混凝土损伤自愈的机理

研究了不同龄期受损混凝土经过相同养护期后的自然愈合现象．混凝土受损后的自愈合实质上是损伤部位未水化或水化不充分的胶凝材料加速水化或进一步水化生成新的水化产物弥合裂缝的过程．以超声波速的变化表征混凝土受压开裂后的损伤程度,建立了混凝土损伤量与愈合状况之间的关系．结果表明,混凝土材料存在一个损伤阈值：当混凝土的损伤低于损伤阈值时,自愈合率随着损伤量的增大而增大;当混凝土损伤超过损伤阈值时,自愈合率随着损伤量的增大而降低．     

基于CT扫描技术的水泥净浆微观结构及水化程度

进行了0.3和0.5两种水灰比的水泥净浆从1~40天龄期的CT扫描试验,成功重构了水泥净浆尺度上的三维微观模型,观测了未水化水泥颗粒、水化产物和孔隙在这期间的形态变化。基于CT扫描的结果量化未水化水泥颗粒体积含量,进行了水化程度的计算。计算结果与常用的TGA方法进行比较,发现可比性好, CT扫描为一项较可靠的量化水泥水化程度的方法,但需要高精度的CT扫描设备。     

Simulation of self-healing by further hydration in cementitious materials

Cracks, caused by shrinkage and external loading, facilitate the ingress of aggressive and harmful substances into concrete and indeed reduce the durability of the structures. It is well known that self-healing of cracks can significantly improve the durability of the concrete structure. In this research, self-healing of cracks was proposed to be realized by providing extra water for further hydration of unhydrated cement particles. In order to provide theoretical guidance for the practice, self-healing by providing extra water to promote further hydration was simulated. The simulation was based on water transport theory, ion diffusion theory and thermodynamics theory. In the simulation, self-healing efficiency under different conditions as a function of time was calculated. The relationship between self-healing efficiency and the amount of extra water from the broken capsules was determined. According to the results of the simulation, the amount of extra water can be optimized by considering self-healing efficiency and other performances.     

Fracture Behavior of Particle Reinforced Metal Matrix Composites

The contributions of the reinforcement volume fraction and annealing temperatures to crack opening force and propagation energy are systematically studied by three point bending tests and by SEM investigations. The bending test data show that for the same reinforcement volume fraction, 2618 and 7075 Al composites require much higher force to open the cracks than 6061 matrix. This relates to the much higher levels of solute elements which causes matrix hardening. Studies reveal that the energy absorption level of the materials during crack propagation depends on both matrix strength and ductility which relates to the reinforcement volume fraction, composition and heat treatment conditions. Large deformation zones are found in front of the crack tip before crack propagation which indicate a ductile failure mode for the composites. Studies also reveal that cracks initiate generally at the particle/matrix interfaces for the low volume fraction reinforced composites. However, for the high volume fraction reinforced composites, crack initiation has been found from both reinforcement/matrix interfaces and broken particles. This indicates that increasing reinforcement volume fraction and matrix strengthening tend to change the fracture mode from interface debonding to particle cleavage cracking.     

Micromechanics-based investigation of the elastic properties of polymer-modified cementitious materials using nanoindentation and semi-analytical modeling

Cementitious materials are modified by the addition of polymers in order to improve the durability and the adhesive strength. However, polymer-modified mortars and concretes exhibit lower elastic moduli in comparison to unmodified systems. The macroscopic properties are governed by microstructural changes in the binder matrix, which consists of both cementitious and polymer components. Herein, different polymer-modified cement pastes were characterized using nanoindentation to better understand the microscopic origin of the macroscopic elastic modulus. By means of the statistical nanoindentation technique, the existence of three micromechanical phases in plain and polymer-modified cement pastes with a water-to-cement mass ratio of 0.40 is evidenced, illustrating that the polymer modification does not induce the formation of additional reaction products. Instead, the polymers adsorb on the hydration products as well as on unhydrated clinker grains and decrease the indentation moduli of the micromechanical phases. The link between the microscopic and macroscopic mechanical properties is established by means of a continuum micromechanics approach. A multiscale model aimed at the prediction of the elastic moduli of polymer-modified cementitious materials is developed with input parameters that are partially obtained from the nanoindentation tests. The comparison of the modeling results with the experimentally determined elastic (macroscopic) moduli at the scales of cement paste, mortar, and concrete is satisfactorily good, underlining the predictive capability of the modeling approach. The improvement of prediction models is essential for the application of polymer-modified cementitious materials in construction and will encourage their integration into design guidelines.     

