Interface Shear Stress of Hollow Core Slabs with Concrete Toppings

Interface Shear Stress of Hollow Core Slabs with Concrete Toppings Izni Syahrizal Ibrahim, B.Eng, M.Eng School of Civil Engineering, University of Nottingham, United Kingdom Kim S. Elliott, B.Tech, Ph.D, CEng, MICE School of Civil Engineering, University of Nottingham, United Kingdom Simon Copeland, HNC in Civil Engineering Tarmac Topfloor Limited, United Kingdom Abstract This paper presents the effect of surface condition on the interface shear stress on hollow core slabs with in-situ concrete topping. Horizontal shear stress at the interface between two different concrete strengths varies between the various codes of practice. Lack of information and inconsistency between these codes of topping construction causes problems to the overall structural behaviour when cast in a monolithic manner. Experimental push-off tests have been carried out to study the effects of surface moisture, surface laitance and roughness at the interface. These parameters are the major concern in the construction of toppings, which shows inconsistency with the various codes of practice. The tests show that there are no significant differences on the interface shear strength between the rough and smooth surface. However, surface laitance and surface moisture have significance effects on the interface shear strength. It was found that excess water at the surface reduces the shear strength at the interface. Keywords: Hollow core slabs; in-situ concrete toppings; composite; interface shear stress; push-off tests 1.0 Introduction Cast in-situ concrete toppings are added to precast reinforced or prestressed concrete floor slabs for the purpose of making a completed floor finish and to enhance the structural performance of the floor by producing a composite structure (Figure 1). The in-situ concrete topping is usually 40 100 mm in thickness and contains a small amount of steel reinforcement (usually a prefabricated welded mesh) as shown in Figure 2. Structural toppings Hollow core slabs Figure 1: Structural toppings on hollow core slabs 1

The joint between the units are usually infill with concrete C30 before the execution of topping to produce continuous slab. The topping would be laid in all weather conditions onto aged concrete of unspecified surface characteristics. The topping is usually a medium strength concrete, where as the base units are repeated precast units manufactured using semi-dry high strength concrete. Figure 3 shows the edge of the end construction of the composite slab. Concrete C30 infill Edge of concrete topping Edge of hollow core slab Figure 2: Prefabricated mesh arranged on hollow core slabs Figure 3: End construction of the composite slab Lack of information of topping construction causes problems to the overall structural behaviour when cast in a monolithic manner. This can be illustrated in Figure 4(a) when moisture content, shrinkage and surface characteristics of the precast units are neglected during topping construction. Some attempts to quantify surface texture are given in the Fédération Internationale de la Précontrainte (FIP) document on interface shear in composite floor structures (FIP, 1982). The topping must be continuously reinforced for the purpose of shrinkage, ideally at mid-depth, and as welded fabric is the preferred choice, there are problems where 3 or 4 sheets are lapped as shown in Figure 4(b). In the Guide to Good Practice (FIP, 1982), the types of surface which a precast unit may have, prior to receiving the in-situ concrete to form a composite section, are identified into ten categories. They were categorised based on the end production of the precast unit and difficult to distinguish between smooth and rough surface. In the Swedish Standard (Swedish Standard, 1981), a specified instrument had been established to measure the roughness at the interface. The surface roughness represented by R a is the arithmetical mean deviation of the profile, which is the average value of the departure of the profile above and below the mean line throughout the sampling length, L. To determine the centre arithmetical mean line of the profile, a straight line PQ is drawn through the lowest profile valley and parallel to the general course of the record. The area, A (Figure 5) between the profile and line PQ is determined and the height, H of the centre arithmetical mean line above PQ is given by: A H = (1) L The value of R a is calculated over the sum area above and below the centre arithmetical mean line by the sampling length. These measurements are based on the procedure given in BS 1134 (BSi, 1988). Although this code appears to be focused on materials in which the surface undulations are significantly smaller than those of roughened concrete, the procedures are relevant to any textured surface. Detail 2

roughness measurement adopted from the Swedish Standard (Swedish Standard, 1981) was investigated by Ros (Ros, Cabo et al., 1994). Three different surfaces were measured; smooth, longitudinal and transverse roughness. Load Surface roughness (see Figure 4(b)) Shrinkage Hollowcore Topping slab Mesh arrangement Strand relaxation Interface shear Tension Debonding Creep (a) Overall problems Mesh Topping Cover Roughness Ponding water Debris, dust Hollowcore slab (b) Surface roughness related problems Figure 4: Problems associated with topping construction Centre arithmetical mean line A H P Q L Centre arithmetical mean line R a P Q H Figure 5: Determination of centre arithmetical mean line 3

