Design and construction application of concrete canvas for slope protection
Graphical abstract
Introduction
Over the past decades, one of the most important research object is to improve the mechanical properties of cement to adapt to the more complex construction environment. New materials are constantly being applied in construction to reduce the environmental impact. Among them, concrete canvas is an ideal construction material for slope protection.
Concrete canvas (CC), invented by Brewin and Crawford in 2004 in the UK, is part of a new class of revolutionary construction materials called geosynthetic cementitious composite mats (GCCMs). CC is a flexible, cement-impregnated fabric that hardens on hydration to form a thin, durable, waterproof and fire-resistant concrete layer [1]. Recently, Bao [2] and Han et al. [[3], [4], [5]] revealed that the mechanical strength and volume stability of CC were directly affected by the properties of the components, including the type of cement matrix, type of fiber, and geometric pattern of the fabric.
Normally, cloth-like CC is rolled up and sealed in a package by the manufacturer. It can be transported to the deployment site for easy utilization. CC allows for construction without the need of mixing/casting/finishing equipment or a framework. The engineer can position CC on the surface of the target easily, and just spray water. After the CC hardens, it forms a rigid protection layer. Compared with traditional materials for slope protection layers (e.g., shotcrete layers), CC is faster, easier, and more cost-effective product to install, and has excellent benefits of reducing the environmental impact, shortening the construction period, and lowering labor costs. For durability, CC is twice as abrasion resistant as standard OPC concrete, has excellent chemical resistance, good weathering performance and will not degrade in UV. CC shows high durability with minimum 50 year expected life. In addition, CC has excellent impermeability, the permeability of material mainly depends on the particles size, the density of particles, the shape of particles or the pore structure around aggregates [[6], [7], [8], [9]]. Since the usage of fine particle cement powder and the fully hydrated of cement powder, the matrix of CC after hydration is very dense and the permeability (k-value) of CC can reach between 10−8 and 10−9 m/s. More important, CC has a significant advantage for the rapid construction of slope protection [10] when the construction environment is severe and complex (i.e., emergency construction of slope protection or heavy rainfall).
At present, slope stability is usually evaluated by using limit equilibrium methods (LEM) or finite element model (FEM), and the seepage field is often estimated by various simplified methods in geotechnical engineering practice. The most difficult problem in using the displacement-based FEM for a stability analysis is in calculating the factor of safety. This difficulty can be overcome by introducing an FEM with a shear strength reduction technique (SSRFEM), which was proposed as early as 1975 by Zienkiewicz et al. [11]. Numerical examples that have appeared in the literature show that SSRFEM is an effective method for assessing the factor of the safety of slope and for locating the failure surface [12,13]. Cai and Ugai [14] utilized SSRFEM to investigate the stability of slopes under rainfall by analyzing the transient water flow through unsaturated-saturated soils. Huang et al. [15] investigated the accuracy of the technique and compared it with a solution from conventional limit equilibrium methods for soil slopes under transient seepage.
CC has been increasingly used for slope protection. However, to the authors' knowledge, little research works on the geotechnical behavior of CC slope protection can be found in the literature. To form a design basis for the application of CC in the field of slope protection, this paper carries out a study on the design and performance evaluation of CC slope protection. First, a series of experiments are carried out to investigate the compressive strength development, tensile strength, and shear strength of CC and FRP-reinforced CC. Then, a brief description of the construction process of CC slope protection is presented. Finally, a finite element model is used to evaluate the stability of a slope protected by CC and FRP-reinforced CC layers. The tensile stress, shear stress, and displacement of CC and FRP-reinforced CC layers are analyzed, the simulation results are compared with the tensile and shear strengths of CC and FRP-reinforced CC. The stability of the slope of CC and shotcrete with different curing times, and the stability of CC slope protection under heavy rainfall, are also investigated.
Section snippets
Mechanical properties
Shotcrete is an engineered material that has been extensively used for the rapid construction of embankment slopes in highway engineering works. In this section, the constitution and preparation process of CC and FRP-reinforced CC are presented. Then, the compressive strength development, the tensile and shear behaviors of CC and FRP-reinforced CC are tested to make a comparison with shotcrete.
Slope stability analysis
Owing to the characteristics of flexibility and light weight, the typical construction speed (as shown in Fig. 6) of CC slope protection is 200 m2/h by a three-man team. The use of AFRP and 3D spacer fabric will inevitably lead to an increase in material costs; but significantly reduce both construction cost and curing cost, for instance, safe and easy to install due to the flexibility of CC, quick construction with very low labor cost, unnecessary extra moisture curing, unnecessary framework
Conclusions
The features of rapid hardening, high early-age strength, and low construction costs of the CC product make it very suitable for application of slope protection. FRP-reinforced CC has high tensile strength and extends the application of CC in slope engineering. In this paper, the compressive strength development of CC and the tensile strength of CC and FRP-reinforced CC are tested to investigate their basic mechanical properties. The stability of CC and FRP-reinforced CC slope protection in
Acknowledgments
The authors gratefully acknowledge the financial support of the National Nature Science Foundation Project of China (Grant No. 518781521 and No. 5146113500), the Ministry of Science and Technology of China's “973 Project” (Grant No. 2015CB655102).
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