THE CENTER FOR CONCRETE COREA AND

ENHANCING THE PERFORMANCE OF SLAB-COLUMN CONNECTIONS

Young-Soo YOON1, Joo-Ha LEE2, Jun-Mo YANG3 and Seong-Woon KIM4

SUMMARY

The Paper presents the brief introduction of the Center for Concrete Corea as a concrete research initiative in Korea along with the more detailed description on the performance of slab-column connection carried out as one of the research topic under the direction of the Center. The Center for Concrete Corea is performing ‘Development and application technology for high performance - multifunctional concrete’ research project, supported by Korea Institute of Construction and Transportation Technology Evaluation and Planning (KICTTEP) under the Ministry of Construction and Transportation (MOCT), from 2006 to 2010. Various structural tests were carried out to investigate the performance of slab-column connections comprising ultra-high-strength concrete (UHSC) columns. The benefits of using “puddled” fibre-reinforced ultra-high-strength concrete (UHSC) for the transmission of loads from UHSC columns through normal strength concrete slabs is demonstrated. Compression tests were performed on two slab-column specimens and four isolated column specimens (without the surrounding slab). The influence of the differences in the physical and mechanical properties between fiber-reinforced polymer (FRP) and conventional steel, concentrated reinforcement in the immediate column region, as well as puddled fibre-reinforced UHSC, on the punching shear behavior of two-way slabs were investigated. Keywords: Ultra-high-strength concrete; Column load; Steel fibers; Fiber reinforced polymers; Concrete slabs; Punching Shear

1 Professor, Department of Civil, Environmental & Architectural Engineering, Korea University, KOREA, e-mail: ysyoon@korea.ac.kr 2 Post-doctoral Researcher, Dept. of Civil Engineering and Applied Mechanics, McGill University, CANADA, e-mail: leejooha@korea.ac.kr 3 PhD Candidate, Department of Civil, Environmental & Architectural Engineering, Korea University, KOREA, e-mail: jmyang@korea.ac.kr 4 Director of Concrete Corea and Managing Director in the Daewoo Institute of Construction Technology, KOREA, e-mail: swk@dwconst.co.kr

INTRODUCTION The objective of the Center for Concrete Corea is to improve the structure’s performance and to decrease the cost of construction and maintenance, through development and practical use of the core techniques of concrete, high flowability, high durability, ultra high strength and multi-functional concrete. Due to recent trends in the construction of taller structures, there is an increased need for using ultra-high-strength concrete (UHSC) in columns. In typical construction, the slab is made of normal-strength concrete and is cast in a continuous fashion through the slab-column joint. For the case of UHSC columns, this results in a layer of lower strength concrete between the upper and lower columns at the floor levels. This lower strength concrete layer can limit the capacity of the column, particularly for UHSC columns and normal-strength concrete slabs. The placement of UHSC in the slab at the location of the joint and in the region around the column (“puddled” concrete) improves the transmission of column loads through the slab, but also has potential to increase the punching shear resistance of the slab, reduce crack widths and reduce slab deflections. The response of slab-column connections may be further improved if the puddled UHSC contains steel fibers. Furthermore, the concentration of the top flexural reinforcement in the slab around the column increases the reinforcement ratio in this critical region and has the potential of improving the performance of the connection. This paper investigates these effects. In addition, fiber-reinforced polymer (FRP) bars were used in the punching shear test specimens. Although there has been a rapid increase in the use of FRP materials as non-corrodible reinforcements for concrete structures, the use of FRP reinforcement is still a formidable challenge for engineers due to the substantial differences in the physical and mechanical properties between FRP and conventional steel. Especially, the punching shear of two-way slabs reinforced with FRP bars has not been fully explored. In this study, therefore, the influence of the differences between FRP and conventional steel on the punching behavior of two-way slabs is also investigated.

ONGOING RESEARCH OF THE CENTER FOR CONCRETE COREA The research in progress of the Center for Concrete Corea is categorized into 3 themes, ‘Development of core and application technology for high performance - multifunctional concrete,’ ‘Development & application of structural members by using ultra high performance concrete,’ and ‘Design and field trial application for high performance - multifunctional concrete.’ Each theme is in the management of Daewoo Construction, Korea Institute of Construction Technology, and Korea Expressway Corporation, and 60 firms and 10 universities are also joining. The details of each theme and sub-theme are summarized in Table 1. Especially in 2010 when the research will have been completed, it is anticipated to have techniques equivalent or superior to those in the advanced countries and utilizing it.