The study shows that for smooth surface, R a ranges between 0.40 0.49 mm, where as the longitudinal and transverse roughness ranges between 1.97 4.00 mm. Similar work had also been carried out by Gohnert (Gohnert, 2003) showing an R a ranging from 0.89 0.94 and 3.41 4.22 for smooth and rough surfaces, respectively. Within the FIP Commission itself there is a popular theory that smooth (clean) interfaces have better overall bond than roughened (often dusty and dirty) surfaces where localised bond failures occur. FIP (FIP, 1982) 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 interface will become full of dust. Other than roughness of the interface, surface treatment plays a major role with regard to the transfer of shear stress across the interfaces because: (a) (b) (c) Laitance skin (Figure 6), 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 less susceptible it is to the quality of workmanship in cleaning and preparation. If the surface of the precast member, before casting, is very dry, this member will absorb water from the in-situ concrete, so that the quality adjacent to the interface is governing for the capacity of the interface. If the surface is very wet, i.e. ponded, the water-cement ratio at the interface will be very high, resulting in weak bond strength in the immediate strata. Figure 6: Laitance skin formed on top of hollow core slab Repairs to damage caused to the topping, e.g. by relative shrinkage, can be as costly as the topping itself. Compared with traditional forms of reinforced concrete floor construction, laying a structural topping onto precast floor units could not be further removed yet similar technology and construction methods exist. 2.0 Related Works A composite member is designed to act monolithically. As the member bent in flexure, the precast member and in-situ concrete tend to slide relative to each other as illustrated in Figure 7. Horizontal shear transfer along the interface between precast unit and insitu concrete topping is an essential requirement to ensure composite action of the two members. When in-situ concrete is cast on a precast unit there is usually no mechanical key in 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. The texture of 4

the interface and the surface condition of the precast units are the important parameters for determining the horizontal shear strength of the composite member. ACI 318 (ACI, 1999) and FIP (FIP, 1982) specifies two categories of roughness, but no concrete compressive strength is specified in ACI 318 (ACI, 1999) for the given capacities (horizontal shear strength is assumed to be the same regardless of the concrete compressive strength). On the other hand, BS 8110 (BSi, 1997) and EC2 (BSi, 2003), express the shear capacity as a function of compressive strength. Another significant difference is categorising the degrees of roughness. BS 8110 (BSi, 1997) merely states the type of instrument used to create the roughness, where as EC2 (BSi, 2003) assigns measurable properties, i.e.3 mm for rough surfaces and 5 mm or greater for intended surfaces. Comparison of the codes cited above are illustrated in Figure 8, which indicates a lack of congruency. Tests carried out by Scott (Scott, 1973), Ros (Ros et al., 1994) and Ueda (Ueda & Stitmannaithum, 1991) on precast hollow core slabs with concrete toppings were to ensure monolithic behaviour up to the ultimate load. The bond between hollow core slabs and the topping is essential and therefore it has to be checked in design and ensured in construction. As shown in Figure 7, the distribution of horizontal shear stresses along the contact surface in a composite member will reflect the distribution of shear along the member. This horizontal shear can be modelled using a small-scale push-off test. The push-off test results by Gohnert (Gohnert, 2000; Gohnert, 2003) found that the scatter of data is in better correlation with surface roughness than with the compressive strength of the concrete. It was also found that for every millimetre change in roughness, the shear capacity changes by 0.2 N/mm 2. Although various relationships were found by the previous researcher, there is still some lack of information on the surface condition in the production process and upon casting and also the degree of roughness which may affect the overall performance of the composite slab. These parameters which include surface roughness, surface moisture and bond of the contact area using the push-off test method will be studied in detail and comply with the recommendation from the various international codes. P kn/m Concrete topping Hollow core slab Horizontal shear stress P kn Shear stress P kn Figure 7: Horizontal shear along the interface of a composite member bent in flexure 5

Shear stress in N/mm 2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 BS 8110-Extruded BS 8110-Brushed BS 8110-Washed FIP-Category (i)-(ii) FIP-Category (iii)-(vi) FIP-Category 2 EC2-Very smooth EC2-Smooth EC2-3mm rough EC2-5mm rough ACI-Roughened ACI-6.4mm rough 0.4 0.2 0 25 30 35 40 Concrete strength in N/mm 2 Figure 8: Comparison of horizontal shear stress with in-situ topping strength 3.0 Experimental Program The hollow core slabs were supplied with two different surfaces; smooth and rough. Smooth surface is as-cast surface, whereas rough surface was formed by means of a stiff wire brush in the transverse direction. However, in differentiating between the two surfaces, roughness was measured in the longitudinal direction and readings were taken at the centre where the surface of the concrete topping is to be cast. Roughness was measured using an instrument developed by Bensalem (Bensalem, 2001) as shown in Figure 9. Two linear potentiometers, LP1 and LP2 were attached to the frame to record vertical and horizontal movements. LP1 runs perpendicular to the concrete surface and detects peaks and valleys of the slab surface, while LP2 runs parallel to the surface and indicates the position of LP1 along the sampling length. 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 freely without displacing the instrument itself. Readings are taken at an increment of 1 mm. LP2 F3 F5 Dry surface F1 F6 LP1 300 mm F4 F2 300 mm 100 mm Figure 9: Surface roughness instrument Figure 10: Casting of concrete topping Three different types of surface conditions were considered before the execution of concrete topping i.e. dry, wet and ponded. Compared to wet, ponded means excess water were purposely left on the surface with a thickness of 2 mm (equivalent to a 20p 6

