Rammed earth construction techniques have gained much popularity across the world, due to its varied sustainable benefits such as availability of construction soil locally, low consumption of energy, easy construction procedure, eco-friendly etc., (1–10). Study on properties of soil, its suitability, and behaviour of structural rammed earth elements such as walls, columns etc., have been made (1, 2, 4, 5, 10–18). Further, in the literature use of 5–10% cement for the construction of cement stabilised rammed earth (CSRE) has been reported (3, 4, 14, 19–21).

Attempts have also been made to study the pullout behaviour of deformed and plain rebars and bamboo in unstabilised and cement stabilized rammed earth (CSRE) (22, 23).

Structural behaviour (e.g. load and deformation capacities, load eccentricity effect) of unstabilised columns and unreinforced CSRE (i.e. UCSRE) walls have been studied (12, 14, 24). Gupta (18) reported the effect of diagonal and horizontal stirrups on the load-capacity (P_us ) of steel reinforced CSRE columns. Studies have shown that bamboo can be a potential substitute to steel in structural concrete elements such as beams and columns (25 – 27) and can improve the ductility of rammed earth wall under horizontal load (28). However, it is observed that only few countries have developed standard guidelines, handbooks etc., for earth construction that include various aspects such as selection of soil, equipment and techniques, testing procedures (29 – 32).

Although significant progress has been made in the understanding of rammed earth techniques, the application of rammed earth in the construction sector remains limited, mainly arising from the lack of systematic study in their structural performances of rammed earth members such as beams, columns etc. With increasing number of modern building designers, seek to use earth materials in a more challenging and innovative ways such as the one shown in Figure 1 (33) the development of reliable and robust design guidelines for rammed earth construction, has become relevant. In this context, in this paper, an attempt has been made to investigate systematically, the structural behaviour (especially, load capacity; axial and lateral deformation; failure modes) of steel reinforced CSRE columns, under axial compression. The results of steel reinforced CSRE columns are further compared with the results obtained from unreinforced and bamboo reinforced CSRE columns.

Properties of soil confirming to Indian Standard - IS 2720 Part 4 (34), IS 2720 Part 5 (35) and IS 2720 Part 7 (36) is shown in Table 1 and Figure 2, adopted from earlier works by the authors (8, 10, 23). Many researchers (3, 4, 11, 13, 24, 37) and standards (30, 38) also suggested varying soil types for rammed earth constructions. Likewise, mechanical properties such as compressive and bond strength of rammed earth specimens are adopted from the works by the authors and other researchers (22, 23). Furthermore, Tripura and Sharma (23) reported the tensile strength of bamboo (bambusa balcooa – locally known as Uabang) to be 315 MPa. About 10% ordinary Portland cement (by mass of dry soil) conforming to IS 8112 (39) was used as stabilizer for production of test specimens throughout the experimental program. The initial and final setting time of cement are 40 and 180 minutes respectively having specific gravity of 3.09. Deformed steel bars (F_e 500 grade) of 6 mm and 8 mm diameters confirming to IS1786 (40) having 558 MPa tensile strength was used as lateral and longitudinal reinforcement respectively.

A wooden mould (Figure 3a) of 150 mm x 150 mm in cross-section (inner dimension) of 20 mm thickness and 1.5 m height was used for making of columns. A 5 kg mild-steel rammer with 25 mm diameter solid handle, 1.02 m length and 70 mm x 70 mm ramming face (Figure 3b) was used for compaction. Further, a 20 mm thick mild steel plate of size 148 mm x 148 mm with 12 mm diameter holes at four corners at about 30 mm away from the edge was employed (Figure 3b) to enable the insertion of vertical reinforcement. More details of reinforcement and compaction procedure have been described in the earlier work by the authors (27). Figure 3c shows a sample of a steel reinforced column.

CSRE columns of size 150 mm x 150 mm x 1500 mm (width x thickness x height) reinforced with steel (composite) were prepared for the experimental program. Lateral reinforcement at 200 mm, 100 mm and 50 mm centre-to-centre spacing (i.e., a tie spacing of about 133.3%, 67% and 33.3% respectively to the column width) were provided and denoted as SR200, SR100 and SR50, respectively comprising of at least three specimens for each series of columns. Two legged steel ties of 90 mm x 90 mm size bent at 90°and 4 longitudinal bars of 8 mm diameter were provided as lateral and longitudinal reinforcement as shown in Figure 4, which is similar to the earlier work by the authors (27). Table 2 outlines the reinforcement data where longitudinal reinforcement ratio (ρ_l) of about 0.89% and lateral reinforcement ratio (ρ_w) of about 0.63%, 1.26% and 2.51% were determined as follows [1, 2].

where (A_sl = area of longitudinal reinforcement; A_st = cross sectional area of steel; A_g = gross area of section; A = cross sectional area of CSRE core bounded by centerline of outer tie; s = spacing of tie; c = side dimension of CSRE core)

Mass of dry soil-cement mix and compaction on each layer was controlled, through prior experimentation, to provide the equivalent of standard Proctor effort. Compaction energy was calculated using the formula given in ASTM D-698 (41) as follows [3]:

Where E_c = compaction energy (kg.cm/cc); n = number of compacted layer; N = number of blows per layer; W = weight of rammer (kg); H = height of fall of rammer; and V = volume of mould.

