Innovative composite concrete filled plastic tubes in compression

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1 Innovative mposite ncrete fied pastic tubes in mpression * Water O. Oyawa 1), Naftary K. Gathimba 2), and Geoffrey N. Mang'uriu 3) 1) Department of Civi, Construction & Environmenta Engineering, Jomo Kenyatta University of Agricuture and Technoogy, P.O. box , Nairobi, Kenya 1) oyawa@eng.jkuat.ac.ke ABSTRACT Kenya has recenty experienced worrying apse of buidings during nstruction argey attributabe to the quaity of in-situ ncrete and poor workmanship. The situation is further mpounded by rapid deterioration of infrastructure, necessitate that measures be taken to deveop enhanced structura materias and systems. The work herein presents the findings of experimenta and anaytica work which investigated the structura response of mposite ncrete fied pastic tubes under mpressive oad regime. Key variabes in the study incuded the strength of infi ncrete, the ength to diameter ratio (L/D) of the pastic tube, as we as the diameter to thickness ratio (D/2t) of the pastic tube. Pastic tubes having varying diameters and heights were used to nfine ncrete of different strengths. Resuts obtained in the study ceary demonstrate the effectiveness of pastic tubes as a nfining medium for infi ncrete, attributabe to enhanced mposite interaction between the pastic tube and infi ncrete medium. It was determined that mpressive strength of the mposite umn specimens increased with increased ncrete strength whie the same decreased with increased umn height, abeit by a sma margin since a the umns nsidered were short umns. Most importanty, the experimenta nfined ncrete strength increased significanty when mpared to unnfined ncrete strength; the strength increased between 1.18 to 3.65 times the unnfined strength. It was noted that ower strength infi ncrete had the highest nfined strength possiby due to enhanced mposite interaction with the nfining pastic tube. The study further proposes an anaytica mode for the determination of nfined strength of ncrete. A in a, the study shows the great potentia of mposite ncrete fied pastics tubes for use in new buiding nstructions or for retrofitting deteriorated structures. 1. INTRODUCTION The drive towards sustainabe nstruction practices around the word has triggered a specia interest in the structura stabiity of od structures, especiay in view of frequenty occurring catastrophic natura and manmade disasters. Acrdingy, there is increased demand for assessment and rehabiitation or retrofitting of existing structures. 1) Professor 2) Senior Lecturer 3) Teaching Assistant 1

2 One of the deficiencies in reinforced ncrete (RC) umns is the ack of atera nfinement and ow energy absorption capacity when subjected to massive atera forces ike earthquakes, hence necessitating use of structura stee tubes to retrofit such structures (Gupta et a. 2007; Schneider, 1998; Oiveira et a. 2009; Oyawa, et a., 2001). The strength, robustness, and ductiity capacity of new ncrete umns during nstruction can be enhanced by providing externa nfinement by empoying stee tubes or wrapping with fibre reinforced pastics. A potentia aternative to stee tube retrofitting technique is the pastic tube or Poy-Viny-Choride (PVC) tube. The use of PVC tubes to nfine RC umns is sti at infancy stage, and very few studies have been done in this area (Marzouck & Sennah, 2002). Taking note of recent apse of buidings under nstruction in Kenya, argey attributabe to poor reinforced ncrete works, this study investigated the use of un-pasticized Poy-Viny-Choride (UPVC) tubes as the nfining medium of mposite ncrete fied pastic tubes in mpression. UPVC tubes are rrosive resistant and are rather inexpensive as mpared to stee tubes or fibre reinforced pastic wraps. Though having ower strength mpared to stee, UPVC are highy eastic undergoing arge dispacement under oad, and provide the added advantages of being used as formwork during nstruction stage as we as being adverse-weather resistant when exposed to high energy regimes ike marine and saine environments. 2. EXPERIMENTAL PROGRAM 2.1 Materia properties Aggregates Ordinary river sand and crushed rocks were used as fine (FA) and arse aggregates (CA) respectivey in the manufacture of ncrete. Aggregates were washed with cean water to making sure that they are free from organic impurities such as cay, organic ntent, sit and other inferior materias. In order to ascertain the aggregates suitabiity and aso to faciitate the design of ncrete mixes some of essentia physica properties were determined foowing the standard aid down aboratory procedures, resuts of which are summarized in Tabe 1. Partice size distribution of river sand, a very significant attribute associated with fine aggregates, was aso determined from the sieve anaysis using British standard test sieves having sizes; 5.00mm, 2.36mm, 1.18mm, 600 microns, 300microns,150microns and the resuts presented in a graphica manner, as shown in figure 2 beow, which is generay referred to as the grading curve. The grading curve of the sand was within the Zone 2 enveope of British Standard thus suitabe for use in ncrete production without need for bending it. Aso the fineness moduus (FM) of fine aggregates, which is the weighted average of a sieve on which the materia is retained; the sieves being unted from the finest, was determined from sieve anaysis and found to be The higher the FM, the arser the fine aggregates. FM is variabe for measuring sight variations in the aggregates from the same source (i.e. a day to day check). Typica vaues of FM are in the range