Furthermore, it is expected to have a market of concrete related field at the size of 2,000 billion won as well as decrease of construction and maintenance cost by making structures highly performed, durable and multi functional. 1st Theme : Development of Core and Application Technology for High performance & Multi-functional Concrete The first theme is divided into four research group, guide and manual, high durability, high flowability, and multifunction Research Group for providing Guide and Manual The research group for providing Guide and Manual has two sub-themes. The first sub-theme is ‘Development of guide and manuals for the design and construction of high performance concrete materials,’ performed by Korea Concrete Institute. This sub-theme is to develop the code with performance based design concept in design and construction for concrete mixture and to report technical road map 2030 for better control of research progress. The second sub-theme is ‘Development of structural design code for high performance concrete structures,’ performed by Hanyang University. The objective of this sub-theme is to develop a constitutive model of HPC in terms of material level, and to extend the model to structural level in order to obtain performance evaluation model for HPC structures. These results will be supplied to foundation of performance based design method. Research Group for High Durability The research group for high durability has three sub-themes. The first sub-theme is ‘Development of evaluation system for deterioration of concrete structures subjected to severe environment,’ performed by Yonsei University. The objective of this sub-theme is to develop an assessment system to predict the deterioration of reinforced concrete structures in order to assist in evaluating the durability of the structures prior to design and execution. The fundamental concept is based on performance based design criteria, specification and guidance. Based on the developed system, the service life of those structures will also be satisfied. The second sub-theme is ‘Development of corrosion control system using the improvement rebar and corrosion inhibitor,’ performed by Hanyang University. Corrosion control system of reinforced concrete was used only for changing a property of matter such as gaining high strength, density, covering thickness. So it was considered as a passive measure. But this sub-theme is to improve corrosion protection performance of RC itself and develop active corrosion control system using Cr-bearing rebar, Zn-Al thermal spray cathodic protection system to apply concrete surface and development of corrosion new inhibitor. The third sub-theme is ‘Development and its practical application of high-durability concrete,’ performed by Byucksan Engineering. To have the technology producing high-durable concrete for more than 100-years under the severe environmental condition is the objective of this sub-theme. And final goal of the research is to put the developed technology to practical use. To this end, theoretical and experimental studies are in progress to secure the mix proportioning and curing skills to ensure concrete the high resistance to chloride attack, carbonation and chemical attack. At present, this research group gives their attention on the

clarification of the deterioration mechanism through the experiments under different environmental conditions. At the end of this study, realization of the high-durability concrete to the severe environment is expected. Research Group for High Flowability The research group for high flowability has three sub-themes. The first sub-theme is ‘Development and application of pre-cast bridge member using high flowing self - compacting concrete,’ performed by Semyung University. Recently, buildings are becoming larger, taller and are being built to meet specific purpose and high-performance concrete superior to existing concrete is being used experimentally in construction. In the case of bridges, due to noise, dust created in construction and prolonged construction period, support construction method has reduced in number. Recently, pre-cast construction is rapidly becoming more popular because of its shorter construction period and suitability to urban context. In bridges, in order to ensure safety in flexural behavior, high-density reinforced members are being produced and applied. So, this sub-theme is to develop high flowing self - compacting concrete (HSCC) for produce high-density reinforced members. The second sub-theme is ‘Development of structural systems using High flowability & self-compacting concrete filled steel tube,’ performed by Kyungpook National University. The objective of this sub-theme is to propose a practical application and development of structural system using the high flowability & self-compacting concrete filled steel tube for shortening of construction time, enhanced quality of finished surface, good quality of a new concrete and quality management of concrete constructions. The third sub-theme is ‘Improvement of concreting property for ultra high flowability concrete and field application,’ performed by Daewoo Construction. Development trend of domestic ultra flowing concrete and actual field application will be analyzed by field investigation. Then, backgrounds, problems, field application results, improvement directions and prospects afterward of ultra high flowability concrete will be reviewed. In addition, the material properties such as variation of fresh concrete properties, pumpability, plasticity and concreting characteristics of high fluidity fresh concrete ranged of 60MPa will be tested by the field application. Research Group for Multi-function The research group for multi-function has five sub-themes. The first sub-theme is ‘Development and field application of fire resistance high performance concrete for securing of human life,’ performed by Cheongju University. The objective of this sub-theme is to develop the functional concrete, in order to perform fire resistance, securing building structures using high strength concrete in fire. Thus, the ultimate object of this sub-theme is first protect the concrete, which has the compressive strength range of 40MPa to 100MPa with currently increase of demands, from fire for three hours. With development of various methods, the developed fire resistance concrete can have spalling protection and secure more than 70% residual compressive strength, and then will be applied in fields with management guide for design and construction. This research will eventually ensure the protection of human life, individual and national property in fire. The second sub-theme is ‘Development and application of green concrete,’ performed by Hanseo University. This sub-theme is composed of two parts; light-transmitting concrete and

