Influence of Roughness, Cohesion and Friction on the Interface Shear Strength of Composite Concrete-to- Concrete Bond

Transcription

1 APSEC-ICCER 212 Norhazilan Md. Noor et. al. (Eds.) 2 4 October 212 Surabaya, Indonesia Influence of Roughness, Cohesion and Friction on the Interface Shear Strength of Composite Concrete-to- Concrete Bond Mazizah Ezdiani Mohamed 1, Izni Syahrizal Ibrahim, A. Aziz Saim & Ahmad Baharuddin Abdul Rahman Faculty of Civil Engineering, Universiti Teknologi Malaysia 8131 Johor Bahru, Johor, Malaysia 1 ezy1188@hotmail.com Abstract Interface shear strength between precast concrete element and in-situ concrete topping is an essential requirement to ensure composite action of the two members. The surface roughness alone does not contribute to the effective bond between the two concretes. Other factors include friction, adhesion, shrinkage and creep. An experimental test was carried out to determine the interface shear strength of concrete-to-concrete bond using the push-off method. Each specimen was made of concrete blocks with two different concrete strengths i.e. 4 N/mm 2 for the base and 25 N/mm 2 for the topping. The top surface of the concrete base was treated with four different textures; smooth as-cast, roughened in the longitudinal and transverse direction, and deep groove. For each surface textures, concrete topping of mm deep were cast onto the concrete base of mm deep. Since the purpose of this test is to evaluate the relationship between interface shear strength and normal stress, failure envelope for n = N/mm 2,.5 N/mm 2 and 1. N/mm 2 were developed. The proposed friction coefficient and concrete cohesion were then determined and compared with the value in Eurocode 2. Further comparison was also made for the interface shear strength showing an acceptable range using the proposed friction coefficient and concrete cohesion. The results are significant to improve the existing interface shear strength equation available in the Code of Practice. Keywords-interface shear strength; surface roughness; friction; concrete cohesion; composite action I. INTRODUCTION Composite construction has been an economical way of integrating precast concrete slab together with concrete topping while performing its monolithic behaviour. To achieve full composite action, the shear strength at the interface depends on the concrete cohesion, friction and dowel action (if shear reinforcement crossing the interface is provided). In this research, contribution from concrete cohesion and friction to the interface shear strength was investigated experimentally. It is known that the type of surface texture or roughness at the interface influence the cohesion and friction to the bond strength between the concrete layers. The value of friction coefficient, µ given in the Code of Practice [BSI 1997 & Eurocode 2 24] for different type of interface textures is in the range of.5 to 1.. However, the value given is not conclusive since the roughness is assessed qualitatively by observing the surface and classifying them from very smooth to very rough. The main objective of this study is to determine the friction coefficient at the interface of composite concrete-to-concrete bond by carrying out experimental test using the push-off method. In order to determine the contribution of normal load to the interface shear strength, normal stresses of N/mm 2,.5 N/mm 2 and 1. N/mm 2 were applied in the test. The top surface of the concrete base was treated in four different types of textures i.e. (i) smooth as-cast, (ii) transversely roughened, (iii) longitudinal roughened, and (iv) deep groove. In the experimental work, 36 push-off tests were carried out to determine the shear transfer mechanism of composite concrete-to-concrete bond. The contribution of shear friction is the main discussion in this paper. II. LITERATURE REVIEW Several researchers (Gohnert, M. 23, Santos et. al. 27), revealed that for the design of composite slab, friction is one of the major parameters in estimating the shear resistance at the interface. Adequate interface shear strength will ensure that the full composite action does achieve either at the design stage or during the construction work. Static friction coefficient, s is the friction that keeps on increasing before sliding occurs and before the kinetic friction, k starts. This requires sufficient bonding strength between the two concretes to provide enough resistance since in composite construction we usually ignore the occurrence of kinetic friction and this could lead to the total failure of the composite action. Friction is an essential parameter to ensure adequate shear resistance whereby composite structures depend on the design of the connections between the two components to serve their 137

