Structural Behaviour of Unreinforced and Reinforced Cement Stabilised Rammed Earth Columns under Axial Compression

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The present study aims to determine the suitability of locally available soil (Agartala,India) for the production of cement stabilised rammed earth (CSRE) columns with a proposed CSRE specimen making technique, followed by investigations on the effects of (1) slenderness ratios on the axial column strengths of both unreinforced rectangular and circular CSRE columns, (2) cross-sectional aspect ratios on unreinforced rectangular CSRE columns, and (3) lateral steel reinforcement ratios for CRSE columns longitudinally reinforced with steel and bamboo splints. The properties of locally available soil and its suitability was determined in terms of density, strength, compaction energy and durability in both cured and uncured conditions using the proposed block-making equipment and technique. Test result shows that the average characteristic strength of cured samples is about two times higher than that of uncured samples. The compressive strength and density of CSRE blocks (with 10% cement content) is quite sensitive to the variations in compaction energy up to 16.94 kg-cm/cc. Increase in compaction energy from 7.26 to 16.94 kgcm/ cc tends to increase the compressive strength and density by about 23% and 11% respectively than that obtained at an energy level close to the standard Proctor effort. It is concluded that the local soil used for the production of CSRE blocks using the proposed block-making equipment and technique satisfy the design criteria outlined in various standards such as NZS 4297 (1998), IS 2110 (2002) and AS HB 195 (2002) in terms of compressive strength and density. The influence of structural parameters such as slenderness ratio (λ), cross-sectional aspect ratio (width/thickness ratio, Ф), and cross-sectional shapes (circular, square and rectangular) on load-capacity (Pu), reduction factor (k), failure pattern etc., on concentrically loaded unreinforced CSRE columns are presented. Columns of crosssectional sizes: 150 mm x 150 mm, 150 mm x 190 mm, and 150 mm x 230 mm for rectangular; 150 mm diameter for circular cross sections, with a height of 900 mm, 1200 mm and 1500 mm were considered for the experimental programme. Experimental results on ultimate compressive strength (σu) of columns were compared with the results obtained with Engesser's tangent modulus theory (Bleich, 1952); and the validity of using masonry design rules for the design of CSRE columns was evaluated. Safety factors were determined based on stress reduction factors obtained from the study. Test result shows that the lateral and vertical deformation (at comparable load) increases with increasing values of λ. Typically, columns tested failed by formation of vertical cracks initially at the platen-column interface followed by shearing and splitting at the later stages of loading. The value of σu decreases with increase in λ. The values of k at λ = 8 and 10 were determined to be 0.92 and 0.84 for rectangular columns and 0.90 and 0.82 for circular columns respectively. At increasing values of Ф (keeping thickness constant at 150 mm) the values of Pu for rectangular columns increases. It was observed that when Ф is increased from 1.0 to 1.27 and 1.0 to 1.53, the values of Pu increased by about 20% and 40.5% respectively, for λ = 6, 8 and 10 of all the column sets, agreeing with the increase in cross-sectional area as Ф increases. The k values outlined in earthen standards such as NZS 4297 (1998) and AS HB 195 (2002), and masonry standard IS 1905 (2002) are in a close range to the experimental values by about ~4.0% at λ = 8 - 10. However, in the case of circular columns, the codal values are found to be higher by about ~7%, thus showing relatively un-conservative nature. Structural behaviour of CSRE columns reinforced with steel (4 numbers of 8 mm diameter steel as longitudinal and two-legged 6 mm diameter as transverse reinforcements; Fe500 steel) under concentric axial loading, for a fixed column height of 1500 mm was assessed. Effects of key variables such as transverse reinforcement ratio, total reinforcement (i.e. combined longitudinal and transverse) ratio etc., were studied. Test result shows that the behaviour of CSRE columns reinforced with closer tie spacing (e.g. 50 mm) is characterized by gradual spalling of cover at the failure zone leading to a loss of axial capacity before the transverse confinement becomes effective. The values of Pus (ultimate load for steel reinforced column) for SR200, SR100 and SR50 (where SR = steel reinforced, and 50, 100, 200 represents tie spacing in mm) columns are about 4.6%, 16% and 33% higher than that of unreinforced CSRE (UCSRE) column, respectively. A linear increase in Pus is observed as the total reinforcement ratio was increased from 0% to 3.41%. A gradual increase in Pus has been seen when the total reinforcement ratio is increased from 0% (i.e. unreinforced) to 1.52%, 2.15% and 3.41% by about 4.5%, 16% and 36% respectively. As the transverse reinforcement is increased from 0.63% to 1.26% and 0.63% to 2.51%, Pus is found to increase by about 11% and 30% respectively. Axial deformation at peak load (δuv) increased by about 6.3% and 19.1%; whilst lateral deformation at 60 kN (δl60) (i.e., pre-peak/ultimate load) by about 13.6 % and 100%, when the tie spacing decreased from 200 mm to 100 mm and 100 mm to 50 mm respectively, suggesting an improved ductility with core confinement. Structural behaviour of CSRE columns reinforced with bamboo-steel under concentric axial loading, for a fixed column height of 1500 mm was also assessed. In this study, the vertical reinforcement steels have been replaced by approximately rectangular bamboo splints (with an equivalent cross-sectional area as that of 8 mm diameter bar). It was found that Pubs (ultimate load for bamboo-steel reinforced column) of bamboo-reinforced CSRE columns are about 3.7% to 15% higher than that of unreinforced CSRE column when total (bamboo and steel) reinforcement ratio is increased from 1.52% to 3.41%. An increase in Pubs of about 6% to 12% has been seen when the transverse reinforcement ratio increased from 0.63% to 1.26% and 0.63% to 2.51% respectively. It was observed that Pubs of SR50 column is about 17% higher than that of BSR50 (BSR = bamboo-steel reinforced), however only minor differences are seen for larger tie spacing, i.e. 200 mm. Similar failure patterns as those of steel reinforced CSRE columns are observed for bamboo-steel reinforced CSRE columns, except for post-ultimate snapping or breaking of buckled bamboo longitudinal reinforcement. A nearly linear increase in δuv and δl60 have been observed i.e. around ~10% and 14% respectively when the tie spacing decreased from 200 mm to 50 mm, thus showing an improvement in ductility.
Supervisor: Konjengbam Darunkumar Singh