11783047.pdf

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Abstract:

Abstract: This paper presents the effect of surface roughness (smooth and rough) and surfaceThis paper presents the effect of surface roughness (smooth and rough) and surface condition (ponded and optimum wet) on the bending capacity of precast prestressed hollow core condition (ponded and optimum wet) on the bending capacity of precast prestressed hollow core slabs with in-situ concrete toppings by a series of full-scale experimental tests. Interface slip was slabs with in-situ concrete toppings by a series of full-scale experimental tests. Interface slip was also measured throughout the test to observe the composite behaviour of the test specimens. The also measured throughout the test to observe the composite behaviour of the test specimens. The tests result show that the ultimate bending capacity for the ponded condition for the smooth and tests result show that the ultimate bending capacity for the ponded condition for the smooth and rough surfaces was 3 to 5% less than that of the optimum wet even though they were still higher rough surfaces was 3 to 5% less than that of the optimum wet even though they were still higher than the calculated values. A slip of 0.08 mm was observed for the smooth-ponded specimen. It than the calculated values. A slip of 0.08 mm was observed for the smooth-ponded specimen. It was later found that by roughening the top surface of the precast prestressed hollow core slab, full was later found that by roughening the top surface of the precast prestressed hollow core slab, full composite action can be achieved if the interface slip is reduced or eliminated. A theoretical composite action can be achieved if the interface slip is reduced or eliminated. A theoretical method to predict the deflection with partial interaction is also presented in this paper based on the method to predict the deflection with partial interaction is also presented in this paper based on the experimental strain gradient. From the theoretical approach, the findings suggest that an interface experimental strain gradient. From the theoretical approach, the findings suggest that an interface slip still may had occurred near the mid-span region although it was not detected by the slip gauge slip still may had occurred near the mid-span region although it was not detected by the slip gauge located at both ends of the specimen.

located at both ends of the specimen. Keywords:

Keywords: Hollow Hollow core core slabs; slabs; In-situ In-situ concrete concrete toppings; toppings; Composite Composite action; action; Bending Bending capacity;capacity; Interface s

Interface sliplip

1.0

1.0 IntroductionIntroduction

Precast concrete flooring offers an economic and versatile solution to ground and Precast concrete flooring offers an economic and versatile solution to ground and suspended floors in any type of building construction. There is a wide range of precast suspended floors in any type of building construction. There is a wide range of precast flooring types available to give the most economic solution for all loadings and spans flooring types available to give the most economic solution for all loadings and spans which includes hollow core slab (HCU), double-tee slab, solid composite plank and beam which includes hollow core slab (HCU), double-tee slab, solid composite plank and beam & composite plank. Cast in-situ concrete toppings are added to precast slab for the & composite plank. Cast in-situ concrete toppings are added to precast slab for the

B

ENDINGENDING

C

APACITY OFAPACITY OF

P

RECASTRECAST

P

RESTRESSEDRESTRESSED

H

OLLOWOLLOW

C

OREORE

S

LABS WITHLABS WITH

C

ONCRETEONCRETE

T

OPPINGSOPPINGS

Izni S. Ibrahim

1,*1,*

, Kim S. Elliott

22

and Simon Copeland

33 1

1

Lecturer, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Lecturer, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai,

Malaysia Malaysia 2

2

Associate

Associate ProfessoProfessor, School r, School of Civil Engineof Civil Engineering, Unering, University of iversity of Nottigham, UnNottigham, University Pariversity Park,k, Nottingham N

Nottingham NG7 2RD, UKG7 2RD, UK 3

3

Technical Design Manager, Tarmac Topfloor Limited, Weston Underwood, Ashbourne, Technical Design Manager, Tarmac Topfloor Limited, Weston Underwood, Ashbourne,

Derbyshire

Derbyshire DE6 4PH, DE6 4PH, UKUK Corresponding author: I

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purpose of making a complete floor fin

purpose of making a complete floor finish, or to enhance the structural performish, or to enhance the structural performance of theance of the floor by producing a composite structure (Fig. 1). The in-situ concrete toppings are floor by producing a composite structure (Fig. 1). The in-situ concrete toppings are usually 40 to 100 mm in thickness, and contain a small amount of steel reinforcement usually 40 to 100 mm in thickness, and contain a small amount of steel reinforcement (usually a prefabricated welded mesh) to control shrinkage. The concrete toppings would (usually a prefabricated welded mesh) to control shrinkage. The concrete toppings would be

be laid laid in in all all weather weather conditions conditions onto onto aged aged HCU HCU of of unspecified unspecified surface surface characteristics.characteristics. The concrete toppings compressive strength usually varies between 25 and

The concrete toppings compressive strength usually varies between 25 and 40 N/mm40 N/mm22 and and lay onto repeated HCUs manufactured using semi-dry high strength concrete. Each year lay onto repeated HCUs manufactured using semi-dry high strength concrete. Each year the UK industry constructs around £30m of composite hollow core floors slabs with no the UK industry constructs around £30m of composite hollow core floors slabs with no bona

bona fide fide information information about about their their design, design, surface surface preparation preparation and and construction. construction. RelativeRelative movement between the wet cast concrete toppings on the HCU, and the injudicial movement between the wet cast concrete toppings on the HCU, and the injudicial placement

placement of of mesh mesh reinforcement reinforcement and and construction construction joints, joints, causes causes delamination, delamination, edgeedge restraint, curvature and loss of serviceability. This is illustrated in Fig. 2. Ultimate failure restraint, curvature and loss of serviceability. This is illustrated in Fig. 2. Ultimate failure modes could be brittle, especially on precast prestressed floors that have a high modes could be brittle, especially on precast prestressed floors that have a high strength-stiffness ratio. Costly remedial work, lost utilities during repair and reduced thickness of stiffness ratio. Costly remedial work, lost utilities during repair and reduced thickness of concrete toppings could save about € 25 – 35 million per year in Europe alone (based on concrete toppings could save about € 25 – 35 million per year in Europe alone (based on about 2% of concrete toppings requiring attention) (Elliott 2002).

about 2% of concrete toppings requiring attention) (Elliott 2002).

Figure 1 Concrete toppings on hollow core Figure 1 Concrete toppings on hollow core slabsslabs

Hollow core slabs

Concrete toppings

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Shrinkage Shrinkage Concrete Concrete toppings toppings HCU HCU Debonding Debonding Strand relaxation Strand relaxation Long-term deflection Long-term deflection Creep Creep Load Load Mesh Mesh arrangement arrangement Tension Tension roughness (see (b)) roughness (see (b)) Surface Surface

Interface shear stress Interface shear stress

(a) (a) Overall problems Overall problems Roughness Roughness Concrete topping Concrete topping

Debris & dust

Debris & dust _{Ponding water}_{Ponding water}

HC HCUU

(b) Surface roughness related

(b) Surface roughness related problemsproblems Figure 2 Problems related

Figure 2 Problems related to concrete toppings constructionto concrete toppings construction