2 purposes. For composite concrete-to-concrete bond, the surface textures at the interface influence the friction coefficient apart from concrete cohesion and adhesion. Static friction coefficient, μ s keeps increasing as long as the bonding between the two concretes is still resisting by not allowing sliding to take place. The applied horizontal load keeps increasing until the bond between the two concretes is broken. Then, as horizontal load is further applied, it will be constant or even drop, because not much force is needed to cause sliding of the concrete topping. This shows that the bond strength between the two concretes is an important factor and is only resisted by friction and interface shear capacity. In composite concrete-to-concrete bond, friction coefficient is related to surface roughness because it consists of several asperities. In addition, the roughness theory assumed that the frictional force is equal to the force required to climb up the asperity of the slope, θ and the friction coefficient is given by μ = tan θ (Suh 1986). Frictional force between concrete base and concrete topping is accompanied by the horizontal movements induced by the variation of the temperature and moisture in the composite concrete elements. The frictional force acts in the opposite direction of the horizontal movement, thus developed stresses in the composite elements. (Wesevich, J. W. 1987) stated that the components of friction are made of adhesion and shear friction. shear capacity in the failure plane is provided by particle to particle friction and interlocking. In convention physics from the Leonardo da Vinci-Amontons Law, it is assumed that the linear relationship between the normal weight of the object to slide and the amount of frictional force to resist sliding is constant as shown in Fig. 1. Therefore, the value of longitudinal stress can be derived as: τ = μ.σ n (1) where τ is the longitudinal shear capacity, μ is the friction coefficient and σ n is the normal stress. Friction, Sliding movement Figure 1. Linear relationship between coefficient of friction and sliding displacement The roughness at the interface have been described in a qualitative manner and they are clearly stated and defined in the design codes of Eurocode 2, ACI 318 and BS 811 (Santos et al. 27). Eurocode 2 indicates that for shear at the interface between two concretes cast at different times, surfaces maybe classified as very smooth, smooth, rough, and indented. ACI 318 specifies two categories of roughness and BS 811 only considers the equipment that should be used to create the desired roughness. This type of roughness evaluation has obvious disadvantages due to the inconclusive and scattered results. According to Eurocode 2 (24), the shear capacity between two concretes cast at different times is a combination of three main components and can be determined from the following equation: Rdi = Rd1 + Rd2 + Rd3.5 f cd (2) where τ Rdi is the shear capacity of the composite structure, Rd1 is the capacity component resulting from element adhesion in the interface layer given as c f t where c is the adhesion coefficient and f ct is the concrete tensile strength for lower class strength concrete, Rd2 is the capacity component resulting from friction on the interface surface given as μ σ n where µ is friction coefficient and σ n is normal stress, and Rd3 is the capacity component resulting from the presence of shear reinforcement crossing the interface given as ρ f yd (µ sin α + cos α). However, in this study, this term is taken as zero () because shear reinforcement was not provided in the specimens. A. Test Setup III. EXPERIMENTAL STUDY The composite member is designed to act monolithically. In composite concrete-to-concrete bond, the horizontal shear between the two layers of different concrete strength is resisted by the shear capacity at the interface. To ensure the bond at the interface fails under constant normal and horizontal force, an experimental work was carried out using the push-off test method. This method had been used by several researchers to study the composite action between the two members and determining the interface shear strength (Choi et. al. 1999, Lam et. al. 1998, Gohnert 23, Bass et. al. 1989). A schematic diagram of the