STRUCTURAL BEHAVIOUR OF A TIE CONFINED SELF COSOLIDATING PERFORMANCE CONCRETE (SCPC) UNDER AXIAL COMPRESSION

Transcription

1 STRUCTURAL BEHAVIOUR OF A TIE CONFINED SELF COSOLIDATING PERFORMANCE CONCRETE (SCPC) UNDER AXIAL COMPRESSION P.Rathish Kumar Associate Professor, Department of Civil Engg, NIT Warangal, A.P, India drrateesh@gmail.com Prasad M.L.V Research Scholar, Department of Civil Engineering, NIT Warangal, Andhra Pradesh, India Radhika K.L Research Scholar, Department of Civil Engineering, NIT Warangal, Andhra Pradesh, India ABSTRACT The construction of modern structures calls for the attention of the use of materials with improved properties in respect of strength, stiffness, toughness and durability. The typical methods of compaction and vibration of normal concrete generates delays and additional costs in concrete. This has necessitated the research and development of a Self Consolidating Concrete with better Performance. This paper aims to develop an analytical model for predicting the stress-strain behavior of Self Consolidating Performance Concrete (SCPC). The results of specimens tested under strain control rate of loading are presented. The behavior of tie confined SCPC is used in formulating a constitutive relationship. INTRODUCTION Self-Consolidating Concrete (SCC) is considered as a concrete which can be placed and compacted under its self-weight with little or no vibration effort, and which is at the same time, cohesive enough to be handled without segregation or bleeding. It is used to facilitate and ensure proper filling and good structural performance of restricted areas and heavily reinforced structural members. Self Consolidating Concrete reduces the intensive labor demand for vibration of highly congested sections. As this concrete can spread easily without any mechanical consolidation, the risk of separation of material constituents is also not present. The performance of such a SCC can be ensured by preparing carefully to exhibit low yield value and a moderate viscosity to maintain high deformability and filling capacity of the form work, with minimum segregation and flow blockage. In order for such a performance concrete to have a wider acceptance for casting complex and congested structural elements, particularly in seismic areas, with heavy reinforcement more knowledge of a good performance SCC is required. The main objective of the study here is to develop a stress-strain model for Self Consolidating Performance Concrete (SCPC). Self Consolidating Performance Concrete (SCPC) is a Self Consolidating Concrete with good durability characteristics. As the focus was to develop concrete with good performance, SCPC is different from SCC. Further, this work is a part of a main project aimed at developing the complete behaviour of a High Performance SCC. The main parameters involved in this phase of the investigation are the strength, spacing and diameter of lateral ties, strength of concrete and core dimensions of the specimen. These parameters controls the behaviour of tie confined SCPC. A single non-dimensional parameter called Confinement Index (C i ) is identified involving all the parameters influencing the behavior of SCPC. The Confinement Index is defined as C b ( P P )( f f ) s = (1) i b b v c where, b P is the ratio of the volume of ties to the volume of concrete, b P is the ratio of the volume of ties to the volume of concrete corresponding to a limiting pitch (1.5 times the least lateral dimension) b is the breadth of the prism and s is the spacing of ties. The stress in the steel binder is given by