Numerical modeling of mechanical regain due to self-healing in cement based composites

Recently there has been an increasing interest in self-healing materials which have the ability to retrieve their physico-mechanical properties once the material is damaged. This paper presents a numerical model for the self-healing capacity of cementitious composites capable of simulating the recovery of mechanical properties of the damaged (cracked) material. The recent SMM (Solidification-Microprestress-Microplane model M4) model for concrete, which makes use of a modified microplane model M4 and the solidification-microprestress theory, is able to reproduce the concrete time-dependent behavior, e.g. creep, shrinkage, thermal deformation, aging, and cracking from early age up to several years. The moisture and heat fields, as well as the hydration degree, are obtained from the solution of a hygro-thermo-chemical problem which is coupled with the SMM model. This numerical framework is extended to incorporate the self-healing effects and, in particular, the effect of delayed cement hydration, which is the main cause of the self-healing for young concrete. The new update model can also simulate the effects of cracking on the permeability and the opposite restoring effect of the self-healing on the mechanical constitutive law, i.e. the microplane model. A numerical example is presented to validate the proposed computational model employing experimental data from a recent test series undertaken at Politecnico di Milano. The experimental campaign has dealt with a normal strength concrete, in which (by means of three-point-bending tests performed up to controlled crack opening and up to failure, respectively before and after exposure to different conditioning environments) the recovery of stiffness and load bearing capacity has been evaluated.     

An investigation of the hydration chemistry of ternary blends containing cement kiln dust

During the manufacture of Portland cement, dust is generated composed of particles of feedstock and condensed volatilised inorganic salts. Due to its highly alkaline soluble fraction, the dust can be used as an activator in blends containing pozzolanic materials or hydraulic slags, allowing them to undergo cementitious reactions. The inclusion of Portland cement in such blends enhances strength development further, although careful proportioning of materials will be required to obtain optimum performance. Ternary systems containing cement kiln dust, pulverised-fuel ash and Portland cement were characterised in this paper in terms of strength development and hydration products. For a given Portland cement content, optimum strength was achieved in blends containing approximately 10% cement kiln dust for Portland cement levels up to 80%. Beyond this level a CKD/PFA ratio of one was optimal. Isothermal conduction calorimetry results and measurements of calcium hydroxide levels indicated that this was due to an acceleration of the reactions of the blend constituents by the dust. Additionally, the chemical composition of the optimal blends promoted the production of calcium aluminate and ferrite hydrates of a type conducive to maintaining the integrity of the cementitious matrix.     

Failure of cemented granular materials under simple compression: experiments and numerical simulations

We investigate the strength and failure properties of a model cemented granular material under simple compressive deformation. The particles are lightweight expanded clay aggregate beads coated by a controlled volume fraction of silicone. The beads are mixed with a joint seal paste (the matrix) and molded to obtain dense cemented granular samples of cylindrical shape. Several samples are prepared for different volume fractions of the matrix, controlling the porosity, and silicone coating upon which depends the effective particle–matrix adhesion. Interestingly, the compressive strength is found to be an affine function of the product of the matrix volume fraction and effective particle–matrix adhesion. On the other hand, it is shown that particle damage occurs beyond a critical value of the contact debonding energy. The experiments suggest three regimes of crack propagation corresponding to no particle damage, particle abrasion and particle fragmentation, respectively, depending on the matrix volume fraction and effective particle–matrix adhesion. We also use a sub-particle lattice discretization method to simulate cemented granular materials in two dimensions. The numerical results for crack regimes and the compressive strength are in excellent agreement with the experiments.     

