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.     

Effect of metakaolin on creep and shrinkage of concrete

The effect of metakaolin (MK) on the creep and shrinkage of concrete mixes containing 0%, 5%, 10%, and 15% MK has been investigated. The results showed that the early age autogenous shrinkage measured from the time of initial set of the concrete was reduced with the inclusion of MK, but the long-term autogenous shrinkage measured from the age of 24 h was increased. At 5% replacement level, the effect of MK was to increase the total autogenous shrinkage considered from the time of initial set. While at replacement levels of 10% and 15%, it reduced the total autogenous shrinkage. The total shrinkage (autogenous plus drying shrinkage) measured from 24 h was reduced by the use of MK, while drying shrinkage was significantly less for the MK concretes than for the control concrete. The total creep, basic creep as well as drying creep were significantly reduced particularly at higher MK replacement levels. Compared with estimated values by the CEB 90 model, total creep of all concretes was overestimated, especially in the mixes containing the higher levels of MK. For basic creep, estimates for low levels of MK were acceptable but, for the higher levels, creep was overestimated.     

Strength,drying shrinkage,and water permeability of concrete incorporating ground palm oil fuel ash

In this study, palm oil fuel ash (POFA) was used as a pozzolanic material in concrete. The POFA was ground to obtain two different finenesses: coarse (CP) and fine (FP). A portion of ordinary type I Portland cement (OPC) was replaced by CP and FP at 10%, 20%, and 30% by weight of binder to cast concrete. Compressive strength, modulus of elasticity, drying shrinkage, and water permeability of concretes containing ground POFA were measured. The results showed that the compressive strength of the concrete increased with the fineness of the POFA. With 10% and 30% replacement of OPC by CP and FP, respectively, the compressive strength of the resulting concrete was as high as that of OPC concrete at 90 days. Moreover, the use of 10–30% of FP as a cement replacement in concrete reduced its drying shrinkage and water permeability. Finally, there was also a strong correlation between the compressive strength and the water permeability of ground POFA concrete.     

Effect of supplementary cementitious materials on shrinkage and crack development in concrete

The autogenous and drying shrinkage of Portland cement concrete, and binary and ternary binder concretes, were measured and compared. The binary and ternary binder concretes were formed by replacing part of the cement with fly ash, very fine fly ash and/or silica fume. Restrained shrinkage test was also performed to evaluate the effect of binder type on early age cracking. After the cracking of the restrained ring samples, crack widths were measured and compared with the results of an R-curve based model, which takes post-peak elastic and creep strains into account.The incorporation of fly ash and very fine fly ash decreased the autogenous shrinkage strain but increased the drying shrinkage strain. Since the total shrinkage strains of both the ternary and the binary concrete mixtures were similar, the strength development became an important factor in the cracking. The lower strength of the concrete with ternary binders led to earlier cracking compared to the binary binder concrete. Portland cement concrete cracked the earliest and had the greatest crack width. Measured crack widths were in accordance with the crack widths calculated with the R-curve model.     

Stress distribution and crack formation in full-scaled ultra-high strength concrete columns

Full-scaled model columns were placed both in summer and winter with an ultra-high strength concrete with a compressive strength more than 150?MPa, and stress distribution and cracking were experimentally evaluated. Specimens placed in summer exhibited cracks around the steel reinforcements which sometimes joined together. Specimens placed in winter showed, in addition to the cracks around the reinforcement, an internal crack perpendicular to the column axis as well as at the specimen surface. These cracks were found to be dependent on the temperature history due to hydration heat liberation and associated autogenous shrinkage strains. It was shown that the autogenous shrinkage of concrete increased when temperature after mixing was low and the maximum temperature during temperature history was high. This accounts for the numerous cracks found in the specimen placed in winter. Strain perpendicular to the axial direction was smaller than that of the axial direction implying the tensile stress due to autogenous shrinkage acting perpendicular to the axial direction as far as the autogenous shrinkage is isotropic. Finite element analysis confirmed the lateral stress due to autogenous shrinkage. Possible influences of autogenous shrinkage of ultra-high strength concrete on the structural performance include (1) early spalling of cover concrete and degradation of flexural strength, (2) degradation of bond and shear strength, and (3) prospect of longitudinal crack in center of the column and degradation of flexural strength.     

