BEHAVIOUR OF SPIRAL REINFORCED LIGHTWEIGHT AGGREGATE CONCRETE COLUMNS M. H. Myat*, National University of Singapore, Singapore T. H. Wee, National University of Singapore, Singapore 32nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28-29 August 27, Singapore Article Online Id: 13239 The online version of this article can be found at: http://cipremier.com/13239 This article is brought to you with the support of Singapore Concrete Institute www.scinst.org.sg All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information www.cipremier.com
32 nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28 29 August 27, Singapore BEHAVIOUR OF SPIRAL REINFORCED LIGHTWEIGHT AGGREGATE CONCRETE COLUMNS M. H. Myat*, National University of Singapore, Singapore T. H. Wee, National University of Singapore, Singapore Abstract This paper presents an experimental observation into the behaviour of spiral reinforced lightweight aggregate concrete columns under axially concentric loading. Leca lightweight coarse aggregate (expanded clay) was used to produce lightweight aggregate concrete (LWAC). Compressive strength of concrete was 45.8 MPa. A total of 11 columns were tested under short-term loading: 1 of which were spiral reinforced columns, while 1 was plain column without any reinforcements. The columns were 9 mm high with a diameter of 25 mm. Study parameters include diameter and pitch of transverse reinforcement, area of longitudinal reinforcement and amount of steel fibre addition. From the test results, it has been observed that as the pitch of spiral reinforcement decreases, the ductile behaviour of columns increases noticeably while the ultimate load capacity of columns decreases. Test results also indicate that area of longitudinal reinforcement does not affect the ductility of columns. Addition of steel fibres in spiral reinforced columns enhances the ultimate load capacity although it has little influence on the ductility. Keywords: Spiral reinforced columns, lightweight aggregate concrete, discrete steel fibres, strength and ductility of columns 1. Introduction Lightweight aggregate concrete (LWAC) has been regarded as an efficient construction material for structural applications. The high strength to weight ratio of LWAC significantly reduces the dead load of structure, thus reducing the cost involved for special ground treatment and foundation in common engineering structures. The application of long-span construction technology in structures especially in bridges also becomes possible with the usage of LWAC. Moreover, the buoyancy of material provides economical solutions for offshore floating platform construction 2. In fact, LWAC is less ductile than normal weight concrete of the same compressive strength. Only limited information is available in literature regarding the behavior of structural members especially columns constructed with this material. Hence, this study attempts to explore the strength and ductile behavior of LWAC columns under axially concentric loading. Ductility is an important characteristic for any structural members as it guards the members against sudden collapse due to any unforeseen overloading 4. But, it is of great importance in columns that support the large part of structure and would lead to the progressive failure of structure. In this study, spiral reinforcement is used as the lateral reinforcement due to its promising confining effect such as uniform confining pressure and large effectively confining area to core concrete.
2. Experimental Program A total of 11 circular columns were tested in this study; 1 of which were spiral reinforced columns while 1 was plain column without any reinforcements. They were 9 mm high with a diameter of 25 mm. Based on the slenderness ratio, they were classified as short columns. The diameter of core concrete was 21 mm measured from out to out of the spiral. The concrete cover for all columns was 2 mm. Lateral reinforcement was equally spaced throughout the length of columns. Parameters investigated are diameter and pitch of lateral reinforcement, area of longitudinal reinforcement and amount of steel fibre addition. The details of reinforcement in columns are described in Table 1.The comparative studies among columns are shown in Table 2. 2.1 Material Properties The material used in this study was produced by using ordinary Portland cement, natural sand and Leca lightweight coarse aggregate (expanded clay). The maximum size of coarse aggregate was 15 mm. A superplasticizing admixture was used in all concrete mixtures to achieve the desirable workability. In fibre reinforced columns, round and hook-end steel fibres with 3 mm in length and.375 mm in diameter were added. Equivalent aspect ratio of fibre was 8. Tensile strength and specific gravity of fibres were 23 MPa and 785 kg/m 3. Volume fractions of fibre used were.5%,.75% and 1% by volume of concrete. The cylindrical compressive strength of plain and fibre concretes used in columns were shown in Table 3. The oven-dried densities of materials were determined with 1 mm cube and also described in Table 3. 2.2 Specimen Preparations 2.2.1 Initiating the weakening zone The cross-section of columns was purposely narrowed down by 1 mm over the length of 2 mm in central portion of columns (Fig 1b). This portion of columns was monitored by electrical resistance strain gauges and transducers. Hence, narrowing down the section in central portion of columns ensures that failure would occur in the monitoring zone 3 (Fig 1d). 