push-off test setup is shown in Fig. 2. The concrete base was fixed to the test frame and the concrete topping was pushed by load cell with capacity of 5 kn. A roller support is placed on top of the specimen to prevent the instabilities inherent during the test. Normal load was then applied at the top of the roller support using a load cell capacity of 2 kn. Linear variable displacement transducer (LVDT) was used to measure the horizontal displacement or interface slip throughout the test. The LVDT was positioned as close as possible to the interface as indicated in the diagram. Shear reinforcement crossing the interface was not provided in all the specimens and therefore the failure is dramatic and well defined. 138

3 Normal load Hydraulic jack Horizontal load Roller Load cell as-cast roughened LVDT Concrete topping Concrete base Horizontal load Horizontal load Figure 2. Schematic diagram of the push-off test setup roughened Deep groove Figure 3. Different types of interface textures B. Test Specimen A total number of 36 specimens were prepared, where each specimen consists of two concretes, known as concrete base and concrete topping. The specimens were 3 3 mm with thickness of 1 mm and 75 mm for the concrete base and concrete topping, respectively. In order to study the contribution of friction to the interface shear strength, four different textures were prepared at the top surface of the concrete base. This include smooth as-cast, longitudinal roughened, transverse roughened and deep groove. The concrete base was first cast, and after about one hour of casting, the top surface was prepared to the desired texture. For example, to get the transverse and longitudinal roughened texture the top surface was raked using a stiff brush. While for the deep groove, a 16 mm bar was pierced into the concrete base to create three longitudinal wavy lines. The prepared surface texture of the concrete base is shown in Fig. 3. For each type of surface texture, 12 specimens were prepared to be tested under variable normal stresses of n = N/mm 2,.5 N/mm 2 and 1. N/mm 2. The concrete topping was then cast onto the concrete base after achieving the 7 days cube compressive strength. Wet burlap was used for curing and continuously monitored until the test day. IV. RESULTS AND ANALYSIS A. Horizontal Load-Interface Slip Relationship Figs. 4, 5 and 6 shows the relationship between horizontal load and interface slip for n = N/mm 2,.5 N/mm 2 and 1. N/mm 2. The interface slip was measured and this was related to the bond between the two concretes. In general, the horizontal load increased steadily with little slip until the bond failure was reached. The bond failure load was defined as the load at which the cohesion bond at the interface was broken [Scott, J 21]. However, when normal stress was increased to.5 N/mm 2 and 1. N/mm 2, there was a delay to the peak horizontal load. This was expected indicating some degree of friction mobilization as it reaches its peak strength. After the bond failure, the load maintained as interface slip increases. During the early stages of test, there was a slight increase in interface slip as the horizontal load increases indicate the occurrence of kinetic friction but were very small and can be neglected. This is because the concrete base and concrete topping was in static mode and the load was not constantly applied. This constant load and motion must be taken into account when determining kinetic friction. Therefore, the friction coefficient in this study is considered as static friction. This will be further discussed in the following section. The horizontal load at peak and interface slip are summarised in Table I, II and III for applied normal stress of N/mm 2,.5 N/mm 2 and 1. N/mm 2, respectively. This research work is funded by the University Grant Scheme for Vote Number and MyBrain 15 scholarship from the Ministry of Higher Education Malaysia. 139