Improper surface preparations and construction techniques causes problems to

Improper surface preparations and construction techniques causes problems to the overallthe overall structural behaviour when cast in a monolithic manner. The contractor often neglected the structural behaviour when cast in a monolithic manner. The contractor often neglected the effect of moisture content, shrinkage or surface characteristics of the HCU during effect of moisture content, shrinkage or surface characteristics of the HCU during concrete topping construction. Some attempts to quantify surface texture are given in the concrete topping construction. Some attempts to quantify surface texture are given in the Fédération Internationale de la Précontrainte (FIP) document on interface shear in Fédération Internationale de la Précontrainte (FIP) document on interface shear in composite floor structures. The topping must be continuously reinforced, ideally at composite floor structures. The topping must be continuously reinforced, ideally at mid-depth, and as welded fabric is the preferred choice, there are problems where 3 or 4 sheets depth, and as welded fabric is the preferred choice, there are problems where 3 or 4 sheets are lapped. Delays of up to 5 days can also accrue by not knowing when the conditions are lapped. Delays of up to 5 days can also accrue by not knowing when the conditions are right for laying. The research aims are to study the effects of surface preparation in are right for laying. The research aims are to study the effects of surface preparation in terms of (i) roughness (smooth (as-cast) and roughened), (ii) pre-soaking before the terms of (i) roughness (smooth (as-cast) and roughened), (ii) pre-soaking before the casting of concrete toppings (ponded

casting of concrete toppings (ponded and optimum wet = approximately 9.5 litres/mand optimum wet = approximately 9.5 litres/m22 until until the surface was light dark grey in colour).

the surface was light dark grey in colour). 2.0

2.0 Background Problems and Related WorksBackground Problems and Related Works

A composite member is designed to act monolithically. Nevertheless, as the member is A composite member is designed to act monolithically. Nevertheless, as the member is bent

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other as shown in Fig. 3. Horizontal shear transfer along the interface between the HCU other as shown in Fig. 3. Horizontal shear transfer along the interface between the HCU and the in-situ concrete topping is an essential requirement to ensure composite action of and the in-situ concrete topping is an essential requirement to ensure composite action of the two members.

the two members.

I

I nnt t e e r r f f aac c e _e s_s h_he_e a

ar r s s t t r r e e s s s s _HCU_HCU Concrete topping Concrete topping

w kN/m w kN/m

Figure 3 Horizontal shear stress along the interface of

Figure 3 Horizontal shear stress along the interface of a composite member bent in flexurea composite member bent in flexure When in-situ concrete is cast on a precast unit there is usually no mechanical key in the When in-situ concrete is cast on a precast unit there is usually no mechanical key in the form of reinforcement provided between the two types of concrete. Reliance has to be form of reinforcement provided between the two types of concrete. Reliance has to be made on the bond and shear strength between the contact surfaces. In the FIP Guide to made on the bond and shear strength between the contact surfaces. In the FIP Guide to Good Practice (FIP 1982), the types of surface which a precast unit may have, prior to Good Practice (FIP 1982), the types of surface which a precast unit may have, prior to receiving the in-situ concrete are identified into ten categories. They were categorised receiving the in-situ concrete are identified into ten categories. They were categorised based

based on on the the end end production production of of the the precast precast unit unit and and it it is is difficult difficult to to distinguish betweendistinguish between “smooth” and “rough” surface. Within the FIP Commission itself there is a popular “smooth” and “rough” surface. Within the FIP Commission itself there is a popular theory that smooth (clean) interfaces have better overall bond than roughened (often theory that smooth (clean) interfaces have better overall bond than roughened (often dusty and dirty) surfaces where localised bond failures occur. FIP (FIP 1982) dusty and dirty) surfaces where localised bond failures occur. FIP (FIP 1982) recommends that contaminants should be removed either by water flushing, compressed recommends that contaminants should be removed either by water flushing, compressed air or vacuum cleaning. Sweeping is not sufficient as the small depressions of the air or vacuum cleaning. Sweeping is not sufficient as the small depressions of the interface will become full of dust. Other than roughness, surface treatment plays a major interface will become full of dust. Other than roughness, surface treatment plays a major role with regard to the transfer of shear stress across the interfaces because:

role with regard to the transfer of shear stress across the interfaces because: (a)

(a) Laitance skin, dust, debris, water etc. are commonly found in the crevices of theLaitance skin, dust, debris, water etc. are commonly found in the crevices of the surface, where, as the tops tend to be less affected; the rougher the surface, the surface, where, as the tops tend to be less affected; the rougher the surface, the less susceptible it is to the

less susceptible it is to the quality of workmanship in cleaning and preparation.quality of workmanship in cleaning and preparation. (b)

(b) If the surface of the precast member, before casting, is very dry, this member willIf the surface of the precast member, before casting, is very dry, this member will absorb water from the in-situ concrete, so

absorb water from the in-situ concrete, so that the quality adjacent to the interfacethat the quality adjacent to the interface is governing for the capacity of the interface.

is governing for the capacity of the interface. (c)

(c) If the surface is very wet, i.e. ponded, the water-cement ratio at the interface willIf the surface is very wet, i.e. ponded, the water-cement ratio at the interface will be very high, resulting in weak bo

be very high, resulting in weak bond strength in the immnd strength in the immediate strata.ediate strata.

When the surface pores are full treated, it is said to be surface-dry and saturated (wet When the surface pores are full treated, it is said to be surface-dry and saturated (wet conditions). If the precast surface was left to stand free in dry air before the casting of conditions). If the precast surface was left to stand free in dry air before the casting of concrete toppings, some of the water contained in the unit will evaporate and be less than concrete toppings, some of the water contained in the unit will evaporate and be less than saturated, i.e. air dry (dry conditions). To produce a bone-dry surface condition, saturated, i.e. air dry (dry conditions). To produce a bone-dry surface condition, prolonged

prolonged drying drying in in an an oven oven or or in in a a closed closed hot hot compound compound would would reduce reduce the the moisturemoisture content in the concrete until no moisture is left. However, this condition is not achievable content in the concrete until no moisture is left. However, this condition is not achievable for large scale production of precast units and is not considered in this study. The various for large scale production of precast units and is not considered in this study. The various stages of surface conditions are shown in Fig. 4. For an extreme condition, surface stages of surface conditions are shown in Fig. 4. For an extreme condition, surface

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moisture is left ponding, making it saturated and moist (ponded conditions). Walraven moisture is left ponding, making it saturated and moist (ponded conditions). Walraven (Walraven 1991) stated that if the surface of the precast unit before casting is very dry it (Walraven 1991) stated that if the surface of the precast unit before casting is very dry it will absorb water from the concrete toppings, where the quality adjacent to the interface will absorb water from the concrete toppings, where the quality adjacent to the interface is governed by the capacity of the interface. In contrast, if the surface was ponded the is governed by the capacity of the interface. In contrast, if the surface was ponded the large amount of free water on the surface will weaken the bond at the interface, thus large amount of free water on the surface will weaken the bond at the interface, thus reducing the capacity of the

reducing the capacity of the composite slab.composite slab.