2.2.2 Wrapping FRP sheets To prevent the premature end zone failure at interfaces between concrete column and steel plates, the two ends of columns over the length of 2 mm each were strengthened by wrapping two layers of FRP sheets (Fig 1c). After wrapping the glass fibre sheets, the columns were left to dry the epoxy resin in laboratory environment for about five days. 2.2.3 Controlling the external eccentricity To eliminate or control the external eccentricity in testing, special attention was paid to ensure that columns were loaded under concentric loading. Plaster-Of-Paris was applied to top and bottom surfaces of columns. A small loading was applied before setting the capped material and maintained during its setting to ensure that even surfaces were achieved. Moreover, at the start of testing, preloading was carried out up to 1 KN. 2.2.4 Instrumentation and testing In each column, three 5-mm electrical resistance strain gauges were installed on longitudinal bars and another three strain gauges were installed on spiral reinforcement (Fig. 1a). Three 6-mm strain gauges were installed vertically at mid-height of column, about 12 o apart from each other around the circumference, to measure the vertical strain of column until the failure of cover concrete. Four transducers with gauge length 5 mm, held in place by transducer cage, were mounted over the length of 4 mm in central portion of column to measure the vertical deformation of column. The other four transducers were installed at the base of ball-seat, 9 apart from each other, to measure the downward movement of loading head of machine (Fig 1c). Finally, all the wires were connected to the data logger system to capture the required data throughout the testing. All columns were tested under axially concentric loading in 5, KN servo-controlled testing machine (Fig 1c). The tests were conducted in displacement-controlled mode. Instead of using monotonic rate of axial displacement, different rates of axial displacement with a range from.5 mm/min to.2 mm/min were used. The rate was reduced just prior to peak load level and immediate post-peak region of load-
strain curve to control the sudden failure of columns. Hadi M. N. S. (25) also reported the similar slow rate of loading adopted in column testing. The duration of each test was about 2 hours and 3 minutes. 3. Observation and Discussion 3.1 Ductile Behavior of Columns Normalized load and normalized strain are used to balance out the differences in ultimate load capacity and corresponding strain at ultimate load level among columns. Load history and strain history have been normalized against the ultimate load capacity and the corresponding strain at ultimate load level respectively. As described in Table 2, there were 3 comparisons available to observe the influence of spiral pitch on ductile behavior when all other columns parameters were kept constant (Table 1). It can be seen from Fig. 2a that column C3 with 5 mm pitch shows the most ductile behavior, while column C4 with 75 mm pitch exhibits a less ductile behavior and column C7 with 1 mm pitch indicates a sudden drop of loading in immediate post-peak region. This sudden drop of loading relates to the small effectively confining area to core concrete along the longitudinal axis when using large spiral pitch. Test results indicate the improvement in ductile behavior with spiral pitch from 1 mm to 75 mm to 5 mm. Similarly, there were 3 comparisons available to investigate the effect of spiral diameter on ductile behavior while other parameters were kept the same (Table 1 and Table 2). It can be seen from Fig. 2b that column C3 with 1 mm wire diameter shows ductile behavior with a gradual loss of load capacity as expected, while both column C2 with 8 mm wire diameter and column C1 with 6 mm wire diameter illustrate sudden drop of loading in post-peak region. It is apparent that this brittle behavior is more pronounced in column C1. With the large size of wire, high flexural stiffness of lateral reinforcement can be obtained. It makes lateral reinforcement effectively restrain the longitudinal bars from buckling under loading. In turn, the disturbing effect from buckling of longitudinal bars on lateral reinforcement reduces. It causes lateral reinforcement to confine the core concrete efficiently. Hence, size of wire plays a role in confining effect of lateral reinforcement, with larger size of wire providing higher confining effect. From the test results, it is apparent that increasing the number of longitudinal bars from 4 to 8 does not affect the function of lateral reinforcement and the ductile behavior of column (Fig 2c). It is probably due to the small size of column in which the curvature of lateral reinforcement is quite large. Hence, lateral reinforcement can effectively restrain the longitudinal bars from buckling under loading. In this study, addition of loose steel fibres in spiral columns has little influence on ductility of columns (Fig. 2d). With the use of vibrator instead of vibrating table, it may be difficult to obtain the adequate compaction of concrete during placing when adding discrete steel fibres in concrete mixtures. For the material used in this study, the optimum amount of fibres to enhance the ductile behavior is.75% by volume of concrete (Fig. 2d). 