4 Horizontal Load, P (kn) Surface Type Specimen Horizontal Load at Peak (kn) Interface Slip at Peak Horizontal Load (mm) Pre-Crack Interface Shear Strength Horizontal Load, P (kn) Horizontal Load, P (kn) Figure 4. Horizontal load-interface slip relationship for n = N/mm 2 up to 6 mm Interface Slip (mm) Interface Slip (mm) Figure 5. Horizontal load-interface slip relationship for n =.5 N/mm 2 up to 6 mm S1 S2 S3 L1 L2 L3 T1 T2 T3 G1 G2 G3 S1 S2 S3 L1 L2 L3 T1 T2 T3 G1 G2 G3 and S3, respectively. This gives an average value of kn. In comparison for n =.5 N/mm 2, the horizontal load at peak was kn, kn and 9.6 kn for specimen S1, S2 and S3, respectively; giving an average value of kn. This shows an increase of only 1.13 kn, which clearly indicate minimal contribution of normal stress to the ultimate horizontal load. However, a huge increase to the horizontal load at peak was observed for n = 1. N/mm 2 with an average value of kn. This was an increase of 39.7 kn compared with the average horizontal load at peak for n = N/mm 2. For the other surface types, the same behaviour was also observed where there was an increase to the horizontal load at peak when the normal stress was increased from N/mm 2 to 1. N/mm 2. However, from n =.5 N/mm 2 to n = 1. N/mm 2, there was no specific pattern on the increase or decrease of the horizontal load at peak. One of the reasons is because there was no specific measurement made to determine the degree of roughness for the roughened and deep groove surfaces. Since the surface texture was made manually using a stiff brush and reinforcement bar for the roughened and deep groove surfaces, respectively, it was not possible to fix the degree of roughness. The interface slip at ultimate horizontal load for n = N/mm 2 were in the range of 1.57 mm to 5.8 mm for all types of surface as given in Table I. For n =.5 N/mm 2 given Table II, the interface slip was in the range of.84 mm to 4.93 mm. While for n = 1. N/mm 2, a less interface slip was observed between 1.1 mm and 3.74 mm (see Table III). This is because as normal stress increases the interface will increase its resistance to reduce the interface slip. However, in general the study shows that the smooth experiencing less interface slip compared with the other surfaces. TABLE I. SUMMARY OF TEST RESULTS FOR n = N/mm Interface Slip (mm) Figure 6. Horizontal load-interface slip relationship for n = 1. N/mm 2 up to 6 mm Table I, II and III show that as n increases from N/mm 2 to 1. N/mm 2, there was no specific pattern on the increase or decrease of the horizontal load at peak. For example, for n = N/mm 2, the horizontal load at failure for the smooth surface was 69.4 kn, kn and 16.5 kn for specimen S1, S2 S1 S2 S3 L1 L2 L3 T1 T2 T3 G1 G2 G3 S S S L L L T T T G G G Note: 1. Cube compressive strength at test day, f cu: Concrete base = N/mm 2 and Concrete topping = N/mm 2 2. Concrete splitting tensile strength at 28 days, f t = 1.82 N/mm 2 3. The concrete properties in Note (1) and (2) were taken as an average of three samples 14

5 Surface Type Specimen Horizontal Load at Peak (kn) Interface Slip at Peak Horizontal Load (mm) Pre-Crack Interface Shear Strength Surface Type Specimen Horizontal Load at Peak (kn) Interface Slip at Peak Horizontal Load (mm) Pre-Crack Interface Shear Strength, TABLE II. SUMMARY OF TEST RESULTS FOR n =.5 N/mm 2 S S S L L L T T T G G G Note: 1. Cube compressive strength at test day, f cu: Concrete base = N/mm 2 and Concrete topping = 2.9 N/mm 2 2. Concrete splitting tensile strength at 28 days, f t = 2. N/mm 2 3. The concrete properties in Note (1) and (2) were taken as an average of three samples TABLE III. SUMMARY OF TEST RESULTS FOR n = 1. N/mm 2 before the structure losses its composite behaviour. Figs. 4 6 show that the interface slip occurred during the early stages of applied load. The study by Cholewicki, A. and Szulc, J. (27) suggested a limited interface slip of 2 mm. The study by Scott, J (21) determined the interface shear strength by its postcracking load. This means that the strength was determined after the bond fails where a drop of shear load was observed, thus giving lower interface shear strength. In this study, the interface shear strength was determined by dividing the horizontal load at peak over the contact area. This peak load when bond failure occurred was referred as the precrack interface shear strength. The results are summarised in Tables I III. General observation found that the interface shear strength increases as the normal stress increases from N/mm 2 to 1. N/mm 2. in both the longitudinal and transverse and also deep groove surfaces for n =.5 N/mm 2 and n = 1. N/mm 2 produced the highest interface shear strength of more than 2. N/mm 2. As for n = N/mm 2, the interface shear strength for all surfaces (except for specimen L2) were below 2. N/mm 2. For the smooth surface, the interface shear strength was almost consistent even though the normal stress was increased. The reason why other surfaces show variable values may be due to the preparation of the interface where it was done manually without specifying a particular roughness. C. Determination