porous concrete. The light-transmitting concrete which has constituents of cement, water, fine aggregate (stone powder) and optic fibers is to use the principle of reflection and transmitting of light through optic fibers embedded in concrete. Main key of this part aims at developing the concrete economically and technically useful. Porous concrete known as having a high permeability can be applied to concrete pavement blocks. The difficulty of this development is to find an appropriate and satisfactory point associated with strength of concrete and permeability which have quite different characteristics each other. The third sub-theme is ‘Development and practical use of high performance lightweight aggregate concrete,’ performed by Hanseo University. Today, It is necessary to use light weight aggregate concrete which overcomes problems and is taken advantage of. This sub-theme, therefore, suggests the standard of materials composition and mixing design and construction guide to acquire construction technique about primary and secondary products of light weight concrete having high performance like high light-weight, high flowable and high durable. The fourth sub-theme is ‘Development of antimicrobial concrete,’ performed by Chung Cheong University. This sub-theme is to develop concretes which can be variously utilized from a comfortable house to social base facilities. The concretes are an antimicrobial concrete which has an antibiosis such as insecticide, deodorant and etc., a resistance to sulfuric acid concrete, a moisture control concrete for humidity controlling, an absorption concrete for nitrogen oxide that is an atmospheric pollution source generated from transportation means such as automobile, air, ship, and etc., an industrial boiler, an incinerator, an electric furnace, and so forth. The fifth sub-theme is ‘The application and development of the decorative concrete,’ performed by Sun Engineering. This sub-theme is to face the creation of the space, agreement with the nature, and various requirements and develop the decorative concrete. Moreover, this sub-theme is to use and commercialize the decorative concrete. 2nd Theme : Development & Application of Structural Members having Ultra-High Strength Concrete The second theme has three sub-themes. The first sub-theme is ‘Development and application of structural members comprised by ultra high strength concrete,’ performed by Korea Institute of Construction Technology. This sub-theme is to develop ultra high strength fiber-reinforcement concrete from 150 MPa to 200 MPa. And this sub-theme is to develop and apply practically structural members that reduced cross section about 30% and extended life twice or more. The second sub-theme is ‘Development of structural high strength concrete members using hybrid reinforcing techniques,’ performed by Korea University. This sub-theme is to optimize the hybrid reinforcing method using headed bar, high strength bar, FRP and fiber reinforcement, and to apply it to general member, stress disturbed member, slab-column joint, and beam-column joint. Furthermore, this sub-theme is to develop new material and new construction method for hybrid reinforcement. The third sub-theme is ‘Practical technique development of monitoring system on high performance concrete properties from very early age to long-term service period,’ performed by Kumoh National Institute of Technology. This sub-theme is to find a practical sensor which

can monitor deformation and temperature of HPC members stably, continuously and reliably. Then based on it, a practical monitoring system is planed to be developed, which can monitor high performance concrete structures especially high-rise building and large-scale bridge from the very early age after casting to long term period. The monitoring system to be developed can provide data for deformation and temperature continuously, the hidden damage inside the HPC member can be found timely. Then maintenance measures can be taken in advance, hence plenty of costs on serious damage maintenance will be saved. 3rd Theme : Practical Utilization and Application for High Performance & Multi-functional Concrete The third theme has three sub-themes. The first sub-theme is ‘Field application to bridge structures of high performance concrete and establishment of process improving measures of concrete placing in situ.,’ performed by Korea Expressway Corporation. If the high performance concrete is applied to the bridge structures, it will be possible to extend the service life of the structures, reduce the sectional area, lighten and slim structural member. Thus, this sub-theme is to do development and field application of high performance concrete for Bridge deck and girder, development and field application of high performance marine concrete, and establishment of improving measures of production, transportation and construction process of high performance concrete. The second sub-theme is ‘The integrated information system development for field concrete process improvement of the high performance concrete,’ performed by Jaram-Tech. This sub-theme is to do construction of quality characteristic data base for the high performance concrete, and information system development of production, transportation and construction work process for the high performance concrete. The third sub-theme is ‘Application of high strength concrete to high-rise residential building,’ performed by Korea National Housing Corporation. HSC is one of the major structural materials built in high-rise buildings. And the development of HSC and the analysis and design technique of high-rise buildings is essential for the improvement of our national construction competitive power. Therefore, this sub-theme is to develop 80MPa HSC, to test and evaluate of the material properties, safety of structural member, workability and placement characteristics of HSC to propose the Guideline of design(concrete mix design, structural design etc.), manufacture and quality control in the fields of HSC.

TRANSMISSION OF COLUMN LOADS THROUGH SLAB-COLUMN CONNECTIONS

The benefits of using “puddled” fibre-reinforced UHSC for the transmission of loads from ultra-high-strength columns through slabs is demonstrated. Compression tests were performed on two slab-column specimens and four isolated column specimens (without the surrounding slab). For the tests on the slab-column specimens, vertical slab loading was applied to simulate realistic conditions at the slab-column joint. Predictions using current North American code approaches are compared to the experimental results.