Concrete topping Concrete topping

HC

HCUU

Air- Air-drydry SurfSurface-ace-dry adry a ndnd HC

HCUU

C

Coonnccrreette e ttooppppiinngg CCoonnccrreette e ttooppppiinngg

HCU HCU Saturated and Saturated and Bone-dry Bone-dry HC HCUU Concrete topping Concrete topping (Dry) (Dry) Free water Free water Abs

Absorbeorbed wd water ater

s

saattuurraatteed d ((WWeett)) momoiisst t ((PPoonnddeedd))

Figure 4 Conditions at the interface with different types of surface

Figure 4 Conditions at the interface with different types of surface preparationpreparation

Tests carried out by Scott (Scott 1973), Ros (Ros, Cabo et al. 1994), Ueda & Tests carried out by Scott (Scott 1973), Ros (Ros, Cabo et al. 1994), Ueda & Stitmannaithum (Ueda and Stitmannaithum 1991) and Girhammar & Pajari (Girhammar Stitmannaithum (Ueda and Stitmannaithum 1991) and Girhammar & Pajari (Girhammar and Pajari 2008) on HCU with concrete toppings were to ensure monolithic behaviour up and Pajari 2008) on HCU with concrete toppings were to ensure monolithic behaviour up to the ultimate loading capacity. The bond between hollow core slabs and the topping is to the ultimate loading capacity. The bond between hollow core slabs and the topping is essential and therefore it has to be

essential and therefore it has to be checked in design and ensured in construction. The testchecked in design and ensured in construction. The test parameters

parameters include include surface surface roughness, roughness, load load transmission transmission between between a a group group of of two two slabs,slabs, pre-stressing

pre-stressing force, force, tension tension reinforcement reinforcement ratio, ratio, shear shear span-to-effective span-to-effective depth depth ratio ratio andand concrete topping depth. In 2007, a more extensive work was carried out by Ajdukiewicz concrete topping depth. In 2007, a more extensive work was carried out by Ajdukiewicz et. al. (Ajdukiewicz, Kliszczewicz et al. 2007) with two different test setup; (i) short-term et. al. (Ajdukiewicz, Kliszczewicz et al. 2007) with two different test setup; (i) short-term loading subjected to instantaneous bending tests until failure and (ii) 6 months long-term loading subjected to instantaneous bending tests until failure and (ii) 6 months long-term loading followed by ultimate bending tests until failure. The same conclusion was made loading followed by ultimate bending tests until failure. The same conclusion was made by these researchers

by these researchers where the cracking where the cracking and ultimate load increased and ultimate load increased between 10 and 42%between 10 and 42% compared to the HCU alone. Although there was an improvement to the ultimate load, compared to the HCU alone. Although there was an improvement to the ultimate load, there is still some lack of information on the surface condition during casting, the degree there is still some lack of information on the surface condition during casting, the degree of roughness and interface slip which plays a major role to the overall performance of the of roughness and interface slip which plays a major role to the overall performance of the composite slab. These parameters is studied in detail using full-scale test and presented in composite slab. These parameters is studied in detail using full-scale test and presented in this paper.

this paper. 3.0

3.0 Test Specimen and Experimental SetupTest Specimen and Experimental Setup

Precast slabs with circular hollow cores of constant 72 mm diameter, 1200 mm in width Precast slabs with circular hollow cores of constant 72 mm diameter, 1200 mm in width and 6300 mm in length were made. The units were pre-tensioned with nine number and 6300 mm in length were made. The units were pre-tensioned with nine number 7-wire helical strands of 9.3 mm and 12.5 mm diameter, placed with a 40 mm bottom wire helical strands of 9.3 mm and 12.5 mm diameter, placed with a 40 mm bottom cover. They were pre-tensioned to 70% of the ultimate strength,

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the concrete was cast. The maximum aggregate size was 10 mm and 20 mm for the unit the concrete was cast. The maximum aggregate size was 10 mm and 20 mm for the unit and concrete toppings, respectively. The length and strands arrangement produces design and concrete toppings, respectively. The length and strands arrangement produces design service and ultimate bending moments of 53.59 kNm and 89.01 kNm, respectively. The service and ultimate bending moments of 53.59 kNm and 89.01 kNm, respectively. The HCU concrete compressive cube strength was 85.5 N/mm

HCU concrete compressive cube strength was 85.5 N/mm22 at 28 days based on the at 28 days based on the average of 3 cubes of 100

average of 3 cubes of 100 ×× 100 100 ×× 100 mm dimension. Two types of surface finishes 100 mm dimension. Two types of surface finishes were studied; (i) rough (by raking the top surface with a stiff brush in the transverse were studied; (i) rough (by raking the top surface with a stiff brush in the transverse direction), and (ii) smooth (as-cast).

direction), and (ii) smooth (as-cast).

The concrete toppings depth was 75 mm over a length of 6 m, leaving 150 mm at The concrete toppings depth was 75 mm over a length of 6 m, leaving 150 mm at each end in order to measure interface slip. The concrete topping compressive strength each end in order to measure interface slip. The concrete topping compressive strength was designed to achieve 30 N/mm

was designed to achieve 30 N/mm22 at 7 days. The concrete topping properties during the at 7 days. The concrete topping properties during the test day are given in Table 1. A142 (R6–200 mm) prefabricated mesh with 25 mm cover test day are given in Table 1. A142 (R6–200 mm) prefabricated mesh with 25 mm cover were laid on top of the HCU to increase the flexural strength and also to distribute the were laid on top of the HCU to increase the flexural strength and also to distribute the flexural cracks into the shear span. All sheets were adequately lapped at 300 mm in the flexural cracks into the shear span. All sheets were adequately lapped at 300 mm in the mid-span region. The overall nominal geometry of the specimens and test layout are mid-span region. The overall nominal geometry of the specimens and test layout are illustrated in Fig. 5 and 6, respectively. A three load cyclic test was applied and removed illustrated in Fig. 5 and 6, respectively. A three load cyclic test was applied and removed incrementally before the final load was applied until failure. The testing procedure incrementally before the final load was applied until failure. The testing procedure follows that specified in BS 8110: Part 2 (BSI 1985). The cyclic load was applied until follows that specified in BS 8110: Part 2 (BSI 1985). The cyclic load was applied until the calculated cracking moment,

the calculated cracking moment, M M crack calccrack calc was attained (see Fig. 8). At the end of eachwas attained (see Fig. 8). At the end of each

cycle, the prestressed recovery was determined during 1 hour and 24 hours recovery cycle, the prestressed recovery was determined during 1 hour and 24 hours recovery period

period from from the the measured measured mid-span deflection mid-span deflection at at the the end end of of the the time time period. period. The The rate rate ofof loading was kept constant at 10

loading was kept constant at 10 kN/min (or bending stress of 0.02 N/mmkN/min (or bending stress of 0.02 N/mm22/sec)./sec).

46.5 46.5 1197 1197 115.5 115.5 138 138 138 138 138 138 138 138 138 138 138 138 138 138 115.5 115.5 47.5 47.5 137.5 137.5 138 138 138 138 138 138 138 138 138 138 138 138 137.5 137.5 1 1 5 5 2 2 7 7 5 5 Concrete topping Concrete topping HCU HCU

Figure 5 Specimen section and detailed dimension for the bending test Figure 5 Specimen section and detailed dimension for the bending test

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Strain gauge Strain gauge 800 800 800 800 6300 6300 7 7 5 5 200200 _{potentiometer}_{potentiometer} Roller load Roller load Load cell Load cell Load jack Load jack 6000 6000 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 potentiometer potentiometer Mid-span deflection Mid-span deflection Left deflection Left deflection Right deflection Right deflection Spherical bearing Spherical bearing RHS section RHS section Vertical slip Vertical slip

Figure 6 Bending test setup (

Figure 6 Bending test setup (all dimensions are in mmall dimensions are in mm))

Table 1 Concrete topping properties Table 1 Concrete topping properties

S S p p e e c c i i m m e e n n S S u u r r f f a a c c e e r r o o u u g g h h n n e e s s s s S S u u r r f f a c a c e e c c o o n n d d i i t t i i o n o n