2 v = ε v. E s and is always limited to maximum yield strength. v f elasticity of the binder steel. RESEARCH SIGNIFICANCE: ε and E s are the strains and modulus of The construction of heavily reinforced concrete members, such as columns and beams in moment resisting frames in seismic areas, makes the placement of concrete quite difficult. SCPC can be used in structural performance and durability. The present paper contributes mainly in the structural behavior and development of a model for such a performance SCC. This should be of interest to engineers considering the use of SCC for various structural applications. EXPERIMENTAL PROGRAM: The experimental program consisted of casting and testing cubes, cylinders, prisms to examine the mechanical properties. In addition to this the main part consisted of casting and testing 15 x 15 xmm specimens for evaluating the stress-strain characteristics. Hence, the research was done in two steps. The first step involves the study of the fresh and hardened properties of SCC including performance aspects, while the second part consisted of validating the structural applications and developing an analytical model for SCPC. Materials used Ordinary Portland cement with a compressive strength not less than 5 MPa [named 5 grade cement], at the end of 28 days was used in the study. The Fine Aggregate (F.A) used was standard river sand confirming to Zone-II as per IS-286. Crushed granite was used as Coarse Aggregate (C.A). The aggregate was properly graded through standard sieves before using in the concrete works. The fly ash available locally was used as a partial replacement for cement, Conplast SP 7 superplasticizer (water reducing admixture) and Viscosity Modifying Agent (VMA) were added in optimum dosages for improving the properties of SCC. Two concrete strengths Viz., M2 (compressive strength not less than 2 MPa at the end of 28 days) & M4 (compressive strength not less than 4 MPa at the end of 28 days) were tested in the study. Nansu method of mix design was followed for the designing the above mixes with some modifications. The details of the mix proportions adopted per m of concrete are shown in Table-1. Table-1 Details of Mix Proportions MIX CEMENT (KG) FLY ASH (KG) FINE AGGREGATE (KG) COARSE AGGREGATE WATER (LIT) SUPERPLASTCIZER (LIT) VISCOSITY MODIFYING AGENT (LIT) M M All the concrete mixes were mixed for about 5 minutes as per standards in a rotating drum type mixer. The fresh properties were determined Self Consolidating Performance Concrete (SCPC) Fresh SCC must possess the key properties including filling ability, passing ability and resistance to segregation at required levels. The filling ability is the ability of the SCC to flow into all spaces within the formwork under its own weight. Without vibrating the concrete, SCC has to fill any space within the formwork and it has to flow in horizontal and vertical directions without keeping air entrapped inside the concrete or at the surface. Passing ability is the ability of the SCC to flow through tight openings such as spaces between steel reinforcing bars, under its own weight. Passing ability is required to guarantee a homogenous distribution of the components of SCC in the vicinity of

3 obstacles. The resistance to segregation is the resistance of the components of SCC to migration or separation and remains uniform throughout the process of transport and placing. To satisfy these conditions EFNARC has formulated certain test procedures. The slump flow equipment is currently widely used in concrete practice, and the method is very simple and straightforward. Thus the slump flow combined with T5 was selected as the first priority test method for the filling ability of SCC. The V-funnel or Orimet tests are recommended as second priority alternatives to the T5 measurement. The passing ability of fresh SCC can be tested by L-box or J-ring. Table-2 Basic Test Results of Self Consolidating Performance Concrete(SCPC) S.NO METHOD UNIT M2 M4 1. SLUMP FLOW BY ABRAMS CONE MM T5CMSLUMP FLOW SEC V-FUNNEL(TIME FOR COMPLETE DISCHARGE) SEC V-FUNNEL AT T5MINUTES SEC J-RING MM L-BOX(H 2/H 1) MM U-BOX(H 2-H 1) MM As explained earlier as part of the first step 24 cubes of 15 x 15 x15mm, 24 cylinders of 15mm diameter and mm height and 24 prisms of 1 x 1 x 4 mm were cast for investigating the compressive strength, split tensile strength and the flexural strength. Of the above cubes 12 each correspond each of 2MPa and 4MPa strengths. In addition to this to evaluate the durability properties viz Acid resistance, chloride permeability, shrinkage and sorptivity additional specimens were cast and tested. Mechanical and Durability Properties of SCPC For determining the Compressive strength the cubes were tested under a standard 1 KN Servo Controlled Dynamic Testing Machine tested under displacement control. While testing, precautions were taken to ensure axial loading. For flexural strength, standard three points loading was adopted as per IS 516. The specimens were tested at the end of, 7 and 28 days and the values of the compressive strength, split tensile strength and the flexural strength are given in Table Table- Mechanical Properties of SCPC GRADE OF COMPRESSIVE STRENGTH (MPa) FLEXURAL STRENGTH (MPa) SPLIT TENSILE STRENGTH (MPa) MODULUS OF ELASTICITY SCPC (MPa) M M A modified British method was used and 7 x 7 x 285mm concrete specimens were prepared for the drying shrinkage tests. After removing the specimens from the curing tank after 28 days of curing, the initial length of the specimens were measured. After the initial reading the specimens were kept in a drying chamber with a temperature of 55 C and a relative humidity of 95% until further measurements at,7 and 28 days after the initial measurement. Before each measurement was taken on the scheduled day, the specimens were first removed from the drying chamber and the kept in a cooling chamber for about 4 hours in order to cool the specimens to a temperature of 25 C and a relative humidity of 75%.The length of each specimen was then measured within 15 minutes before delivering the specimens back to the drying chamber for the subsequent drying process. The procedure of drying, cooling and measuring continued until the final length measurement was recorded at 28 days. Both the SCPC mixes satisfied the required shrinkage strain

