Study on effects of solar radiation and rain on shrinkage, shrinkage cracking and creep of concrete

In this paper, the effects of actual environmental actions on shrinkage, creep and shrinkage cracking of concrete are studied comprehensively. Prismatic specimens of plain concrete were exposed to three sets of artificial outdoor conditions with or without solar radiation and rain to examine the shrinkage. For the purpose of studying shrinkage cracking behavior, prismatic concrete specimens with reinforcing steel were also subjected to the above conditions at the same time. The shrinkage behavior is described focusing on the effects of solar radiation and rain based on the moisture loss. The significant environment actions to induce shrinkage cracks are investigated from viewpoints of the amount of the shrinkage and the tensile strength. Finally, specific compressive creep behavior according to solar radiation and rainfall is discussed. It is found that rain can greatly inhibit the progresses of concrete shrinkage and creep while solar radiation is likely to promote shrinkage cracking and creep.     

Prediction equation of drying shrinkage of concrete based on composite model

In this study, the prediction equation of drying shrinkage of concrete is obtained with two-phase composite model as aggregate and matrix. In order to obtain the input values for this prediction equation easily, the experimental formula of drying shrinkage and Young's modulus of cement paste are obtained, and the estimation method of Young's modulus of aggregate are proposed with easy test using cement paste, mortar and concrete. According to the experimental results, this equation can predict the drying shrinkage at any age in error by less than about 100 μm.     

Tensile basic creep of early-age concrete under constant load

Viscoelastic behavior of early-age normal and high-strength concrete has been investigated. The study shows that concrete exhibits high tensile creep strain if loaded at an age less than or equal to 1 day. The investigations furthermore show that the creep strain is not proportional to the stress level in the specimen when loading occurs at 1 day. Creep experiments were also carried out on concretes with different w/c ratios and some qualitative comments are made. Finally, an approach for mathematical modeling of early-age creep for normal concrete was explored.     

Tensile creep due to restraining stresses in high-strength concrete at early ages

This paper reports an experimental study on the early-age tensile creep behavior of high strength concrete (HSC) comprising of silica fume concrete, fly ash concrete and plain concrete under uniaxial restraining stresses. A series of restraint shrinkage tests were carried out adopting semi-adiabatic and isothermal conditions to determine the effects of temperature history on the tensile creep properties for young concretes. Furthermore, the effects of restraining stress history on creep were also discussed under three different degrees of restraint conditions. It was found that the initial thermal dilation deformation delayed the development of tensile creep and weakened the creep potential of early age concretes. It was also observed that the young concrete subjected to a lower restraining tensile stress history had a higher potential of visco-elastic response in tension at early ages.     

Coupling between leaching and creep of concrete

In a radioactive waste disposal, concrete containment structures must be studied over extended periods during which it is necessary to account for a possible degradation by calcium leaching due to on-site water. An experimental investigation is described where the effects of an accelerated calcium leaching process of concrete on creep of concrete are highlighted. The comparison with a creep test where the sample is immersed in water shows that leaching generates tertiary creep and rupture of the specimen. A Dirichlet series coupled to a mechanical damage are used to model the coupled tertiary creep. With this method we can evaluate the lifetime of concrete structures subjected to chemical and mechanical loading.     

Experimental study for the evaluation of creep in concrete through thermal measurements

This investigation, conducted at the Non-Destructive Testing Laboratory at the Politecnico di Torino, made it possible to establish an experimental relationship between specific creep values measured in concrete test pieces obtained from different mixes and the decrease in temperature observed in identical test pieces at the end of low-intensity short-duration compressive tests.This relationship is independent of type of mix and therefore has general validity: it can be used to make qualitative predictions about creep without having to perform long duration tests. It may lead to interesting developments in various applications, especially in the field of structural diagnosis.     

Characterization of creep and crack growth interactions in the fracture behavior of concrete

Creep and crack growth govern the long-term deformability of concrete and thus its service behavior and its durability. The mechanisms involved in the couplings between these effects are not yet clearly identified. The aim of this research was to investigate experimentally and numerically at both the macro and the meso-level (heterogeneous material) the interactions between creep and crack growth in concrete and identify the main phenomena acting on the overall viscoelastic response of concrete, at various stages of fracture, during relaxations.     