3 Tabe 1: Summary of some physica properties of aggregates Specific gravity on oven Dry Basis Specific gravity on saturated surface dry (SSD) Basis Apparent Specific Gravity Water absorption (%) FA CA Cement Cement is widey used as the binder materia in the ncrete matrix in many appications. The cement mmony used in Kenya is categorized as 32.5or 42.5 aso referred to as power pus. Cement Cass 42.5 (Ordinary Portand cement) was used in this research. Concrete Three series of ncrete grade C20, C25 and C30 were designed for use in this research work. Whie the grades were based on the standard cube test, the unnfined strength of the ncrete specimens ( ) was determined on specimens of the same height and inner diameter as the encasing pastic tube for mposite stub umns. The method of ncrete mixing design appied here was in acrdance to the Department of Environment of British Research Estabishment (BRE), United Kingdom (1988) and the determination of various parameters necessary for the design of the mixes was based on the design tabes and figures drawn from the BRE manua. The mixing of ncrete was done acrding to the BS, ASTM and JIS procedures given in aboratory guideines. The BRE mix design procedure from the DOE, UK was adopted for this research. A summary of the cacuated mix proportions and the ncrete's mpressive strength, f ck, at 28 days is as given in Tabe 2. Cass of mix C20 C25 C30 Water*/cement ratio Tabe 2: Summary of ncrete mix proportions Water* (kg/m3) Cement (kg/m Sand (kg/m3) Aggregate (kg/m 3 ) 5/10 10/ Compressive strength at 28 days f ck, (MPa) *Incudes free water for cement hydration and water for absorption of aggregates to make them SSD. Pastic tubes Unpasticized poyviny choride (UPVC) tubes were used to nfine pain ncrete stub umns. UPVC pipes nforming to Kenyan standards KS ISO 3633 and KS were procured from Masterpipe Kenya (LTD) and Agrofow irrigation systems Limited, pastic pipes manufacturing mpanies based in Nairobi, Kenya. To determine the tensie strength of the UPVC tube materia, a upon was 3

4 prepared with dimensions as shown in Figure 1. This sampe was tested in the UTM SERVO-PLUS EVOLUTION machine with a oading rate of 2.0 Mpa/sec. Nomina properties of UPVC tube are given in Tabe NB: A dimensions in mm Figure 1 Dimensions of UPVC specimen for tensie strength test Tabe 3 Nomina physica properties of UPVC pipes Parameter Density Eastic moduus Fexure strength Tensie strength Compressive Strength Poisson ratio Eongation Vaue KN/m MPa 65.5 MPa 40 MPa 59 MPa % 2.2 Instrumentation and Testing A tota of 144 short umns (i.e. 48 umn sets repicated three times, for statistics) were cast i.e. 72 unnfined ncrete umns and 72 UPVC-nfined ncrete umns. UPVC tubes with three different outer diameters: 55mm, 83mm and 110mm were used to nfine ncrete. The thicknesses of the tubes were 2.5mm, 3.0mm and 2.5mm respectivey. The three predesigned ncrete mixes namey C20, C25, and C30 were used to fi the UPVC tubes. For each cass of ncrete and for each diameter and height seected, three unnfined and three UPVC-nfined umns having the same ength were cast. To obtain unnfined nditions for a ncrete umn of a simiar diameter and height as a nfined umn, empty UPVC tube was spit on one side aong the ength and tied with binding wires which was then used as a moud for casting of umns. The outer pastic casing was then removed after 24 hrs, eaving an unnfined umn specimen. Typica nomencatures for the specimen are used to designate them. For instance, C/C25/110/3 represent a nfined (C) ncrete umn cast with ncrete grade C25, having a diameter of 110mm and a senderness ratio of 3. Freshy cast specimens were aowed to stand for 24 hrs unti ncrete was fuy hardened. They were then marked for ease of identification then cured by vering them with sisa sacks which were wetted daiy with water unti the testing date. The specimens were cured for 28 days prior to testing (Figure 2). The experimenta work was nducted utiizing mpression machines at Structures and Materias aboratory of Civi Engineering Department of Jomo Kenyatta University of Agricuture and Technoogy in Kenya. The specimens were aowed to dry prior to testing. A specimens were tested up to faiure under monotonic oads. The 4

