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The mechanisms of moisture ingress & migration in rammed earth walls.

The ingress and migration of moisture in rammed earth walls can be a particular problem for contractors and property owners. To date there have been no comprehensive research projects aimed at understanding moisture ingress in rammed earth materials. A much more detailed understanding of how soil-grading parameters affects the moisture ingress performance of rammed earth walls is required along with appropriate suggestions for optimising soil mix designs. Quarry materials have been blended to produce a variety of rammed earth soil mix recipes with accurately controlled grading characteristics. Ten different unstabilised mix recipes were established and tested, followed by three mix recipes that were tested with different levels of cement stabilisation. Rammed earth cube samples were used for laboratory experiments that determined capillary suction and pressure-driven moisture ingress properties, and full-scale test walls were used in the SHU climatic simulation chamber. The experimental data was analysed and found to be in good agreement with existing theories on non-saturated flow theory.Moisture ingress in rammed earth is generally very low and typically equal to or less than that of vibration-compacted C30 concrete. A positive relationship exists between the rate of capillary suction and the volume fraction porosity (f) of rammed earth. The mass of absorbed water (m w) increases linearly against the square root of elapsed time (t[0.5]). The extended Darcy equation can be used to describe capillary moisture ingress in rammed earth and so the gradient of the slope i/t[0.5] is used to define the parameter S, known as sorptivity. Static pressure-driven moisture ingress occurs at a rate that is significantly higher than S, and it does not obey the extended Darcy equation. The effective hydraulic pore radius (r) of rammed earth is typically very small which indicates high levels of constriction and tortuosity within the pore structure. The surface inflow velocity (u[0]) of capillary moisture ingress decreases linearly against t[0.01]. The gradient of the slope u0?t[0.01] can be used to provide a value for the parameter o, defined here as the surface receptiveness. The value o effectively quantifies the surface finish of the material. When r is increased, the sorptivity (S) and surface receptiveness (o) also increase, but the rate of decline in S becomes greater due to a more rapid water logging of the facade, i.e. the 'overcoat' effect.The ratio between the total specific surface area (SSAt) of the soil aggregate fraction in a mix, and the relative clay content (CC), expressed as a proportion of the total soil mass, is defined here as the SSAt/CC ratio. A positive relationship exists between the SSA/CC ratio and r of a given mix recipe. Where r is less than 0.65 nm a mix recipe should have optimised moisture ingress resistance. The ratio between the mass of soil particles in a mix whose diameter is greater than 3.35mm, and those whose diameter is less than 3.35mm, is defined here as the 3.35 ratio. When the 3.35 ratio of a mix recipe is 5 or less, and the clay content (CC) is approximately 0.1, the mix recipe appears to be optimised for low sorptivity (S) and small effective hydraulic pore radius (r). The climatic simulation of pressure-driven rainfall applied to stabilised rammed earth walls gives a calculated sorptivity (S) of approximately zero, and a very low initial surface absorption that appears to be independent of soil grading. A correctly graded soil mix recipe can make the capillary and pressure-driven moisture ingress resistance of rammed earth significantly exceed that of vibration-compacted C30 concrete without the need for chemical admixtures or surface treatments. This is the most sustainable and potentially cost effective approach to enhancing the moisture ingress resistance of rammed earth materials. Rainfall penetration in rammed earth walls may not be a problem due to the low levels of absorption from run-off water. However, capillary ingress through basal dampness or faulty rainwater goods/plumbing could be a significant cause of damp ingress. This research provides guidance on how to optimise the moisture ingress resistance in a rammed earth mix recipe, which can be specified according to the level and nature of exposure the wall is likely to encounter.