Experimental Program Two slab-column and four isolated column specimens were tested to investigate the transmission of axial loads. The specimens were divided into two series: a normal strength concrete slab series (N Series) and a series with puddled fiber-reinforced UHSC in the slab at the column location (F Series), as shown in Fig. 1. Specimen NT was constructed with UHSC (90 MPa) stub columns that extended above and below the 150 mm thick normal-strength (45 MPa) concrete slab. Isolated column Specimen NC is a UHSC column with a 150 mm thick layer of 45 MPa slab concrete at its mid-height. Control Specimen C1 is a normal strength concrete column, constructed with 45 MPa concrete. The F series contained puddled fiber-reinforced UHSC in the slab. One of the main advantages of using fiber-reinforced UHSC for the puddled concrete is the higher plastic stiffness of this concrete and hence, the concrete is much more capable of maintaining its shape while the normal-strength concrete is being placed around it, thus eliminating the need for containing the puddled concrete and simplifying the construction procedure (McHarg et al. 2000). Slab-Column Specimen FT contained fiber-reinforced UHSC puddled over the entire depth of the 150 mm thick slab in the immediate vicinity of the column (within a distance twice the effective depth, 2d, of the slab from the column faces). Isolated column Specimen FC is a UHSC column with a 150 mm thick layer of fiber-reinforced UHSC at its mid-height. Control column Specimen C2 is a UHSC column, constructed with 90 MPa concrete. Test Setup, Loading and Material Properties Figures 2 and 3 give the details of the slab-column specimens, NT and FT, as well as the reinforcing details and the instrumentation. After the application of an initial axial load of 400 kN in the column of Specimen NT, the total slab loading was increased to 132 kN to produce strains in the uniformly distributed top slab reinforcement at the column face of 2,000 με. This same slab load was applied to Specimen FT. The slab loads were kept constant and the axial load in the column was increased to failure. The properties of the concrete are summarized in Table 2. At an age of 180 days, the average compressive strength of the HSC and normal-strength concrete was 95 MPa and 52 MPa, respectively. Table 3 gives the material properties of the reinforcing steel. A volume of steel fibers of 0.5% was chosen for this test series. The 30 mm long hooked steel fibers used in the specimens had a diameter of 0.5 mm and an ultimate tensile strength of 1200 MPa. Figure 4 gives details for the geometry, reinforcement and instrumentation of the isolated column specimens, NC, FC, C1 and C2. Test Results Both the ACI Code (2005) and the CSA Standard (2004) define the cross-sectional capacity, P0, of a column under concentric load as:

stystgc AfAAfP +−′= )(10 α (1) where, gA : Gross area of the column

stA : Area of longitudinal reinforcement

cf ′ : Concrete compressive strength

yf : Yield strength of the steel

1α : Constant equal to 0.85 in the ACI Code (2005) and a variable dependent on the concrete strength in the CSA Standard (2004)

The effective concrete compressive strength, cef ′ , for a maximum measured axial load, testP , can be determined from:

)(85.0 stg

stytestce AA

AfPf

−

−=′ (4)

The ratio of the effective column strength to the specified compressive strength of the column concrete, ccf ′ , was calculated to be 1.07 and 1.04 for the control column Specimens C1 and C2, respectively. Therefore, Eq. (4) is a good indicator of the effective strength for these tests. In all specimens, the strains measured in the columns indicated that the vertical bars yielded in compression. The test results including values of cef ′ for all column specimens are given in Table 4. All the column specimens reached peak compressive loads greater than the concrete compressive strength governing the capacity of specimens. In addition, the effective compressive concrete strength of 91.1 MPa of Specimen NT was higher than the predicted compressive strengths of 82.6 and 71.3 MPa from the ACI Code (2005) and the CSA Standard (2004), respectively. The experimentally determined effective concrete strength varied from 1.04 to 1.34 times the design strength of the codes. Figure 5 illustrates the cracking patterns on the top surface of the slabs. Specimen FT had fewer cracks and smaller crack widths than Specimen NT, due to the puddling of fiber-reinforced UHSC in the region around the column. Transverse strains were obtained from gages placed on the top slab reinforcement (Fig. 3). For both specimens, immediately after the slab loads have been applied, the strain at the face of the joint is higher than near the center of the joint. As loading is increased, however, the strain at the centerline approaches the strain measured at the column face. For Specimen FT, the transverse strains at the slab-column joint did not yield during testing, while Specimen NT exhibited strains of about twice the yield strain. Figure 6 shows the influence of several key parameters on the axial load versus strain responses of the specimens. The isolated column specimen (NC), with a layer of normal-strength concrete representing the slab, showed higher strength than the normal-strength concrete control column specimen (C1) due to the restraint by shear stresses at the interfaces between the UHSC column stub and the normal-strength concrete layer (Fig. 6(a)). The layer of slab concrete reduced the strength of the isolated column specimen (NC) below the strength of the control column (C2) constructed entirely with UHSC (Fig. 6(b)). The strength of Specimen NC was approximately 64% of the strength of the control column (C2). Figure 6(c) compares the axial load versus strain responses of Specimens C2 and FC. Specimen FC has virtually the same strength (within 4%) as Specimen C2. Figure 6(d) illustrates the influence of puddled fiber-reinforced UHSC on the axial load versus average strain responses at the joint for the slab-column specimens. It is evident that the puddled fiber-reinforced UHSC increased both the strength and the axial stiffness of the slab-column specimens. Figures 6(e) and (f) show the effect of the confinement provided by the surrounding slab on the responses. Specimen NT, with the additional confinement provided by the slab surrounding the normal-strength slab layer, resulted in a strength increase of about 45% over the companion isolated column Specimen NC. Furthermore, Specimen NT had an effective strength that was about double the strength of the weaker concrete of the slab, as shown in

Table 4. Specimen FT with puddled fiber-reinforced UHSC had little effect from the slab confinement when compared with companion isolated column Specimen FC because the capacity was governed by the column strength.

