In this work, we propose to extend the Ovarlez and Roussel model, for estimating lateral form pressure exerted by fresh concrete, to low casting rates and long duration formwork filling. In this ...case, the assumption of a linear increase of the concrete yield stress with time at rest is imprecise. This assumption is valid for approximately 1 h at rest after placing and may lead to an overestimation of the lateral pressure for a longer period. Based on chemical arguments and experimental observations, it has been shown that the yield stress increases exponentially over time. The same fundamental physical assumptions made by Ovarlez and Roussel are used to predict the lateral pressure exerted on formwork. Comparison of the proposed model with experimental measurements shows that, pressure can be predicted with an accuracy comparable to the Ovarlez and Roussel model for casting times less than 1 h.
The mixture design of cement-based materials for casting and three-dimensional (3D) printing processes are very different because of their workability requirements. For 3D printing, a compromise has ...to be found between pumpability and printability to build a structure able to resist its own weight. Specific admixtures can be used to guarantee specific properties and avoid printing deffects. This study is part of an ongoing work about the characterization of the tensile behavior of fresh cementitious materials governing deposit-induced cracks during the printing process. During printing, the material layer can be bent, in a sharp corner of a printed structure or during gravity-driven deposition, inducing tensile cracks in part of the outgoing extruded section. This paper presents a new device to test cementitious materials under tension to analyze visco-elastoplastic properties and damage mechanisms. Special attention is paid to the influence of viscosity-modifying admixtures on tensile behavior to improve material printability. Keywords: Double Dumbbell Device for Tensile Test (3DTT); microcracks; mixture design; plasticity; tensile behavior; three-dimensional (3D) printing; underwater printing; viscosity-modifying admixture (VMA); workability.
Fresh cement paste permeability is a key parameter to understand the hydro-mechanical behavior of cement-based materials, i.e., rhelogical properties and static stability. However, its permeability ...measurement is not easy to assess. The porous medium is not rigid and tends to change due to hydration kinetics. Two measurement methods, with 70
mm and 20
mm initial height specimens respectively, are presented and compared in this paper. The first uses a basic cell of soil permeability measurement and consists of simultaneous consolidation and percolation tests. The second uses a displacement-controlled oedometer cell equipped with pore water pressure transducers, and consists in inducing consolidation to a given void ratio first and, consecutively, in accurately measuring the permeability. A good correlation of results is observed. A comparison with theoretical models confirms that, from one fitted parameter relative to particle characteristics, a relationship between permeability and void ratio can be established.
•(Semi-)analytical solution of Luikov equations.•Considering time-periodic boundary conditions.•Using the Laplace transform and Gaussian quadrature.•Evaluation of moisture and temperature ...distribution within the porous material.
The paper addresses the problem of coupled heat and moisture transfer in porous materials with the time-periodic boundary conditions applied. The solution of Luikov equations 1, which describe coupled heat and moisture transfer, is presented. Laplace transform is used, where some terms of the inverse Laplace transform ought to be solved by Gaussian quadrature, meaning that the solution is semi-analytical. The time-periodic boundary conditions are applied to simulate the humidity and temperature oscillations of natural environment. Therefore, the proposed solution is appropriate to evaluate the distribution of moisture and temperature within the porous material exposed to everyday natural cycles. The paper presents convergence tests, validation of semi-analytical solution and application to different building materials are presented in the paper.
A novel oedometric cell has been used to study the permeability and compressibility of freshly made cement-based pastes. This cell allows rapid tests providing an accurate estimation of ...compressibility, permeability and thus consolidation coefficient with no cement hydration effect. The influence of the test-induced flow on the results obtained is discussed in order to find an optimized procedure. The effects of common mix-design parameters such as W/C ratio and polycarboxylate amount are discussed. Especially, it is shown that polycarboxylate high range water reducing admixture reduces the cement paste permeability and does not influence the compressibility of the cement grain network. The effect of W/C and admixture amount on paste bleeding is also discussed.
•Fresh cement paste permeability is largely reduced by polycarboxylate.•Mechanism of bleeding of paste containing polycarboxylate is highlighted.•Polycarboxylate content has no effect on the compressibility of fresh cement paste.