5 specimens were subjected to ncentric axia mpressive oad appied at the specimens center by means of a mpression testing hydrauic machine through uniform stee end pates to achieve a nstant stress distribution at the ncrete cross section (Figure 3). The oad was appied to the entire umn section. Ata E-kareim (2011) in his study on behaviour of ong ncrete umns nfined by means of pastic tubes noted that when the oad was appied to the entire section, the ntribution by the ncrete re and tube to the tota axia force was nstant aong the height of the umn, and was not affected by the bond strength. Furthermore, Gardner & Jabson (1976) in their study on structura behaviour of ncrete fied stee tube observed that the bond strength had no infuence on the structura behaviour of the umn. The umns were tested using an incrementa oading procedure. Load ce of 500 kn capacity was used to monitor the appied oad vaues. Due to the extensiveness and the magnitude of sampe specimens being handed in this research work, ony mpressive strength data is presented in this paper. The appied oad was kept nstant at each oad stage to aow for measurements and observations. The oad ce was nnected to TDS 303 data ogger where the resuts were rerded for further anaysis. Figure 2.Sampes of stub umns ready for testing Load ce Data Logger LVDT Specimen Strain gauge attached to the specimen Figure 3. Specimen testing using mpression machine 5

6 2. RESULTS AND DISCUSSIONS 3.1 Load-deformation response, and faiure modes Figure 4 gives a typica oad versus axia deformation of the tested mposite stub umns. It is observed that the mposite umns demonstrate enhanced ductiity attributabe to mposite interaction. Ductiity is observed to increase with reduced senderness ratio and vice versa. On the other hand, the strength of the mposite umns increases with increased tube diameter as woud be expected. With regard to faiure modes, a specimens behaved more or ess in the same manner during the oading process. Sounds were heard during the eary or midde stages of oading, which may be attributed to the micro cracking of the ncrete. Bursting of the pastic tube was aso witnessed toward the end of the oading process, for those specimens which were tested unti tota apse (Figure 5(d)). It was aso observed that the faiure of the pastic tube was preceded by fow of resin which manifested itsef by white patches at highy stressed sections (Figure 5(c)). Load, P, (kn) C/C25/110/3/S3 C/C25/110/2/S2 C/C25/83/4/S3 C/C25/83/3/S3 C/C25/83/2/S3 C/C25/55/4/S1 C/C25/55/3/S3 C/C25/55/2/S1 Axia deformation (mm) Figure 4. Typica oad-deformation curves for mposite umns (nstant ncrete strength) (a) Shearing (b) Mid-height buging (c) Top buging (d) Bursting (e) Loca bucking of empty tubes Figure 5. Typica mposite and empty UPVC tube specimens after test - faiure modes 6