自密实混凝土圆环约束收缩试验研究

进行了密封与沿环外侧面干燥两种条件下的自密实混凝土圆环约束收缩试验,分别研究自密实混凝土试件在自生收缩单独作用下,以及在干燥与自生收缩共同作用下钢环应变随混凝土龄期的发展规律及开裂性能,并与普通混凝土进行比较。配合比参数包括粉煤灰掺量、粉煤灰和矿渣复掺掺量、水胶比。揭示了配合比参数在两种不同条件下对钢环应变和开裂龄期的影响规律,并提出以“标准化”的钢环应变曲线作为开裂趋势曲线,以开裂系数为0.95时的龄期为“名义开裂龄期”作为圆环试验中混凝土开裂指标的建议。研究结果还表明,同一配合比参数对干燥约束收缩与自生约束收缩的作用有可能一致,也有可能完全相反,因此有必要对两种收缩分别研究;该文提出的“开裂趋势曲线”和“名义开裂龄期”能综合混凝土开裂中“作用”和“抗力”两个方面的影响,从而对混凝土的开裂性能进行动态、定量、综合的评价。     

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.     

Influence of clays on the shrinkage and cracking tendency of SCC

The influence of different types of clay on the shrinkage and cracking tendency of fly ash modified self-consolidating concrete (SCCF) for the application of slipform paving were investigated in this study. The mortar phase of each mix was tested for autogenous shrinkage, total free shrinkage under drying and restrained shrinkage cracking. The mechanical properties (flexural strength, compressive strength, and modulus) were studied to supplement the results of the shrinkage and cracking tests. The plain SCCF mix was compared against the clay-modified SCCF mixes, as well as conventional SCC and slipform concrete (SFC) mixes. The results showed that the very early-age autogenous shrinkage of SCCF mortar was increased by the addition of clays due to adsorption effects. The effects of the clays on total shrinkage under long-term drying were found to depend mainly on the pozzolanic reactivity, but these effects were very slight at low dosages of about 1% by mass of binder. The early-age cracking tendency was aggravated by the clays composed of purified magnesium alumino silicate and metakaolin, but little influenced by the clay composed of kaolinite, illite and silica. Overall, the SCC mixture modified with both fly ash and a small amount of clay showed comparable shrinkage and early-age cracking performances as conventional SFC.     

高吸水树脂对混凝土水化及强度的影响

高吸水树脂(Super-absorbent polymer,SAP)作为混凝土内养护材料可有效抑制混凝土自收缩,提高混凝土抗裂性,但其对混凝土是否具有负面影响有待研究。利用XRD和DTA-TG研究了不同SAP掺量净浆在不同养护龄期的水化产物量,并测试其抗压强度,定量分析高吸水树脂对混凝土水化和强度的影响。实验结果表明:掺加SAP会延缓混凝土早期(0~7d)的水化反应,降低混凝土的抗压强度,但对混凝土中后期(7~28d)水化的进行及强度发展的影响不大。当高吸水树脂的掺量为1kg/m~3(占胶凝材料的质量分数为0.2%)和1.5kg/m~3(占胶凝材料的质量分数为0.3%)时,混凝土28d抗压强度可达基准组的100%和96%,56d抗压强度可达基准组的107%和96%。针对C50混凝土,推荐掺量为1kg/m~3。     

A viscoelastic approach for the assessment of the drying shrinkage behaviour of cementitious materials

A new model for the drying shrinkage of concrete is presented. In this model, drying shrinkage strains are regarded as being a spherical elastic and creep response of the material under rising pore pressures during the drying process. Therefore, a basic creep model which allows to incorporate these pore pressures is developed on the basis of microscopic considerations of the role of water in the creep mechanism. Then, the model response is compared to experimental results performed on a cement paste specimen subjected to drying. The developed model is able for describing the main features of the shrinkage behaviour of cement based materials.     

