Paste Volume Research Articles

Influence of initial water content of aggregates on the compressive strength and the shrinkage of High Performance Concretes

The time-dependent deformations of concretes, particularly in the case of high performance concrete (HPC) must be taken into account precisely when measurement of civil engineering works. theses deformations is essential for predicting the deflections, the stress distribution in statically indeterminate structures and the loss of pre-stressing force in elements of pre-stressed concrete. However, the experience shows a certain dispersion of values of instantaneous and delayed deformations measured and some significant differences with the values calculated using the building codes. The objective of the present paper focuses on the parameter which is not taken into account in building codes, which could causes the inaccuracies of their predictions: the initial water content of aggregates. The experimental program includes a comparative study of the mechanical behaviours (instantaneous deformations and shrinkage) of HPC with the same basic formulation (water cement ratio, volume of paste), differing principally in the initial water content of aggregates. The experimental results show that HPC made with wet aggregates has a higher resistance than others (dry aggregates). The pre-saturated aggregates would be a water reservoir in which the dough could slow the self-drying due to hydration in the capillary pores at the level of the Interfacial Transition Zone (ITZ). This would increase the amount of hydrates and improve the mechanical behaviour of this interface through better adhesion and less porosity. If the evolution of shrinkage of HPC with wet or dry aggregates can be distinct during the first months, the effect of this formulation parameter is not clear since it differs according to the type of aggregate. Moreover, in the long term, convergent shrinkage shows that this influence becomes insignificant. The comparisons with estimations of Eurocode 2 model were also performed and showed significant differences with the experimental values.

Effects of calcium leaching from high volume fly ash cement paste and mortar

Calcium leaching is a deterioration problem of concrete that has long been in contact with water such as dam or reservoir. This paper investigated the effects of calcium leach out on high volume fly ash mixtures of paste and mortar. The samples, composed of fly ash to cement ratio, of 100:90 (by weight), were immersed in 6 M ammonium nitrate solution for 1- 28 days. Then, the leaching depths of samples were investigated and characterized by several techniques for porosity and microstructural studies namely; scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), mercury intrusion porosimetry (MIP) and Vickers hardness test. The results from SEM/EDS showed that the calcium ion was leached out as high as 42% and 36% for paste and mortar, respectively. The hardness of leaching zone was reduced by 27% for paste and 22% for mortar. It was found that pore diameter and pore volume of both paste and mortar were significantly increased. The results indicated significant changes in porosity and microstructure.

Influence of Soil-Cement Composition on its Selected Properties

The paper discusses the results of mechanical and technological tests of soil-cement composites made with cohesive soil. The compositions of analysed soil-cement mixtures differed in terms of their cement paste volume fractions and water-cement ratios. Limiting values of these technological parameters that enable the application of the soil-cement mixtures obtained in real life conditions for the purposes of the Deep Soil Mixing (DSM) method were determined. Based on the test results obtained, it was found that mechanical properties of the materials analysed were very sensitive to changes in their compositions. Variations in the volume fraction of cement paste within the range analysed caused mechanical properties to change even by an order of magnitude.

Low cement content SCC (Eco-SCC) – the alternative for ready-mix traditional concrete

Self-Compacting Concrete (SCC) is often regarded for in-situ applications as a more expensive and less environmentally friendly version of traditional concrete for use in places where the latter is highly inconvenient for various reasons. To counteract this belief, a new type of SCC has been proposed, called Eco-SCC or green-SCC. In comparison to typical SCC this building material has lower powder content resulting in low cement content and a slightly lower paste volume. The disadvantage of this approach is high water content and low viscosity resulting in high vulnerability of fresh concrete to sedimentation. Additionally, comparing to EN-206 limits, mixes with too low cement content are often obtained. This is why a research program has been undertaken to check the possibility of obtaining fresh Eco-SCCs of higher viscosity, thereby fulfilling all EN-206 requirements. It was possible to obtain concretes of C25/30-C35/45, SF2, VF2, PL2, containing 265-300 kg/m3of cement and 165-190 kg/m3of water owing to the use of limestone powder and typical rounded aggregate of local origin with very low sand to a total aggregate ratio ofs/a= 0.4. Based on Global Warming Potential (GWP) analysis, a modification of Eco-SCC definition is proposed: Eco-SCC is an SCC having cementitious materials volumetric content no greater than 100 dm3/m3(or mass content no greater than 315 kg/m3).

Online control of stencil printing parameters using reinforcement learning approach

This research proposes a novel approach to control the stencil printing process (SPP) parameters online in surface mount technology (SMT) of printed circuit boards (PCBs). Several external variables induce variations in stencil printing quality including environment conditions, operator faults, and others. This research aims to build an optimal adaptive controller that captures these variations and consequently adjusts the controllable and significant printing parameters to enhance the solder paste volume transfer efficiency (TE) during actual production run. Q-learning which is a reinforcement learning (RL) approach is used to control the main printing parameters (printing speed and pressure, and the separation speed) online. The results show that Q-learning converges to the optimal policy for the SPP problem, and the optimal sets of actions for different states are retrieved using Q-table. Moreover, the developed controller is capable to reach the terminal state for several testing examples with taking few actions.