7 Generay, two faiure modes were observed. The principa faiure mode was a typica shear faiure of the pastic tube (Figure 5(a)). A typica characteristic of shear faiure is that the re ncrete is damaged by shear stress in one direction due to weak nfinement effect of the tube. The shear crack direction can be judged by the appearance of the specimen. The same type of mode of faiure was aso observed by Gupta (2007) and Wang & Yang (2011) in their experimenta program. This type of shear faiure is affected by D/t ratio, and in order to avoid this type of shear faiure, the ratio shoud be reduced, i.e., the wa thickness of the pipe shoud be increased. The send type of faiure mode observed was buging which ead to umn specimen crushing under mpression. This buging was observed to occur either near the bottom, top or mid-height of the specimen (Figure 5(b & c)). This is a typica faiure mode of short umns under pure ncentric axia mpressive oad. At the end of the oading process, the encased ncrete is totay crushed and amost puverized. It was aso observed that no ncrete was attached to the remainder of the tube, and a smooth interface was disvered. This ed to the ncusion that no bond is deveoped between ncrete and UPVC tube. For empty tubes, faiure was by oca bucking at the ends (Figure 5(e)). It is hence cear that the presence of infi ncrete prevents or deays oca bucking of the pastic tubes, thus nverting the pastic tube to a restraining medium against atera expansion of the infi materia. This is the essence of mposite interaction where each materia enhances the other e.g. the pastic tube nfines and restrains the infi ncrete from bursting whie the infi ncrete deays the bucking of the pastic tube. 3.2 Effect of varying diameter and height effects on oad and mpressive strength of specimen Figure 6 iustrates mpressive oad test resuts on mposite ncrete fied UPVC tubuar stub umns. The Figure presents the variation of Load capacity, P ut of ncrete fied UPVC tubuar umns with change in infi ncrete strength, encasing tube diameter and height for nfined specimens. As expected, the oad carrying capacity of the mposite umns increases with increase in the grade of ncrete, as is evidenced by the resuts in group I through III. It is further rerded that as the specimen diameter increases so does the oad capacity, whie as the height to diameter ratio (senderness ratio) increases the oad capacity of the mposite umn aso decreases. (a): Group I specimen (ncrete CN20) (b): Group II specimens-cn25 (c): Group III specimens-cn30 7

8 Figure 6: Variation of Load capacity, P ut of ncrete fied UPVC tubuar umns with change in infi ncrete strength, encasing tube diameter and height 3.3 Experimenta nfinement effectiveness Tabe 4 and Figure 7 report finding on how nfinement of infi ncrete is affected by varying ncrete strength, tube thickness and senderness ratio. The measure of how we a certain materia nfines ncrete is referred to as nfinement effectiveness which is defined as cc /, where cc = mpressive strength of nfined ncrete; =mpressive strength of unnfined ncrete. From the resuts as presented in Tabe 4 and Figure 7, it is cear that pastic pipes are effective in nfining ncrete, as evidenced by the increased mpressive stress. The enhancement in strength due to nfinement of circuar umns is substantia (Figure 7). Depending on the eve of nfinement, strength is increased anywhere from 1.18 to 3.65 times the unnfined strength. It aso observed from Figure 7 nfinement effectiveness curve goes down with increase in the unnfined strength of ncrete. This is typica for a umn specimens tested regardess of their diameter, height or tube thickness, and is attributabe to the fact that the higher the strength of the ncrete, the more britte and ess expansive it is. It is postuated that ower strength ncrete is ess stiff (as indicated by its ow eastic moduus which is usuay reated to the mpressive strength of ncrete), and is therefore abe to 'fow' thus interacting with ncrete tube more effectivey, with nsequent increase in mposite action. Another observation from Figure 7 is that the nfinement effectiveness decreases with increase in senderness ratio. This is depicted by the downward shift of the curves when the senderness ratio is increased with diameter kept nstant. This can be expained in the ntext that as senderness ratio increases, the oad carrying capacity of the umn reduces, especiay that of the encasing pastic tube hence reducing mposite interaction. A notabe observation is how the nfinement effect for the 83mm-diameter umns set was higher than the other umn. This is attributed to the fact that the tube thickness was higher (3.0mm) as mpared to the other umns (2.5mm). Generay, cnfinement action is dependent on the tendency of the ncrete infi to diate when oaded, as we as the radia stiffness of the nfining member to restrain the diation. For an axiay oaded unnfined ncrete eement, transversa strains are induced resuting into radia ncrete expansion (Poisson's effect). Under ow oading nditions, the transverse strains are proportiona to ongitudina strain, and associated by the Poisson's efficient which for ncrete usuay varies between 0.15 to After reaching a certain critica stress (typicay between 60% and 80% of the ncrete strength), micro-cracking formation occurs in ncrete, transversa strains increases quicky eading to arge transversa strains for reativey sma ongitudina strain. These micro-cracks evove to macro-cracks that eventuay ead to ncrete rapture with cracks parae to the oading. The rupture of unnfined ncrete can be deayed by nfining the ncrete appropriatey. The nfinement mechanism of ncrete is reated to the use of materias that provides tensie strength to restrict this increase in transversa strain. The effect of nfinement of ncrete at high eves of oading eads to a triaxia mpression stress state in ncrete, which provides a superior behaviour in both strength and ductiity than ncrete which is uniaxiay mpressed. Concrete umns can be nfined by: atera reinforcement in the form of stee ties or spiras; encasing ncrete in stee tubes; externa fiber mposite wraps; encasing ncrete in fiber mposite tubes; or encasing ncrete in pastic tubes (a new technoogy). A these means of nfinement produce a so-caed passive state of nfinement, in which the nfining effect is a function of the atera expansion of the ncrete re. 8

