Permeability measurement of fresh cement paste

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.     

Early-age acoustic emission measurements in hydrating cement paste: Evidence for cavitation during solidification due to self-desiccation

In this study, the acoustic emission activity of cement pastes was investigated during the first day of hydration. Deaired, fresh cement pastes were cast in sealed sample holders designed to minimize friction and restraint. The majority of acoustic emission events occurred in lower water to cement ratio pastes, while cement pastes with higher water to cement ratios showed significantly less acoustic activity. These acoustic events occurred around the time of setting. A layer of water on the surface of the cement pastes substantially reduced acoustic emission activity at the time of setting. According to these experimental results, the acoustic emission measured around setting time was attributed to cavitation events occurring in the pores of the cement paste due to self-desiccation. This paper shows how acoustic emission might be used to indicate the time when the fluid–solid transition occurs in a cement paste, often referred to as time-zero. Knowledge of time-zero is fundamental for determining when mechanical properties develop and in calculations of residual stresses.     

Lateral stress exerted by fresh cement paste on formwork: Laboratory experiments

In the present paper, experimental results from laboratory testing on cement pastes are presented. The validity of both the predictions of the model proposed by Ovarlez and Roussel [Mat. and Struct. 39(2) (2006)] and the assumptions on which the model is based in the case of rough interfaces between the cement paste and the formwork surface are validated. The consistency of the underlying theory is tested in several different configurations (plate test with large and narrow gaps and a column test). Finally, the concept of a new test allowing the measurement of the structuration at rest of a cementitious material is proposed. Such a simple test might be used to assess the input parameters needed to predict formwork pressure without the need for a rheometer.     

Observations of air-bubbles escaped from fresh cement paste

Recent experimental work is presented using a new technique to observe bubbles that have escaped from cement pastes and suspended in the bleed water. These experiments suggest that the stability of an air-entrained bubble may be related to the integrity of the hydration shell. This paper also reviews literature dealing with changes to air bubbles with time in fresh air entrained cement paste and concrete and to the existence of a shell surrounding these bubbles.     

A volumetric technique for measuring the coefficient of thermal expansion of hardening cement paste and mortar

Knowledge of the coefficient of thermal expansion (CTE) is of paramount importance for the determination of the cracking risk of concrete structures at early ages. This paper presents a novel technique which is suitable to measure the CTE of hardening materials with high accuracy starting from casting time.The technique consists of casting a small amount of cement paste or mortar into flexible membranes. The specimens are immersed in an oil bath, whose temperature is rapidly changed and then kept constant in repeating cycles. By suspending the sample from a high-precision balance and reading the change of mass after each temperature step, the CTE is calculated with high accuracy from the measured temperature and strain.Results on cement pastes and mortars (water/cement 0.3) showed a good repeatability. In particular, a sudden decrease in the CTE at setting time, followed by a gradual increase as the cement paste self-desiccates, was measured.     

A study on the applicability of vibration in fresh high fluidity concrete

The introduction of superplasticizers (Sp) in the production of concrete has produced highly flowable mixtures with enhanced viscosity. In cases of optimum flowability and viscosity, for example self-compacting concrete (SCC), no vibration is necessary for placement. However, such ideal conditions are not practically easy to achieve and deviations are possible. This paper reviews the results of a study to investigate the vibration of such high fluidity concrete. Two criteria were used to characterize the fresh mix, that is, slump flow and V-funnel time (V-time). Firstly, the feasibility of vibration on such mixes was studied. Then, the significance of flowability and viscosity was determined. Next, the relationship between workability and its segregation tendency was investigated. Finally, concrete mixes that missed SCC criteria were vibrated. Three different scenarios of vibration were concluded: namely, mix that accept vibration freely, mix that required controlled vibration and mix that needed prior treatment of viscosity enhancing agent (VEA) before vibration.     

Role of disjoining pressure in cement based materials

During the hydration process of Portland cement a nanoporous gel is formed. The high internal surface of the hydration products, mainly calcium-silicate hydrates, and the numerous adsorbed cations interact with water. At an equilibrium moisture content below 50% adsorbed water molecules reduce the surface tension of the solid particles. Water adsorbed at higher relative humidity is at the origin of disjoining pressure acting in small gaps between particles. Both processes are major mechanisms of shrinkage and swelling of hardened cement paste. Results of investigations into the disjoining pressure are summarised. The relevance of disjoining pressure for the behaviour of cement-based materials will be discussed.     