C C o o n n c c r r e e t t e e c c u u b b e e c c o o m m p p r r e e s s s s i i v v e e

s s t t r r e e n n g g t t h h ( ( N N / / m m m m 2 2 ) ) E E l l a s a s t t i i c c M M o o d d u u l l u s u s ( ( k k N N / / m m m m 2 2 ) ) C C y y l l i i d n n d e e r r s s p p l l i i t t t t i i n n g g s s t t r r e e n n g g t t h h ( ( N N / / m m m m 2 2 ) ) F F l l e e x x u u r r a a l l s s t t r r e e n n g g t t h h ( ( N N / / m m m m 2 2 ) ) SS1

SS1 Smooth Smooth Ponded Ponded 35.2 35.2 29.2 29.2 2.82 2.82 3.473.47 SS2

SS2 Optimum Optimum wet wet 30.8 30.8 28.1 28.1 2.64 2.64 3.273.27 SR1

SR1 Rough Rough Ponded Ponded 36.5 36.5 30.7 30.7 2.95 2.95 3.623.62 SR2

SR2 Optimum Optimum wet wet 37.3 37.3 28.8 28.8 2.98 2.98 3.653.65

Note:

Note:_{Average of 3 tests}_{Average of 3 tests}

4.0

4.0 Surface Roughness and Surface ConditionSurface Roughness and Surface Condition

Roughness was measured using an instrument developed by Bensalem (Bensalem 2001) Roughness was measured using an instrument developed by Bensalem (Bensalem 2001) as shown in Fig. 7. The instrument is placed on top of the slab to measure the roughness as shown in Fig. 7. The instrument is placed on top of the slab to measure the roughness along a sampling length of 200 mm. Once the instrument is in place, the slider moves along a sampling length of 200 mm. Once the instrument is in place, the slider moves freely without displacing the instrument itself. Roughness was measured at three freely without displacing the instrument itself. Roughness was measured at three locations in the longitudinal direction; mid-span, left and right end of the HCU. The locations in the longitudinal direction; mid-span, left and right end of the HCU. The average was taken of the three measured locations.

average was taken of the three measured locations.

Two different types of surfaces were prepared before casting the concrete Two different types of surfaces were prepared before casting the concrete topping, i.e. optimum wet and ponded. The prepared condition was based on the results of topping, i.e. optimum wet and ponded. The prepared condition was based on the results of

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the “push-off” test where the ponded was the least value in terms of interface shear the “push-off” test where the ponded was the least value in terms of interface shear strength (Ibrahim and Elliott 2006). The optimum wet was the preferred condition as strength (Ibrahim and Elliott 2006). The optimum wet was the preferred condition as mentioned in the FIP document (FIP 1982) with the HCU being light dark grey in colour mentioned in the FIP document (FIP 1982) with the HCU being light dark grey in colour with no standing surface water. Compared to being wet, ponded means excess water was with no standing surface water. Compared to being wet, ponded means excess water was purposely left

purposely left on on the the surface surface with a with a depth depth of of approximately 1.8 approximately 1.8 mm. Preliminary mm. Preliminary studiesstudies were also carried out to produce the optimum wet and ponded conditions on a 1 m

were also carried out to produce the optimum wet and ponded conditions on a 1 m ×× 1 1 mm surface area. The time required for the water to evaporate was measured and for the surface area. The time required for the water to evaporate was measured and for the surface to change to the light dark grey in colour for the optimum wet, in contrast to the surface to change to the light dark grey in colour for the optimum wet, in contrast to the ponded

ponded condition. condition. The The preliminary preliminary studies studies show show that that for for ponded ponded surface surface conditions, conditions, itit requires 45 litres/m

requires 45 litres/m22 of water as compared with the optimum wet surface which only of water as compared with the optimum wet surface which only requires 9.5 litres/m

requires 9.5 litres/m22 of water. Another method of measurement was also carried out of water. Another method of measurement was also carried out using a moisture meter (given in percentages). For the ponded condition, the surface using a moisture meter (given in percentages). For the ponded condition, the surface moisture was between 41 and 43% and the optimum wet was between 26 and 28% moisture was between 41 and 43% and the optimum wet was between 26 and 28% without any standing water on the surface. All the results from the preliminary studies without any standing water on the surface. All the results from the preliminary studies were then used as guidance to prepare the required surface conditions.

were then used as guidance to prepare the required surface conditions.

Figure 7 Surface

Figure 7 Surface roughness instrumentationroughness instrumentation 5.0

5.0 Experimental ResultsExperimental Results 5.1

5.1 Bending Moment and Deflection Rela Bending Moment and Deflection Relationshiptionship

The bending moment and mid-span deflection relationships are shown in Fig. 8. The bending moment and mid-span deflection relationships are shown in Fig. 8. Deflections recorded at 0.8 m from the left and right supports were the same, proving that Deflections recorded at 0.8 m from the left and right supports were the same, proving that the loading and test rig position was centrally aligned throughout the test. During the the loading and test rig position was centrally aligned throughout the test. During the cyclic load test up to

cyclic load test up to M M sr calcsr calc, , the the deflection deflection was was within within the the limit limit of of . . TheThe

M

M sr calcsr calcwas calculated at two stages of loading – Stages 1 and 2 before and after hardeningwas calculated at two stages of loading – Stages 1 and 2 before and after hardening LP1

LP1

LP2 (200 mm)

LP2 (200 mm) _{Bridge supply}_{Bridge supply} unit and unit and monitor monitor HCU HCU

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of concrete topping as described by Elliott (Elliott 2002). This shows that all specimens of concrete topping as described by Elliott (Elliott 2002). This shows that all specimens were behaving elastically before cracking first occurred. Cracking occurred in specimen were behaving elastically before cracking first occurred. Cracking occurred in specimen SR1 and SR2 during the first cyclic

SR1 and SR2 during the first cyclic load increment at 107.7 and 119.8 kNm, rload increment at 107.7 and 119.8 kNm, respectively.espectively. It was also observed that for specimen SR1, the first crack occurred below the calculated It was also observed that for specimen SR1, the first crack occurred below the calculated M

M crack calccrack calc (Elliott 2002). The prestressed recovery results for each loading cycle are given (Elliott 2002). The prestressed recovery results for each loading cycle are given

in Table 2. The recovery was consistent between 80 and 84% in the 1

in Table 2. The recovery was consistent between 80 and 84% in the 1stst and 2 and 2nd nd cycle. In cycle. In the 3

the 3rd rd cycle, the recovery dropped below 80% for all specimens. The results found that cycle, the recovery dropped below 80% for all specimens. The results found that the recovery for all specimens was less than the required 85% recovery for prestressed the recovery for all specimens was less than the required 85% recovery for prestressed Class 1 and 2 category stated in BS 8110: Part 2 (BSI 1985). The reason for the low Class 1 and 2 category stated in BS 8110: Part 2 (BSI 1985). The reason for the low percentage

percentage recovery recovery was was because because the the load load applied applied was was more more than than the the service service load. load. ThisThis was taken into consideration since BS 8110: Part 2 (BSI 1985) stated that if the measured was taken into consideration since BS 8110: Part 2 (BSI 1985) stated that if the measured deflection is very small (e.g.