of Friction Coefficient S S S L L L T T T G G G Note: 1. Cube compressive strength at test day, f cu: Concrete base = 39.61N/mm 2 and Concrete topping = 26. N/mm 2 2. Concrete splitting tensile strength at 28 days, f t = 2.32 N/mm 2 3. The concrete properties in Note (1) and (2) were taken as an average of three samples B. Interface Shear Strength Interface shear strength is calculated by determining the shear load before interface slip occurs. This is because once interface slip occurs, full composite action is lost and therefore interface shear strength does not exist. However, there is still yet no conclusive evidence on the allowable interface slip From the pre-cracked interface shear strength given in Tables I III, the friction coefficient of concrete-to-concrete bond was determined from Eq. (2). The tensile strength of the concrete topping, f t was determined from the splitting tensile strength test. The normal stress, n was determined from the normal load divided by the surface area. Therefore, the two unknowns from Eq. (2) which are concrete cohesion, c and friction coefficient, µ can be determined. According to Eurocode 2 (24), the interface shear strength can be determined from Eq. (3) below: τ = c f t + µ σ n + ρ f yd (µ sin α + cos α) (3) However, the term ρ f yd (µ sin α + cos α) is the presence of shear reinforcement crossing the interface, in which was not presented in this study. Therefore, Eq. (3) can be simplified as: τ = c f t + µ σ n (4) The relationship between interface shear strength and normal stress is shown in Fig. 7. Best fit line from the linear relationship for each surface type was used to determine the concrete cohesion, c. The term c.f t is the value crossing the y- axis, and dividing them over the splitting tensile strength, f t will give the concrete cohesion, c. This method is vastly used in geotechnical engineering to determine the cohesion factor for different type of soil (Budhu, M. 2, Joseph, P. G. 29, Terzaghi, K. 1942). An example of the term c.f t for smooth surface is shown in Fig. 7. Taking an average value of f t = 2.5 N/mm 2 (refer Note 2 in Table I, II and III), the concrete 141

6 Experimental (from Fig. 7) Eurocode 2 Experimental (from Fig. 7) Eurocode 2 Surface Type Normal Stress Average Interface Shear Strength from Experiment, exp Interface Shear Strength cohesion for the smooth surface is c smooth =.59. As for the other surface type, the concrete cohesion, c is given in Table IV. The friction coefficient was then determined from the slope of the best fit line in Fig. 7. The friction coefficient is given in Table IV. Figure 7. Relationship between interface shear strength and normal stress TABLE IV FRICTION COEFFICIENT FOR DIFFERENT TYPES OF SURFACE TEXTURE Surface Type c smooth.f t = 1.2 N/mm Normal Stress Concrete Cohesion, c Friction Coefficient, coefficient in Table IV was taken as the indented surface provided in Eurocode 2. V. DISCUSSION In this section, comparison on the interface shear strength was made using the proposed concrete cohesion, c and friction coefficient, in Table IV. The interface shear strength was then calculated using Eq. (4) and the results are summarised in Table V. The interface shear strength from experiment was taken as the average for every surface type. In general, the comparison was acceptable between the experimental and calculated values. For the smooth surface, the difference in percentage was 19%, 8% and 5% for normal stress of N/mm 2,.5 N/mm 2 and 1. N/mm 2, respectively. As for the roughened surface, only the transverse with n =.5 N/mm 2, was higher (3% difference) in comparison with the calculated values. For the deep groove surface, the comparison shows good relationship with the calculated value since the difference in percentage was less than 1%. TABLE V. COMPARISON BETWEEN THE EXPERIMENT AND CALCULATED INTERFACE SHEAR STRENGTH Calculated Interface Shear Strength, calc Deep Groove The highest concrete cohesion, c was found for the longitudinal roughened surface, where as the lowest was the smooth. As for the friction coefficient, the highest was the transverse roughened, followed by deep groove and longitudinal roughened surfaces. The lowest friction coefficient was the smooth surface. Comparison was also made from the value given in Eurocode 2 (24) for both the cohesion factor and friction coefficient. The concrete cohesion, c provided in Eurocode 2 was lower than the proposed value for all surface types. As for the friction coefficient, except for the smooth, other surfaces