2007_infratech_kaedah

8/8/2019 2007_infratech_Kaedah

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FLEXURAL BEHAVIOUR OF PRECAST SLAB MADE OF OPS LIGHTWEIGHTCONCRETE

S.P. Narayanan1, Doh Shu Ing2, Z. Ideris2, V. J. Kurian1, Lillian Gungat2

1Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia.2Universiti Malaysia Sabah, 88999 Kota Kinabalu, Sabah, Malaysia.

Corresponding author: [email protected]

Abstract

Palm oil production is a major agriculture based industry in Malaysia. The palm oil extraction produceslarge quantities of waste namely fibre, shell and effluent which are usually disposed by incineration or left torot. The utilization of waste material in places where it is abundant has been an emerging aspect whichgives much lower cost of construction. OPS easily achieves a characteristic strength of 30 N/mm2 which is

20% greater than the required design strength of floor slab. Besides that, OPS is a lightweight material thatreduces the overall design loads of the construction. The results reported here are from a research projectfunded by the Construction Industry Development Board (CIDB) Malaysia that aims to utilise processed OilPalm Shell (OPS) as aggregate in making OPS concrete precast flooring slabs. Utilising OPS in concretesaves the problem of disposing the solid waste while conserving the natural aggregates that would havebeen used otherwise. This paper discusses the behaviour of OPS slab precast slab in comparison withsandstone and granite precast slabs. Data presented include the characteristics of deflection, end rotation,strain and cracking behaviour. The flexural behaviour of all the slabs complied with the requirementsspecified by the current codes of practice. The precast slab with OPS concrete has an added advantagethat it is much lighter.

Keywords: cracks, lightweight concrete slab, low cost construction, Sustainable material development.

Introduction

Precast technology is being widely used in construction of commercial and residential buildings.This technology has great potential in quality control as it utilizes highly controlled precastcomponents. OPS concrete has bright potential in this area as it is very light compared to theconventional sandstone and granite concrete (Mannan, 2003; Teo et. al, 2006). This lightweightslab will reduce the dead load of a building and therefore will reduce the cost of construction(Mannan, 2003). Only small capacity machinery is required for lifting and handling the lightweightslabs made of OPS concrete.

Currently, Malaysia is producing more than 4 million tonnes of oil palm shell (OPS) solid wasteannually. Normally, this waste is either disposed through incineration or dumped to decay inhuge pile. Unless this solid waste is further recycled, it would be detrimental to the natural

environment. Therefore, the usage of this waste material as sustainable building material helpsto preserve the natural non-renewable resources and also conserve the ecology.

Literature Review

OPS concrete produced by Teo has successfully achieved the 28 days compressive strength of28.12 N/ mm2 which < 25 N/ mm2 than the minimum required strength for structural residentialslab (Teo et. al, 2006). Mannan (2003) suggested that the floor slab cast in OPS concretesatisfies the limit state condition for residential structure. Like other organic material, OPSaggregates although are porous in nature, however the resulting OPS concrete was reasonablyimpermeable (Teo et. al, 2006).The modulus of elasticity of OPS at 28 days is relatively lowranging between 0.70-0.76x104 N/mm2 compared to the conventional concrete made of granite.

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Properties of Coarse Aggregate

Both sandstone and granite are non-organic materials while OPS is an organic material.

Generally, OPS aggregate is porous in nature and has low bulk density and high ability to absorbwater. The low bulk density characteristic is an advantage in producing light weight concretecompared to sandstone and granite. The aggregate impact value (AIV) for OPS aggregates isrelatively very low at 7.51% compared to sandstone 13.58% and granite 13.95%. In other words,it has the characteristic to absorb greater shock. The properties of aggregates are shown inTable 1. Since the water absorption of OPS is very high (33.0%), therefore the aggregates shouldbe soaked 24 hours to ensure the aggregates are water saturated before it can be used in theconcrete.

Table 1: Properties of Aggregates

Properties Sandstone OPS Granite

Maximum Size 15 mm 15 mm 15 mm

Shell thickness N/A 0.5-3.0 mm N/A

Bulk Density 1450 kg/m3 590 kg/m3 1490 kg/m3

Specific gravity (SSD) 2.58 1.17 2.63Fineness Modulus 6.34 6.08 6.68

Los Angeles Abrasion Value 21.20 4.90 % 20.30 %

Aggregate Impact Value 13.58 % 7.51 % 13.95 %

Aggregate Crushing Value 21.20 % 8.00 % 19.00 %

24-hour Water Absorption 1.53 % 33.0 % 0.97 %

Properties of Concrete

The mix proportions of sandstone concrete, OPS concrete and granite concrete used arrived aton the basis of trial mixes is shown in Table 2.

Table 2: Mix Proportion of Concrete

Material OPS Concrete SandstoneConcrete

GraniteConcrete

Cement, (kg) 450 350 350Water, (kg) 171 151 176

Coarse Aggregate, (kg) 488 1053 1241

River Sand, (kg) 629 306 361

Crushed Sandstone Sand (kg) 0 219 257

Super plasticizer (litres) 7.50 3.88 4.20

The density of precast concrete made of granite, sandstone and OPS are 2385kg/m3,2083kg/m3 and 1745 kg/m3 respectively. Hence it is observed that the use of OPS as coarseaggregate resulted in the weight reduction of 28.75% for the whole slab. OPS slab iscategorised as lightweight slab as its density is less than 1800 kg/m3.