9 Tabe 4: Summary of experimenta resuts in terms of Coumn oad capacity, umn strength and nfinement effectiveness Coumn abe (Dia./sen. ) Tube thickness, t, (mm) C-110/2 2.5 C-110/3 2.5 C-83/2 3.0 C-83/3 3.0 C-83/4 3.0 C-55/2 2.5 C-55/3 2.5 C-55/4 2.5 Concrete designatio n Coumn Load capacity (kn) Confine d P cc Unnfine d, P Coumn strength, Mpa Confine Unnfine d d cc Confinement effectiveness ( cc / ) C C C C C C C C C C C C C C C C C C C C C C C C Confinement effectiveness cc / Unnfined ncrete strength - (N/mm 2 ) C-110/2 C-110/3 C-83/2 C-83/3 C-83/4 C-55/2 C-55/3 C-55/4 Figures 7: The reationship between nfinement effectiveness and unnfined ncrete strength 9

10 3.4 Anaytica estimation of nfined strength of ncrete Mechanics of Confinement Mechanics of nfinement in ncrete fied tubuar members has been studied by severa researchers in an attempt to deveop modes to quantify the effects of nfinement on strength and ductiity of ncrete. Concrete infi surrounded by an ensing restraining medium and subjected to axia oad wi deveop passive atera pressure as it expands under the infuence of axia mpression. The expansion of the infi ncrete creates a mutiaxia state of stress which is dependent on severa factors incuding size and strength of the nfining and restraining mediums, Poisson ratio of the ncrete fi and restraining medium, amongst others. For the case of ncrete fied stee tubuar members, Gardner (1967) proposed that as the stee tube restrains the diating ncrete re, and interna pressure deveops between the stee tube and ncrete, creating hoop stresses as given in Figure 8. t t sh sh r i (a) Concrete stress (b) Stee stress Figure 8. Interaction of ncrete and stee in CFT umn under mpression Confinement action thus requires mutua simutaneous interaction and is dependent on the tendency of the ncrete infi to diate when oaded, as we as the radia stiffness of the nfining member to restrain the diation. When this interaction is active, it is rationa to assume that there wi be geometric (strain) mpatibiity between the re and the she, and aso the equiibrium of forces in the free-body diagram for any sector of the nfined section. As iustrated in Figure 8, equiibrium of forces acting on the stee tube is obtained as summation of forces on the sma eement subtended by ange from the centre i.e.; F y t 2t sh (2t) ri sin d sh sh (1) r D 0 i i Anaytica nfined strength of ncrete The strength of infi materia with noninear and non-homogeneous characteristics under a muti-axia state of stress may be difficut to estabish theoreticay, necessitating the use of test data to deveop empirica or semi empirica approaches. Eary 10