PUNCHING SHEAR BEHAVIOR OF SLAB-COLUMN CONNECTION Punching shear behavior of two-way slabs reinforced with steel rebar or FRP bars was investigated. The influence of the differences in the physical and mechanical properties between FRP and conventional steel, concentrated reinforcement in the immediate column region, as well as using puddled fiber-reinforced UHSC in the slab in a region close to the column were investigated. The performance of the slab-column specimens tested was investigated, including the punching shear capacity, cracking on the top of the slabs around the columns, and the stiffness of the slab-column connections. Experimental Program and Test Setup Fig. 7 shows the test setup of two-way slab specimens. The slab was loaded with eight equal concentrated loads around the perimeter to simulate a uniformly distributed load on the test specimen. Four identical hydraulic jacks were connected to a single hydraulic pump to ensure eight equal loads were applied to the slab. The deflection of each loading point was measured with a linear voltage differential transformer (LVDT). Electrical resistance strain gages were glued to the reinforcing bars in the top mat in line with the column faces in the two principal directions of the slab. Fig. 8 shows the layout of the top and bottom slab reinforcements. The main variables were the slab reinforcement material (steel or GFRP), the concentration of slab reinforcement around the column, and the steel fibers in the slab concrete. The letters U and B in the specimen names indicate the concentration of slab reinforcement around the column (U for uniform spacing of top reinforcement and B for banded reinforcement). Also, the number in the designation of specimens indicates the slab reinforcement ratio in the immediate column region (within a distance 1.5h from the column faces). For example, GFB3 indicate the GFRP-reinforced slab with a banded distribution of approximately 3% reinforcement ratio. Specimen GFBF3 had the same reinforcing detail as Specimen GFB3, but SFRC was placed to 2d (twice the effective depth of the slab) into the slab from the column faces. For all specimens, the average effective depth was 110 mm. Specimens SU1 and SB2 were previously tested by Ghannoum (1998). Specimens SU1 and SB2 had the same reinforcement layouts as GFU1 and GFB2, respectively. Therefore, Specimens SU1 and SB2 provided benchmark results for comparison with the responses of slabs reinforced with GFRP bars. Table 5 summarizes the material properties of the concrete used in the slab-column specimens. In this study, a 0.5% volume of steel fibers was chosen for the SFRC of the slab within the immediate column region of Specimen GFBF3. Table 6 gives the mechanical properties of the steel and GFRP bars used in the construction of the test specimens. For the GFRP bars, the manufacturer’s guaranteed design properties were used in this study.

Test Results Influence of Reinforcement Material Fig. 9 shows the total shear load versus average slab deflection for all slabs. Table 7 summarizes the measured total shear load and average deflections at first cracking, first yielding, and at the peak loads. Since the GFRP bars exhibit no yield characteristic, the load and deflection at a GFRP bar’s strain of 0.0045, which is based on the allowable in service stress of GFRP, were adopted in this study to correspond to the yield (CAN/CSA S806-2002; El-Ghandour et al. 2003). As shown in Fig. 9, all the specimens exhibited abrupt punching shear mode failure. At peak load, the column suddenly penetrated through the slab, with an immediate and significant drop in load. Although all slabs were designed to have similar flexural strength, those reinforced with GFRP bars had significantly lower punching strengths compared with slabs reinforced with steel bars. This was due to the lower elastic modulus of GFRP bars, which leads to a smaller area of concrete in compression. In addition, the GF series demonstrated larger deflections due to the lower modulus of elasticity of GFRP, resulting in a reduced effective moment of inertia of the slabs. As shown in Table 7 and Fig. 9, the stiffness of the specimens was a function of the elastic modulus of the reinforcing bars and reinforcement ratio in the immediate column region. All slabs behaved similarly in the uncracked state, but the stiffness of the GF series greatly decreased after the first cracks occurred, compared with the S series. Fig. 10 shows the crack patterns at peak load for the GF series. In all slabs reinforced with GFRP bars, the first cracks occurred along the line perpendicular to the weak direction of reinforcement passing through the slab-column joint, shortly followed by the forming of similar cracks along the lower layer top bars. Radial cracks then occurred in the column region and propagated towards the edges of the slab. At the same time, many circumferential cracks, connecting the radial cracks, formed especially in the immediate column region. While Specimen SU1 had similar cracking patterns to the GF series, for Specimen SB2, the first cracking occurred at the edges, where the reinforcement ratio and the flexural stresses were lower, and then the cracks propagated towards the column corners. Contrary to the crack pattern of SB2, the cracks in the slabs with banded distribution of GFRP bars propagated from near the columns to the edges, as mentioned above. This phenomenon could be explained by noting that the equivalent reinforcement ratio, ρ Ef /Es, of all specimens with GFRP was considerably low, resulting in decreased stiffness. According to the strains measured at the reinforcing bars, it was seen that no anchorage failure occurred. In addition, the surface condition of the GFRP bars embedded in the specimens was investigated after testing; as shown in Fig. 11, where the concrete did not split from the surface of the GFRP bars, even after failure. This implies that GFRP sand-coated reinforcing bars have good bond characteristics, preventing anchorage failure. Influence of Concentration of Flexural Reinforcement in the Column Vicinity Comparing slabs reinforced with the uniform and banded distributions of flexural reinforcement, the specimens with the banded distribution had higher punching shear capacities and stiffness. In the specimens with banded reinforcement, the area of reinforcement was better distributed to resist the slab moments, resulting in lower, more uniform strains on the slab reinforcement. However, it was noted that GFB3 and GFB2 showed similar ultimate loads.

