Determination of length changes due to moisture variations in autoclaved aerated concrete

Length changes of autoclaved aerated concrete due to moisture variations have been investigated by tests on a Danish brand of autoclaved aerated concrete. The investigation have shown that autoclaved aerated concrete with delivery moisture content exhibits swelling when initially conditioned at relative humidities above 43%, whereas initial conditioning below 43% relative humidity leads to shrinkage. After the initial conditioning has been performed, length changes are shown to be almost proportional to the variation in relative humidity. The investigation has shown that the present Danish Standard in this area is misleading, and the principle of a new NORDTEST method, NT Build 444(1996), is presented, Furthermore this test method is compared with the European Standard EN 680,Determination of the drying shrinkage of autoclaved aerated concrete, and a draft for another European Standard prEN 772-12:1992,Determination of length change during moisture movements in autoclaved aerated concrete masonry units.     

Shrinkage of plain and silica fume cement concrete under hot weather

Supplementary cementing materials (SCMs) are widely used these days to improve the durability of concrete. Silica fume has gained world wide acceptance due to its high pozzolanic reactivity compared to other SCMs. While silica fume cement concrete has several advantages over other blended cement concretes its main draw back is increased plastic and drying shrinkage, particularly under hot weather conditions. This paper reports results of a study conducted to assess these properties of plain and silica fume cement concrete specimens cast and cured in the field under hot weather conditions. The effect of specimen size and method of curing on plastic and drying shrinkage and some of the mechanical properties of silica fume and plain cement concrete specimens were evaluated. Results indicated that the type of cement significantly affected both the plastic and drying shrinkage of concrete in that these values in the silica fume cement concrete specimens were more than those in the plain cement concrete specimens. As expected, the shrinkage strains in both the plain and silica fume cement concrete specimens cured by continuous water-ponding were less than that in similar concrete specimens cured by covering them with wet burlap. The results point to the importance of selecting a good quality silica fume and good curing for avoiding cracking of concrete due to plastic and drying shrinkage, particularly under hot weather conditions.     

Experimental investigation of drying shrinkage cracking of composite mortars incorporating crushed tile fine aggregate

Drying shrinkage is generally classified as an important hardened concrete property. It expresses the strain occurring in hardened concrete due to the loss of water. During the drying process, free and absorbed water is lost from the concrete. When the drying shrinkage is restrained, cracks can occur, depending on the internal stresses in the concrete. The ingress of deleterious materials through these cracks can cause decrease in the compressive strength and the durability of concrete. In this study, being as a fine aggregate in mortars, crushed tile (CT) effect on drying shrinkage and drying shrinkage cracking is investigated. Thus, compressive and flexural strength, modulus of elasticity, and free and restrained drying shrinkage tests are conducted on mortar specimens produced with and without crushed tile fine aggregate. The ring test has been used in order to investigate the cracks induced by restrained drying shrinkage. In this way, free drying shrinkage strain, along with the number and development of drying shrinkage cracks, of the crushed tile fine aggregate mortar composites are quantified and observed.     

Structure and properties of aerated concrete: a review

Aerated concrete is relatively homogeneous when compared to normal concrete, as it does not contain coarse aggregate phase, yet shows vast variation in its properties. The properties of aerated concrete depend on its microstructure (void–paste system) and composition, which are influenced by the type of binder used, methods of pore-formation and curing. Although aerated concrete was initially envisaged as a good insulation material, there has been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilisation of wastes like pulverised fuel ash. The focus of this paper is to classify the investigations on the properties of aerated concrete in terms of physical (microstructure, density), chemical, mechanical (compressive and tensile strengths, modulus of elasticity, drying shrinkage) and functional (thermal insulation, moisture transport, durability, fire resistance and acoustic insulation) characteristics.     

Specific surface area of aggregate and its relation to concrete drying shrinkage

This paper deals with the influence of aggregate properties on the shrinkage of concrete during drying. The drying shrinkage strains of concretes with various types of aggregates were measured and their influences on the fundamental properties of the different types of aggregates were investigated. Furthermore, the specific surface areas (SSAs) of aggregates were obtained by the BET method using both nitrogen (N₂) and water vapour (H₂O). The SSAs determined by using H₂O exhibited higher values than those by using N₂. The drying shrinkage strains of concretes increased with the H₂O SSAs of the aggregates used. Our results suggest that the SSA determined by using H₂O is an effective index for evaluating the influence of the aggregate type on the drying shrinkage of concrete.     