Influence of Cementing Paste Volume on Properties of Controlled Low Strength Materials

AbstractControlled low strength materials (CLSM) are gaining increasing importance with the growing emphasis on sustainability in the construction industry. Controlled low strength material can be ...

Optimizing mortar extrusion using poplar wood sawdust for masonry building block

The wood industry generates some 12 million tons of waste in France, 60% of which basically stems from sawmills. The reuse of this waste in the most common construction materials (e.g., concrete) may offer a sustainable solution. In the literature, the optimization of composites formulations generally focuses on aggregates plant pretreatment regardless of classical optimization parameters and the implementation process that can afford to get a high strength composite. The work presented herein is intended to optimize the mix design of a mortar made from poplar wood by-products, without involving any pretreatment. The conventional mix design parameters (i.e., paste volume, W/C ratio) have been varied. An optimal implementation technique (i.e., extrusion) was used. The wood mortar mix designs have been optimized in order to obtain composites compliant with French standard NF EN 771-3/CN, with respect to criteria regarding both dimensional tolerance and mechanical strength for structural elements (i.e., above 8 MPa at 7 days).A mortar mix design, optimized by means of extrusion, has been used as a reference. Poplar wood particles have been introduced through substitution for the sand volume. Test results indicate that the blocks produced from two wood mortar mixes with 60 and 70% wood aggregate-for-sand substitution rates are in line with the normative requirements relative to dimensional tolerances and mechanical strength. This material compliance underscores the potential use of such blocks as load-bearing masonry elements, thus facilitating the building of structures with a limited environmental impact.

Filler technology for low-carbon high-dimensional stability concrete

In a concrete mix, a minimum paste volume is needed to ensure that there is sufficient paste to fill the voids between aggregate particles and provide excess paste for forming paste films on aggregate surfaces. However, in order to decrease the carbon footprint and increase the dimensional stability of the concrete, it is desirable to reduce the cementitious paste volume and cement content to as low as possible. Herein, it is proposed to employ the filler technology of adding a filler, which is as fine as cement, to fill into the voids so that the cementitious paste volume could be reduced to lower than the usual minimum. For feasibility study, a number of trial concrete mixes with different amounts of limestone fines added as a filler to replace an equal volume of cementitious paste were produced for testing. By so doing, some low-carbon high-dimensional stability concretes with the cementitious paste volume lowered to 20% or 18% of concrete volume, carbon footprint substantially reduced, and dimensional stability significantly improved are developed.

Viscoelastic properties of self-consolidating concrete: Influence of the sustainable approach

The service life of concrete structures is primarily affected by cracking. Concrete with a lower cracking potential is consequently more sustainable. In order to estimate the risk of shrinkage-induced cracking, at least four properties should be studied: shrinkage, Young's modulus, creep/relaxation and tensile strength. A mixed experimental and numerical approach, taking into account these properties is presented and used in this paper to improve our understanding of this complex phenomenon. The study is focused on restrained shrinkage of thin structures exposed to drying. Based on the viscoelastic and solidifying theories and on the experimental results from an adapted ring test some explanations of the observed cracking tendencies can be given. The efficiency of this method is shown when it is applied on two experimental studies linked with the sustainability issues. The first one concerns the greener mix-design which was found to have a positive effect on the cracking resistance, with two ways of reducing cement content. The reduction of the paste volume as well as the substitution of Portland cement by supplementary cementitious materials (SCMs) delayed cracking with different mechanisms. The second study aims to analyze the effect of the sealed curing duration on cracking, where the extracted aging-viscoelastic Young's modulus decreases with the curing duration and explains the observed results.

Hydrate failure in ITZ governs concrete strength: A micro-to-macro validated engineering mechanics model

Ever since the early days of Féret (1892) and Abrams (1919), concrete research has targeted at relating concrete composition to uniaxial compressive strength. While these activities were mainly characterized by empirical fitting functions, we here take a more fundamental approach based on continuum micromechanics. The loading applied at the concrete level, is first concentrated (“downscaled”) to maximum stresses related to cement paste volumes which are directly adjacent to the aggregates, i.e. to the interfacial transition zones (ITZ). These maximum stresses are further “downscaled” to the micron-sized hydrates, in terms of higher-order stress averages. The latter enter a Drucker-Prager failure criterion with material constants derived from nanoindentation tests. The model is successfully validated across the hydrate-to-concrete scales. Strength magnitude is governed by ITZ stress concentrations, and the water-to-cement ratio is its dominant mixture design parameter.