This paper deals with the processing of lime hemp composite blocks by compaction. A rigid compression die has been developed to reach relatively high compression pressure. This has been fitted with ...force and strain transducers to measure the stress state during compaction. It provides some means of understanding the effect of mix design and stress level on the compressibility and homogeneity of hardened composites. It is shown that moisture globally increases both friction and packing compressibility. Proper moisture content and paste-to-shiv mass ratio (P/S) can be chosen to reduce the heterogeneity. The combination of friction at wall level and stress transmission is the most detrimental for a moisture content of 20% and a P/S=1.
A rigid compression die has been developed to reach high pressure during LHC moulding. The die is fitted with force and strain transducers to measure the stress state during compaction. It allows understanding the influence of mix design and stress level on the compressibility and homogeneity of produced composites. Display omitted
•An original instrumented die able to strongly compact hempcrete was developed.•Water and lime paste do not always act as a lubricant at wall level.•Water and lime paste make packing more cohesive and reduce radial stress transmission.•A parameter to evaluate stress uniformity into the mould is proposed.•The heterogeneity parameter shows a maximum at 15% moisture and Paste Shiv Ratio of 1.
Hydraulic conductivity or permeability is a key parameter that affects the stability of fresh cement-based materials. For example, bleeding and water retention are among the most important issues ...governed by concrete permeability. Whereas this parameter is relatively easy to measure at the cement paste scale, it is difficult to evaluate with the addition of sand or gravel due to the sample size needed to obtain a representative sample. For ordinary permeability measurement, the test duration is linearly linked to the sample height. The aim of this study is to relate the concrete permeability to the cement paste permeability for various aggregate volume fractions in mortar or concrete. The proposed model uses an adaptation of Darcy’s law to compute concrete permeability from measured cement paste permeability and mix-design parameters. This model has been successfully tested with kaolin and cement pastes with spherical aggregates (glass beads) and sand.
It is shown herein that the bending problem of a discrete damage system, also called microstructured damage system or lattice damage system, can be rigorously handled by a nonlocal continuum damage ...mechanics approach. It has been already shown that Eringen’s nonlocal elasticity was able to capture the scale effects induced by the discreteness of a microstructured system. This paper generalizes such results for inelastic materials and first presents some results for engineering problems modelled within continuum damage mechanics. The microstructured model is composed of rigid periodic elements connected by rotational elastic damage springs (discrete damage mechanics). Such a discrete damage system can be associated with the finite difference formulation of a continuum damage mechanics problem, i.e. the Euler–Bernoulli damage beam problem. Starting from the discrete equations of this structural problem, a continualization method leads to the formulation of an Eringen’s type nonlocal model with full coupling between nonlocal elasticity and nonlocal continuum damage mechanics. Indeed, the nonlocality appears in this continualized approach both in the constitutive law and in the damage loading function. A comparison of the discrete and the continuous problems for the cantilever shows the efficiency of the new micromechanics-based nonlocal continuum damage modelling for capturing scale effects. The length scale of the nonlocal continuum damage mechanics model is rigorously calibrated from the size of the cell of the discrete repetitive damage system. The new micromechanics-based nonlocal damage mechanics model is also analysed with respect to available nonlocal damage mechanics models.
The objective of the study was to evaluate the effect of ligation of the ipsilateral common carotid artery (CCA) combined with various antimycotic treatments for the prevention of epistaxis in horses ...with guttural pouch mycosis. For each case, ipsilateral ligation of the CCA was performed, followed by application of various topical medications under endoscopic guidance. Frequency and number of treatments, outcome and recurrence of haemorrhage were retrospectively recorded. Twenty-four horses were included. Topical medication was administered by detachment of the diphtheric membrane and spraying (n=16) or by intralesional injection directly in the plaques using a transendoscopic needle (n=8). Epistaxis recurred in five horses (20.8 per cent), causing death of four horses (16.6 per cent). The mean number of treatments was 6.3±4.0 (range 2-14) for all topical treatments. Ligation of the ipsilateral CCA and topical medication carries a fair prognosis for avoidance of recurrent episodes of epistaxis, but fatal haemorrhage can occur. Removal of the fungal plaque and topical treatment of the underlying lesion appeared to speed up resolution of the mycotic mucosal lesions. The described technique is a salvage procedure when financial or technical constraints prevent the use of transarterial catheter occlusion techniques.
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