An experimental approach for the analysis of early-age behaviour of high-performance concrete structures under restrained shrinkage

This paper presents a systematic approach for the experimental testing and analysis of the early-age thermo-mechanical behaviour of large prismatic high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint in order to enable the characterisation of high-performance concrete specimens without premature termination of the test in case of significant restrained shrinkage. The instrumentation system can measure the strains and other parameters from the setting time with high accuracy and reliability. The analysis method takes into account the temperature effects on the measured properties and provides equations to determine the time-evolution of shrinkage, thermal expansion, stiffness and creep of the concrete. Results from the testing of 200 × 200 × 1000 mm specimens made with a 0.34 water-cement ratio concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.     

Predictions and verifications of early-age stress development in hydrating blended cement concrete

A procedure for calculating the early-age stress development in concrete incorporating the aging viscoelastic effects is presented in this paper. The important features of the present procedure are the use of tensile creep and inclusion of heat of hydration. The latter is used as an aging parameter and incorporates the effect of temperature on age-dependent material parameters. To validate stress predictions, experiments to measure early-age stress development in concrete mixes made of blended cements were conducted. The predictions were found to be accurate and could be improved when the effect of temperature was included. Effects of using mineral additives (fly ash, slag, and silica fume) appeared to be beneficial in reducing the risk of cracking at early ages. The effect of temperature gradient was also studied when the stress calculation was applied to pavement or foundation slabs resting on a very stiff subgrade.     

Induced anisotropic permeability due to drying of concrete

Structural strength, porous space, and permeability of concrete are strongly affected by mechanical, hydrous, and thermal loading. These various loadings may lead to drying shrinkage, one of the main characteristics of this type of material, which has to be involved in the behaviour modelling and experimental investigations being the subjects of this paper. Experimental devices and principal parameters studied are first presented. Drying shrinkage and loss of mass in time were measured on prismatic samples while uniaxial compression tests were performed on cylindrical samples. Gas permeability tests, carried out on a concrete cylinder 30 mm in diameter, form the second part of this study. The samples used for these measurements were cored from each prismatic sample at the end of 10 months or 2 years of drying, either from the transverse direction of sample (privileged direction of drying) or from the longitudinal direction. Gas permeability procedure, using micropulse test technique, is described as well as the experimental process. Experimental results are finally commented on and discussed with a view on induced anisotropy due to desiccation. Such an anisotropy is clearly observable in permeability, which is also increasing with drying time.     

Stability of ancient masonry towers: Stress redistribution due to drying, carbonation, and creep

Extending the analysis of the evolution of pore humidity and carbonation presented in a preceding paper by Ferretti and Baz?ant, this paper analyzes the redistribution in time of vertical normal stresses across the multiple-leaf wall of ancient towers, using the example of collapsed Pavia Tower. It is shown that stress redistribution due to nonuniform shrinkage and nonuniform creep across the wall, including the additional redistribution due to cracking, produces in the masonry cladding of the wall very large compressive stresses that reach, after centuries, the level of compression strength of the cladding.     

Investigation on key properties controlling early-age stress development of blended cement concrete

Age dependent mechanical and kinetic properties including Young's modulus, early-age creep, autogeneous and thermal deformations, and heat of hydration were investigated for concrete made of blended cements. These are among the key properties that control the early-age cracking behavior in hydrating concrete members. Among the main goals of the investigation were to provide the experimental data and to study the effect of adding mineral additives such as fly ash (FA), ground granulated blast furnace slag (GGBF), and silica fume (SF) on the aforementioned properties. The age-dependent behavior of Young's modulus, creep compliance, and autogeneous shrinkage as functions of heat of hydration were modeled. We emphasized on mathematical modeling the viscoelastic properties of concrete. The equations obtained can be used as inputs needed to calculate the early-age stress development in concrete members.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

Microstudy on creep of concrete at early age under biaxial compression

An interesting phenomena of crack restoration and increasing strength of concrete under biaxial compression creep were described in this paper. A small loading apparatus was prepared and a long work distance optical microscope with variable focus was used for studying the cracks. It was found from the micrographs that these cracks diminished under biaxial compression creep. There were increases in the strength of the creep specimens under the sustained biaxial compression load compared with the free companion ones. Multiaxial compression caused by the early temperature rise inside the mass concrete may strengthen the concrete and reduce the tensile cracks during and after temperature drop.     