How Water-Aggregate Interactions Affect Concrete Creep: Multiscale Analysis

AbstractCustomary micromechanics models for the poroelasticity, creep, and strength of concrete restrict the domain affected by the hydration reaction to the cement paste volume, considering the la...

Rapid Prototyping Tape Stencils for the Application of Solder Paste

Abstract This works describes a promising method for rapid prototyping tape stencils for the application of solder paste. This process is appropriate for research settings requiring developmental flexibility and the ability to deal with small device dies. This work compares the volume of solder paste deposited versus aperture volume for several common tape materials and several common printed circuit board (PCB) stencil materials. The solder deposits are then reflowed to identify which aperture and solder paste parameters can generate successful solder bumps. Electrically conductive solder bonds for small bond pads (100 μm and larger) are demonstrated between silicon device dies and glass dies using this process.

Filler technology for improving robustness and reducing cementitious paste volume of SCC

The flowability, cohesiveness and passing ability of self-consolidating concrete (SCC) are very sensitive to changes in free water content, aggregate grading and superplasticizer dosage, among which the free water content is the most difficult to control because of intrinsic variations in water absorption and moisture content of the aggregate. Hence, there is the robustness problem of maintaining the required performance during large scale production. Moreover, there is also the problem of large cementitious paste volume, which leads to low dimensional stability and high carbon footprint. In this study, the possible use of filler technology to overcome these two problems has been explored by adding limestone fines and ground sand as cementitious paste replacement and evaluating their effects on the performance and robustness of SCC. It was found that such filler technology can significantly improve the robustness of SCC and the key factors are to have sufficient mortar volume and cohesiveness. However, this technology is not suitable for application to concrete mixes for long distance pumping.

The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate

Efforts to tune the bulk physical properties of concrete are hindered by a lack of knowledge related to the atomic-level structure and growth of calcium silicate hydrate phases, which form about 50–60% by volume of cement paste. Here we describe the first synthesis of compositionally uniform calcium silicate hydrate phases with Ca:Si ratios tunable between 1.0 and 2.0. The calcium silicate hydrate synthesized here does not contain a secondary Ca(OH)2 phase, even in samples with Ca:Si ratios above 1.6, which is unprecedented for synthetic calcium silicate hydrate systems. We then solve the atomic-level three-dimensional structure of these materials using dynamic nuclear polarization enhanced 1H and 29Si nuclear magnetic resonance experiments in combination with atomistic simulations and density functional theory chemical shift calculations. We discover that bridging interlayer calcium ions are the defining structural characteristic of single-phase cementitious calcium silicate hydrate, inducing the strong ...

The Evaluation of Self Compacting Concrete Robustness Based on the Rheology Parameters

The fresh properties of Self Compacting Concrete (SCC) might be more susceptible to quality and quantity changes of ingredients than conventional concrete because of a combination of detailed requirements, more complex mix design, and inherent low yield stress and viscosity. In spit of the low robustness of SCC, there are a few methods available to assess the SCC robustness that the accuracy of these methods has not been fully agreed. The current study provides an index for SCC robustness based on the rheology parameters. Thus, an experimental program was undertaken to evaluate the robustness of eight selected SCCs. For doing this, water content of each SCC was changed slightly and their fresh and hardened properties were measured. The results indicated that the length of rheology parameters curve due to variation of mixing water is able to assess the SCC robustness that is comparable with combined performance based on the workability tests changes. According to this index, the robustness of SCC increases about 10% using air-entraining admixture (AEA) and decreases considerably by reduction the paste volume (up to about 5 times). In addition, the most appropriate single workability test to assess the robustness is sieve segregation test. Moreover, the scattering of compressive strength results show that there is a level of robustness in fresh state that after that the scattering of results in hardened state can be affected.

Mix Design and Recycled Aggregates Effects on the Concrete’s Properties

An experimental program was carried out to study the properties of concretes with C25/30 and C35/45 resistance classes designed using natural aggregates and recycled ones provided from construction and demolition waste (C&DW). Eight mixtures were prepared with different incorporation ratios of fine and coarse recycled aggregates. The physical and mechanical characteristics were measured and their evolution regarding to the paste volume, the equivalent replacement ratio and the quality of the interface between recycled aggregates and new paste has been investigated. Scanning Electron Microscopic observations have revealed a good quality of the interface. It was found that water porosity is more dependent on the equivalent replacement ratio than on the volume of the new paste. The same conclusions are reported for the electrical resistivity which has been related to the water porosity. Based on experimental results, it has been found that the Archie’s law predicts the electrical resistivity as a function of water porosity for all type of concrete. The effect of mix design parameters and aggregates properties on the compressive strength has been investigated and a model is proposed on the basis of Feret’s expression through experimental results and data points available in the literature. The modified expression is related to the aggregate fragmentation resistance expressed by the Los Angeles coefficient (LA). The Feret’s coefficient can be taken equal to 6.4 if LA ≤ 20% and it is inversely proportional to LA otherwise. With regard to the splitting tensile strength, it can be correlated to the electrical resistivity through a power law relationship. The elastic modulus, the peak and the ultimate strains are than rewritten as functions of the compressive strength and the replacement ratio to model the full stress–strain curve under uniaxial compression. Furthermore, the validity of using EC2 to model the evolution of all mechanical properties with age is ensured.

