Influence of Cement Particle-Size Distribution on Early Age Autogenous Strains and Stresses in Cement-Based Materials

The influence of cement particle-size distribution on autogenous strains and stresses in cement pastes of identical water-to-cement ratios is examined for cement powders of four different finenesses. Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal relative humidity development; autogenous deformation; and eigenstress development, using a novel embedded spherical stress sensor. Because the latter three measurements are conducted under sealed conditions, whereas chemical-shrinkage measurements are made under "saturated" conditions, the National Institute of Standards and Technology cement hydration and microstructure development model is used to separate the effects of differences in hydration rates (kinetics) from those caused by the different initial spatial arrangement of the cement particles. The initial arrangement of the cement particles controls the initial pore-size distribution of the cement paste, which, in turn, regulates the magnitude of the induced autogenous shrinkage stresses produced by the water/air menisci in the air-filled pores formed throughout the hydration process. The experimental results indicate that a small autogenous expansion (probably the result of ettringite formation), as opposed to a shrinkage, may be produced and early age cracking possibly avoided through the use of coarser cements.     

Internal curing of high-performance concrete with pre-soaked fine lightweight aggregate for prevention of autogenous shrinkage cracking

The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m³ of saturated LWA in HPC, providing 27 kg/m³ of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.     

Electrical conductivity method to assess static stability of self-consolidating concrete

The objective of this study is to evaluate the applicability of the electrical conductivity method to assess the stability of self-consolidating concrete (SCC) at early age. The method consists in inserting four electrode pairs at different depths of concrete to monitor local change in ionic concentrations with time. Such variations can reflect migration of bleed water along concrete column during the plastic stage. The experimental set-up consisted of a rectangular column measuring 1005 mm in height and 250 × 250 mm in cross section. The variations in ionic concentrations were exploited to derive stability indices with regards to bleeding and homogeneity of concrete. Derived stability indices included bleeding coefficient, segregation coefficient, and homogeneity index.Various SCC mixtures made with a fixed water-to-cementitious materials ratio (w/cm) of 0.42, different aggregate gradations, and slump-flow values of 650 ± 10 and 700 ± 10 mm were evaluated. Analysis of changes in ionic concentrations along column samples with time provided adequate evaluation of stability of SCC. For example, the increase in the concentration of viscosity-modifying admixture from 1% to 2% was shown to decrease the homogeneity index from 0.36 to 0.27, reflecting better stability. Validation procedure was carried out by correlating stability indices derived from electrical conductivity measurements to physical variations of coarse aggregate concentrations determined on plastic concrete sampled from the tested column elements at the end of electrical conductivity monitoring period. Good correlations between stability indices and aggregate concentrations are established.     

Effects of lithium nitrate admixture on early-age cement hydration

Although the benefits of lithium admixtures for mitigation of alkali-silica reaction (ASR) have been well documented, the potential ancillary effects of lithium compounds on cement and concrete remain largely uncharacterized. To examine the effects of the most common lithium admixture — lithium nitrate — on early-age behavior, the admixture was introduced at dosages of 0% to 400% of the recommended dosage to six cements of varying composition and to a cement-fly ash blend. Behavior was examined by isothermal calorimetry and measurements of chemical shrinkage, autogenous shrinkage, and setting time. Results indicate that lithium nitrate accelerates the early hydration of most cements but may retard hydration after 24 h. In the lowest alkali cement tested, set times were shortened in the presence of lithium nitrate by 15-22%. Higher dosages appeared to increase autogenous shrinkage after 40 days. The replacement of cement by Class F fly ash at 20% by weight appeared to diminish the early acceleration effects, but later hydration retardation and autogenous shrinkage were still observed.     

Micromechanics analysis of thermal expansion and thermal pressurization of a hardened cement paste

The results of a macro-scale experimental study of the effect of heating on a fluid-saturated hardened cement paste are analysed using a multi-scale homogenization model. The analysis of the experimental results revealed that the thermal expansion coefficient of the cement paste pore fluid is anomalously higher than the one of pure bulk water. The micromechanics model is calibrated using the results of drained and undrained heating tests and permits the extrapolation of the experimentally evaluated thermal expansion and thermal pressurization parameters to cement pastes with different water-to-cement ratios. It permits also to calculate the pore volume thermal expansion coefficient α_? which is difficult to evaluate experimentally. The anomalous pore fluid thermal expansion is also analysed using the micromechanics model.     