The compressive strengths of concretes made from OPS, sandstone and granite aggregateswere determined according to BS at the ages 3, 7 and 28 days. Figure 1 shows thedevelopment of compressive strength of concrete made of sandstone, OPS and granite ascoarse aggregates. Granite concrete showed higher strengths at all ages 30 MPa at 3 days and60 MPa at 28 days) whereas OPS showed strengths of 15 MPa and 30 MPa.

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0

10

20

30

40

50

60

70

0 5 10 15 20 25 30

Age of Concrete (Days)

Compressive

Strength(N/m2)

OPS Sandstone Granite

Figure 1: Development of Compressive Strength of Concrete with Age

Experimental Programme and Set Up

The test programme included 3 precast slabs made of sandstone, granite and OPS. The slabswere 3 m long, 1 m wide and 130 mm thick. The reinforcement consisted of four high tensile bar(Y-10) at the bottom and two high tensile bar Y-10 at top, connected with R-6 diagonals to formtwo lattice girders at the spacing of 500 mm (Figure 2). B5 mesh was used as bottomreinforcement.

Figure 2: Arrangement of reinforcement

Slab Test

The slabs were tested simply supported over 3.0 m span with 2-line load applied as shown inFigure 3. The load was applied and controlled using hydraulic jack. The measurements wererecorded using the data logger. Figure 4 shows the photograph of the test set up.

Maximum Loads

The type of aggregate used in slab had a significant effect on its load capacity. The performanceof different types of slab is tabulated (Table 3). The table indicates that the first crack of OPSconcrete slab occurred at 5 kN while that for sandstone and granite concrete slabs occurred at 14kN and 16 kN respectively. Granite concrete slab had the highest failure load followed closely bysandstone concrete slab while OPS concrete slab could carry only a much lower load. At failureload, OPS concrete slab had the highest midspan deflection while the granite and sandstoneconcrete slabs deflected to only about a quarter of the OPS concrete slab value. The load-midspan deflection diagrams for the different aggregate slabs are shown in Figures 5.

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Figure 3: Experimental set up for slab test

Figure 4: Photograph of test set-up

Table 3: Performance of Different Slabs

SlabType of Coarse

Aggregate

Initial Crack(kN)

Failure Load(kN)

Displacement atFailure Load

(mm)

S1 OPS 5 31.76 200.57

S2 Sandstone 14 59.54 59.31

S3 Granite 16 62.14 69.12

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70

Load

(kN)

Figure 5: Load-Displacement Diagram Concrete Slab

During the slab testing, at every 5 kN increment of loading, the cracks were observed usingmagnifier. The cracks began to form when the load reached a value of 10-15 kN. The number ofcracks occurring between the loading points and outside the loading points was recorded atevery interval of loading until the slab eventually failed. The spacing of cracks was recorded atthe failure load. The crack behaviour of the different slabs is shown in Table 4. The slab withgranite as coarse aggregate had the highest average crack spacing followed by sandstone andOPS slabs.

Table 4: Crack Behaviour in Different Slabs at failure loads

Slab

Type of

CoarseAggregate

Average CrackSpacing (mm)

No. ofcracks

betweenload

points

No. ofcracks

outsideloadingpoints

Average crack

distance from theedge(mm)

S1 OPS 82.0 19 6 554.0

S2 Sandstone 148.0 10 5 498.5

S3 Granite 160.3 8 6 498.2

Deflection

The deflections of the slabs were measured using four LVDTs. Two LVDTs (100 mm maximumdisplacement) were placed at mid span. The deflections of the slab were recorded at load

increments of 2 kN up to 12 kN load and at load intervals of 5 kN after that. The moment at midspan versus deflections at mid span of precast slabs with different types of coarse aggregate isshown in Figure 6.

5

A

CB

A-First Cracking

B- Steel YieldingC-Concrete Failure


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35

Mom

ent(k

Nm)

Figure 6: Moment at midspan Vs deflection for different precast slabs

Strain Distribution

Strain gauges were fixed on the slabs at mid span at 20 mm, 32.5 mm, 97.5 mm and 110 mmfrom the bottom of the slab thickness. From the strain gauge readings, the strain distributionalong the depth of the slab at the mid span is plotted in Figures 7 to 9. From the figures, it canbe observed that the distance of the neutral axis from the top of the slab is about 43 mm forOPS precast slab, about 63 mm for sandstone precast slab and about 60 mm for graniteprecast slab.

Figure 7: Strain distribution along the depth at mid span for OPS slab

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Figure 8: Strain distribution along the depth at midspan for sandstone slab

Figure 9: Strain distribution along the depth at midspan for granite slab

The theoretical ultimate load can be calculated using the equationM= Asfy(d-0.5a)

Where As = Area of tension reinforcementfy = characteristic tensile strength of reinforcementd = effective depth of tension reinforcementa = depth of neutral axis

The theoretical ultimate loads are calculated based on the depths of neutral axis obtained fromexperimental strain measurements and are shown in column (3) of table 5. These are compared

with the experimental ultimate loads (column 4) and the percentage difference is indicated incolumn (5).

Table 5: Comparison between Theoretical and Experimental Ultimate Loading

1 2 3 4 5

SlabType of Coarse

Aggregate

TheoreticalUltimate Load

(kN)

ExperimentalUltimate Load

(kN)

% difference

100)3(

)4()3(

S1 OPS 48.94 31.76 -31.1

S2 Sandstone 56.18 59.54 6.0

S3 Granite 54.39 62.14 14.2

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2007_infratech_kaedah

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