Effects of drying conditions, admixtures and specimen size on shrinkage strains

The paper presents the results of an experimental investigation on the effects of drying conditions, specimen size and presence of plasticizing admixture on the development of shrinkage strains. The measurements are taken in a harsh (50 °C and 5% R.H.) and a moderate environment (28 °C and 50% R.H.). The results include strain development at various levels of cross sections of concrete prisms. The drying conditions are found to be the dominant parameter affecting the shrinkage strain development particularly in specimens of smaller sizes. The effect of plasticizing admixture on shrinkage strains is negligible.     

Stress states in plasma-sprayed thermal barrier coatings upon temperature cycling: Combined effects of creep,plastic deformation,and TGO growth

To ascertain material parameter effects on the stress states is beneficial to comprehend the crack growth behavior and delamination mechanism in thermal barrier coatings (TBCs). In this work, numerical models are established to explore the combined effects of material parameters including creep, plastic deformation, and thermally grown oxide (TGO) growth on the stress states upon temperature cycling. For all layers, thermal-physical properties reliant on temperature are incorporated into the model. The process of bond coat (BC) oxidation, namely TGO growth, is materialized by changing material properties with cycles. Based on the principle of a single variable, the residual stress states are explored using many different material combinations. The results indicate that the tensile stress in the ceramic top coat (TC) decreases with the increase in the TGO lateral strain distribution gradient. Increasing the BC yield strength or decreasing the TGO growth stress can reduce the tensile stress in TC if there is no creep in the model. When BC yield strength is relatively high (≥150 MPa), BC creep will strengthen the TC tensile stress. TGO creep can decrease the tensile stress in TC irrespective of TGO growth stress and BC creep. When TGO creep rate is higher than 10B^tgo, an exceedingly small tensile stress can always be achieved. This work could provide significant theory direction for material selection and composition control towards advanced TBCs with prolonged lifetime.     

Influence of shrinkage-reducing admixtures on the reduction of plastic shrinkage cracking in concrete

Fresh concrete exposed to high evaporation rates is prone to plastic shrinkage cracking, especially in structures with large surface area/volume ratios. The present work shows that the reduction of the surface tension of the mixing water is an effective way for decreasing such cracking. In this study, conventional and high strength concretes with superplasticizers and shrinkage reducing admixtures (SRAs) were exposed to drying in the plastic state. Continuous monitoring of the surface displacement facilitated the identification of the different stages of plastic shrinkage cracking. Measurements of capillary pressure, settlement, internal temperature and evaporation rate were also made. The results show the effectiveness of SRAs in reducing plastic shrinkage cracking, even in high strength concrete. This is attributed to the reduction in the evaporation rate, delay of the peak capillary pressure due to the development of menisci in the pores and lower settlement.     

Effect of curing temperature and type of cement on early-age shrinkage of high-performance concrete

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.     

How to study drying shrinkage microcracking in cement-based materials using optical and scanning electron microscopy?

Three different ‘destructive’ microscopy methods were tested on their ability to show drying shrinkage microcracks on a specimen cross-section. The first two were methods in which the microcracks were impregnated with a fluorescent epoxy and examined with fluorescence microscopy. In one method, the impregnation was applied before making the cross-section and in the other after making the cross-section. In the third method, the sample was kept wet constantly and examined in an environmental scanning electron microscope (ESEM). It was concluded that the method in which the dried specimen was impregnated before making the cross-section was the most reliable method to record drying shrinkage microcracks. With this method, it was possible to impregnate the complete drying shrinkage microcrack pattern in the studied cement-based materials from the surface, and there was no risk of recording microcracks introduced by sample preparation.     

Changes in the size of pores during shrinkage (or expansion) of cement paste and concrete

The often-argued and seldom-resolved issue of relating the bulk shrinkage of concrete to changes in the size of pores is analyzed using a simple model. It is shown that the pores in a composite containing both shrinking and nonshrinking solid phases can themselves either shrink or expand when the matrix shrinks, depending on the amount of restraint in the system. The analysis also applies to expansive deformations, which occur with some types of chemical attack of cement paste. A mathematical relationship between the degree of restraint in a composite and the relationship between bulk volume changes and pore volume changes is given. These observations provide guidelines for interpreting the meaning of gaps that often form between aggregate and paste in concrete.