Elasto-plastic model for sand including time effect

Time effects on granular soils have been observed in the laboratory and by in-situ tests, but these cannot be reproduced by classical elasto-plastic models. To address these concerns, existing specific modelling approaches were based on the theory of viscoplasticity formulated by Perzyna or on a viscous evanescent relationship. This work explores an alternative elasto-plastic modelling framework formulated in a multiaxial structure space. The proposed elasto-plastic model is associated with a thixotropic-type framework through the use of a structure parameter, the evolution of which illustrates the competition between two effects: the time-dependent tendency of the granular system to reach its stable configuration - restructuration - and its destructuration under external perturbations. The structure parameter is linked to the existence of a stress-dependent target structure towards which the current granular material structure evolves. The timescale is explicitly introduced by postulating a rate for this structure evolution. The modelling of the material behaviour has shown good similarities with the response of granular soils observed in monotonic loading, as well as during creep and variable strain rate loading experiments

Cob, a vernacular earth construction process in the context of modern sustainable building

The will of reducing environmental and social impact of building industry has led to a renewed interest in earth construction. Most of earth construction literature dealt with rammed earth or adobe techniques, but very little with cob. Yet, cob participates in the diversity of vernacular earth construction processes that value local materials and is an alternative to rammed earth and adobe in specific geographical conditions. Conservation of cob heritage also requires a better knowledge of this vernacular construction process. This bibliographical analysis gathered extensive data on cob process and summarized the different cob process variations, attempting to take into account their diversity. This analysis allowed us to provide novel data on cob process, and more specifically, (1) a clear definition of cob with regard to other earth construction processes, (2) a first summarized description of cob process that clearly distinguished its variations, (3) a list of fibres traditionally employed, (4) values and, if possible, average and standard deviation for fibre length, fibre content, manufacture water content, drying times, lift heights and wall thicknesses, (5) a summary of the strategies to manage shrinkage cracks, (6) a criterion on the quality of implementation and/or earth for cob, based on slenderness ration of lifts and (7) a discussion on the evolution of cob process with regard to societal evolutions

Influence of the water and aggregate contents on the concrete mixing evolution

A new image analysis technique coupled with the power consumption were employed in this paper and proved to be a relevant method to detect the different characteristic times of the mixing evolution, i.e. the cohesion time, the fluidity time and the half-life time of the mixture evolution after fluidity. It was shown that the “effective water-to-powder ratio in the mortar” which excludes from the effective water the quantity of water needed to fill the less compact packing in the vicinity of the coarse aggregates, governed the mixing behaviour before fluidity and the interval of water content where the concrete mixture behaves as a homogenous and fluid granular suspension. It was also shown that the mixing behaviour was adversely affected by the amount of water into the mixture. The fluidity and cohesive times were faster to be obtained when the water proportion was increased. At opposite, it was more difficult to obtain the final consistency for a more fluid mixture, at given super-plasticizer proportioning. As a general remark, the behaviour seemed controlled by the mixture composition before the fluidity point and by the mixture consistency after the fluidity point. To end, the image analysis technique coupled with the mixing power technique can be employed at the end of the mixing to complete the information obtained with the other methods of analysis in order to monitor the consistency of the mixture. But, the use of the cohesion and fluidity times, easy to detect with the texture analysis, could be more efficient monitoring tools from an industrial point of view

Differences between laboratory and in-situ conditions leading to bad estimation of the water absorption capacity of recycled concrete aggregates

Poor estimation of the water absorption coefficient (WA) of recycled concrete aggregates (RCA) engenders inadequate effective water for the recycled aggregate concrete (RAC), leading to poor mechanical properties or poor workability. Pre-saturation of RCA before mixing could appear as a potential technical solution. However, pre-saturation before mixing is a complex task that is most generally not applied in RAC manufacturing. Therefore, better knowledge of in-situ RCA water absorption capacity is necessary. This paper focuses on the influence on the water absorption capacity of i) the initial moisture of RCA, and ii) their heterogeneity. For this purpose, 6.3/10 mm RCA were pre-wetted using different methods simulating long-term (LT) and short-term (ST) pre-moistened aggregates. It is shown that this pre-moistened history (LT or ST) leads to different levels of total water absorption for a same initial moisture level. The difference lies between 0 and 1%, depending on the initial moisture level. In addition, the heterogeneity of RCA was investigated by separating samples of RCA into classes of densities using a water jig. The WA is about 5% for heterogeneous 6.3/10 mm RCA, but for the homogenous specimens separated by density WA ranges from 2% to 9%

Mixing recycled aggregates concrete – old methods for new concrete

There have been a great number of laboratory studies on mixing approach for recycled aggregate concrete (RAC) even if in practice, the simplest procedures are generally employed. Some of these methods give good results comparing with the traditional mixing method. The present paper resume some main mixing methods proposed in the literature, specifically for RAC. The two-stage mixing approach (TSMA) splitting the required water into two parts in divers proportions, is a development of the previously Japanese proposed sand enveloped with cement (SEC) mixing method. Other methods were proposed, like for instance the sand enveloped mixing approach (SEMA) or the loading of (recycled) aggregates into a preprocessed mortar or paste (cement paste encapsulating aggregate method). This short literature review also discusses the effect of the initial moisture of the RCA and the concept of effective water which seems still valid for RAC. It is not clarified today the extent to which the improvements resulted from alternative mixing methods are specific to the RAC mixing, or are more general phenomena. In the light of recent works, two points are underlined here. First, the quantity of water absorbed by the aggregates depends of factors not sufficiently elucidated yet. For instance, the pre-wetting method potentially could conduct to different RCA water absorption after mixing. Second, the mechanical benefits of the hardened RAC produced by TSMA it is classically explained by an increase of the ITZ strength. We consider that a complementary explanation should be looked in the higher attrition of the RCA produced by TSMA. This phenomenon is supposed to induce better compacity of the mixture and higher quality of the RCA