3.2 Strength enhancement in columns Increasing the amount of fibre content increases the ultimate load capacity of column (Fig. 3a) although it has little influence on the ductility (Fig. 2d). Moreover, ultimate load of column increases with increasing diameter of spiral reinforcement (Fig. 3b). It has been observed that as the pitch of spiral increases, the ultimate load of columns increases (Fig. 3c), while the ductility of columns decreases (Fig 2a). With increasing pitch of spiral, the congestion of steel cage decreases (Fig. 1a). It results in decreasing the effect of plane of separation between cover and core concrete 6. Thus, cover concrete becomes less disturbed by the plane of separation and its contribution to load capacity of column becomes larger. It may be the reason why the ultimate load of column increases with increasing pitch of spiral. 3.3 Spiral Reinforcement Spiral reinforcement was supposed to yield prior to ultimate load level in order to confine the core concrete effectively. In this study, it has been observed that even at ultimate load level, stress developed in spiral reinforcement was always well below its yield strength. Spiral reinforcement only became yielded in post-peak region which was far below ultimate load level. It indicates that the core concrete do not obtain adequate confining pressure from lateral reinforcement in immediate post-peak region which leads to the sudden drop of load capacity to a lower level.
It is known that the lateral expansion of LWAC is low in pre-peak region. In fact, transverse reinforcement is the passive confinement which requires to be activated first in order to provide the confining pressure to core concrete. Actually, this activation is achieved from lateral expansion of core concrete under loading. Since the lateral expansion of core concrete is low in LWAC, it causes the delay in activating and utilizing the transverse reinforcement. It is probably the reason why the transverse reinforcement can not provide adequate confining pressure to core concrete in immediate post-peak region. All the columns were tested until the rupture of spiral reinforcement. In this study, the rupture of spiral always occurred over longitudinal bar (Fig.1e). Similar observation was reported by Hadi M. N. S. (25). It shows that spiral reinforcement was subjected to tensile stresses not only from core concrete but from the buckling of longitudinal bars. Hence, it is necessary to make sure that longitudinal bars are bundled adequately to avoid from excessive buckling under loading. 3.4 Ratio of Plain Column strength to Cylinder strength This ratio would have an influence on determining the strength reduction in columns due to size effect, segregation and bleeding, and the strength enhancement in columns due to confining effect of transverse reinforcement. For the material used in this study, the ratio was.96. Although only one plain column was tested, the result agrees with the observation of Basset (1986). The failure mode of plain column was sudden failure in an explosive manner. 4. Conclusions From this experimental observation, the following conclusions can be drawn: (1) As the pitch of spiral reinforcement decreases, the ductility of columns increases (Fig. 2a), while the ultimate load capacity decreases (Fig. 3c). (2) Increasing the diameter of spiral increases the ductility (Fig 2b) as well as the ultimate load capacity of columns (Fig. 3b). (3) Increasing the number of longitudinal bars from 4 to 8 does not affect the ductile behavior of column (Fig 2c). (4) Increasing the fibre content in spiral columns increases the ultimate load capacity (Fig. 3a), although it has little influence on ductility (Fig. 2d). In this study, the optimum amount of fibres to enhance the ductile behavior is.75% by volume of concrete (Fig. 2d). (5) Test results show that spiral reinforcement only became yielded in post-peak region which was far below the ultimate load level. It indicates that the core concrete did not obtain adequate confining pressure from lateral reinforcement in immediate post-peak region which leads to the sudden drop of load capacity to a lower level. (6) For the material used in this study, the ratio of plain column strength to cylinder strength was.96. References (1) Basset R. and Uzumeri S. M., 1986, Effect of confinement on the behavior of high-strength lightweight concrete columns, Canadian Journal of Civil Engineering, V. 13, No. 6, Dec., pp. 741-751. (2) Berra M. and Ferrara G., 199, Normal weight and total-lightweight high-strength concretes: A comparative experimental study, High Strength Concrete: Second International Symposium, ACI, SP 121, pp. 71-733. (3) Foster S. J. and Attard M. M., 1999, Behaviour of fibre reinforced high strength concrete columns, Mechanics of structures and materials: proceedings of the 16 th Australasian Conference on the Mechanics of Structures and Materials, Sydney, New South Wales, Australia, Dec., pp. 21-26. (4) Foster S. J., 21, On behavior of high-strength concrete columns: cover spalling, steel fibres, and ductility, ACI Structural Journal, V. 98, No. 4, July-August, pp. 583-589. (5) Hadi M. N. S, 25, Behaviour of high strength axially loaded concrete columns confined with helices, Construction and Building Materials, 19, pp. 135-14. (6) Hadi M. N. S, 27, Using fibres to enhance the properties of concrete columns, Construction and Building Materials, V. 21, Issue 1, Jan., pp. 118-125.
