than out, as illustrated in Figure 3 This could reduce on the required rib width from 155mm to the minimum allowed of 125mm, thus saving on the concrete, making the calculation of concrete shear stress easier, while at the same time providing the key for holding the hollow blocks safely in the slab. 2.0 C O M P A R A T I V E ANALYSIS Two sets of slabs were designed; one set using hollow blocks while the other used solid blocks. For each set, one side of the slab was kept at 8m while the other was varied from 2m, 3m, 4m, 5m, 6m, 7m, up to 8m. The imposed and partition loads were assumed to be 2 2 2.5N / mm and 1.0N / mm respectively. The floor finish and underside plaster was 3 each assumed to be 25ram and of unit weight 24.0kN / m ; giving a dead load from partitions and finishes of "DL(paF ) = 1.0 + 0.05x24 = 2 . 2 k N / m 2 . The dead load for the hollow block slab is given by: DL(stab~ = 24(h - N b V ~ !+ N b W b , where: h 3is the overall slab depth in meters, N b is number of blocks per m Vb is volume in m of a hollow block and Wb is Weight of a block in k N . The slab was assumed to be an interior panel in a building with over 3 panels in either direction. The corresponding beams, columns, and pad footings were designed. Comparative analyses of the design loading, Moments, reinforcement, shear forces and costs of construction were carried and a few of these are given below.
2.1 Design Loads per Square Meter
2. As the span and thus loading increases, design loads in k N / m increases for both solid and hollow blocks slabs. Figure 4 shows a comparison of design loads for hollow blocks and solid slabs. For hollow blocks slabs less than 4m span, the design load is constant because the slab thickness used is dictated by topping requirements and depth of available blocks. For this depth and span (175ram &<_ 4m ), deflection is not critical. On the other hand the design depth increases with span in solid slabs because slab thickness varies as per allowable deflection requirements.
86
Kyakula, Behangana & Pariyo
Figure 4: Variation of design loads for Solid and hollow blocks Floor slabs 20 A
--B--- Hollow blocks slab
i.
Solid slab
~6
--'~
J
.a4 Z
~2
~o
U
0
1
m
2
3
m
4 5 Span Length (m
6
7
8
9
2.2 Moments and Reinforcement From Figure 5 it is seen that, despite the fact that the solid slab has a greater load and thus greater applied moment it has a greater reserve capacity, its ratio of applied to ultimate moments is less than that of hollow blocks for all spans greater than 3m. Also its areas of 2 reinforcement in mm per m width of slab are less than that for hollow blocks slab for all spans. This is because for lower than 4m, even where required area of reinforcement is small, one must provide the minimum allowed, the hollow blocks slab is treated as a Tee beam and one is required to provide a minimum of area of steel given by
(100A~/b w h ) - 0.26 fy - 460N / mm
2
for
flanged
beams
with
the
flange
in
compression
and
as per table 3.25 BS8110. On the other hand, Solid slabs are provided
with a minimum of ( 1 0 0 A ~ / b h ) - 0 . 1 3 in both directions. Also for hollow slabs it is preferable to provide one bar per rib, thus the next bar size has to be provided even where required area of steel has exceeded the previous bar size by a small value.
Figure 5: Variation of ratio of applied moment to Moment of resistance for solid and Hollow blocks Slabs 0.25
E 0
0.20
=E o r
C .~
Hollow blocks slab J
- - A - - Solid
~
0.15
0 =En~
0.10
9-r r
0.05
<
0.00 0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
Span Length (m)
87
International Conference on Advances in Engineering and Technology
2.3 Applied and Concrete Shear Stresses The value of applied shear stress v and concrete shear stress vCobtained depends on the value of b v used. The usual practice is to stop hollow blocks at about 500mm to 1000mm from the support and for this length the slab is made solid. This serves to increase the shear resistance of slab close to the support It is also the practice to ignore the keys, then. b v -155mm and . ( v c > v ) . However if the keys are not ignored and b v - 7 5 r a m , then as shown in Figure 6, for span greater than 3m (vc < v), thus necessitating shear reinforcement or using a solid slab up to .a length when the applied shear stress is no longer critical. On the other hand, the design concrete shear stress for the solid slab was greater than the applied shear stress for all length of span.