The origin of early age expansions induced in cementitious materials containing shrinkage reducing admixtures

Studies on the early-age shrinkage behavior of cement pastes, mortars, and concretes containing shrinkage reducing admixtures (SRAs) have indicated these mixtures frequently exhibit an expansion shortly after setting. While the magnitude of the expansion has been noted to be a function of the chemistry of the cement and the admixture dosage; the cause of the expansion is not clearly understood. This investigation uses measurements of autogenous deformation, X-ray diffraction, pore solution analysis, thermogravimetry, and scanning electron microscopy to study the early-age properties and describe the mechanism of the expansion in OPC pastes made with and without SRA. The composition of the pore solution indicates that the presence of the SRA increases the portlandite oversaturation level in solution which can result in higher crystallization stresses which could lead to an expansion. This observation is supported by deformation calculations for the systems examined.     

Internal curing by superabsorbent polymers in ultra-high performance concrete

To limit self-desiccation and autogenous shrinkage that may lead to early-age cracking of ultra-high performance concrete (UHPC), internal curing by means of superabsorbent polymers (SAP) may be employed. Cement pastes and UHPC with water-to-cement ratio below 0.25, with or without SAP, were studied. The absorption capacity of a solution-polymerized SAP was first determined on hardened cement pastes by SEM image analysis. It was observed that the SAP cavities become partially filled with portlandite during cement hydration. Isothermal calorimetry showed that water entrainment with SAP delays the main hydration peak, while after a couple of days it increases the degree of hydration in a manner similar to increasing the water-to-cement ratio. Internal curing by SAP is effective in reducing the internal relative humidity decrease and the autogenous shrinkage. Although the mechanical properties are affected by SAP addition, it is possible to reach compressive strengths of almost 150 MPa at 28 days.     

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.     

Effect of Chloride‐Based Accelerator in the Presence of Water‐Reducing and Retarding Admixture on Autogenous Shrinkage

Rapid repair concrete mixtures commonly used for full‐depth concrete pavement repair sections can use large dosages of accelerating admixtures to increase strength gain rates and decrease the time to traffic opening. Most often, these mixtures also contain water‐reducing and retarding admixtures (WRRAs) to allow for the use of a low water–cementitious material ratio in order to meet strength requirements. The use of large dosages of accelerating admixtures in combination with retarding admixtures could have significant side effects on concrete. Autogenous shrinkage of low water–cementitious concrete can contribute to high tensile stresses and cracking problems. The effect of calcium chloride‐based accelerating admixture dosage, when used with WRRAs, on autogenous shrinkage was measured. It was found that the inclusion of calcium chloride‐containing accelerating admixtures has a nonlinear effect on the pore size distribution and consequently a nonlinear increase on the autogenous shrinkage.     

Oxygen diffusion through hydrophobic cement-based materials

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The obtained results show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.     

Integrative Study on the Effect of Internal Curing on Autogenous and Drying Shrinkage of High-Strength Concrete

High-strength concrete generally has a low water-to-cement ratio, which in turn increases the possibility of early-age cracking due to its high autogenous shrinkage. In this article, the effect of internal curing using presoaked lightweight aggregate (PSLWA) in high-strength concrete on shrinkage and interior humidity is investigated by continuously measuring the deformation and interior humidity of the test specimen under plastic film sealing and surface drying conditions since specimen cast. Four mixture proportions with an induced curing water-to-cement ratio (W _IC/C) of 0, 0.04, 0.08, and 0.12 were used in experiments. The experimental results show that the decrease in interior humidity was gradually reduced with an increase in W _IC/C. Accordingly, both autogenous and drying shrinkage of concrete gradually decreased with an increase in W _IC/C. Internal curing cannot completely eliminate autogenous shrinkage because part of the autogenous shrinkage is developed within the humidity-saturated stage in early-age concrete.     

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.     