Efficient utilization of cementitious materials to produce sustainable blended cement

To achieve sustainable development of cement industry, cementitious efficiency of different cement clinker and supplementary cementitious materials (SCMs) fractions, in terms of hydration process and strength contribution ratio, was characterized. The results show that blast furnace slag and steel slag should preferably be arranged in fine fractions due to their desirable hydration processes and high strength contribution ratios. Cement clinker should be positioned in intermediate fraction (8–24 μm) due to its proper hydration process. Replacement of cement clinker by SCMs with low activity or inert fillers in coarse fractions was also suggested, because coarse cement clinker fractions gave very low hydration degrees and little strength contribution. Both early and late properties of gap-graded blended cements prepared can be comparable with or higher than those of Portland cement, indicating both cement clinker and SCMs were used more efficiently. These blended cements also give additional cost savings and reduced environmental impact.     

Physico-chemical deformations of solidifying cementitious systems: multiscale modelling

At early stages of hydration and in autogenous conditions (no mass transfer with the outside), solidifying cementitious systems exhibit dimensional variations following two main processes: Le Chatelier contraction (also called chemical shrinkage) and self-desiccation shrinkage causing autogenous shrinkage. Chemical shrinkage results from the difference between the specific volumes of reactants (anhydrous cement and water) and hydration products. Early-age autogenous shrinkage is generally attributed to the development of a negative capillary pressure in the porous network related to the water consumption by the hydration reactions. If restrained, deformations associated to these shrinkages can induce the development of internal stresses high enough to generate cracking of the hardening material. The purpose of this study is to propose a multiscale approach to model the rate of self-desiccation shrinkage of cementitious materials at very early-age, between 0 and 48 h. Within the first hours, Le Chatelier contraction is computed from a formulation suggested in a later work which is based on the chemical equations of hydration and the specific volume of each phase. Then, when the setting of the cement paste takes place, the autogenous shrinkage is calculated according to the evolution of the capillary pressure and the stiffness of the cement paste. The stiffness is calculated by applying a classical homogenization method. Computed results are discussed and analyzed. Good agreements between experiments and simulations are achieved and a sensitivity study is performed to assess the influence of the cement fineness and the aggregate volume fraction on early-age autogenous strain.     

高延性纤维增强偏高岭土-粉煤灰基地聚合物在不同环境下的自愈合性能

在1%、2%及3%不同程度预加单轴直接拉伸应变破坏下,研究了3天、7天及28天龄期的高延性聚乙烯醇（PVA）纤维增强偏高岭土-粉煤灰基地聚合物（PVA/MK-FA EGC）在空气中和干湿循环条件下的裂缝分布及自愈合性能。结果表明：PVA/MK-FA EGC结合了传统高延性纤维增强水泥基复合材料（ECC）及地聚合物二者的优点,表现出了明显的多缝开裂特性和应变硬化行为。2~5 mm的裂缝间距、小于25 μm的最大残余裂缝宽度给裂缝的自愈合提供了更加有利的条件。带缝试件在不同环境中自愈合后,裂缝数量大大下降,极限拉伸应变可达3.8%以上,大部分试件的极限拉应变及最终应力均能超过对比试件,空气中的养护环境更加有利于PVA/MK-FA EGC材料的自愈合。裂缝内颗粒表面覆盖有凝胶状的地聚合产物,可能增强了体系中的纤维/基体界面,使力学性能恢复。     

