Pullout Response of Ferrocement Members Embedded in Soil

Ferrocement, a reinforced cementitious composite, may attain its optimal reinforcing capability for soil reinforcement applications owing to its synergetic action from two components of wire mesh and mortar. This paper presents some experimental results on the performance behavior and failure modes of ferrocement elements reinforced with woven square mesh and chicken mesh under pullout tests in two types of backfill materials. It is demonstrated that the failure modes of ferrocement depend on the surface properties of ferrocement, mortar composition, and type of mesh. Tests results reveal that square mesh-reinforced ferrocement with smooth surface shows frictional failure only under normal stresses, whereas with chicken mesh-reinforced ferrocement, there is mortar failure at higher normal stresses. It is also observed that the pullout resistance of ferrocement is improved significantly by using some small parallel channels on the surface of ferrocement elements transversely to the pullout direction. In the case of ferrocement containing two, four, and six channels, only the mortar failure is observed for square mesh-reinforced ferrocement, whereas the mesh failure is apparent for chicken mesh-reinforced ferrocement. A discussion regarding the corrosion of ferrocement elements as embedded in soil and a comparison between this technique and other soil reinforcement methods, such as geogrids or geosynthetics, in terms of cost, strength, and durability, are presented.

Keywords: failure modes; ferrocement; pullout response.

INTRODUCTION

Generally, conventional reinforcement used for reinforcing soil contains only one type of material, such as geogrid, geosynthetic, or wire mesh. It is known that the material used in soil reinforcement applications must be safe against tension failure and adhesion failure for its effective use in the field and reliable design of earth structures.1 A single type of material can provide limited reinforcement capability in reinforced soil structures due to its low frictional resistance and poor cohesion. For an optimal response, therefore, different types of reinforcement that fulfill both requirements such as possessing adequate tensile strength and frictional resistance, are getting considerable attention lately. Ferrocement, a thin reinforced-mortar composite consisting of evenly distributed fine wire mesh as the reinforcement and cement-sand mortar as the matrix can be a prospective complementary material for this perspective. The enhanced performance of ferrocement over conventional reinforcement comes from its synergetic action of mesh with mortar and mortar with soil. In ferrocement, high-tensile steel wire mesh provides adequate tensile and pullout resistance, whereas the sand-cement mortar provides adequate frictional resistance and improved cohesion owing with its relatively greater surface area and roughness as compared with conventional soil reinforcing materials. If properly designed, the rough surface of ferrocement elements can grip the soil particles and the frictional resistance needed for optimal design against pullout failure can be significantly improved. Very little or no research attention has been given to investigating the pullout response of ferrocement elements embedded in soil. The author is aware of only two such investigations on ferrocement thus far.2,3 In the first investigation, ferrocement-soil interface shear behaviors were studied under shear tests of two types of ferrocement panels with varying shear speed. It was shown that the ferrocement panels with plain surfaces had less frictional resistance and cohesion than the ferrocement panels with rough surfaces both in sandy and clayey soils, and it was concluded that the ferrocement-soil interface shear strength was decreased with the increase in shear speed. The second investigation presented a comparative study on the fundamental behavior between shear and pullout tests of ferrocement elements. It was concluded that the pullout strength was higher than the shear strength of ferrocement elements embedded in soil. An in-depth clarification on any particular test method such as detailed failure mechanisms of ferrocement elements under pullout tests, however, have not been clarified yet. The lack of research interest of ferrocement in this concern may be due to the fact that ferrocement is usually used as compression, flexural, and tension members of building components and structures, which preclude pullout response of ferrocement embedded in soil. Another possible cause of the scarcity of research on ferrocement as soil reinforcement may be the due to the lack of a bridge between the two professions such as geotechnical and structural engineers or soil and building engineers. Using ferrocement for reinforcing soil is not a difficult task as compared with using conventional soil reinforcement materials such as geogrids or geosynthetics. Ferrocement, a composite material made of sand-cement mortar reinforced by mesh, is especially suitable for applications in reinforcing soil structures such as earth dams and embankments, which are usually constructed in a layer-by-layer method and when the layers are particularly made in horizontal plane. Generally, in such soil works, reinforcements are horizontally placed between the two soil layers with a vertical spacing of approximately 800 to 1000 mm (31.4 to 39.3 in.). In these construction works, ferrocement panels either in precast form or in-place fabrication can be easily used. For example, in the case of in-place fabrication, after compaction of a soil layer, thin mortar (sand-cement mixture) layer (first mortar layer) can be spread over the soil and then the mesh can be placed on the first mortar layer. After that, another mortar layer (second mortar layer) can be spread on the mesh layer placed. No formwork nor skilled labor would be needed for this task. Application of geocomposites made with two separate material such as strip and grid, strip and anchor, as well as steel bars and anchor plates are some of the examples.4-6 In this way, the ferrocement would provide a composite that derives benefits from each of the individual reinforcement and exhibits a synergetic action between soil and mortar, as well as between mortar and mesh. Reinforcement of soil with ferrocement, however, still remains a science in its infancy, and ideas are still evolving toward assessing the optimal technique for soil reinforcement applications. Nevertheless, owing to the aforementioned distinct advantages of ferrocement and recent developments that broaden the scope of application of ferrocement, pullout response of ferrocement elements embedded in soil may become a critical design consideration for soil reinforcement applications.