Tensile basic creep versus compressive basic creep at early ages: comparison between normal strength concrete and a very high strength fibre reinforced concrete

The paper presents experimental results concerning the comparison of tensile and compressive basic creep behaviours at early ages of two different concretes: a normal strength concrete (NSC) and a very high strength fibre reinforced concrete (HPFRC). This research project has been done in the context of a bilateral collaboration between Polytechnique Montreal and IFSTTAR. Observations on the HPFRC showed specific compressive creep similar to the specific tensile creep. Moreover, the specific creep curves obtained under compressive and tensile loading had always positive values, i.e. they were in same direction of the applied load on specimens. Measurements made on the NSC revealed specific compressive creep with positive values (in the loading direction). However, specific tensile creep presented negative values (opposite direction of loading) for a long period. A physical explanation based on the existence of two mechanisms with opposite effect is proposed to describe these basic creep results. The first mechanism is a coupling between the microcracking process and the water transfers that lead to additional self-drying shrinkage; the second mechanism is the self-healing of concrete induced by the microcracking.     

Practical prediction of time-dependent deformations of concrete

Proposed is a practical model for predicting creep and shrinkage of concrete from the composition of concrete mix, strength, age at loading, conditions of environment, size and shape, etc. The main features are: double power law for basic creep, square-root hyperbolic law for shrinkage, diffusion-type size dependence of humidity effects, additive drying creep term related to shrinkage, and activation energy treatment of thermal effects. Optimization techniques are used to fit numerous test data available in the literature. The work is a continuation of previous investigations and consists of several parts. This first part deals with shrinkage.     

Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Creep and drying shrinkage of a blended slag and low calcium fly ash geopolymer Concrete

The main purpose of this research is to study the time dependent behaviour of a geopolymer concrete. The geopolymer binder is composed of 85.2 % of low calcium fly ash and only 14.8 % of ground granulated blast furnace slag. Both drying shrinkage and creep are studied. In addition, different curing conditions at elevated temperature were used. All experimental results were compared to predictions made using the Eurocode 2. The curing regime plays an important role in the magnitude and development of both creep and drying shrinkage of class F fly ash based geopolymer concrete. A minimum of 3 days at 40 °C or 1 day at 80 °C is required to obtain final drying shrinkage strains similar to or less than those adopted by Eurocode 2 for ordinary Portland cement (OPC) concrete. Creep strains were similar or less than those predicted by Eurocode 2 for OPC concrete when the geopolymer concrete was cured for 3 days at 40 °C. After 7 days at 80 °C, creep strains became negligible.     

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.     

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.     

Constitutive modeling of creep-fatigue interaction for normal strength concrete under compression

Conventional approaches to model fatigue failure are based on a characterization of the lifetime as a function of the loading amplitude. The Wöhler diagram in combination with a linear damage accumulation assumption predicts the lifetime for different loading regimes. Using this phenomenological approach, the evolution of damage and inelastic strains and a redistribution of stresses cannot be modeled. The gradual degration of the material is assumed to not alter the stress state. Using the Palmgren–Miner rule for damage accumulation, order effects resulting from the non-linear response are generally neglected.In this work, a constitutive model for concrete using continuum damage mechanics is developed. The model includes rate-dependent effects and realistically reproduces gradual performance degradation of normal strength concrete under compressive static, creep and cyclic loading in a unified framework. The damage evolution is driven by inelastic deformations and captures strain rate effects observed experimentally. Implementation details are discussed. Finally, the model is validated by comparing simulation and experimental data for creep, fatigue and triaxial compression.     

Short-term tensile creep and shrinkage of ultra-high performance concrete

The tensile creep and free shrinkage deformations of ultra-high performance concrete (UHPC) were examined through short-term testing to assess the influences of stress/strength ratio, steel fiber reinforcement, and thermal treatment. The use of fibers and the application of thermal treatment decreased 14-day drying shrinkage by more than 57% and by 82%, respectively. Increasing the stress-to-strength ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by volume decreased the tensile creep coefficient by 10% and the specific creep by 40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 90 °C, after initial curing, decreased its tensile creep coefficient by 73% and the specific creep by 77% at 7 days, as compared to ordinarily cured companion mixes. Comparison of tensile creep behavior to published reports on compressive creep in UHPC reveal that these phenomena differ fundamentally and that further evaluation is necessary to better understand the underlying mechanisms of tensile creep in UHPC. Results from this study also showed that the effects of both thermal treatment and fiber reinforcement were more pronounced in tensile creep behavior than tensile strength results of different UHPC mixes. This emphasizes the importance of conducting tensile creep testing to predict long-term tensile performance.     

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%.