coin thickness). Six concrete toppings (F1 F6) of 300 x 300 x 100 mm deep were cast on each slab as shown in Figure 10. A layer of steel mesh (A142) with 6 mm high yield steel bars welded in a grid with spacing of 200 mm was provided for the toppings and the concrete cover to the main reinforcement is 25 mm. The reinforcement is used to control any sudden differential shrinkage and to distribute the load equally within the concrete. To simulate dust or debris effect at the interface, a thin polythene sheet (0.01 mm thick) was laid on the hollow core slab for the half and full debonding parameters. The concrete strengths are cube compressive strength measured on 100 100 100 mm cubes. The test setup is shown in Figure 11(a). To avoid any local failure to the toppings, a custom made U type shape load spreader was used to distribute the load (Figure 11(b)). The load spreaders were located at the end of the loading setup, which is perpendicular to the surface of the toppings. Dial gauge was positioned at the end of the topping to record any slip movement at the interface. Loading was applied at an increment of 5 kn until failure, which is considered when the topping separated from the base. Load spreader Concrete topping (a) Side view (b) Plan view Figure 11: Push-off test setup 4.0 Experimental Results and Discussions In all the specimens, sudden failure was observed when the topping separated from the hollow core slab. This sudden failure is illustrated typically in Figure 12. However, this behaviour was not observed for the full-debonding parameters since the failure load was zero. This is because there is no interaction between the hollow core slab and the concrete topping and the test was not continued for the other specimens. Shear stress is calculated by dividing the load against the contact area. The slip between the interface is relatively small for all specimens except at failure, where large slips were observed. The relationship between shear strength and concrete strength for smooth and rough surfaces are shown in Figure 13(a) and 13(b). The results were also compared with the allowable shear strength given in BS 8110 (BSi, 1997). In general, for half-bond interface, the shear strength is slightly lower compared to the full-bond interface. It was also found that rough surface has higher shear strength compared to smooth surface; however, this is not significant to the overall observation. As mention in the previous section, three types of surface conditions were considered in this study, i.e. dry, wet and ponded. Figure 14 illustrates the relationship between 7

these three parameters with the shear strength. The overall result shows low shear strength for the half-bonded surface, which can be seen throughout the dry and wet surface. However, for ponded surface, the shear strength is slightly lower with no influence to the full and half-bonded surface. The behaviour clearly shows that any excess water should be removed before the execution of concrete topping. Although roughness was categorised as smooth and rough, the test shows no effect to the shear strength for different parameters. In fact, most of the data lies below the BS 8110 (BSi, 1997) values. This was further proved by looking into the relationship between the shear strength and the measured roughness as illustrated in Figure 15. For smooth surface, roughness ranges between 0.10 0.60 mm, whereas for rough surface, it ranges between 0.10 1.00 mm. The scatter of data is so broad that any trend or correlation is hardly distinguishable. The work by Gohnert (Gohnert, 2003) shows similar pattern where roughness amplitude of the brushed surface does not exceed 1 mm, the results were poor and below the codes values. However, when roughness amplitude exceeded 3 mm, the results were significantly better. The fault lies within the code itself for not specifying a minimum roughness amplitude. Surface conditions before and after testing are shown in Figure 16(a) 16(c). For fullbond with dry, wet and ponded surface, it can be seen that most of the concrete from the topping still intact to the hollow core slabs. This can be furthered illustrated in Figure 17 showing the different surface profile before and after testing. The figure also shows an increase in roughness of 0.16 mm after testing. For the half-bond parameters only half of the surface is concrete free of which was covered with polythene sheet. This also proved a reduction of shear strength up to half from the full-bond parameters. Although the use of polythene sheet may not reflect the actual behaviour of surface laitance, debris etc., it does show the importance of surface cleaning before casting. 1.2 1.0 Shear stress in N/mm 2 0.8 0.6 0.4 0.2 F1 F3 F5 F2 F4 F6 0.0 0 1 2 3 4 5 6 7 Slip in mm 8 Figure 12: Typical relationship between shear stress and slip 8