Effects of sorbed water on properties of low and high molecular weight PMMA: 1. Deformation and fracture behaviour

The influence of sorbed water on deformation and fracture behaviour of both low and high molecular weight poly(methylmethacrylate) has been investigated. In the low molecular weight polymer, addition of water produces a rise in internal friction in the ?100°C region but it has no apparent effect on the β-relaxation process. The tensile strength falls gradually with increasing concentration of water and more rapidly at high concentrations. Ductility initially increases with increasing water content but it reduces at high concentration. In the high molecular weight polymer, in contrast to the low molecular weight material, a yield maximum is observed for both dry and air-equilibrated samples of low moisture content and some samples show both necking and cold drawing prior to fracture. Observed deformation modes include shear bands, crazes, and diamond-shape surface cavities. However, water saturated samples fail in a brittle manner. From the observed deformation behaviour, as well as from observation of fracture surface morphology, it is suggested that sorbed water acts as a mild plasticizer for PMMA up to a concentration of about 1.1 % and, at higher water concentration, water clustering occurs.     

硫酸钠掺量对混凝土早期收缩开裂的影响

研究了分别掺加占胶结料质量分数1％,2％的硫酸钠对混凝土早期收缩开裂的影响。用非接触式混凝土收缩测量仪及板式混凝土开裂架进行实验。用电子显微镜对硫酸钠混凝土试件早期亚微观结构进行较为直观的研究分析。结果表明：硫酸钠对混凝土早期收缩开裂在不同孔径中的作用不尽相同。硫酸钠对混凝土早期收缩的影响与水胶比及硫酸钠的掺量有一定的关系。硫酸钠掺量相同情况下,水胶比较小混凝土早期收缩值较大;在相同水胶比情况下,硫酸钠掺量较大混凝土早期收缩值较小。硫酸钠对混凝土早期开裂的影响与养护方式相关,空气中养护与密封条件下养护这2种养护制度相比,在空气中养护混凝土表面出现裂纹的时间较早,裂纹较宽且较多。在这2种养护制度下,水分蒸发速度的不同决定了硫酸钠结晶初始深度的不同,是硫酸钠对混凝土早期开裂的主要影响机理。     

Influence of a thermal cycle at early age on the hydration of calcium sulphoaluminate cements with variable gypsum contents

Hydration of a belite calcium sulphoaluminate cement was investigated over one year as a function of its initial gypsum content (variable from 0 to 35%). Particular attention was paid to the influence of the thermal history of the material at early age on its subsequent evolution. Pastes and mortars (w/c 0.55) were either cured at 20 °C or submitted for one week to a thermal treatment simulating the temperature rise (up to 85 °C) and fall occurring in drums of cemented radwastes. The thermal cycle accelerated the early stages of hydration and mainly decreased the proportion of AFt versus AFm hydrates, especially at low initial gypsum contents (≤ 20% by weight of cement). It also strongly reduced the compressive strength of gypsum-free specimens (by 35% after one year), and doubled their expansion under water. These results were explained by mineralogical evolutions towards a more stable phase assemblage which included retarded ettringite formation.     

水泥基材料水化过程中通过缓凝作用达到促凝效果(英文)

"通过缓凝作用达到促凝效果"的含义是通过抑制石膏的溶解并用除石膏以外的外加剂抑制铝酸三钙(C3A)的水化这2个过程,配制一种加速水泥凝结的外加剂组分。小掺量(水泥含量的0.02%～0.03%)的邻苯二酚导致水泥的闪凝,这很可能是由于抑制了硫酸钙的分解以及C3A的快速水化。而这些小剂量的邻苯二酚好像对硅酸三钙(C3S)的水化没有影响。通过水化热和物理性能测试表明:由邻苯二酚导致的"闪凝",而其他比石膏掺量小的缓凝剂(例如:葡萄糖酸钠)可以减缓"闪凝",使样品较快凝结(即加速凝结)。但这些缓凝剂会延迟C3S的水化,从而也延迟水泥浆体强度的发展。如果假定上述机理是正确的,那么被抑制溶解的石膏会在水化后期发生反应,导致延迟钙矾石生成并使水泥浆体产生膨胀开裂。制备了掺加或不掺加这种外加剂的混凝土,测定了这些混凝土的线性膨胀和体积膨胀,以检验其发生膨胀的可能性,但是两个月后没有发现混凝土表现出非正常膨胀。     