deflection is very small (e.g. span/1000), estimates of recovery become meaningless. Thisspan/1000), estimates of recovery become meaningless. This means that the measured deflection must be more than 6 mm in order to comply with the means that the measured deflection must be more than 6 mm in order to comply with the code requirement. In addition, specimen SR1 and SR2 had developed flexural cracks code requirement. In addition, specimen SR1 and SR2 had developed flexural cracks during the load increment, which had affected the specimen recovery.

during the load increment, which had affected the specimen recovery. The experimental curves at elastic or before reaching

The experimental curves at elastic or before reaching M M crack calccrack calc follow the follow the

calculated deflection for the uncracked section,

calculated deflection for the uncracked section, EI EI uu. The ultimate bending moment. The ultimate bending moment

capacity for specimen SS1 and SR1 were 190.3 and 178.2 kNm, respectively, which were capacity for specimen SS1 and SR1 were 190.3 and 178.2 kNm, respectively, which were 4.4 to 9.5% lower than specimen SS2 (196.8 kNm) and SR2 (188.1 kNm). These were the 4.4 to 9.5% lower than specimen SS2 (196.8 kNm) and SR2 (188.1 kNm). These were the ponded

ponded condition condition specimens specimens with with smooth smooth and and rough rough surfaces, surfaces, respectively. respectively. This This is is aa possible

possible explanation explanation of of the the excess excess water water that that was was not not removed removed during during the the casting casting of of thethe concrete toppings, which reduced the ultimate bending moment capacity compared to the concrete toppings, which reduced the ultimate bending moment capacity compared to the optimum wet. However, the results were still higher than

optimum wet. However, the results were still higher than M M ur calcur calc (see Table 2). The (see Table 2). The M M ur calcur calc

was also calculated using the two-stage approach rather than the one-step approach which was also calculated using the two-stage approach rather than the one-step approach which is obviously less conservative. The method is to calculate the area of steel,

is obviously less conservative. The method is to calculate the area of steel, A A ps1 ps1 required required

in Stage 1, and to add the area,

in Stage 1, and to add the area, A A ps2 ps2 required in Stage 2 using an increased lever arm required in Stage 2 using an increased lever arm

(Elliott 2002). The ratio with the calculated values were more than 1.00, except for (Elliott 2002). The ratio with the calculated values were more than 1.00, except for

specimen

specimen SR1, SR1, where where the the = = 0.960.96 ≤≤ 1.00. Although the ratio was less than 1.00. Although the ratio was less than

1.00

1.00 but but at at failure failure . . This This shows shows that that the the ultimate ultimate bendingbending moment capacity of specimen SR1 was still 17% higher than

moment capacity of specimen SR1 was still 17% higher than M M ur calcur calc although the earlier although the earlier

cracking occurred below the expected

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Figure 8 Bending moment vs. mid-span deflection relationship Figure 8 Bending moment vs. mid-span deflection relationship

For all specimens, the first crack occurred at the mid-span. As further loading was For all specimens, the first crack occurred at the mid-span. As further loading was applied, more cracking develops in the tension region and moves toward the compression applied, more cracking develops in the tension region and moves toward the compression region before crossing the interface into the concrete toppings. The same cracking pattern region before crossing the interface into the concrete toppings. The same cracking pattern was observed at both sides for each specimen. Except for specimen SS1, although was observed at both sides for each specimen. Except for specimen SS1, although cracking crossed the interface and into the concrete toppings, there was no failure in the cracking crossed the interface and into the concrete toppings, there was no failure in the interface (zero slippage). This is shown in Fig. 9 for specimen SR2. For specimen SR1 interface (zero slippage). This is shown in Fig. 9 for specimen SR2. For specimen SR1 and SR2, the test was stopped when concrete crushing occurred on the top surface at the and SR2, the test was stopped when concrete crushing occurred on the top surface at the mid-span region.

mid-span region.

Table 2 Specimen self

Table 2 Specimen self weight and recovery during the cyclic weight and recovery during the cyclic loadingloading

S S p p e e c c i i m m e e n n Recovery (%) Recovery (%) F F i i r r s s t t c c r r a a c c k k i i n n g g m m o o m m e e n n t t , , M M c c r r a a c c k k e e x x p p ( ( k k N N m m ) ) U U l l t t i i m m a a t t e e m m o o m m e e n n t t o o f f r r e e s s i i s s t t a a n n c c e e , , M M u u r r e e x x p p ( ( k k N N m m ) ) 1 1stst Cycle Cycle 2 2nd nd Cycle Cycle 3 3rd rd Cycle Cycle SS1 SS1 83.6 83.6 83.2 83.2 79.1 79.1 128.7 128.7 190.3 190.3 1.15 1.15 1.251.25 SS2 SS2 84.1 84.1 83.5 83.5 78.5 78.5 133.2 133.2 196.8 196.8 1.19 1.19 1.291.29 SR1 SR1 81.7 81.7 82.0 82.0 77.5 77.5 107.7 107.7 178.2 178.2 0.96 0.96 1.171.17 SR2 SR2 80.7 80.7 81.0 81.0 80.8 80.8 119.8 119.8 188.1 188.1 1.07 1.07 1.231.23 M

Mcrack calccrack calc= 116.2 kNm= 116.2 kNm

L/250 = 24 mm L/250 = 24 mm EI EIuu Selfweight Selfweight M Mur calcur calc = 145.5 kNm= 145.5 kNm M Msr calcsr calc= 85.8 kNm= 85.8 kNm

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Figure 9 Flexural cracking pattern at failure for specimen SR2 Figure 9 Flexural cracking pattern at failure for specimen SR2 5.2

5.2 Bending Moment and Concrete Str Bending Moment and Concrete Strains Relationshipains Relationship

A typical bending moment vs. concrete strains relationship at the top, bottom and A typical bending moment vs. concrete strains relationship at the top, bottom and interface is shown in Fig. 10 and the strain distribution is shown in Fig. 11. The interface is shown in Fig. 10 and the strain distribution is shown in Fig. 11. The calculated neutral axis depth shown in the figure from the uncracked,

calculated neutral axis depth shown in the figure from the uncracked, x xuu and cracked, and cracked, x xcc

section were calculated using the f

section were calculated using the following equation:ollowing equation:

(1) (1)

(2) (2) where

where A Ahcuhcu is the cross sectional area of the hollow core unit, is the cross sectional area of the hollow core unit, A Atoptop is the cross sectional is the cross sectional

area of the concrete topping,

area of the concrete topping, A Ass is the total reinforcement area, is the total reinforcement area, y y is the centroid of the area is the centroid of the area

considered from the point of reference,

considered from the point of reference, bbeff eff is the effective breadth and is the effective breadth and mm is the steel to is the steel to

concrete modular ratio.

concrete modular ratio. For specimen For specimen SS1 and SS1 and SS2, the SS2, the measured strains measured strains were actingwere acting elastically until

elastically until M M crack calccrack calc. During this behaviour, the difference in the calculated strain,. During this behaviour, the difference in the calculated strain,

