Dynamic characterization of adobe in compression: the effect of fibre fraction in soil matrix

Abstract

Adobe is one of the most ancient forms of masonry. Adobe bricks are sundried mixtures of clay, silt, sand and natural fibres locally available joined together using mud mortar. Adobe structures are largely spread in areas of the world prone to earthquakes or involved in military conflicts. Unfortunately, almost no literature concerns the dynamic assessment of soil-based masonry components. From earlier research, it was derived that the mechanical behaviour of adobe in statics fits in the class of quasi brittle materials. Its resemblance with cementitious materials concerns the main failure modes and the constitutive models in compression. This study deals with the experimental characterization of adobe components response in dynamics. It is aimed to study and quantify the rate sensitivity of adobe material from bricks at a wide range of strain rates, from statics up to impact conditions. In particular, the influence of fiber reinforcement in the mixture on the mechanical behaviour of the material has been addressed. Adobe bricks are commonly mixed using organic content locally available in the field, from straw to chopped wood. Fibres are added to prevent shrinkage cracks during the air drying process. In modern materials such as concrete, inclusion of artificial fibres is originally meant to enhance the mechanical performance of the material, benefiting from the selective properties of reinforcement and binder. An experimental campaign was carried out in a collaboration between Delft University of Technology, Dutch Ministry of Defence, TNO and the Joint Research Centre (JRC) of the European Commission. Two types of bricks were tested. They both had the same soil composition in terms of mineralogical family and soil elements proportions but only one was mixed using straw and wood. Cylindrical samples were subjected to compression tests at different rates of loadings in compression: low (˙ 1 = 3 10 −4 s −1), intermediate (˙ 2 = 3 s −1) and high (˙ 3 = 120 s −1). High strain rate tests were performed using the split Hopkinson bar of the Elsa-HopLab (JRC). For each test, high resolution videos registered the failure process and force-displacement plots were

Dynamic simulations of traditional masonry materials at different loading rates using an enriched damage delay: Theory and practical applications

“…In quasi brittle materials, spatial and temporal progression of micro-cracking processes from the first flaws is found to be dependent on the intrinsic properties of the material and on the external load applied [32,33]. It is experimentally observed that the nucleation time leading to the bridging process for adobe as well as for other masonry materials is significantly influenced by the external rate [16]. In statics, if propagating flaws encounter stiffer areas, they have the time to deviate around these zones, bridging into macro-cracks, and fracture along a crack path with minimum energy demand is followed.…”

“…3 (Figure 1), with loading profiles at the upper boundary of the quasi static regime (1-5 mm/s). Next, modifications are validated against real dynamic tests recently performed by the authors on various soil based masonry materials [16]. Numerical simulations of experimental tests using eq.…”

“…However, besides a decay of the strength, the shape of the resulting softening slope remain the same using constant parameters. Instead, the mineralogical properties and inter-particle interactions influence the micro-flaws size and distribution which dictate the speed and progression of the micro-cracks coalescing into the structural macro-crack up to failure [16,33].…”

“…The opportunity of the proposed solutions are interpreted in light of the theory on the dynamic behaviour of quasi brittle materials commonly used in masonry. For both cases, they are firstly numerically demonstrated via simulations of theoretical problems and validated via numerical simulations of two real dynamic compression tests recently performed by the authors on different types of masonry bricks [16]. Many of these tests concerned the material characterization of adobe components, sundried mixtures made of silt, sand and clay.…”

Dynamic behaviour of adobe bricks in compression: The role of fibres and water content at various loading rates

“…Mean values are shown as a log-scale function of the applied strain rate in Fig. 10 and 11 and standard deviations are included in symbol thickness [35]. The validity of the essential assumption of stress equilibrium in the specimen during split Hopkinson bar tests has been checked and calculated average relative errors of the order of 5% in the non linear force equilibrium conditions were considered as acceptable (Fig.…”