1.2 1 Full bond Shear strength in N/mm 2 0.8 0.6 0.4 BS 8110 washed BS 8110 brushed BS 8110 as-cast Half bond 0.2 0 20 25 30 35 40 45 Concrete strength in N/mm 2 Figure 13(a): Relationship between shear strength and concrete strength for smooth surfaces 1.2 Full bond 1 Shear strength in N/mm 2 0.8 0.6 0.4 BS 8110 washed BS 8110 brushed BS 8110 as-cast Half bond 0.2 0 20 25 30 35 40 45 Concrete strength in N/mm 2 Figure 13(b): Relationship between shear strength and concrete strength for rough surfaces 9

1.2 Smooth Rough Smooth Rough Smooth Rough 1.0 Shear strength in N/mm 2 0.8 0.6 0.4 Full bond 0.2 Half bond 0.0 Dry Wet Ponded Figure 14: Relationship between shear strength with different surface preparation 1.2 Smooth Rough 1.0 Shear strength in N/mm 2 0.8 0.6 0.4 0.2 0.0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Roughness in mm Figure 15: Relationship between shear strength and roughness 10

(a) Before testing Bond area No bond (b) After testing Full-bond (c) After testing Half-bond Figure 16: Surface condition before and after testing 2.500 2.000 1.500 After testing Roughness in mm 1.000 Before testing 0.500 0.000 0 10 20 30 40 50 60 70 80 91 101 111 121 131 141 151 161 171 181 191-0.500-1.000-1.500-2.000-2.500 Length in mm Figure 17: Surface profile before and after testing R a before = 0.30 mm R a after = 0.46 mm 11

5.0 Conclusion Horizontal shear strength at the interface between hollow core slabs and concrete toppings were investigated in this paper. Parameters of surface roughness, surface moisture and contact surface were studied in details, which are a major concern in the construction of toppings. Close inspection of the results of small scale push-off tests implies that the surface should be properly wet before the execution of concrete topping. Dry and too much excess water will weaken the bond at the interface and thus reduce the shear strength. Ponded surface shows the critical states due to the weaken bond at the interface. There is also a reduction of shear strength due to the presence of surface laitance, debris etc. This was proved by debonding half of the contact area with a very thin polythene sheet. The scatter of data between shear strength and roughness indicates poor correlation and seems it does not differentiate between smooth and rough surface. Raking or brushing can produce vast range of roughness values depends on the amount of pressure applied by the operator and the viscosity or age of the mix. Therefore, it is necessary for the codes to give a minimum roughness amplitude as a guide to manufacturer. Acknowledgement The author would like to acknowledge the Precast Flooring Federation (PFF) and Tarmac Topfloor for its support and financial assistance during its research. References ACI. ACI 318: Building Code Requirements for Structural Concrete and Commentary. 1999. American Concrete Institute. Bensalem, K. (2001). The Structural Integrity of Precast Concrete Floor Systems used as Horizontal Diaphragms. Ph.D Thesis. University of Nottingham. BSi. BS 1134: Assessment of Surface Texture - Part 1: Methods and Instrument. 1988. British Standard Institute. BSi. BS 8110: Structural Use of Concrete - Part 1: Code of Practice for Design and Construction. 1997. British Standard Institute. BSi. DD ENV 1992-1-1: Design of Concrete Structures - Part 1: General Rules and Rules for Buildings (Draft). 2003. British Standard Institute. FIP. Guide to Good Practice - Shear at the Interface of Precast and In-situ Concrete. 1982. Fédération Internationale de la Précontrainte. Gohnert, M.. "Proposed Theory to Determine the Horizontal Shear Between Composite Precast and In-situ Concrete" in Cement and Concrete Composites. Vol: 22. No: 6. pp: 469-476. 2000. Elsevier Applied Science. Gohnert, M.. "Horizontal Shear Transfer across a Roughened Surface" in Cement and Concrete Composites. Vol: 25. No: 3. pp: 379-385. 2003. Elsevier Applied Science. Ros, P. S., Cabo, F. D. et al. "Experimental Research on Prestressed Hollow Core Slabs Floors with Insitu Concrete Topping" in FIP 12th International Congress Proceedings. pp: C33-C41. 1994. Fédération Internationale de la Précontrainte. Scott, N. L. "Performance of Precast Prestressed Hollow Core Slab with Composite Concrete Topping" in PCI Journal. Vol: 18. No: 2. pp: 64-77. 1973. Precast /Prestressed Concrete Institute. Swedish Standard. SIS 81 20 05: Concrete Surfaces - Determination of Surface Roughness. 1981. Swedish Standard Institute. 12

Ueda, T. and Stitmannaithum, B. "Shear Strength of Precast Prestressed Hollow Slabs with Concrete Topping" in ACI Structural Journal. Vol: 88. No: 4. pp: 402-410. 1991. American Concrete Society. 13