Shrinkage performance of Crumb Rubber Concrete (CRC) prepared by water-soaking treatment method for rigid pavements

This investigation deals with the shrinkage properties of rubberised concrete pavement. Arrays of concrete samples were prepared with different water–cement ratios and rubber content. The experimental results revealed that the introduction of rubber into concrete mixes results in the control of shrinkage cracks if the optimised content of rubber is selected. Accordingly, the optimised rubber content was determined based on the mix characteristics, mechanical properties and the results of plastic and drying shrinkage tests. The mechanical strength, toughness, bleeding, plastic shrinkage and drying shrinkage tests were conducted in this experimental program. Analysing the results revealed that the most promising performance results were achieved for samples prepared with the rubber contents of 20% and 25% of fine aggregates, and water–cement ratios of 0.45 and 0.40, respectively.     

养护方式对磷渣加气混凝土性能的影响

利用磷渣的潜在活性,研制了以磷渣为原材料的加气混凝土。研究了标准养护和蒸压养护两种养护方式对磷渣加气混凝土体积密度、抗压强度、干燥收缩、抗冻性和导热系数等性能的影响。结果表明,与标准养护相比,蒸压养护可以降低磷渣加气混凝土的体积密度、干燥收缩值和导热系数,提高加气混凝土的抗冻性能。     

Modeling of drying shrinkage of concrete specimens at the meso-level

In this paper, an existing mesomechanical model for cementitious materials is extended to the domain of diffusion-driven phenomena. The model is based on the Finite Element Method, and uses zero-thickness interface elements equipped with a fracture-based constitutive formulation to represent cracks. The new developments presented in this paper consist of the application of the model to the hygro-mechanical coupled analysis of drying shrinkage in concrete specimens, explicitly taking into account the influence of (micro) cracks on the diffusion of moisture. In a first part of the paper, the model is presented in some detail, especially the new aspects regarding moisture diffusion including effects of cracks, and H-M coupling. The model predictions are then quantitatively compared with classical drying shrinkage experiments on concrete specimens. The consideration of different assumptions for the relation linking shrinkage strains and weight losses is discussed in some detail. Finally, the effect of size and volume fraction of the main heterogeneities of concrete on the drying process and drying-induced microcracking is also addressed.     

Restrained shrinkage cracking: the role of shrinkage reducing admixtures and specimen geometry

Early-age cracking may occur in concrete elements if shrink-age is prevented by the surrounding structure. The risk of early-age shrinkage cracking in any given structure is influenced by many factors including the magnitude of shrinkage, rate of shrinkage, degree of stress relaxation, degree of structural restraint, and rate at which material properties develop. In addition to the aforementioned factors, this paper highlights the fact that shrinkage cracking is also influenced by geometry. This paper compares two series of experiments to better understand the role of specimen geometry. In the first series ring specimens of varying size were prepared to undergo the same rate of shrink-age and maximum strain development. Although the maximum residual tensile stress that developed was nearly identical for all geometries, the age at which cracking occurred varied with specimen thickness. In the second series of experiments the combined effect of moisture gradients and specimen geometry was investigated. In these experiments specimen thickness was varied while the surface area exposed to drying was maintained constant. The age of cracking was measured, as was the development of a moisture gradient. Again the influence of specimen size/geometry was apparent with smaller specimens cracking at an earlier age. This paper provides an explanation for this geometry dependence through the use of fracture mechanics concepts.     

Shrinkage characteristics of heat-treated ultra-high performance concrete and its mitigation using superabsorbent polymer based internal curing method

The shrinkage and cracking risk of heat-treated ultra-high performance concrete (UHPC) can be mitigated by using the superabsorbent polymer (SAP)-based internal curing method. The heat treatment (HT) accelerates the hydration reaction and resulting self-desiccation of UHPC; consequently, the UHPC experiences severe shrinkage during the HT. This study experimentally demonstrates that the shrinkage is effectively resolved by adopting the SAP-based internal curing method during the HT period as well as early-ages. This method also reduces the strain rate resulting from dimensional change, without showing an increase in drying shrinkage. The accurately conducted experiments herein can help to better understand the shrinkage characteristics of heat-treated UHPC and broaden the application of various internal curing agents.     