Contribution of mixture design to chemical and autogenous shrinkage of concrete at early ages

In this work, autogenous shrinkage at early ages (<24 h) was accurately measured by linear displacements on slabs simulating field constructions. The best correlation of the amount of chemical to autogenous shrinkage was found at the time of 4 h after the final setting time. It was possible to account for test arrangement artifacts, such as thermal dilation, to get a measure of pure autogenous shrinkage. Many material parameters, such as superplasticizer (SP) and aggregate amount, effected the magnitude of autogenous shrinkage in secondary ways. These consequential effects, such as amount of bleed water and time of setting, were accounted for in the slab measurements. Recommendations are given for reducing the likelihood of cracking due to early age chemical and autogenous shrinkage.     

The effect of undrained heating on a fluid-saturated hardened cement paste

The effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6 MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement paste pore fluid is back analysed from the results of the undrained heating test.     

Impact of time-dependant thermal expansion coefficient on the early-age volume changes in cement pastes

Volume changes and time-dependent thermal expansion coefficient were determined at early stages and the measured total strain was separated into thermal strain and autogenous strain. Cement paste specimens were subjected to temperature histories that imitated hydration-induced temperature rise of the mass concrete. It was shown that the thermal expansion coefficient increased significantly with the development of hydration and became more conspicuous when the ground granulated blast furnace slag was added. The time-dependant increase of thermal expansion coefficient, due to self-desiccation, could result in considerable shrinkage strain at the end of the temperature history. The impact of the time-dependant increase of thermal expansion coefficient might be taken into account as one of the necessary factors in the crack control design from now and cannot even be neglected within the range of the water to binder ratio of this study, because the shrinkage originated in that effect sometimes exceed the autogenous shrinkage.     

Early-age behavior of materials with a cement matrix

Materials with a cement matrix classically present early-age volume variations (shrinkage and/or swelling). This intrinsic early-age behavior strongly influences the length of time the buildings and structures will last because of the micro-cracking and cracking that results from it. One explanation for the macroscopic shrinkage is the presence of pore pressure in the porous medium. In this study, fine modeling of the coupling mechanism behind these internal strains is proposed. The chemical reaction associated with hydration is considered as the main force behind the hydric and mechanical evolutions in an endogenous configuration. Thus, the influence of chemical contraction, porosity, pore-size distribution and pore pressure are central to the study in the light of the numerical and experimental results obtained. A self-leveling layer of mortar of sulfo-aluminous concrete base was used.     

Evolution of penetration resistance in fresh concrete

The objective of this research was to examine the setting of concrete through its penetration resistance; an experimental device, especially developed for this purpose was used, which consists of a system that lets a sphere fall on concrete from a certain height and then measures the depth of the crater. Forty-five samples were made with four different types of cement varying its quantity, water-cement ratio, at ambient temperature and humidity. A semiempirical model which explains the penetration resistance evolution in fresh concrete was proposed and experimentally demonstrated. The results are compared with the consolidation curves for soils represented by the logarithmic method. The proposed setting time was defined as the elapsed time between the placement of the concrete and the time when the depth of the crater is 18% of the initial one.     

Rheology and vibration of fresh concrete: Predicting the radius of action of poker vibrators from wave propagation

The compaction of fresh concrete by an internal poker vibrator has been analysed using closed-form solutions for the propagation of the shear and compressive waveforms, assuming that concrete conforms to the Bingham model. In the inner liquefied zone around the vibrator the flow is due to shear whereas in the outer unsheared zone propagation is due to compressive waves. The analysis gives a method of predicting the radial position at which the flow changes, which coincides with the radius of action of the vibrator. Theory and experiment agree well and confirm that the peak velocity of the vibration governs its efficacy, with radius of action increasing with increasing velocity. The radius of action increases with decreasing yield stress and with increasing plastic viscosity. The work offers the potential to optimise the design and use of vibrators.     

A neutron diffraction study of ice and water within a hardened cement paste during freeze–thaw

A 1-month-old, saturated rod of hardened Portland cement paste with w/c=0.40 was formed with, and stored under, heavy water. This rod was studied undergoing two freeze–thaw cycles over the range 227–297 K, using neutron diffraction, at slow rates of heating and cooling. Neutron diffraction gives a direct and independent quantification of the amount of ice and liquid water in the pore system of hardened cement paste as a function of temperature. The amount of ice that formed was totally reproducible over two freeze–thaw cycles, implying negligible changes to the pore size distribution and geometry. An analysis of some of the factors that may contribute to the freeze–thaw hysteresis is given. There is significant broadening of the diffraction peaks of the ice, which corresponds to a small correlation length of crystalline order in the ice. Water was observed to be “irreversibly” expelled during the first freeze, above ca. 250 K.     