11 investigators showed that the strength of nfined ncrete ( cc ) and the rresponding ongitudina strain ( cc ) at the strength of ncrete nfined by an active hydrostatic fuid pressure can be represented by the foowing simpe reationships (Gupta 2013): cc = +k 1 (2) cc = (1+k 2 / ) (3) Where is the atera nfining pressure on fi materia, and are the unnfined fi materia strength and strain at utimate strength, respectivey, whie k 1 and k 2 are nstants. Richart et a. (1928), one of the eariest researchers in this fied, found k 1 =4.1 and k 2 =5k 1. Since then a number investigators have basicay modified this simpe inear mode, to suit the various nditions of their tests (Mander et a. 1988). These incude Newman & Newman (1971), Saatci ogu et a. (1992), the Architectura Institute of Japan-AIJ (1997) and Cusson (1995) as is shown in Tabe 5. Based on current work, a proposed mode is aso proposed. Tabe 5. Existing and proposed nfined strength and strain modes Researcher Confined strength Strain at nfined strength Richart (1928) Newman & Newman (1971) Saatciogu & Razvi (1992) Architectura Institute 2t cc = +k 1 where k 1 =4.1 and sh D cc = +k 1 where 2t k 1 =3.7{ / ) (-0.14) and D cc i 2t sh D 6.7( ) (4.1)(0.19)2t cc of Japan (AIJ) (1997) D 2t 0.17 sy i sh i where cc = (1+k 2 / ) where k 2 =5k 1 5 cc 1 cc cc , cc when cc 1.5 cc, when 1.5 Cusson (1995) t cc where sh Di cc

12 Proposed Oyawa (2015) cc = +4.1 where 2t sy D 2t Nomencature cc Confined strength of fi materia Unnfined strength of fi materia Latera nfining pressure on fi materia sy stee yied strain cc Strain at nfined strength of fi materia Strain at unnfined strength of fi materia In this study, the existing modes and a proposed mode given in Tabe 5 are evauated for suitabiity vis-à-vis experimenta resuts. Confinement effectiveness is defined as cc/ ; where cc = mpressive strength of nfined ncrete, and = mpressive strength of unnfined ncrete. The nfining pressure is reated to the interna diameter of the encasing tube (D int = D-2t), tensie strength of encasing tube ( sy ) and encasing tube wa thickness(t) by Eq. (4) beow as given by various authors; 2t 2t sy sy (4) Dint D 2t However, it is herein argued that the above equation for the nfining atera pressure fais to capture the effect of the properties of ncrete such as Poisson s ratio on the atera pressure. It is therefore proposed that the equation beow be adopted for the atera nfining pressure. 2t int sy 2t sy D D 2t (5) In Eq. (5), is a dimensioness factor that depends on Poisson s ratio, buk moduus and strains at utimate strength for the infi ncrete materia (oyawa et a. 2001). Tabes 6 & 7 give a mparison of experimenta nfined strength of the infi ncrete versus anaytica nfined strength of the infi ncrete. The Tabes revea that the proposed equation with =4 gives the best estimation of the anaytica strength. That is to say, the determined Anaytica/Experimenta nfined strength of ncrete is 101% for the proposed mode, whie the others are 93%, 97%, 109%, 56% and 49% respectivey. It aso observed from Tabe 7 that the proposed mode gives the owest standard deviation as mpared to the other existing anaytica modes. Unike the other equations proposed by Richart, Newman & Newman, Saatciogu & Razvi, Architectura Institute of Japan (AIJ) and Cusson, the proposed equation takes into nsideration the 12

13 properties of the infi materia in the cacuation of the atera pressure on the infi materia. The Tabes aso show that anaytica equations by AIJ and Cusson are very nservative and probaby usefu for nfined strength of high strength ncrete or for sender mposite stub umns. It is remmended that future studies investigate the effect of creep and ncrete shrinkage on mposite interaction between the encasing pastic tube and infi ncrete. Tabe 6. Comparison of experimenta nfined strength and anaytica nfined strength of ncrete Coumn abe (Dia./sen.) C-110/2 C-110/3 C-83/2 C-83/3 C-83/4 C-55/2 C-55/3 C-55/4 Exte r-na Dia m-et er (mm ) Tube thick - ness, t, (mm ) Experimenta strength, Mpa Confined ncrete strength, cc Propose d Anaytica strength, Mpa Confined ncrete strengths by researchers Richart Newma n Saatciogu AIJ Cusson