EI

EI uu was below 10%. After cracking, the specimens behaved non-linearly. When the was below 10%. After cracking, the specimens behaved non-linearly. When the

specimens were close to their ultimate capacity, large bottom strain increments of more specimens were close to their ultimate capacity, large bottom strain increments of more than 2000

than 2000 μεμε were observed for were observed for a small increment of load. a small increment of load. Specimens SR1 and SR2 actedSpecimens SR1 and SR2 acted non-linearly after the first crack occurred, which was below

non-linearly after the first crack occurred, which was below M M crack calccrack calc. Beyond this point. Beyond this point

and up to ultimate, the strains were behaving similarly to specimen SS1 and SS2. The and up to ultimate, the strains were behaving similarly to specimen SS1 and SS2. The strain distribution changes rapidly when multiple flexural cracks start to occur. At the strain distribution changes rapidly when multiple flexural cracks start to occur. At the same time, the interface strain

same time, the interface strain changes from negative (compression) to positive (tension).changes from negative (compression) to positive (tension). At uncracked, the neutral axis was below the interface (in the HCU) and as the At uncracked, the neutral axis was below the interface (in the HCU) and as the section cracked, the neutral axis moved above the interface into the concrete toppings. At section cracked, the neutral axis moved above the interface into the concrete toppings. At failure, the neutral axis (measured from the top of the composite slab) reduced from 102.4 failure, the neutral axis (measured from the top of the composite slab) reduced from 102.4

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to 33.0 and 49.8 mm for specimen SS1 and SS2, respectively. The same was observed for to 33.0 and 49.8 mm for specimen SS1 and SS2, respectively. The same was observed for specimen SR1 and SR2, where the neutral axis reduced to 38.9 and 52.8 mm at failure. specimen SR1 and SR2, where the neutral axis reduced to 38.9 and 52.8 mm at failure. The neutral axis reduced considerably for specimen SS1 and SR1, where the surface was The neutral axis reduced considerably for specimen SS1 and SR1, where the surface was ponded. However, the difference was only 3.7% compared to the calculated

ponded. However, the difference was only 3.7% compared to the calculated x xcc = 34.2 mm = 34.2 mm

(measured from the top composite slab) suggesting the interface was not disturbed at (measured from the top composite slab) suggesting the interface was not disturbed at failure.

failure.

Figure 10 Typical concrete strains relationships for specimen SR1 up to 10000 Figure 10 Typical concrete strains relationships for specimen SR1 up to 10000 μεμε

Figure 11 Typical strain distribution diagram for

Figure 11 Typical strain distribution diagram for specimen SR1specimen SR1

M

Mcrack calccrack calc

EI EIu bottomu bottom M Mur calcur calc M Msr calcsr calc EI EIu topu top x xcc = 34.2 mm = 34.2 mm x xuu = 116.5 mm = 116.5 mm 180 kN 180 kN 177 kN 177 kN 128 kN 128 kN 20 kN 20 kN 100 kN 100 kN 60 kN 60 kN C C o o n n c c r r e e t t e e t t o o p p p p i i n n g g ( ( m m m m ) ) H H C C U U ( ( m m m m ) ) Selfweight Selfweight

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5.3

5.3 Surface Roughness and Interface SlipSurface Roughness and Interface Slip

The roughness profile at the mid-span of each specimen is shown in Fig. 12(a) and 12(b). The roughness profile at the mid-span of each specimen is shown in Fig. 12(a) and 12(b). Higher amplitude can be seen in the rough surface profile showing higher peaks and Higher amplitude can be seen in the rough surface profile showing higher peaks and valleys compared to the smooth surface. In fact, the

valleys compared to the smooth surface. In fact, the smooth surface profile seems to showsmooth surface profile seems to show equal heights throughout the whole sample length. Roughness is usually represented as equal heights throughout the whole sample length. Roughness is usually represented as the average roughness,

the average roughness, R Raa which is the average value of the departure of the profile above which is the average value of the departure of the profile above

and below the mean lines throughout the sampling length,

and below the mean lines throughout the sampling length, ll (see Fig. 13(a)). A recent (see Fig. 13(a)). A recent study by Santos et. al. (Santos, Julio et al. 2007) suggested to use other alternatives like study by Santos et. al. (Santos, Julio et al. 2007) suggested to use other alternatives like the mean peak-to-valley height, total roughness height or maximum valley depth to the mean peak-to-valley height, total roughness height or maximum valley depth to determine the roughness amplitude, since these correspond to the highest coefficient of determine the roughness amplitude, since these correspond to the highest coefficient of correlation obtained. The mean peak-to-valley height,

correlation obtained. The mean peak-to-valley height, R R z z represents the distance between represents the distance between

the five highest profile peaks and the five deepest profile valleys within the sampling the five highest profile peaks and the five deepest profile valleys within the sampling length,

length, ll as shown in as shown in Fig. 13(b). The definition ofFig. 13(b). The definition of R R z z is given as (BSI 1988): is given as (BSI 1988):

∑

= =

=

5 5 1 1

5

1

ii ii z z

z

R

(3)(3)

The total roughness height,

The total roughness height, R R y y is given by ( is given by (BSI 1988):BSI 1988):

R

R y y = = p pmaxmax – – vvmaxmax (4)(4)

where

where p pmaxmax is the maximum peak height and is the maximum peak height and vvmaxmax is the maximum valley depth. is the maximum valley depth.

(a)

(a) Smooth Smooth surface surface (b) (b) Rough Rough surfacesurface Figure 12 Surface profile measured at the mid-span Figure 12 Surface profile measured at the mid-span

SR1 SR1 SR2 SR2 SS1 SS1 SS2 SS2

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R Raa z z11 2 2 z z z z33 z z44 z z55 ll llcc llcc llcc llcc llcc

(a) Average roughness,

(a) Average roughness, R Raa (b) Mean peak-to-valley height,(b) Mean peak-to-valley height, R R z z

Figure 13 Determination of

Figure 13 Determination of R Raa and and R R z z The values of roughness for the different definition of

The values of roughness for the different definition of R Raa,, R R z z and and R R y y are given in Table 3. are given in Table 3.

The findings show that when

The findings show that when R Raa is considered to define roughness, there was not much is considered to define roughness, there was not much

difference to differentiate between the smooth and rough surfaces. However, if

difference to differentiate between the smooth and rough surfaces. However, if R R z z and and R R y y

are considered, the relationship can be divided as follows; (i) smooth surface when

are considered, the relationship can be divided as follows; (i) smooth surface when R R z z << 1 1

mm and

mm and R R y y << 2 mm and (ii) rough surface when 2 mm and (ii) rough surface when R R z z >> 1.00 mm and 1.00 mm and R R y y >> 2 mm. In fact, in 2 mm. In fact, in

Gohnert (Gohnert 2000; Gohnert 2003), Santos et. al. (Santos, Julio et al. 2007) and Gohnert (Gohnert 2000; Gohnert 2003), Santos et. al. (Santos, Julio et al. 2007) and Ibrahim & Elliott (Ibrahim and Elliott 2006) work, they also found that the scatter of data Ibrahim & Elliott (Ibrahim and Elliott 2006) work, they also found that the scatter of data was so broad when

was so broad when R Raa is considered that any trend or is considered that any trend or correlation is hardly distinguishable.correlation is hardly distinguishable.

Based on this finding and the suggestion by Santos et. al. (Santos, Julio et al. 2007),

Based on this finding and the suggestion by Santos et. al. (Santos, Julio et al. 2007), R R z z is is

considered in this study when obtaining the roughness value. considered in this study when obtaining the roughness value.