The variations in the maximum crack widths in the immediate column region, as a function of the slab deflection for all slabs, are shown in Fig. 12. Regardless of the reinforcement material, the banded distribution of reinforcement resulted in a larger number of cracks in the immediate column region with smaller crack widths, leading to a much better crack distribution, as shown in Figs. 10 and 12. All slabs failed in the classical punching mode. The specimens failed along a sloping surface extending from the compression surface of the slab at the face of the column to the tension surface some distance from the column face. The shear failure plane was affected by the concentration of flexural reinforcement in the column vicinity. It was apparent that for specimens with banded reinforcement, the punching shear failure plane surfaced at a greater distance from the column faces. Influence of Steel Fiber-Reinforced Concrete The addition of steel fibers in concrete was more effectively pronounced on the punching shear strength than the concentration of slab reinforcement. Slab GFBF3 had considerably a higher punching strength than GFB3, and even higher than SB2. This implies that SFRC could be the solution to the problems of substituting FRP reinforcement for conventional steel. Specimen GFBF3 displayed excellent performance with regard to crack control in the column region. The presence of steel fibers in concrete led to smaller crack widths and better crack distribution, indicating that the steel fibers in the concrete matrix bridge the cracks and limit their growth with increases in loading. Furthermore, SFRC dramatically increased the ductility of slab while the effect of the reinforcement ratio in the immediate column region, as well as the reinforcement material, on the ductility of the specimens was not as evident. Although Specimen GFBF3 was cast with SFRC only near the column face, GFBF3 was 63% more ductile than GFB3. However, the inclusion of steel fibers in the concrete did not affect the slab stiffness, such that the load-deflection curves for GFBF3 were very similar to GFB3 until failure, as shown in Fig. 9.

CONCLUSIONS 1. Providing puddled fiber-reinforced UHSC in the slab around the column for the distance of

2d from the column faces results in a significant improvement in performance, including an increase in the axial compressive strength, greater loading stiffness, smaller cracks in the slab at all levels of loading, and smaller transverse strains at the slab-column joint. This resulted in an increase of 16% in the failure load compared to the slab-column specimen with a normal-strength concrete slab.

2. The confining effects of the slab around the column increased the axial compressive

strength of the column by about 45% and increased the ductility for the specimen having the normal-strength joint concrete slab layer.

3. For these tests, both the ACI Code (2005) and the CSA Standard (2004) give conservative predictions for the transmission of column loads through slab-column connections even when the slab loads were applied during the tests.

4. Due to the lower elastic modulus of GFRP bars, the GFRP-reinforced slabs with a similar

flexural strength to that of the steel-reinforced slabs have significantly lower punching shear capacity, lower post cracking stiffness and higher deflection, but produced more cracks in the immediate column region than slabs reinforced with steel bars.

5. Concentrating the top mat of flexural reinforcement within a distance 1.5h from the column

faces resulted in higher punching shear strength, greater post cracking stiffness, more uniform distribution of strains in the top flexural bars and better crack distribution compared to the companion slab with a uniform distribution of the same amount of reinforcement. In addition, for the specimens with banded reinforcement, the punching shear failure plane surface occurred at a greater distance from the column faces.

6. The presence of the steel fibers in concrete leads to remarkable improvement in the

punching shear capacity as well as in the overall cracks control capability. Furthermore, steel fibers in concrete enhance the ductility of slabs, while the effect of the reinforcement material and the concentration of flexural reinforcement on the ductility of the specimens is not as evident. However, SFRC has little influence on the post cracking stiffness and strains on GFRP bars on maximum loading. The use of SFRC in GFRP-reinforced slabs can make up for the weak points observed when GFRP reinforcements are substituted for conventional steels.

ACKNOWLEDGEMENT

The work presented in this paper was funded by Center for Concrete Corea(05-CCT-D11), supported by Korea Institute of Construction and Transportation Technology Evaluation and Planning (KICTTEP) under the Ministry of Construction and Transportation (MOCT).

REFERENCES

McHarg, P. J., Cook, W. D., Mitchell, D., and Yoon, Y.-S. (2000) “Improved transmission of high-strength concrete column loads through normal-strength concrete slabs”, ACI Struct. J., 97(1), 157-165.

American Concrete Institute (ACI). (2005) “Building code requirements for structural concrete and commentary.” ACI 318-05 and ACI 318R-05, Farmington Hills, Mich.

Canadian Standards Association (CSA). (2004) “Design of concrete structures.” CSA A23.3-04, Mississauga, Ont., Canada.