4 25 Shrinkage Strain(1-4 ) Age in Days Figure 1 Age in Days Vs Shrinkage Strain Figure.2 Sorptivity Test setup Sorptivity is an indirect method of measuring the permeability. It is defined as the tendency of a porous material to absorb and transmit water by capillarity. Specimens of size 1 1mm were used for sorptivity tests. The Sorptivity was determined measuring the capillary rise absorption rate on reasonably homogenous materials. Water was used as the test fluid. The sample was rested on rods or pins to allow free access of water to the inflow surface. The water level was kept not more than 5mm above the base of the specimen. The quantity of water absorbed in a time period of minutes was measured by weighing the specimen. The schematic diagram of the test setup is shown in Fig.2. The sorptivity is given by i t, where i = w/ad and t is the soaked time in minutes, w is the increase in weight of the specimen, A is the surface area of the specimen through which the water penetrated, d is the density of the fluid medium(water). Fig. shows the plot for the variation of sorptivity with age for different mixes. The water sorptivity decreased with increase in age of mortars for all the mixes. The values of the sorptivity of both the SCPC mixes were with in the limits and the values were more for 4MPa SCPC concrete as against 2MPa SCPC concrete. The values for the same grade of concrete decreased with the increase in age of concrete Sorptivity ACID ATTACK TEST A Age in Days Figure Age in Days Vs Sorptivity In acid attack studies on SCC, the effect of 2% and 5% Sulfuric acid and Hydrochloric acid were studied. Standard cube specimens of 15 x 15 x15 mm cube specimens were dipped in the acid solutions and the mass loss as given in Table 4 was noted. The mass loss is lower than that of specimens made with normal concrete as investigated by the author in the previous paper [Reference 8].

5 Table 4 Percentage Mass Loss for various acid immersions PERCENTAGE MASS LOSS % NO OF DAYS 2% H 2SO 4 5% H 2SO 4 2% HCl 5% HCl OF IMMERSION M7 M4 M7 M4 M7 M4 M7 M STRESS-STRAIN BEHAVIOR OF SCPC After examining the strength and durability aspects of the SCPC, the behaviour under axial compression was investigated. A structural reinforced concrete member can be theoretically analyzed if the stress-strain behaviour of its constituent materials is known. Stress-Strain relation of steel is not a big problem as there is very less material variation compared to that of concrete. Concrete being produced at site has very much uncertainty; moreover, there is significant variation in the behaviour of vibrated concrete (VC) and SCC. Also, there is much variation in behaviour of confined and unconfined concrete as well. Generally, in practice we use code specified stress-strain relation in the analysis and design, but generally they are recommended for normal concrete only. Now as the advancement in concrete technology has been promoting the use of SCC, the stress-strain relation for SSC is to be used in design. The most common practice for confining concrete is by the use of lateral ties, thus our study is being done for the tie confined concrete. For the prediction of stress-strain relation of vibrated concrete confined with lateral ties, many empirical confinement models based on experimental investigation have been reported in the literature during last three decades. But there is no model for self compacting concrete, so to predict the stress-strain relation of SCC confined with lateral ties, an empirical confinement model was developed based on experimental work. The experimental program was designed to study the behaviour of confined SCC under axial compression by testing prisms of size 15 mm x 15 mm x mm. The variables in the study are grade of concrete, confinement index (Ci) of lateral steel reinforcement, which indicates the degree of confinement provided by laterals. Required companion specimens were cast to evaluate the concrete compressive strength. Similarly, prisms of size 15 x 15 x mm (15 for M2, and 15 for M4) were cast and tested after 28 days curing for examining the stress-strain behavior of M2 & M4 grades. The mix was designed as per modified Nansu method of mix design. The details of the specimens tested are shown in Table 5. Table5 Details of Prisms Specimens Tested SL.NO DESG LONG.STEEL LATERAL PLAIN PRISM (G.I WIRE) STEEL CUBE STRENGTH NO. OF CI STRENGTH DIA DIA SPACING (MPa) PRISMS NO (MPa) (mm) (mm) (mm) 1 NA NA NA NA NA NB NB NB NB NB Note: N=Natural Aggregate; A=M2 Grade of Concrete; B=M4 Grade of Concrete