The study found that as roughness increases, the interface slip can be minimised. The study found that as roughness increases, the interface slip can be minimised. This was observed during the test where only interface slip occurs in specimen SS1 as This was observed during the test where only interface slip occurs in specimen SS1 as shown in Fig. 14. The maximum slip at failure at the left and right position was 0.08 and shown in Fig. 14. The maximum slip at failure at the left and right position was 0.08 and 0.03 mm, respectively. The slip only occurred for the smooth surface with ponded 0.03 mm, respectively. The slip only occurred for the smooth surface with ponded condition, which was the least roughness values and in contrast to the rough surface (see condition, which was the least roughness values and in contrast to the rough surface (see Table 3). Furthermore, with excessive standing water,

Table 3). Furthermore, with excessive standing water, the interface is easily disturbed andthe interface is easily disturbed and maybe subjected to sli

maybe subjected to slippage. ppage. However, by roughening tHowever, by roughening the top surface of the HChe top surface of the HCU, thisU, this will eliminate or reduce any interface slip that may occur. This is observed for specimen will eliminate or reduce any interface slip that may occur. This is observed for specimen SR2 (rough – ponded) where

SR2 (rough – ponded) where R R z z = 1.87 mm compared to 0.51 mm for specimen SS1. = 1.87 mm compared to 0.51 mm for specimen SS1.

Table 3 Surface roughness results Table 3 Surface roughness results

S S p p e e c c i i m m e e n n S S u u r r f f a a c c e e r r o o u u g g h h n n e e s s s s S S u u r r f f a a c c e e c c o o n n d d i i t t i i o o n n

A A v v e e r r a a g g e e r r o o u u g g h h n n e e s s s s , , R R a a ( ( m m m m ) ) M M e e a a n n p p e e a a k k - - t t o o - - v v a a l l l l e e y y h h e e i i g g h h t t , , R R z z ( ( m m m m ) ) T T o o t t a a l l r r o o u u g g h h n n e e s s s s h h e e i i g g h h t t , , R R y y ( ( m m m m ) ) SS1 SS1 SSmmooootthh PPoonnddeedd 00..3322 00..5511 11..6600 SS2 SS2 OOppttiimmuum m wweett 0..20 255 00..6644 11..8833 SR1

SR1 RoughRough PPoonnddeedd 0..40 422 11..5511 22..9922 SR2

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Figure 14 Bending moment vs. interface sl

Figure 14 Bending moment vs. interface slip relationship for specimen SS1ip relationship for specimen SS1 6.0

6.0 Deflection with Partial InteractionDeflection with Partial Interaction

The test results show that interface slip was only observed in specimen SS1. However, for The test results show that interface slip was only observed in specimen SS1. However, for the other specimens with similar strain distribution diagrams pattern shown in Fig. 11, the other specimens with similar strain distribution diagrams pattern shown in Fig. 11, thethe strain gradients above and below the neutral axis were not the same, suggesting an strain gradients above and below the neutral axis were not the same, suggesting an internal slip occurred in the mid-span region (where the strain gauge was positioned). If internal slip occurred in the mid-span region (where the strain gauge was positioned). If this happens, the predicted deflection is based on partial interaction, where the internal this happens, the predicted deflection is based on partial interaction, where the internal interface slip must also be

interface slip must also be taken into account in the calculation. In taken into account in the calculation. In 2000, Lam et.al. (Lam,2000, Lam et.al. (Lam, Elliott et al. 2000) proposed a method to predict the deflection with partial interaction. Elliott et al. 2000) proposed a method to predict the deflection with partial interaction. However, the work was on a composite steel beam with precast slabs. The proposed However, the work was on a composite steel beam with precast slabs. The proposed equation was modified in this paper to suit with the type of slab used. The strain equation was modified in this paper to suit with the type of slab used. The strain distribution from the experimental results and associated with partial interaction is shown distribution from the experimental results and associated with partial interaction is shown in Fig. 15. The shear stiffness,

in Fig. 15. The shear stiffness, K K ss is given as: is given as:

(5) (5) Where

Where qq is the shear force at the interface per unit length, is the shear force at the interface per unit length, llss is the uniform spacing is the uniform spacing

roughness profile and

roughness profile and δ δ ss is the interface slip. Consider a composite slab with axial forces is the interface slip. Consider a composite slab with axial forces

and moments acting on an element shown in Fig. 16. The force and moment for the and moments acting on an element shown in Fig. 16. The force and moment for the concrete toppings act through the centroid of the concrete toppings at a distance

concrete toppings act through the centroid of the concrete toppings at a distance y yt t from from

the interface, and the force and moment for the HCU act at the centroid of the HCU at a the interface, and the force and moment for the HCU act at the centroid of the HCU at a

Selfweight Selfweight

(16)

distance

distance y yss from the interface. The elastic strain at the bottom of the concrete toppings, from the interface. The elastic strain at the bottom of the concrete toppings, ε ε t t

as shown in Fig. 15 is

as shown in Fig. 15 is given as:given as:

(6) (6) where

where E’ E’cc is the Elastic Modulus of the concrete topping. The elastic strain at the top of is the Elastic Modulus of the concrete topping. The elastic strain at the top of

the HCU is given by: the HCU is given by:

(7) (7) where

where E E cc is the Elastic Modulus of is the Elastic Modulus of the HCU and assuming:the HCU and assuming:

F

F int int = = F F toptop = = F F hcuhcu (8)(8)

where

where F F int int = qx = qx and the shear force per unit length of the composite slab, and the shear force per unit length of the composite slab, qq can be written can be written

as: as:

(9) (9)

Combining

Combining Eq. Eq. (5) (5) and and (9), (9), the the interface interface slip slip strain strain in in the the composite composite slab, slab, as as shownshown in Fig. 17, becomes: in Fig. 17, becomes: (10) (10) gradient gradient True strain True strain εε s s εε εεtt εεss tt HCU HCU Concrete Concrete topping topping εε εε

+

_

gradient gradientTrue strain True strain

_

+

d s/dx d s/dxδδ d s/dxd s/dxδδ (a)

(17)

Figure 15 Strain distributions for partial interface interaction Figure 15 Strain distributions for partial interface interaction

Concrete topping Concrete topping HCU HCU s s tt h h h h IItoptop top top A A c c E' E' E E A Ahcuhcu

hcu hcu II cc y y_tt s s y y M Mtoptop top top F F hcu hcu M M hcu hcu F F concrete toppings concrete toppings Centroid of Centroid of Centroid of HCU Centroid of HCU