基于多项材料细观力学的混凝土干缩量预估

混凝土的干缩特性一直是学术与工程界关注的问题之一。该文用细观力学方法对混凝土相对于水泥浆体的干缩率进行研究。假定混凝土为由骨料和水泥浆体组成的二相材料,根据混凝土收缩量与组成混凝土的各相介质收缩量之和相等的原则,分别用基于Eshebly等效夹杂理论的Mori-Tanaka方法和基于载荷叠加思想的细观力学方法推导了混凝土干缩量。结果表明:Mori-Tanaka法、自洽方法和经验公式得到的相对干缩量基本相同,能很好地反映混凝土的干缩特性。混凝土相对干缩随骨料体积分数的增加而下降,随水泥基质泊松比的增大而增加。相对干缩随骨料与水泥基质体积模量比的增大而减小,但是当其值大于5后的影响比较小。用该文的方法估算混凝土的干缩,物理意义明确且计算简单,可以减少实验的工作量,同时也为混凝土结构的设计提供一定的参考。     

蒸压加气混凝土的孔结构及表征方法研究进展

蒸压加气混凝土因其质轻、保温性好、环保等优点而受到人们的重视。作为一种典型的宏观多孔建筑材料,蒸压加气混凝土的孔结构特征与其微观结构和性能有密切的关系。钙硅比、水料比、铝粉及工艺参数是影响蒸压加气混凝土孔结构的重要因素,孔隙率、孔径分布等孔结构特征与蒸压加气混凝土的强度、吸水性、干燥收缩、导热性能及耐久性等性能有紧密的联系。本文重点阐述了蒸压加气混凝土的孔结构特征及影响孔结构的主要因素,孔结构对蒸压加气混凝土性能的影响,并简述了蒸压加气混凝土孔结构现阶段主要采用的表征方法。     

Modelling creep and shrinkage of concrete by means of effective stresses

A novel model of mechanical performance of concrete at early ages and beyond, and in particular, evolution of its strength properties (aging) and deformations (shrinkage and creep strains), described in terms of effective stress is briefly presented. This model reproduces such? phenomena known from experiments like drying creep or some additional strains, as compared to pure shrinkage, which appear during autogenous deformations of a maturing, sealed concrete sample. Creep is described by means of the modified microprestress-solidification theory with some modifications to take into account the effects of temperature and relative humidity on concrete aging. Shrinkage strains are modelled by using effective stresses giving a good agreement with experimental data also for low values of relative humidity. Results of four numerical examples based on the real experimental tests are solved to validate the model. They demonstrate its possibilities to analyze both autogenous deformations in maturing concrete, and creep and shrinkage phenomena, including drying creep, in concrete elements of different age, sealed or drying, exposed to external load or without any load.     

Improving strength, drying shrinkage, and pore structure of concrete using metakaolin

This paper presents the results of an investigation on the use of metakaolin (MK) as a supplementary cementing material to improve the performance of concrete. Two MK replacement levels were employed in the study: 10% and 20% by weight of the Portland cement used. Plain and PC-MK concretes were designed at two water–cementitious materials (w/cm) ratios of 0.35 and 0.55. The performance characteristics of the concretes were evaluated by measuring compressive and splitting tensile strengths, water absorption, drying shrinkage, and weight loss due to the corresponding drying. The porosity and pore size distribution of the concretes were also examined by using mercury intrusion porosimetry (MIP). Tests were conducted at different ages up to 120 days. The results revealed that the inclusion of MK remarkably reduced the drying shrinkage strain, but increased the strengths of the concretes in varying magnitudes, depending mainly on the replacement level of MK, w/cm ratio, and age of testing. It was also found that the ultrafine MK enhanced substantially the pore structure of the concretes and reduced the content of the harmful large pores, hence made concrete more impervious, especially at a replacement level of 20%.     

The assessment of ceramic and mixed recycled aggregates for high strength and low shrinkage concretes

Very few studies on recycled aggregate concretes (RC) have been extended to the use of recycled ceramic and mixed aggregates in relation with high strength concretes. In the main they concentrate only on the analysis of the physical and mechanical properties. This study deals with the investigation of the influence that different percentages (up to 30% substitution for natural aggregates) of high porous ceramic and mixed recycled aggregates have over the plastic, autogenous and drying shrinkage of the concretes. The physical and mechanical properties as well as the chloride resistance were also determine in order to assess the viability of the use of ceramic and mixed recycled aggregates in high strength concretes. The results revealed that the employment of highly porous recycled aggregates reduced the plastic and autogenous shrinkage values of the concrete with respect to those obtained by conventional concrete (CC). Although the total drying shrinkage of the recycled concrete proved to be 25% higher than that of the CC concrete, the CC concrete had in fact a higher shrinkage value than that of the RC from 7 to 150 days of drying. It can be concluded that the RC concrete produced employing up to 30% of fine ceramic aggregates (FCA, with 12% of absorption capacity) achieved the lowest shrinkage values and higher mechanical and chloride ion resistance. In addition, the concrete produced with low percentage (10–15%) of recycled mixed aggregates also had similar properties to conventional concrete.     