Kinetics of formwork pressure drop of self-consolidating concrete containing various types and contents of binder

A comprehensive research study was carried out to determine the effect of binder type and content on the variations in lateral and pore water pressures that can be exerted by self-consolidating concrete after casting and up to early stages of hardening. Test results show that both physical and chemical phenomena can influence the kinetics of the decrease in lateral pressure until cancellation. The former phenomenon occurs mainly during the dormant period of cement hydration, and is significantly affected by the binder type and content. Regardless of the binder type, the effect of increasing the binder content resulted in sharper drops in pressure. The cancellation of lateral pressure depends on a chemical effect and occurs after the end of the dormant period when the rate of cement hydration is accelerated. Beyond the dormant period, the progressive formation of hydration products leads to the creation of a structural network, and the pore water pressure begins to drop abruptly towards negative values.     

Influence of superplasticizers on rheological behaviour of fresh cement mortars

To assure required workability of high performance concrete (HPC), various superplasticizers are used. Only by using superplasticizers can rheological properties of HPC mix be adequately adjusted to the methods and conditions of concrete processing. Thus, the key element in efficient workability shaping is the complex knowledge how superplasticizers influence the rheological properties of fresh concrete in different technological circumstances.In the paper, the methodology and test results of an investigation into the influence of chemically different superplasticizers on the rheological properties of standard mortars are presented and discussed. The rheological parameters of mortars yield value g, and plastic viscosity h were determined using VISCOMAT PC rotational rheometer. In the research, the influence of the performance of superplasticizers was investigated taking into account following factors: chemical origin of superplasticizers (SNF/naphthalene sulfonic acid/, AP/polycarboxylate acid, PC/policarboxylate ester/), superplasticizer dosage, W/C ratio, cement type (CEM I, CEM II and CEM III), cement physical and chemical properties and temperature.The results presented in the paper show that by testing rheological parameters of mortars with rotational viscometer, it is possible to complex and precisely determine the performance of superplasticizers. On the ground of obtained results, it is possible to optimise the composition of mortars and concretes from workability point of view.     

Modeling of hydration reactions using neural networks to predict the average properties of cement paste

This paper presents a hydration model that describes the evolution of cement paste microstructure as a function of the changing composition of the hydration products. The hydration model extends an earlier version by considering the reduction in the hydration rate that occurs due to the reduction of free water and the reduction of the interfacial area of contact between the free water and the hydration products. The BP Neural Network method is used to determine the coefficients of the model. Using the proposed model, this paper predicts the following properties of hardening cement paste: the degree of hydration, the rate of heat evolution, the relative humidity and the total porosity. The agreement between simulation and experimental results proves that the new model is quite effective and potentially useful as a component within larger-scale models designed to predict the performance of concrete structures.     

锂渣掺量对水泥-减水剂浆体流变特性和混凝土性能的影响

本文研究了锂渣掺量对水泥-减水剂浆体流变特性、新拌混凝土工作性及硬化混凝土强度的影响。从紧密堆积理论和固体颗粒体积分数角度解释了锂渣掺量对水泥-减水剂浆体流变性能的影响。结果表明：锂渣水化初始的屈服应力和塑性黏度均随锂渣掺量的增加而增大,流变性能劣化。新拌混凝土的工作性与锂渣掺量呈负相关;随锂渣掺量的增加,7 d抗压强度不断降低,28 d抗压强度呈现出先增后降的趋势。     

Rehydration and microstructure of cement paste after heating at temperatures up to 300 °C

This paper is concerned with the evolution of the microstructure of cementitious materials subjected to high temperatures and subsequent resaturation in the particular context of long-term storage of radioactive wastes, where diffusive and convective properties are of primary importance. Experimental results obtained by mercury intrusion porosimetry (MIP) are presented concerning the evolution of the pore network of ordinary portland cement (OPC) paste heated at temperatures varying between 80 and 300 °C. The consequences of heating on the macroscopic properties of cement paste are evaluated by measures of the residual gas permeabilities, elastic moduli and Poisson's ratio, obtained by nondestructive methods. Resaturation by direct water absorption and water vapour sorption are used to estimate the reversibility of dehydration. The results provide some evidence of the self-healing capacity of resaturated cement paste after heating at temperatures up to 300 °C.     

如何减少P·O42.5水泥新拌混凝土的 坍落度损失

天能水泥有限公司P·O42.5级水泥在加入萘系高效减水剂后混凝土坍落度损失较大。经原因查找分析,认为水泥过粉磨和熟料C3A含量偏高是主要原因。对此采取针对性的措施,包括对配料方案重新调整,操作上采用高温煅烧和快速冷却,及混合材的优选。随机试验结果表明,坍落度达到了工程规定范围。