Figure 16 Force and moment distribution of

Figure 16 Force and moment distribution of composite slabcomposite slab

When interface slip occurs, the strain diagram can be represented as shown in Fig. 17. In When interface slip occurs, the strain diagram can be represented as shown in Fig. 17. In the test setup, electrical strain gauges were positioned at the top and bottom of the HCU the test setup, electrical strain gauges were positioned at the top and bottom of the HCU and the top surface of the

and the top surface of the concrete topping as shown in Fig. 6 representing point A, C concrete topping as shown in Fig. 6 representing point A, C andand D. During the serviceability state and under full-composite action, above and below D. During the serviceability state and under full-composite action, above and below neutral axis strain gradients are the same as shown in Fig. 17(a). If the interface was neutral axis strain gradients are the same as shown in Fig. 17(a). If the interface was disturbed, e.g. interface slip, the strain changed between point B and C. The disturbance disturbed, e.g. interface slip, the strain changed between point B and C. The disturbance to the interface is shown in Fig. 17(b). Therefore, the strain gradient above the neutral to the interface is shown in Fig. 17(b). Therefore, the strain gradient above the neutral axis between point A’ and C’ was not applicable as it does not represent the actual strain axis between point A’ and C’ was not applicable as it does not represent the actual strain behaviour

behaviour in in the the concrete concrete topping, topping, Fig. Fig. 17(c). 17(c). This This is is known known as as the the “apparent “apparent strainstrain gradient”. Therefore, “true strain gradient” can only be determined using the values of gradient”. Therefore, “true strain gradient” can only be determined using the values of point C’ and D’. Here,

point C’ and D’. Here, the terms (the terms (ε ε t t −− ε ε ss) is also the difference between strains at point B’) is also the difference between strains at point B’

and C’,

and C’, ε ε B’C’ B’C’. Combining Eq. (6), (7), (8) a. Combining Eq. (6), (7), (8) and (10) gives:nd (10) gives:

(11) (11) The composite moment,

The composite moment, M M compcomp is equal to: is equal to:

(12) (12)

(18)

(Theoretical) (Theoretical) + + _ _ _ _ + + _ _ + + measured measured N.A N.A gradient gradientTrue strain True strain Strains as Strains as Interface slip Interface slip Full-bond Full-bond C C'' A' A' D' D' D D A A B B C C D D C C B B A

A _{Apparent st}_Appar_{ent strain}_rain

gradient gradient B' B' Interface Interface disturbance disturbance cracked cracked ((aa)) ((bb)) ((cc))

Figure 17 Strain distribution diagram during the ultimate load test Figure 17 Strain distribution diagram during the ultimate load test

For the composite slab to act monolithically and to prevent any separation between the For the composite slab to act monolithically and to prevent any separation between the HCU and concrete toppings, the curvature

HCU and concrete toppings, the curvature κ κ for the concrete toppings and the HCU must for the concrete toppings and the HCU must be the same, so that:

be the same, so that:

(13) (13)

Therefore,

Therefore, and and and and substitution substitution into into Eq. Eq. (11) (11) andand (12) gives: (12) gives: (14) (14) and and (15) (15) where and where and

Combining Eq. (14) and (15) gives: Combining Eq. (14) and (15) gives:

(16) (16)

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Using the parallel axis theorem, Using the parallel axis theorem,

Therefore, Therefore,

(17) (17) Substituting Eq. (17) into Eq. (16) and

Substituting Eq. (17) into Eq. (16) and rearranging them gives:rearranging them gives:

(18) (18) i.e. the interface slip strain is due to the difference between the strain due to the

i.e. the interface slip strain is due to the difference between the strain due to the M M compcomp and and

F

F int int . . Multiplying Multiplying each each term term in in Eq. Eq. (18) (18) with with gives:gives:

(19) (19) and substituting Eq. (17) gives:

and substituting Eq. (17) gives:

(20) (20) Substituting Eq. (10) and (18) into Eq.

Substituting Eq. (10) and (18) into Eq. (20), gives:(20), gives:

(21) (21) For mid-span deflection assuming constant moment along the interface and integrating For mid-span deflection assuming constant moment along the interface and integrating

twice with respect to

twice with respect to x x gives: gives:

(22) (22)

(20)

where

where δ δ part part is the deflection of the composite slab with partial interaction is the deflection of the composite slab with partial interaction

δ

δ full full is the deflection of is the deflection of the composite slab with full interactionthe composite slab with full interaction

ε

ε B’C’ B’C’ is the interface slip strain difference is the interface slip strain difference

The difference between the measured and the calculated interface strain is the interface The difference between the measured and the calculated interface strain is the interface slip strain or the interface disturbance,

slip strain or the interface disturbance, ε ε B’C’ B’C’. The calculated interface strain was from the. The calculated interface strain was from the

“true strain gradient”, which was then applied for the strain gradient above the neutral “true strain gradient”, which was then applied for the strain gradient above the neutral axis. The relationships between

axis. The relationships between EI EI from the “true strain gradient” andfrom the “true strain gradient” and ε ε B’C’ B’C’ for specimen for specimen

SR2 are shown in Fig. 18. SR2 are shown in Fig. 18.

The mid-span deflection comparison with the experimental results,

The mid-span deflection comparison with the experimental results, δ δ full full (based on (based on

the

the EI EI uu of the composite section) and of the composite section) and δ δ part part from Eq. (20) is shown in Fig. 19 for specimen from Eq. (20) is shown in Fig. 19 for specimen

SR2. The

SR2. The δ δ part part and and δ δ expexp curves show linear relationship before reaching the cracking curves show linear relationship before reaching the cracking

moment,

moment, M M crack calccrack calc. However, the deflection for both curves during this stage was 3 to 5%. However, the deflection for both curves during this stage was 3 to 5%

more than the

more than the δ δ full full curve. The possible explanation of the results is that there was an curve. The possible explanation of the results is that there was an

interface slip occurred near the mid-span region even though cracking was not observed interface slip occurred near the mid-span region even though cracking was not observed during this stage. After cracking occurred, non-linear behaviour was observed and as the during this stage. After cracking occurred, non-linear behaviour was observed and as the specimen was near to failure, both

specimen was near to failure, both δ δ part part and and δ δ expexp curves was in agreement showing larger curves was in agreement showing larger

deflection of more than 8 mm

deflection of more than 8 mm per loading increment.per loading increment.

Figure 18 Relationship between stiffness, the measured and calculated interface slip f

Figure 18 Relationship between stiffness, the measured and calculated interface slip f or specimenor specimen SR2

SR2

ε ε B’C’B’C’

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Figure 19 Deflection with partial interface interaction compared to the experimental results Figure 19 Deflection with partial interface interaction compared to the experimental results forfor

specimen SR2 specimen SR2

The relationship between the bending moment and the ratio,

The relationship between the bending moment and the ratio, η η , is shown in Fig. 20, where, is shown in Fig. 20, where η

η is the mid-span deflection ratio with full and partial interaction. If is the mid-span deflection ratio with full and partial interaction. If η η = 1.0, it is = 1.0, it is suggested that full composite action was achieved and as

suggested that full composite action was achieved and as η η decreases, an interface slip decreases, an interface slip near the mid-span region may had occurred. When this occurred, only partial composite near the mid-span region may had occurred. When this occurred, only partial composite action was achieved and if

action was achieved and if η η reaches zero, full separation at the interface may had reaches zero, full separation at the interface may had occurred. In this study, the figure shows a decrease from 1.0 to 0.08 and 0.03 for all occurred. In this study, the figure shows a decrease from 1.0 to 0.08 and 0.03 for all specimens as the bending moment reached its ultimate capacity. However, the worst case specimens as the bending moment reached its ultimate capacity. However, the worst case was observed for specimens with ponded for both the smooth and rough surfaces. was observed for specimens with ponded for both the smooth and rough surfaces. Therefore, the results show obvious influence of surface preparation to the overall Therefore, the results show obvious influence of surface preparation to the overall performance

performance of of the the composite composite slab. slab. The The decrease decrease in in ratioratio η η shows that shows that δ δ part part are higher are higher

than

than δ δ full full after after M M crack crack before following the before following the δ δ expexp curve. curve.

δ δ expexp δ δ partpart M M_{ur calc}_{ur calc} Self weight Self weight M