14 Tabe 7. Average percentage and standard deviation of Anaytica/Experimenta nfined strength Average (%) : Anay. Exper. Standard deviation: Anay. Exper. Proposed Richart Newman et a. Saatciogu et a. AIJ Cusson CONCLUSIONS This paper presents the resuts of an experimenta programme which investigated the structura performance of ncrete fied UPVC tubuar umns under mpressive oad. The study determined that UPVC are effective in nfining ncrete, as evidenced by the increased mpressive strength of nfined ncrete when mpared to unnfined ncrete. The enhancement in strength due to nfinement of circuar umns is substantia and depending on the eve of nfinement, strength is increased anywhere from 1.18 to 3.65 times the unnfined strength vaues. It was evident that nfinement effectiveness is dependent on the strength of ncrete where higher strength ncrete resuts in reduced nfinement effectiveness attributabe to reduced atera expansion hence reduced interaction with the nfining UPVC tube. Low strength ncrete, on the other hand, tends to be more ductie than high strength ncrete, hence enabing enhanced mposite interaction with the nfining medium. The study aso discusses existing anaytica modes for the determination of nfined ncrete strength by UPVC tube, and proposes an improved anaytica mode that, unike existing modes, inrporates the effect of ncrete properties on the nfining atera pressure. The proposed anaytica mode is determined to give the best estimation of nfined mpressive strength of ncrete when mpared to existing anaytica modes. It is generay demonstrated that UPVC tubes have a great potentia for use as the encasing medium in ncrete fied tubuar umns. The tubes wi further serve as permanent formwork during nstruction. With regard to fire, avaiabe iterature nfirms the UPVC pipes do not support mbustion and are sef-extinguishing. They are, therefore, ideay suited for use in buidings and other nstructions. It is aso documented that UPVC is of superb chemica resistance since it is unaffected by most ncentrations of acids, akais, organic chemicas, ois and fats. This resistance to rrosion by most chemicas makes upvc pipes indispensabe for use in marine structures or in rrosive environments. REFERENCES Architectura Institute of Japan (AIJ) (1997), Remmendations for design and nstruction of ncrete fied stee tubuar structures, Tokyo, Japan. Ata E-kareim Shoeib Soiman, (2011). Behavior of ong nfined ncrete umn, Ain Shams Engineering Journa, 2, Cusson, D. and Pautre, P. (1995), Stress-strain mode for nfined high-strength ncrete, J. Str. Engrg., ASCE, 121(3), Gardner, N.J. and Jabson, E.R. (1967), Structura behaviour of ncrete-fied stee tubes, ACI J., 64(11),

15 Gupta (2013), Confinement of ncrete umns with Unpasticized Poy-viny choride tubes, Internationa Journa of Advanced Structura Engineering, Avaiabe onine at: Mander, J.B., Priestey, M.J.N. and Park, R. (1988), Theoretica stress-strain mode for nfined ncrete, J. Str. Engrg., ASCE,114(8), Marzouck M. & Sennah K. (2002), Concrete-fied PVC tubes as mpression members: Composite Materias in Concrete Construction, Proceedings of the internationa ngress chaenges of ncrete nstruction, Newman K, Newman JB (1971), Faiure theories and design criteria for pain ncrete, Proc. Int. Civ. Engrg. Mat. Conf. on Struct., Soid Mech. and Engrg. Des. Wiey Interscience, New York, Oiveira, W., Nardin S., Debs, A. and Debs M., (2009), "Infuence of ncrete strength andength/diameter on the axia capacity of CFT umns", Journa of Constructiona Stee Research 65, Oyawa, W.O., Sugiura, K. and Watanabe, E. (2001), Poymer-ncrete-fied stee tubes under axia mpression, J.Const Buid. Mats., 15, Richart FE, Brandtzaeg A, Brown RL (1928), A study of the faiure of ncrete under mbined mpressive stresses, Engg. Experimenta Station Bu. No.185. University of Iinois, Urbana III. Sastciogu, M. and Razvi, S.R. (1992), Strength and ductiity of nfined ncrete, J. Struct. Engrg., ASCE, 118(6), Schneider, S.P. (1998), "Axiay oaded ncrete fied stee tubes",, Journa of Structura Engineering, 124(10), Wang, J. Yang, Q. (2010), Experimenta study on Mechanica Properties of ncrete nfined with pastic Pipe, Tite no. 107-M17. ACI Mater J: 107,