were higher than the value in Eurocode 2. The difference may be because in Eurocode 2, the interfaces were generally classified as very smooth, smooth, roughened and indented, where as in the current study the roughened was made in two different directions; i.e. longitudinal and transverse. Furthermore, there was no recommened values given for the deep groove, and therefore, the concrete cohesion and friction Therefore, the study recommended that for the roughened surface, the roughness degree should be measured and specified when determining the friction coefficient and concrete cohesion. This will reduce any uncertainties and produce a reliable coefficient. Further comparison was also made for each individual specimen and the results are represented in Fig. 8. A scatter of results was observed especially when n was increased to.5 N/mm 2 and 1. N/mm 2. As for n = N/mm 2, the comparison is acceptable since the results lies along the 1:1 line. 142

7 Interface Shear Strength from Experiment Figure 8. Comparison between the experimental and calculated interface shear strength for all specimens VI. CONCLUSION Experimental work was carried out to study the interface shear strength of concrete-to-concrete bond using the push-off test method. The study can be concluded as follows: (a) (b) (c) (d) (e) (f) Calculated Interface Shear Strength Bilinear behaviour was observed for horizontal loadinterface slip relationships all surface types. For the roughened and deep groove surfaces, there was no specific pattern on the horizontal load at peak (whether increase or decrease) with the increased of normal stress from N/mm 2 to 1. N/mm 2. The concrete cohesion, c for all surface types was higher than the value in Eurocode 2. The friction coefficient, for the smooth surface was lower than the value in Eurocode 2. For the roughened (both transverse and longitudinal) and deep grooves surfaces, the friction coefficients, were higher than the value in Eurocode 2. The comparison of interface shear strength between the experiment and calculated values were in acceptable range. However, the study suggested that the degree of roughness should be measured and specified for the roughened and deep groove surfaces. Improvement to the existing Eq. (4) should include the degree of roughness, apart from the concrete cohesion and friction coefficient..5 n =.5 N/mm 2 1 n = N/mm 2 n = 1. N/mm 2 technicians in the Structural and Material Laboratory, Faculty of Civil Engineering, Universiti Teknologi Malaysia, and the MyBrain 15 scholarship from Ministry of Higher Education Malaysia. REFERENCES British Standard Institute. (1997). BS 811. Structural Use of Concrete Part 1: Code of Practice for Design and Construction. London. Bass R.A. et. al. (1989). Shear Transfer across New and Existing Concrete Interfaces. ACI Structural Journal. 86 (4): Budhu, M. (2). Soil Mechanics and Foundations. New York: John Wiley & Sons. Choi D-U, Jirsa J. O, Fowler D. W. (1999). Shear Transfer across Interface between New and Existing Concretes using Power-driven Nails. ACI Structural Journal. 96 (2): Cholewicki, A and Szulc, J. (27). Interaction in Precast Composite Beams. Concrete Plant + Precast Technology, 5: European Committee for Standardization. (24). Eurocode 2: BS EN : 24. Design of Concrete Structures Part 1-1: General Rules and Rules for Buildings. London. Gohnert, M. (23). Horizontal shear transfer across a roughened surface. Cement and Concrete Composites, 25(3), Joseph, P. G. (29), Constitutive Model of Soil Based on a Dynamical Systems Approach, Journal of Geotechnical and Geoenvironmental Engineering. 135 (8): Lam D, Elliot K. S, Nethercot D. A. (1998). Push-off Tests on Shear Studs with Hollowcore Floor Slabs. Structural Engineering. 76 (9): Santos, P. M. D., Júlio, E. N. B. S., & Silva, V. D. (27). Correlation between concrete-to-concrete bond strength and the roughness of the substrate surface. Construction and Building Materials. 21 (8): Scott, J. (21). Interface Shear Strength in Lightweight Concrete Bridge Girders. Master Thesis. Virginia Polytechnic Institute and State University. USA. Suh, N. P. (1986). Tribophysics. New Jersey: Prentice Hall Inc.: Englewood Cliffs. Terzaghi, K. (1942). Theoretical Soil Mechanics. New York: John Wiley & Sons. Wesevich, J. W. (1987). Stabilized Sub-base Friction Study for Concrete Pavements. Master Thesis. University of Texas at Austin. USA. ACKNOWLEDGMENT This research work is funded by the University Grant Scheme for Vote Number Invaluable appreciation goes to 143