Thoughts about drying shrinkage: Experimental results and quantification of structural drying creep

In this second article, we examine in greater detail the influence of concrete skin micro-cracking, linked with the non-uniformity of water content within the specimen. This cracking is responsible for the drying creep which results from a structural effect, also called “microcracking effect”, and which we prefer to call “structural creep” here. It is defined as the difference between the potential drying shrinkage of a specimen which does not crack and the shrinkage measured experimentally. We propose a simple experimental method making use of experimental curves of drying shrinkage as a function of weight loss and allowing the flanking and specification of certain properties of structural drying creep. We shall see that this method must also deal, in the interpretation of results, with the choice of the mechanical constitutive model allowing the processing of nonlinearities induced by concrete skin micro-cracking. Finally, to validate the hypotheses we have made, we shall base our approach on a probabilistic model of the cracking of concrete resulting from the work of Rossi, “coupled” with consideration of the drying of the concrete material in a sense that we shall specify. We shall see that this model is in satisfactory agreement with the curves of shrinkage as a function of weight loss. In addition, the results of simulations are very instructive, as regards the spacing and the width as well as the depth of cracks which appear in the drying shrinkage and creep tests.     

Shrinkage of concrete stored in natural environments

The paper reports on the influence of the natural environment on the drying shrinkage of a range of concretes, with and without steel fibre reinforcement. A combination of increasing cement content, the addition of silica fume (SF) and reduced water/binder ratio was used to obtain a wide range (C30–C70) of concrete strengths. Both prism and cylinder test specimens were used in the study and a fibre concentration of 2% (by weight) was used in the fibre reinforced concrete (FRC) mixes. The experimental results have been compared with predicted shrinkage strains obtained from the ACI 209 model. The results show that the effect of varying relative humidity and temperature in the natural environment had only a limited effect on the drying/autogenous shrinkage. The addition of 2% fibre to the various mixes had a negligible effect on shrinkage for the lowest strength (C30) but the restraint on the development of shrinkage was enhanced as the strength of the concrete was increased. Comparison between experimental results and predicted shrinkage strains was not good for the high-strength concretes, although good correlation was observed for the lower strengths (C30–C45). Further research is required to improve the prediction models for use with high-strength concretes with particular emphasis being given to the rapid development of drying/autogenous shrinkage during the first month after casting the concrete.     

基于细观损伤模型的混凝土湿度及干缩特性研究

将混凝土视为骨料与砂浆两相非均质复合材料,同时考虑材料损伤的影响。混凝土视为非均质体能较好的体现骨料和砂浆材料特性差异对湿度扩散及干缩应力的影响,内部出现的损伤也与混凝土视为均质体时明显不同。采用考虑材料特性差异及损伤作用的细观模型和计算方法,能更真实的反映混凝土的湿度、干缩变形和干缩应力分布规律。细观研究结果有助于更好地理解混凝土干缩裂缝的产生机理、开裂模式及发展规律。     

Effect of cementitious permanent formwork on moisture field of internal-cured concrete under drying

Drying shrinkage of concrete may still be the main source of cracking in concrete structures, even though the autogenous shrinkage of concrete can be effectively reduced by using internal curing. In the present paper, the effect of internal curing with pre-soaked lightweight aggregate and engineered cementitious composite permanent formwork (ECC-PF) on a moisture distribution in three kinds of concrete in a drying environment are investigated from both aspects of experiments and theoretical modeling. The test results show that the combination use of ECC-PF and internal curing can well maintain the humidity at a relatively high level not only at a place far from drying surface, but also at a place close to the drying surfaces. The developed model can well catch the characteristics of the moisture distribution in concrete under drying and the impacts of internal curing and ECC-PF can well be reflected as well. The model can be used for the design of concrete structures with combination use of internal curing and permanent formwork.     

How do recycled concrete aggregates modify the shrinkage and self-healing properties?

This paper presents the main results of a research carried out to analyze the mechanical properties, intrinsic permeability, drying shrinkage, carbonation, and the self-healing potential of concrete incorporating recycled concrete aggregates. The recycled concrete mixtures were designed by replacing natural aggregates with 0%, 30%, and 100% of recycled concrete gravel (RG) and 30% of recycled concrete sand (RS). The water to equivalent binder ratio was kept constant and recycled concrete aggregates were initially at saturated surface dried (SSD) state. The contribution of the porosity of natural and recycled aggregates to the porosity of concrete was estimated to understand the evolution of the intrinsic permeability and the open porosity. At long term, the maximum variation of drying shrinkage magnitude due to recycled concrete gravels did not exceed 15%. The correlation between drying shrinkage and mass-loss through “drying depth” concept showed that recycled concrete aggregates are affected by drying as soon as concrete is exposed to desiccation. A good correlation between 1-day compressive strength and 18-month carbonation depth was observed. The recycled concrete aggregates presented a good potential for self-healing as the relative recovery of cracks reached up to 60%.