Table 1 Details of reinforcement in columns longitudinal reinforcement Ref: number of bar dia: yield stregth (Mpa) steel ratio (%) dia: lateral reinforcement pitch provided volumetric ratio (%) yield strength (Mpa) volume fraction of fibre (%) C1 6 16 537 2.45 6 5 1.5 617 C2 6 16 537 2.45 8 5 1.84 58 C3 6 16 537 2.45 1 5 2.85 532 C4 6 16 537 2.45 1 75 1.89 532 C5 4 16 537 1.64 1 75 1.89 532 C6 8 16 537 3.28 1 75 1.89 532 C7 6 16 537 2.45 1 1 1.42 532 C8 6 16 537 2.45 1 75 1.89 532.5 C9 6 16 537 2.45 1 75 1.89 532.75 C1 6 16 537 2.45 1 75 1.89 532 1 C11 plain column without any reinforcements Table 2 Comparative studies among columns compared columns purpose C3, C4, C7 to observe the influence of spiral pitch on column behavior C1, C2, C3 to investigate the effect of spiral diameter on column behavior C5, C6 to explore the influence of area of longitudinal bar on column behavior C4, C8, C9, C1 to evaluate the influence of fibre content on column behavior Table 3 Cylindrical strength and oven-dried density of plain and fibre concretes used in columns description volume fraction of fibre (%) cylindrical strength of concrete (f'c, MPa) oven-dried density of concrete (kg/m 3 ) plain concrete. 45.83 1767.5 5.62 1818 fibre concrete.75 51.8 1889 1. 54.44 1911 (a) (b) (c)
(d) (e) Fig. 1 (a) spiral cages with three different pitches: 1 mm, 75 mm and 5 mm, (b) narrowing down the section in central portion of columns, (c) test set-up and instrumentations, (d) appearance of columns after testing, (e) rupture of spiral reinforcement over longitudinal bar normalized load 1.8.6.4.2 (a) normalized load Vs. normalized strain pitch 5 mm pitch 75 mm pitch 1 mm normalized load 1.8.6.4.2 (b) normalized load Vs. normalized strain dia: of spiral 6 mm dia: of spiral 8 mm dia: of spiral 1 mm 2 4 6 8 1 12 14 normalized strain 2 4 6 8 1 12 normalized strain normalized load 1.8.6.4.2 (c) normalized load Vs. normalized strain 4 longitudinal bars 8 longitudinal bars normalized load 1.8.6.4.2 (d) normalized load Vs. normalized strain % fibre.5% fibre.75% fibre 1% fibre 5 1 15 2 normalized strain 2 4 6 8 1 12 14 normalized strain Fig. 2 Ductile behavior of LWAC columns with (a) pitch of spiral reinforcement, (b) diameter of spiral reinforcement, (c) number of longitudinal reinforcement, and (d) amount of steel fibres addition
3 (a) 25 (b) ultimate load of column (KN) 28 26 24 22 2.5.75 1 ultimate load of column (KN) 2 15 1 5 6 8 1 vol: fraction of fibre (%) dia: of spiral (mm) (c) ultimate load of column (KN) 245 24 235 23 225 5 75 1 pitch of spiral (mm) Fig. 3 Strength enhancement in columns due to (a) steel fibre addition, (b) diameter of spiral and (c) pitch of spiral