Mechanical behaviour of rammed earth column: A comparison between unreinforced, steel and bamboo reinforced columns

D. D. Tripura^a*; K. D. Singh^b

a. Department of Civil Engineering, National Institute of Technology Agartala, (Tripura, India).

b. Department of Civil Engineering, Indian Institute of Technology Guwahati-(Assam, India).

* debdulaltripura@gmail.com

1. INTRODUCTIONTOP

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.

2. MATERIAL PROPERTIES AND EQUIPMENT TOP

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.

3. PRODUCTION AND TESTING OF SPECIMENTOP

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.

4. RESULTS AND DISCUSSIONTOP

4.1 Steel reinforced columns with 200 mm tie spacing (SR200)TOP

Figure 6a shows the failure pattern of steel reinforced columns of 200 mm ties spacing (SR200). Columns did not show any sign of distress up to 60 kN load, however at nearly 70 kN load vertical cracks on CSRE close to longitudinal bars generated near the loading end, which eventually led to spalling of cover. This is attributed to outward buckling of steel bars in between the ties, thereby leading to localized failure at the loading end, with little or no failure impact on the rest of the column length. This implies that the lateral tie spacing provided is sufficiently large enough to allow lateral buckling of the longitudinal steel bars reinforcement, thereby leading to ultimate failure of the column. However, unlike bamboo reinforced columns, no rupture of the longitudinal steel bars was observed at the failure load. This may be attributed to higher tensile strength and ductility possessed by steel bars which is capable of resisting the lateral pressure offered by the inner core (CSRE) of the column. Similar pattern of failure was observed in BSR200 columns as shown in Figure 6b (27).

Failure of SR200 columns occurred close to the loading end. This can be attributed to development of localized stress concentration due to weak shear wedge zone resulting from platen effect as shown in Figure 7 in UCSRE column (27), leading to early failure of longitudinal steel reinforcement and preventing further splitting of CSRE in presence of lateral ties. It can also be noticed from Figure 6a that the altitude of shear wedge pyramid is ~ 97–167 mm, which is about ~ 48%-83% of the tie spacing (i.e. lesser than the tie spacing of 200 mm). Hence, it is possible that with relatively large tie spacing of 200 mm (~ 133% of the column width) the confining effect from the ties is not sufficient to prevent such local failure due to formation of shear wedge. Lateral deformation (δ_l) of column is in the range of 0.5 - 2 mm from the axis at the corresponding load of 10 - 60 kN (Figure 8). The location of column failure corresponds nearly to the location where the maximum δ_l occurred (Figs. 6 and 8). The average P_us of the column is determined to be 83.6 kN with a standard deviation of 1.56 kN. No ties were found to yield/rupture. Table 3 shows the summary of column test results.

4.2 Steel reinforced columns with 100 mm and 50 mm tie spacing (SR100 and SR50)TOP

Figure 9a shows the failure pattern of steel reinforced columns of 100 mm tie spacing (SR100). Unlike SR200 columns (see Section 4.1), the sign of distress is not seen in SR100 columns even at 70 kN load. However, as soon as the load was close to 80 kN and above (i.e., at about 85% - 90% of ultimate load), vertical cracks on CSRE close to longitudinal bars were generated near the support end, similar to the SR200 columns. Spalling of cover occurs near the mid-height of column where maximum lateral deformation happens, followed by bending of steel bars leading to ultimate failure. Similarly, no rupture of the longitudinal steel bars was observed at the failure load. Average P_us of column is about 92.9 kN with a standard deviation of 2.37 kN, which is about 11.1% higher than SR200 columns. Lateral deformation (δ_l) of columns ranges from 0.6 to 2.6 mm at the corresponding load of 10 to 60 kN (Figure 8). Relatively there is an enhancement of δ_l as compared to SR200 columns. Similar type of failure was observed in BSR100 columns with slightly closer to the mid-height as shown in Figure 9b (27).

Figure 10a shows the failure pattern of steel reinforced columns of 50 mm tie spacing (SR50). Failure of this column is similar to SR100 columns. δ_l of columns ranges from 0.7 to 5 mm at the corresponding load of 10 to 60 kN (Figure 8). Failure occurs at the point where the maximum deformation occurred (Figures 8 and 9). Average P_us of column is about 109 kN with a standard deviation of 1 kN, which is about 17.3% higher than the SR100 columns. Similar type of failure pattern (compression side crushing and tension side cracking) can be seen for both SR100 and SR50 columns, closer to the mid-height, along with spalling of cover near the support. Compression crushing for both SR100 and SR50 extends to about ~100 mm (i.e. around half the face width) at the face. However, presence of relatively distributed micro-cracks can be seen on the tension side of SR50, in contrast to SR100 where a well-defined macro-crack appears on the tension side of the failure zone. This led to the relatively smoother curvature of the failure zone in SR50 (see Figures 9a and 10a) and this can be attributed to the improved distributed stress with increased confinement effect, resulted from decreasing tie spacing (i.e. with higher ρ_w). The reason for the shifting of localized failure zone (or development of hinge) closer to the mid-height for SR50 column in relation to that of SR100, can again be linked to better or improved distribution of the stresses as a result of increasing confinement effect from the ties. The relatively closer spaced ties in SR50 inhibited the possibility of premature formation of shear wedge failure zone near the supports (ties are known to provide/enhanced capacity), thereby greatly diminishing localized stress concentrations near the supports, which further arrests the possibility of localized cover spalling and buckling of longitudinal reinforcements closer to the supports. Again, no yielding of ties was observed during the entire process of experimentation. Similar type of failure pattern was observed in BSR50 columns as shown in Figure 10b with about 200 mm cover spalling and bending close to the mid-height (27).