6 Stress(Mpa) Ci=. 45 Ci= strain x Ci= Ci=.1 85 Ci= Stress(Mpa) Ci=. Ci=. Ci= strain x 1-6 Figure 4 Experimental Stress-Strain Curves for M2 & M4 From the stress-strain curves explained in Figure 4, the ultimate strength( Ci=.182 Ci=.96 f u ), Strain at ultimate strength( ε u ), strain at 85% of the ultimate on the ascending portion ( ε.85u asc) and 85% of ultimate on the descending portion ( ε des) were obtained. These are shown in Table 6..85u Table 6 Confinement Index, Peak Strength, Peak Strain, and Ductility Factor PEAK STRENGTH f PEAK STRAIN ε ε.85 cu ASCENDING X 1^ - 6 ε.85 cu DESCENDING X 1^- 6 DUCTILITY FACTOR cu cu S.NO DESG C I (MPA) X 1 ^-6 1 NA NA NA NA NA NB NB NB NB NB An examination of the curves in Figure 4 indicates that the behaviour is similar for all the grades, meaning that the stress-strain behaviour is linear up to 8-9% of the ultimate and non linear beyond this. The post peak stress-strain response for all the GFRSCC specimens is gradual and appears to have a consistent and constant gradient. The ductility factor is defined as the ratio of strain at 85% descending portion to that at 85% ascending portion. Also from the study the stress block parameters, the ultimate moment and the corresponding curvature of the confined SCPC section can be determined. CONCLUSIONS 1) The mechanical properties viz compressive strength, split tensile strength and flexural strength were satisfactory in both the grades of concrete M2 and M4. Both the concretes could satisfy the EFNARC specifications 2) Both the shrinkage strains and Sorptivity values of SCPC were low and suggested a better performance SCC ) The mass loss is more in case of 5% based HCl and H 2 SO 4 in both the grades of concrete while a higher mass loss was noticed in M4 grade concrete as compared to M2 grade SCC 4) With the increase in the Confinement Index the peak strain, strain at peak strength increased in both the concretes. 5) The ductility factor defined as the ratio of strains at 85% descending to 85% ascending increased with increase in the Confinement Index.

7 REFERENCES 1) Ozawa K., Kunishima, M., Maekawa, K. and Ozawa, K, Development of High Performance Concrete Based on the Durability Design of Concrete Structures. Proceedings of the second East-Asia and Pacific Conference on Structural Engineering and Construction (EASEC-2), Vol. 1, pp , January ) M.T. Bassuoni, M.L. Nehdi, Resistance of Self-Consolidating Concrete to Sulfuric acid attack with consecutive ph reduction, Cement and Concrete Research 7 (27) ) J.C. gibbs and W. zhu strength and hardened Self Compacting Concrete University of Paisely, Scotland, United Kingdom, 1 st International RILEM Symposium on Self Compacting Concrete, Stocholm, Sweden September1-14, ) P. Dinakar, K.G. Babu, Manu Santhanam, Durability properties of high volume fly ash self compacting concretes, Cement and Concrete Composites Volume, Issue 1, November 28, Pages ) Zhen-Tian Chang, Xiu-Jiang Song, Robert Munn, Marton Marosszeky, Using limestone aggregates and different cements for enhancing resistance of concrete to sulfuric acid attack, Cement and Concrete Research, Volume 5, Issue 8, August 25, Pages ) P.Rathish Kumar and Rao C.B.K., Constitutive Behaviour of High Performance Ferrocement under Axial Compression, International Journal of Magazine of Concrete Research, Volume 58, issue 1, pp , Dec 26. 7) M.L.V.Prasad, P.Rathish Kumar and Toshiyuki Oshima, Development of analytical stressstrain model for glass fiber reinforced self compacting concrete " Vol 4, No.1, 29 pp 25-7, ISSN , International Journal of Mechanics and Solids, Research India Publications. 8) P.Rathish Kumar etal, Resistance of standard grade Self Compacting Concrete and Normal Concrete to Acid Attack, Vol 4, No 12, 29 pp , International Journal of Applied Engineering Research, Research India Publications, ISSN