The production of test specimens is as follows. First of all four steel bars were tied together at four corners of the tie/stirrups respectively at their bottom (Figure 3a) and then placed inside the mould followed by adding of wetted mix and placing of perforated steel plate over it (see Figures 3a and 3b). A rammer was dropped uniformly on the perforated plate from the height of 300 mm to compact the mix. Once, the layer was compacted fully the perforated plate was taken out and successive tie was placed over the compacted layer followed by pouring of wetted mix, replacing the perforated plate over it and compacting. This process continued until the desired height of column was achieved (Figure 3c). After de-moulding, the test specimens were cured (curing) for 28 days under wet gunny clothes followed by drying in ambient laboratory conditions for 4 weeks (as commonly practiced) prior to testing in order to avoid further loss of weight.

Detail of column test set up is shown in Figure 5. The lateral movement at the top of the column in the direction perpendicular to the plane was constrained by the loading system. Adequate care was taken to avoid overturning of columns at higher loads by providing steel bracket at top end of the column (Figure 5), in such a way that the axial movement of the column is not restricted.

A 500 kN motorized hydraulic jack was used to apply vertical load on 250 kN load cell which was placed in between the loading arm of the jack and the articulated plate resting on the test specimen. The loading rate was maintained at about 2.5 kN/min (approximately) until failure (27). Six digital dial gauges were used to measure the lateral movement of column at every 10 kN loading interval. A dial gauge was also fixed on top of each column to monitor the axial movement at every 5 kN loading interval until failure. Due to precautionary measures some instruments were removed before the ultimate failure of the columns.

Figure 14 shows the P_u relationship of BSR and SR columns with respect to ρ_w. It is observed that the P_u increase gradually with the increase in reinforcement ratio in both BSR and SR columns. Furthermore, the SR columns possess higher P_u by about 6% to 11% than BSR columns at the rate of two times increase in reinforcement ratio. Figure 15 shows the influence of longitudinal reinforcement type on P_u. Here the CSRE strength and tie spacing is considered as constant and material type for longitudinal reinforcement as variable. It can be seen that the difference in P_u is negligible between BSR200 and SR200 columns. This shows that both bamboo and steel carries similar amount of load at this reinforcement ratio (i.e., ρ_w= 0.63). However, the difference is highest between BSR50 and SR50 columns by about 17%, which shows that the longitudinal steel becomes more effective and carries more load than bamboo at ρ_w equal to 2.51. The reason for this difference can be due to steel being stronger material than bamboo.

The comparison of ultimate vertical deformation (δ_uv) and lateral deformation at 60 kN load (δ_l60) between SR and BSR CSRE columns are shown in Figs. 16a and 16b respectively. It can be seen that the values of δ_uv and δ_l⁶⁰ for SR are found to be higher than that of BSR by ~8% and ~16 % respectively, for greater tie spacing i.e., 100 mm and 200 mm. However, a relatively sharp improvement in δ_uv and δ_l60 can be seen for SR when the tie spacing is reduced towards 50 mm from 100 mm, in comparison to BSR wherein a gradual trend is maintained for all the tie spacing considered (i.e. 200 mm to 50 mm). The enhanced deformation improvement trend in SR as compared to BSR, may be related to the improved confinement core strength achieved as a result of the higher strength in longitudinal steel (i.e. confinement effect has relatively increasing effect in mobilising the strength/stiffness of higher strength steel as compared to bamboo). Figure 17 shows the comparison of average compressive strength (σ = P_u/ sectional area) of all types of columns. It can be observed that SR50 column possess the highest strength and circular column the least with a value of 3.12 and 4.83 MPa respectively. Table 3 shows the summary of column test results and comparison.

It is important to determine moisture content and density of the structural elements during the time of testing to achieve greater strength and durability of the rammed earth structures. Bui et al., (7) reported that, when the moisture content of rammed earth specimen is greater than 4%, the compressive strength decreases quickly for all types of soil studied, and the effect is more in clayey soil than sandy soil. However, this effect is negligible to soil stabilized with 8% natural hydraulic lime and it was noted that the stabilization by hydraulic lime decrease the sensitivity to water of rammed earth material. Likewise, the effect of moisture content on strength and density of test specimens is expected to be negligible in the present study due to use of sandy soil and 10% cement. Details of moisture content and density of CSRE columns during testing are presented in Table 3. In general, the average moisture content of the CSRE samples varies from 4.25% to 6.89% with a standard deviation of 0.13% to 0.91%; and the average dry density varies from 1790kg/m³ to 1990kg/m³with a standard deviation of 0.003% to 0.016% respectively. Analysis shows that there exists a marginal difference in average dry density and average moisture content between the specimens of the same series of column during testing. Tripura and Singh (8) reported that the difference in moisture content between the test specimens during testing ranging from of 1.79% – 2.65% has negligible variation on strength and density between the test samples. Therefore, in the present study, the difference in moisture content between the test specimens during testing lies within this limit.

This paper presents a comparative study on the behaviour of CSRE column reinforced with steel and bamboo under axial compression. Based on the study following conclusions have been drawn: