Evaluating the flexural strength and failure patterns of cement stabilized rammed earth wallettes reinforced with coir, bamboo and steel

Abstract

This paper investigates the effect of coir along with bamboo and steel reinforcement on the flexural strength and failure patterns of cement stabilized rammed earth wallettes. About 10% cement was used as stabilizer and 1% coir of 25 mm length along with bamboo or steel was used as reinforcement for rammed earth wallettes production. To study the bond strength between rammed earth and reinforcement, 20 rammed earth blocks were prepared with bamboo and steel reinforcement and to study the flexural strength of rammed earth wallettes 36 samples were prepared in two categories. First category consists of vertical samples of size 600 mm $\times$ 230 mm $\times$ 120 mm (height/length $\times$ Width $\times$ thickness) and second category consists of horizontal samples of size 230 mm $\times$ 600 mm $\times$ 120 mm (height/length $\times$ width $\times$ thickness). The test results show that coir reinforcement improved the bond strength and the highest bond strength was obtained with coir and steel reinforced rammed earth blocks. Coir along with bamboo or steel reinforcement increased the flexural load carrying and lateral deflection capacity of the samples by about 13.42–154.88% and 33.78–76.12% respectively. Rammed earth samples undertake 42.59–63.30% more flexural stress when tested perpendicular rather than parallel to the ramming layers. The current technique of reinforcing cement stabilized rammed walls with coir along with bamboo or steel can be used for constructing internal and external load bearing walls in single or two storey rammed earth houses considering the seismic zone of the area. To predict the ultimate load carrying and lateral deformation capacity of samples, a series of regression analysis was performed using the test data correlating the reinforcement type and loading conditions. With coefficient of correlation R² > 0.78, the equations obtained from the analysis represent a strong relation between the actual measured and predicted values.

Appendix A

Flexural strength of wallettes can be calculated using Fig. 

Fig. 10

Typical bending moment and stress diagrams of wallette (dimensions in mm)

10 and the following formulae.

Flexural stress,$f={\sigma }_{T}-{\sigma }_{C}$

Bending stress,${\sigma }_{T}=\frac{M}{Z}$

Bending moment, $M = P a$

Lateral load applied, $W = 2P = {R}_{1}+{R}_{2}$

Section modulus, $Z= \frac{1}{6}b{t}^{2}$

Where,

${\sigma }_{C}$= Pre-compression applied.

P = Load applied (kN).

a = shear span (mm).

R₁ = R₂ = Reaction at supports as per IS 516 [64]

b = Width of specimen (mm).

t = Thickness of specimen (mm).

Example

Considering VCSCRRE sample for demonstration.

$W = 2P = {R}_{1}+{R}_{2}$ = 11.0 kN $\Rightarrow$ P = 5.50 kN.

a = 160 mm, b = 230 mm, t = 120 mm, ${\sigma }_{C}$ = 0.063 N/mm².

$M = P a\Rightarrow$ 5.5 × 1000 × 160 = 880,000 N-mm.

$Z= \frac{1}{6}b{t}^{2}\Rightarrow \frac{1}{6}$ × 230 × 120² = 552,000 mm³.

${\sigma }_{T}=\frac{M}{Z}\Rightarrow$ $\frac{880000 }{552000}$ = 1.594 N/mm².

Flexural stress, $f={\sigma }_{T}-{\sigma }_{C}$ $\Rightarrow$ 1.594—0.063 = 1.531 MPa.

Hence, flexural strength of VCSCRRE wallette is 1.53 MPa.