水泥基材料裂缝自愈合的研究进展

总结了近些年来在水泥基材料裂缝自愈合领域的研究进展，重点论述了两种普通类型的水泥基材料裂缝自愈合的机理、愈合过程、影响因素及评价。普通水泥基材料裂缝愈合机理包括结晶沉淀、结晶渗透，对聚合物水泥基材料，主要包括空气固化愈合、热聚合愈合和温致愈合机理。并提出了进一步的研究方向。     

From microstructure to macrostructure: an integrated model of structure formation in polymer-modified concrete

A model is proposed for the formation of the microstructure in polymer-modified cementitious materials. Cement hydration and polymer film formation were studied, with specific emphasis on the synergetic effect between cement particles and polymer particles. Alterations at the microstructure level result in macroscopic changes in the properties of the modified material. In this paper, the influence of the polymer addition on the appearance of the cement hydrates and the presence of the polymer film through the cement hydrates are presented in relation to the minimum film forming temperature. Owing to the presence of the cement particles and to cement hydration, film formation can take place at lower temperatures, so that a polymer dispersion with a slightly higher MFT (minimum film forming temperature) can be used. This is important for the physical and mechanical properties of the polymer-modified materials. The findings have been included in an integrated model based on the three-step model of Ohama, in which the polymer film formation and the cement hydration processes are integrated in relation to each other. A time-dependent evaluation of both processes was incorporated. The research presented in this paper was part of a PhD research at the Civil Engineering Department, University of Leuven, Belgium [1].     

硬化水泥净浆基于纳米压痕的相态识别与水化程度计算

采用纳米原位压痕手段测量硬化水泥净浆中单一相态的代表性微观力学性能,并采用纳米点阵压痕研究各相态的含量。研究对象囊括水灰比为0.3、0.4、0.5的纯水泥净浆和水灰比为0.3情况下含50%、70%矿渣掺量复合体系,共5种配比,以表征它们的相态分布和微观力学性质的异同点。掺矿渣的试件中含有明显多的复合相,因此提出三相模型测算复合相中未水化物的体积分数。此外,提出基于纳米压痕技术计算纯水泥和掺矿渣水泥试件水化程度的方法,结果吻合于热重分析的结果,其中纯水泥净浆中复合相较少,计算得到的水化程度优于对掺矿渣水泥试件的计算。     

Examination of grinding techniques for the reuse of by-products of cement production

The purpose of the letter is to explore an effective way to substantially utilize by-product of cement production by developing an environmentally friendly, sufficiently performing, and cost-effective cementitious product for future concrete materials. The study involves properly blending fly ash with cement kiln dust to create a cementitious material in which the material deficiencies will be converted into benefits. The activation process chosen, in order to facilitate and enhance hydration of the two materials, is mechanical grinding. Properties are determined through the use of heat of hydration test, particle size test and compressive test. The results show that such a material is feasible with additional study.     

Analytical heat conduction model of particle reinforced tertiary composite materials based on complete spatial randomness of fillers in base matrix and its application in the development of cryosorption pump

In this article, we propose an analytical heat conduction model within a stochastic frame work which estimates the thermal conductivity (TC) value of particle reinforced composite materials comprising of three parent elements i.e. a base matrix along with two different filler element particles randomly distributed in it. The spatial distribution of the filler particles in a sample of specific dimension has been estimated by applying bivariate Poisson distribution. This distribution is then used to arrive at the TC value of the composite. This concept has been applied to predict the TC of the tertiary composite comprised of epoxy as the base matrix, aluminium and zinc particles as filler elements. The TC values obtained from this model for different volume fractions of fillers were extensively compared with experimental results. The model is found to predict the results fairly well with less aberrations up to the total filler volume fraction of ∼20%. The developed model for TC prediction has been used in the design of high efficiency cryosorption pump where the adhesive material used is Epoxy-Aluminium -Zinc composite.     