Ghannoum, C. M. (1998) “Effect of high-strength concrete on the performance of slab-column specimens,” M. Engrg. Thesis, Dept. of Civil Engineering and Applied Mechanics,

McGill Univ., Montréal, Canada. Canadian Standards Association (CSA). (2002) “Design and construction of building

components with fibre reinforced polymers,” CAN/CSA S806-02, Mississauga, Ont., Canada.

El-Ghandour, A. W., Pilakoutas, K., and Waldron, P. (2003) “Punching shear behavior of fiber reinforced polymers reinforced concrete flat slabs: Experimental study,” Journal of Composites for Construction, V. 7, No. 3, pp. 258-265.

Table 1 Details of Center for Concrete Corea

Ⅰ. 1st Theme Development of Core and Application Technology for High Performance & Multi-functional Concrete / Kim, Seong-Woon (Daewoo Construction)

1 Guide / Manual

(1) Development of Guide and Manuals for the Design and Construction of High Performance Concrete Materials / Jeong, Ha-Sun (Korea Concrete Institute) (2) Development of Structural Design Code for High Performance Concrete Structures / Park, Tae-hyo (Hanyang University)

2 High Durability

(1) Development of Evaluation System for Deterioration of Concrete Structures Subjected to Severe Environment / Song, Ha-Won (Yonsei University) - Evaluation on Durability of Reinforced Concrete on the Acceleration/Deterioration Exposed to The Outdoor Environment

/ Jung, Hyun-Oh (Korea Institute of Construction Materials) - Evaluation of Combined Deterioration with Freezing-and-Thawing Resistance of Concrete / Ueda Tamon (Hokkaido University, Japan)

(2) Development of Corrosion Control System Using the Improvement Rebar and Corrosion Inhibitor / Lee, Han-Seung (Hanyang University) (3) Development and Its Practical Application of High-Durability Concrete / Yoon, Seok-Ho (Byucksan ENG.)

3 High Flowability

(1) Development and Application of Precast Bridge Member Using High Flowing Self - Compacting Concrete / Choi, Yun-Wang (Semyung University) (2) Development of Structural Systems Using High Fluidity & Self-Compacting Concrete Filled Steel Tube / Choi, Yeol (Kyungpook National University) (3) Improvement of Concreting Property for Ultra High Fluidity Concrete and Field Application / Kim, Seong-Woon (Daewoo Construction)

4 Multi-Function

(1) Development and Field Application of Fire Resistance High Performance Concrete for Securing of Human Life / Han, Cheon-Goo (Cheongju University) (2) Development and application of green concrete / Kim, Sang-Chel (Hanseo University) (3) Development and Application of High Performance Lightweight Aggregate Concrete / Choi, Soo-Kyung (Hanseo University) (4) Development of Antimicrobial Concrete / Choi, Hong-Shik (Chung Cheong University) (5) Application and Development of the Decorative Concrete / Shin, Dong-An (Sun ENG)

Ⅱ. 2nd Theme Development & Application of Structural Members having Ultra-High Strength Concrete / Kim, Sung-Wook (Korea Institute of Construction Technology)

Ultra High Strength

(1) Development and Application of Structural Member having Ultra High Strength Concrete / Kim, Sung-Wook (Korea Institute of Construction Technology) - Development of Numerical and Analytical Method for Behavior of Structure with Ultra High Strength Concrete / Kwak, Hyo-Gyyoung (KAIST)

(2) Development of Practical High-Strength Structural Member using Hybrid Reinforcing Techniques / Yoon, Young-Soo (Korea University) (3) Practical Technique Development of Monitoring System on High Performance Concrete / Jang, Il-Young (Kumoh National Institute of Technology)

Ⅲ. 3rd Theme Practical Utilization & Application for High Performance & Multi-functional Concrete / Ahn, Tae-Song (Korea Expressway Corporation)

Practical Application

(1) Field Application to Bridge Structures of High Performance Concrete and Establishment of Process Improving Measures of Concrete Placing In Situ / Ahn, Tae-Song (Korea Expressway Corporation)

(2) Integrated Information System Development for Field Concrete Process Improvement of High Performance Concrete / Choi, Young-Min (Jaram Tech.) (3) Application of High Strength Concrete to High-Rise Residential Building / Lee, Bum-Sik (Korea National Housing Corporation)

Table 2 Concrete properties of column test specimens

Specimen ccf ′ (MPa)

csf ′ (MPa)

puddledcf ,′

(MPa)ccε ′

(με)csε ′

(με)puddledc,ε ′

(με)

NT 88.3 (0.8)*

46.9 (1.3) - 2420

(102)1750(65) -

NC 88.3 (0.8)

46.9 (1.3) - 2420

(102)1750(65) -

C1 46.9 (1.3) - - 1750

(65) - -

FT 89.3 (4.5)

46.9 (1.3)

90.1 (5.5)

2225(200)

1750(65)

2270 (221)

FC 88.3 (0.8)

90.1 (5.5) - 2420

(102)2270(221) -

C2 97.9 (5.7) - - 2490

(245) - - * (standard deviation)

Table 3 Reinforcing steel properties

Bar size Area (mm2)

yf (MPa)

yε (%)

shε (%)

uf (MPa)