Characterization of multi-scale porosity in cement paste by advanced ultrasonic techniques

The effectiveness of advanced ultrasonic techniques to quantitatively characterize the capillary porosity and entrained air content in hardened cement paste is examined. Direct measurements of ultrasonic attenuation are used to measure the volume fraction and average size of entrained air voids and to assess variations in intrinsic porosity - as influenced by water-to-cement ratio (w/c) - in hardened cement paste samples. For the air entrained specimens, an inversion procedure based on a theoretical attenuation model is used to predict the average size and volume fraction of entrained air voids in each specimen, producing results in very good agreement with results obtained by standard petrographic methods and by gravimetric analysis. In addition, ultrasonic attenuation measurements are related to w/c to quantify the relationship between increasing porosity (with increasing w/c) and ultrasonic wave characteristics.     

高强流态膨胀混凝土力学性能和膨胀行为

研究了高流态膨胀混凝土的强度性能、膨胀性能以及强度与膨胀的协调发展性能,考察了流态条件下中强和高强混凝土中膨胀剂的作用效应及细掺料对其的影响。研究表明高强流态膨胀混凝土的膨胀行为有其特殊性：在不加磨细矿渣(blast furnace slag,BFS)时,水养1d出现自收缩,整个硬化过程膨胀受到抑制,原因在于高强流态膨胀混凝土较高的强度,特别是较高的早期强度限制了膨胀的发挥,在混凝土内部存在“自约束效应”。掺加BFS后,未发现自收缩现象,限制膨胀率大于对应的未掺BFS时的限制膨胀率。BFS的加入使高强膨胀混凝土早期强度降低,膨胀效应得以较充分发挥。对比中低强膨胀混凝土分析认为：过高的强度(特别是早期强度)会抑制混凝土膨胀的发挥,而较低的强度(特别是早期强度)则会导致更多的膨胀变为无效膨胀。强度与膨胀协调发展这一膨胀混凝土的基本规律对于高强流态膨胀混凝土仍然适用且更为重要。     

Internal curing of blended cement pastes with ultra-low water-to-cement ratio: Absorption/desorption kinetics of superabsorbent polymer

Internal curing by superabsorbent polymer (SAP) is an effective method to mitigate the autogenous shrinkage of cement-based materials with low water-to-cement ratio (w/c). In this study, the water absorption/desorption kinetics of SAP were studied quantitatively in blended cement pastes with ultra-low w/c. An absorption process at a rate of 0 to 6 g/(g h) was calculated at early ages. After that, SAPs showed mainly two distinct water desorption behaviors with a rate of 0 to 1.1 g/(g h), which was mainly governed by the osmotic pressure and capillary pressure triggered by the drop of internal relative humidity (IRH). The size and amount of SAP played a predominant role in controlling its absorption and desorption kinetics in the cement paste. Compared with ordinary Portland cement, a different desorption process with a higher release rate was noticed in binary and ternary cement pastes, primarily due to the changes in osmotic pressure resulting from the acceleration of cement hydration by silica fume at early ages. Overall, the mitigation of autogenous shrinkage is found to be highly dependent on SAP's absorption and desorption kinetics.     

镁渣粉煤灰复掺配制混凝土的自收缩特性

为了探索镁渣与粉煤灰复掺对混凝土自生收缩的作用规律,设计正交试验方案研究了混凝土的自生收缩特性,微观分析掺合料的形貌与作用机理.结果表明:镁渣和粉煤灰对混凝土的自生收缩具有显著的抑制效应,镁渣与粉煤灰的掺量由(10%,15%)增加到(40%,30%),混凝土180 d的自生收缩变形减少约44.0%;混凝土自生收缩在镁渣掺量为30%~40%区间上的极差较镁渣掺量为10%~20%区间上的极差增大3.5倍,混凝土自生收缩变形的敏感性在镁渣掺量高时相对较大;混凝土的自生收缩变形主要发生在早期,28 d就完成了测定龄期内总自生收缩变形的65.0%~80.0%,早期是混凝土收缩变形控制的重要阶段;自生收缩的模型预测值与实测值间的偏差小,可用于复掺镁渣粉煤灰混凝土自生收缩的分析与预测.