A combined SEM–Calorimetric approach for assessing hydration and porosity development in GGBS concrete

A combination of semi-adiabatic calorimetry and Scanning Electron Microscopy (SEM) is employed for characterising the hydration process and pore structure development of cementitious pastes. The efficiency of this method is investigated by obtaining hydration curve parameters for four different concrete mixes manufactured using varying combinations of limestone blended cement (CEM II/A-LL) and ground granulated blast-furnace slag (GGBS) over a period of six months after casting. Embedded thermocouples recorded the internal temperature development associated with heat of hydration released in the first hours after casting. Hydration monitoring was continued by analysing SEM images taken from broken concrete specimens at various time intervals. Reliable hydration quantification using this approach requires the aggregate particles to be identified and filtered out of the image; this is achieved using a semi-automatic image processing methodology developed for detection and segmentation of aggregates from the concrete paste. Grey-level thresholding and the inflection point method are employed to determine the area fraction of the void space and assess porosity. Hydration degrees are then determined by applying thresholding methods to distinguish the hydrated and anhydrous cement particles. Corresponding hydration curve parameters were obtained based on the experimental data, and the resulting curves were compared with those obtained based on commonly used cement composition models.     

The influence of temperature on autogenous volume changes in cementitious materials containing shrinkage reducing admixtures

This research examines the influence of temperature on unrestrained and restrained autogenous volume changes in cementitious systems containing shrinkage reducing admixtures (SRAs). The apparent activation energy of cement hydration is determined using measurements of isothermal conduction calorimetry. Time-temperature (equivalent-age based) transformations are applied to extract the apparent activation energy of cement hydration (reactions). The results indicate that while equivalent-age transformations are a suitable procedure for describing the influence of temperature on chemical reactions, they are an inappropriate approach to describe the evolution of volume changes in cementitious materials cured at different temperatures. It is noted that while SRAs do not substantially alter the temperature sensitivity of hydration reactions, their ability to induce early-age expansions negates the use of maturity (equivalent age) approaches in describing autogenous deformations in these materials. Efforts are made to better describe the thermodynamic-limitations of autogenous RH change (self-desiccation) and the need to account for viscoelastic (i.e., creep) and damage (i.e., microcracking) considerations in interpreting the residual stress development response of cement-based materials cured at different temperatures.     

The influence of blast-furnace slag hydration products on microcracking of concrete

New hydration products of ground granulated blast-furnace slag are formed during the hydration process of Portland slag cement concrete. Spatial distribution of microcracks in concrete is related also to newly formed slag hydrates. The chemical composition of hydration products is variable and unstable. The Si/Ca ratios rise in hydration products near the unhydrated slag core significantly. There is also a certain increase of Mg and Al content in the central parts of hydration rims. The scope of the paper is to find a relation between slag hydration products and the process of microcracking. The amount of microcracks was reduced in concretes with a lower content of vitreous fraction where the slag basicity was high. Neoformation of hydration products is accompanied by volume changes, which can lead to concrete microcracking.     

Effects of mechanical grinding on pozzolanic activity and hydration properties of quartz

To provide basic research into the utilization of mine tailings as supplementary cementitious materials in cement, the pozzolanic activity and hydration properties of quartz-a common mineral phase in mine tailings-after undergoing mechanical grinding were investigated. In this study, a supplementary cementitious material was obtained using the mechanical grinding of quartz. Prolonged grinding resulted in a gradual increase in the pozzolanic activity index and percentage of dissolution in an alkaline solution, as well as a reduction of the relative crystallinity. The particle size appeared to have reached a limit after 80 min of grinding; however, the specific surface area reached its limit after 120 min of grinding, which was mainly due to the continual increase in pore volume of the micropores and mesopores from the grinding process. As an active supplementary cementitious material, the hydration product of ground quartz was an amorphous C-S-H gel in the presence of calcium hydroxide. This study provides a research basis for investigating the pozzolanic activity and hydration properties of ground quartz, which is beneficial to an evaluation of the pozzolanic activity of siliceous mine tailings after mechanical activation.