Figure 6: Comparision of applied shear and Concrete shear stresses for hollow blocks slab (bv =75mm) 1.2 A
t~ :3
O"
r 0.8 E E 0.6 #
0.4
Applied Shear stress v
------Zk--Concrete Shear stress Vc
L
I o.2
0
1
2
3
I
l
i
I
I
l
4
5
Span Length (m)
I
,
6
7
8
9
2.4 Cost Comparisons The cost of various Structural Elements were derived and compared for both solid and hollow blocks slabs. The cost of each element designed using solid slab was divided by that of the hollow blocks slab and this ratio was plotted against span. Figure 7 shows the variation of the cost of solid and hollow blocks slabs with span length. It is seen that for slabs less than 4m and greater than 5m, the cost of hollow slabs is higher than that of solid slabs. This is due to the fact that for spans less than 4m, solid slabs allow smaller depth, as per deflection requirements and hollow blocks slabs dictates slab thickness based on the depth of available blocks and topping. Thus for spans of 2m and 3m, hollow blocks have bigger slab depth than solid slabs with corresponding material requirements. At 4 and 5m, the hollow blocks slab becomes cheaper. Above 5m, the minimum topping (50mm) cannot be use because the available hollow blocks offer few standard depths and in order to meet deflection requirements as the span increases, the only option is to increase the topping. Thus for 8m span, deflection requirements dictate the overall depth of
88
Kyakula, Behangana & Pariyo
340mm, yet maximum depth of available hollow blocks is 225mm, giving uneconomical a topping is 115mm. The comparison of the cost for beams revealed that for spans less than 4m, and greater than 5m beams supporting solid slabs were slightly cheaper. This is because. The current practice of using the beams of the same size even when the hollow blocks have constrained the slab to act as one way spanning maintains rigidity of the structure and reduces effective height for the columns, but offers no reduction in the materials used in beams. The cost of columns were found to be the same for both cases because, the case considered carried little weight and the reinforcement areas were dictated by the minimum requirements rather that loading conditions. This implies that for structures supporting many floors, the columns of one for hollow blocks slabs will be cheaper because it will carry less loads and the bending may be assumed to act about only one axis for all the columns. On the other hand the foundation for a structure supporting hollow blocks slab were found to be cheaper by an average of 10%. This is because the hollow blocks slabs ensured a reduced weight.
2.5 Design and Construction Use of hollow blocks constrains the slab to act as one-way spanning. These are simple to analyse, and design. The Structural drawings are easy to detail and understand. During construction it is easier to lay the reinforcement, thus minimizing mistakes on site. The weld mesh included in the topping ensures distribution of imposed loading to the whole slab. Its ease of construction has contributed to its growing popularity such that it now occupies 75% of the market share.
Figure 7" Variation of the cost of solid and hollow blocks slab with span 1.1
1 A
E -c
09
c ,_1
c
0.8
0.. t~
#
V a r i a t i o n of c o s t y ratio
0.7
0.6 2
3
4
5
6
7
8
Cost of Solid/hollow blocks slab
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International Conference on Advances in Engineering and Technology
3.0 CONCLUSION The current shape of the hollow clay blocks has keys and groves that provide mechanical interlock and friction resistance to hold the blocks firmly in concrete. However, his shape could also decrease the shear resistance of the slab. A shape has been proposed that has all the advantages of the one currently used, while at the same time increases shear resistance of the slab and a saving in the concrete used. The limited range of hollow blocks available on the market makes hollow blocks slabs more expensive than solid slabs for spans less than 4m or greater than 5m. For spans less than 4m the minimum slab depth is 175mm, because the minimum available block depth is 125mm and minimum topping required is 50mm. Yet for solid slabs the depths can vary from 50mm to 150mm for spans varying from lm to 3m depending on loading and deflection requirements. For spans greater than 5m, deflection requirements dictate increasing depth with spans, yet the maximum depth of available blocks available is 225mm, leading to uneconomical depth of the topping. Using the beams of the same size even when the hollow blocks have constrained the slab to act as one way spanning maintains rigidity of the structure and reduces effective height for the columns, but offers no reduction in the materials used in beams. The reduced weight due to use of hollow blocks slabs results in reduced cost of columns and foundations. Moreover since use of hollow blocks constrains the slab to be designed and act as one way spanning, the loading and thus moments from one set of beams framing into the column is negligible compared to the other. Thus the columns experience uniaxially moments, which causes a saving in reinforcement. REFERENCES Balu Tabaaro. W. (2004), "Sustainable development and application of indigenous building materials in Uganda" Journal of Construction exhibition. Issue 1 Page 4-5 BS8110-1 (1985, 1997) Structural use of Concrete- Part 1, code of practice for design and construction MosleyW.H. and Buney J. H. (1989) "Reinforced Concrete Design" 5th Edition Macmillan, London Pariyo Bernard. (2005) "Comparative cost analysis of solid reinforced concrete slab and hollow clay blocks slab construction", Final year undergraduate project, Kyambogo University" Seeley I. H. (1993)"Civil Engineering quantities" 5th edition Macmillan London
Uganda Clays, Kajjansi catalogue (2004) Price lists and weights of suspended floor units.
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