10M 100 443 0.22 1.11 619

15M 200 449 0.23 0.66 702

Table 4 Summary of column test results

Specimen Peak load (kN)

cef ′ (MPa)

ACIcef ,′ (MPa)

CSAcef ,′ (MPa) ACIce

ce

ff

,′′

CSAce

ce

ff

,′′

NT 5138 91.1 82.6 71.3 1.10 1.28

NC 3648 62.7 46.9 46.9 1.34 1.34

C1 2985 50.1 46.9 46.9 1.07 1.07

FT 5920 106.0 89.3 89.3 1.19 1.19

FC 5932 106.3 88.3 88.3 1.20 1.20

C2 5710 102.0 97.9 97.9 1.04 1.04

Table 5 Concrete properties of punching shear test specimens

Specimen cf ′ (MPa) cε ′ (με) rf (MPa) S series (column & slab) 37.2 2248 3.5

GF series (column) 80.3 2490 8.9 GF series (slab) 36.3 1936 4.4

SFRC 33.8 1620 3.9

Table 6 Properties of GFRP and steel bars

Designation Area (mm2) rE (GPa) yf (MPa) fuf or suf (MPa) yε (%) GFRP #5 198 48.2 N/A 683 fuε =1.58 Steel 10M 100 200 454 676 0.25 Steel 15M 200 200 445 588 0.23

Table 7 Summary of punching shear test results

Specimen ρ

(%) Pcr

(kN) Py

(kN) Pu

(kN)Δcr

(mm)Δy

(mm)Δu

(mm)Stiffness (kN/mm)

Ductility(Δu/Δy)

SU1 1.18 56 203 301 0.75 9.82 16.95 16.21 1.73 SB2 2.15 58 211 317 0.80 8.93 15.44 18.82 1.73

GFU1 1.18 81 163 222 0.72 14.16 26.15 6.10 1.85 GFB2 2.15 101 186 246 1.37 14.37 23.39 6.54 1.63 GFB3 3.00 87 166 248 1.23 11.74 20.93 7.52 1.78

GFBF3 3.00 95 169 330 1.36 11.13 32.43 7.57 2.91

90 MPa

45 MPa

Series Slab-Column Isolated Column Control Column

NSeries

FSeries

NT

FT

NC C1

FC C2

90 MPa

45 MPa 45 MPa

90 MPa

90 MPa

45 MPa

90 MPa

90 MPawith

fibers

90 MPawith

fibers2d

Figure 1 Column load test specimens

Hydraulicjack

Loadcell

Reaction floor

Tensionrod

Loadingassembly

Steelclamp

Testing machine head

LVDT

150 mm

1350 mm x1350 mmtwo-way

slab

250 x 250 mmstub column

Figure 2 Column load test setup of slab-column specimens

1 50

6@

100

5 0

2 stra in gages

235

4 stra in gages

2 stra in gages

slab c over= 20 m m10M 15M

top bars: 10-10M eac h way

250

10M tie15M ba r

c olum n c over= 40 m m

250

Sec tion A-A

A A

Figure 3 Reinforcement and strain gage layout for slab-column specimens

Steelclamp

Testing machine head

LVDT

250 x 250 mmcolumn

8010

010

075

8010

010

07575

025

0

250

10M tie15M bar

40 mm cover

(C1, C2,NC, FC)

2 straingages

(NC, FC)

2 straingages

Figure 4 Test setup and reinforcement layout for isolated column specimens

(a) Specimen NT (b) Specimen FT

Figure 5 Crack patterns at full service load on top surface of slab

C1NC

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000average column strain (με)

load

(kN

)

C2NC

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000average column strain (με)

load

(kN

)

(a) UHSC in column stub (b) Normal strength slab layer in UHSC column

FC

C2

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000average column strain (με)

load

(kN

)

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000average column strain (με)

load

(kN

)

FT joint concreteNT joint concrete

(c) Fibers in slab layer of UHSC column (d) UHSC with fibers in slab-column joint

NC

NT

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000average column strain (με)

load

(kN

)

FC FT

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000average column strain (με)

load

(kN

)

(e) Presence of slab in N serie (f) Presence of slab in F Series

Figure 6 Effects of different parameters on column responses

Figure 7 Test setup for punching shear test of two-way slabs

(a) SU1 and GFU1 (b) SB2 and GFB2

(c) GFB3 and GFBF3 (d) Bottom mat for all specimens

Figure 8 Reinforcement layout for punching shear test specimens

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30 35 40Average deflection (mm)

Load

(kN

)

GFBF3

GFU1GFB2

SB2 SU1GFB3

Figure 9 Load versus average deflection responses for punching shear tests

(a) GFU1 (b) GFB2

(c) GFB3 (d) GFBF3

Figure 10 Crack patterns at peak load for punching shear tests

Figure 11 Surface of GFRP bar after testing

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30 35 40

Average delection (mm)

Cra

ck w

idth

(mm

)

SU1SB2GFU1GFB2GFB3GFBF3

Figure 12 Crack width versus average deflection in the immediate column region