4.3 Load-axial deformation (P- δ_v) response of columnTOP

Figure 11 shows the load-axial deformation (P- δ_v) curves of columns. Pre-peak and post-peak deformation was not recorded due to removal of dial-gauges attached to the column at about 60 kN load due to precautionary measures. It can be seen from the curves that with the increase in ρ_w(or decrease in tie spacing), the P_us increases and the ductility property of the column is enhanced. It may be noted that when reinforcements are provided, measurement of post-peak deformations are made possible with the present test setup and unlike UCSRE columns as shown in Figure 7 (10, 27), sudden failure (i.e. brittle) of columns did not occur. δ_v of column is about 71% higher than that of UCSRE columns at the corresponding load of 60 kN.

Unlike UCSRE and SR200 columns, P- δ_v curve behaves differently for SR100 and SR50 columns (Figure 11). It can be observed that the curves possess two peak points. During the ascending part of loading, confinement has little or no effect and the CSRE cover is visually free of cracks up to the first peak load equal to 65 to 75 kN for SR100 columns and 70 to 80 kN for SR50 columns approximately, (i.e., at about 75% to 80% ultimate load). As the loading progressed there was a gradual fall of load by about 5 to 8 kN, which lasted for about 7 to 10 minutes, followed by increase in load up to second peak and beyond this there was gradual decrease in P_us. Sudden fall in load after first peak can be attributed to gradual formation of micro-cracks on the tension side and de-bonding of the CSRE from the reinforcement leading to spalling of cover. Because of the tension cracks and spalling of cover, the effective cross-section available to resist the axial load drops, thereby dropping the P_us. At this stage, lateral CSRE strains increase significantly. As a result, the inner confinement becomes very significant. The CSRE core gains strength, while cover gradually disappears (Figs. 9a and 10a) at the failure zone. Generally, the load-deformation curve for the specimen shows a strength gain and reaches a second peak equal to average load of 92.9 kN (SR100) and 109 kN (SR50) respectively, when the CSRE core reaches the maximum stress. At this load level, the longitudinal steel bars tend to bend, whereas lateral steel or ties shows no sign of distress or deformation. Since, column has been tested under pure axial load; hence the ultimate load exerted on column is insufficient to cause deformation in ties due to shear as observed at failure. Similar type of deformation pattern was observed for BSR100 and BSR50 columns (27)

It can be seen that, unlike the studies made by Cusson and Paultre (42) on high strength concrete columns confined by rectangular ties, the P_us of column at second peak did not fall below the value at first peak, in the present study. On the other hand, specimens with low confinement (specimens SR200) did not show the second peak (UCSRE and SR200 columns), which is attributed to brittle nature of failure.

Figure 12 shows P_us-δ_v curve with respect to variation in reinforcement ratio at the peak load. It is observed that SR50 column possess the maximum P_us as well as undergo maximum δ_v of about 19% to 25% higher than that of SR100 and SR200 columns respectively. This may be attributed to enhancement of greater ductile property due to higher reinforcement ratio.

4.4 Effect of reinforcement on P_usTOP

Figure 13a shows the effect of reinforcement on P_us. P_us is normalized with respect to ultimate capacity of unreinforced column (i.e. P_ucs), as P_ur(= P_us /P_ucs). Positive effect of lateral steel confinement is clearly seen for ρ_w> ~ 0.5. There is an increase in P_us by about 30% when ρ_w is increased by about 300%. The plateauing effect of P_us at higher reinforcement ratio is consistent with the intuition that at vanishing tie spacing, the column would approach a rammed earth filled steel tube (just like concrete filled steel tubes with a finite strength value (43, 44, 45). The increase in P_us with decreasing tie spacing (or increasing ρ_w) is consistent with similar studies done for steel reinforced concrete columns (42). At lower values of ρ_w (i.e. <~ 0.5), when the tie spacing is lesser than 200 mm (or 133% of the column size), the effect of lateral confinement is not very significant on the column strength. Thus it can be seen that the overall response of the column P_us with confinement effect of the steel reinforcement can appears to follow a non-linear S-curve (i.e. a double curvature curve) plateauing at both the ends.

Figure 13b shows the effect of percent increase in reinforcement ratio to percent increase in axial P_us. In this case, P_us is normalized with respect to average P_us of SR200 columns. P_us of columns is significantly affected by ρ_w and it increases by about 11% to 30% when the percent ρ_w was increased from 0.63% to 1.26% and 2.51%, respectively. Again, similar type of effect was observed for BSR columns (27).

5. SR AND BSR COLUMNS COMPARISONTOP

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.

6. MOISTURE CONTENT AND DENSITY OF COLUMNSTOP

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.

7. SUMMARY AND CONCLUSIONSTOP

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:

ACKNOWLEDGEMENTSTOP

The authors would like to thank Pratik, Bandana, Ranjit and Souvik for their constant support during the entire process of casting and testing of the test specimens.

REFERENCESTOP