Effect of the mix design on the robustness of fresh self-compacting concrete

Self-compacting concrete (SCC) has many advantages compared to vibrated concrete. A disadvantage is the lower robustness of fresh SCC. SCC is more sensitive to small changes in the mix design, material properties, and the applied production methods. In an experimental program, the influence of important mix design parameters on the robustness of SCC was studied. First, the influence of the paste volume and the water-to-powder volumetric ratio was investigated. Depending on the mechanisms providing stability in the mixture, different levels of impact were observed. When the yield stress is the main factor providing stability in the mixture, a change in the water content will mainly affect the yield stress, making the stability of the yield stress the most important factor determining the robustness of the mixture and can be improved by lowering the paste volume. Analogue, the sensitivity of the plastic viscosity is determining the robustness of mixtures in which mainly the plastic viscosity is providing stability. The robustness of such a mixture can be improved by increasing the water-to-powder volumetric ratio. The influence of two types of viscosity modifying agents (VMA's) on the robustness of fresh SCC was examined in a second stage. The two used VMA's (diutan gum and attapulgite clay) were especially effective in SCC mixtures having a high yield stress and a low plastic viscosity. In mixtures having a low yield stress and a high plastic viscosity, the inclusion of a VMA in the mix design resulted in a decrease of the robustness.

Roles of packing density and water film thickness in strength and durability of limestone fines concrete

Recent studies have revealed that the use of limestone fines (LF) to replace an equal volume of cementitious paste in concrete could improve the strength and durability of the concrete and reduce the carbon dioxide footprint of concrete production. However, there are different views on how and why the use of LF as a cementitious paste replacement without changing the mix composition of the paste could increase the performance of hardened concrete, bearing in mind that LF are not inherently cementitious. In this work, the effects of the addition of LF on the packing density and water film thickness (WFT) of concrete mixes were investigated, and the roles of packing density and WFT were studied by correlating the strength and durability to the voids ratio (determined from the packing density) and WFT. Based on the results of this study, it is suggested that the use of LF as a cementitious paste replacement improves strength and durability mainly through increasing the packing density (the same as decreasing the voids ratio) and decreasing the WFT of the concrete mix.

Transport properties evolution of cement model system under degradation - Incorporation of a pore-scale approach into reactive transport modelling

This paper describes a multi-scale approach for the modelling of the degradation of model cement pastes using reactive transport. It specifically aims at incorporating chemistry-transport feedback results from a pore-scale approach into a continuum description. Starting from a numerical representative elementary volume of the model cement paste, which was built according to extensive experimental dedicated chacarterizations, this paper provides three separate descriptions of two different degradations: leaching and carbonation. First, 2D pore-scale simulations are performed and predict degradation depths in very good agreement with experiments. Second, 3D pore scale descriptions of how the microstructre evolves provides accurate description of the evolution of transport properties through degradation. Finally, those latter results are incorporated as a feedback law between porosity and effective diffusion coefficient into a 1D continuum approach of reactive transport. This paper provides pore-scale explanations of why reactive transport modelling has encountered mitigated success when applied to cementitious materials, especially during carbonation or degradations consisting of precipitation reactions. In addition to that, different degradation modellings are in very good agreement with experimental observations.

The durability of cementitious composites containing microencapsulated phase change materials

This study investigates the durability of cementitious composites containing microencapsulated phase change materials (PCMs). First, the stability of the PCM's enthalpy of phase change was examined. A reduction of around 25% in the phase change enthalpy was observed, irrespective of PCM dosage and aging. Significantly, this reduction in enthalpy was not caused by mechanical damage that was induced during mixing, but rather by chemical interactions with dissolved SO42- ions. Second, the influence of PCM additions on water absorption and drying shrinkage of PCM-mortar composites were examined. PCM microcapsules reduced the rate and extent of water sorption; the former was due to their non-sorptive nature which induces hindrances in moisture movement, and the latter was due to dilution, i.e., a reduction in the volume of sorptive cement paste. On the other hand, PCM inclusions did not influence the drying shrinkage of cementitious composites, due to their inability to restrain the shrinkage of the cement paste. The results suggest that PCMs exert no detrimental influences on, and, in specific cases, may even slightly improve the durability of cementitious composites.