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.     

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.     

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.     

Cracking control of concretes modified with short AR-glass fibers at early age. Experimental results on standard concrete and SCC

At early ages (less than 24 h), cracking can occur in concrete because it can be subjected to dimensional changes, due to shrinkage, can generate loads which are greater than the low strength capacity of the material at this age. This is especially the case in members with highly exposed surfaces, such as floor slabs or precast panels.

As any other cement based composite, Self Compacting Concrete (SCC) shrinks at an early age and can crack when shrinkage is restrained.

One possible solution to reduce the impact of early age shrinkage on concrete durability is to include low volumetric fractions of short fibers in order to control crack growth. To evaluate the cracking control ability of Alkali Resistant (AR) glass fibers in standard concrete and SCC, an experimental program, developed in accordance with the AR-glass fiber producer, was conducted. Two different types of AR-glass dispersible fibers, two concrete compositions and several volumetric fractions of fiber have been studied.

The experimental program included a mechanical characterization of the different concrete compositions (compression and flexural strength tests), free shrinkage tests, with and without air flow over the samples, and double restrained slab cracking tests (Kraai slab modified test).

The results obtained show that the inclusion of low volumetric fractions of the two types of AR-glass fiber under study can control the cracking produced due to very early age shrinkage on both standard concrete and SCC in two different ways: reducing the total cracked area and the maximum length of the cracks. Although, a non-linear dependence of cracked area on AR-glass fiber amount was found. A microscopic study of the cracked surface confirms the favorable effect of the presence of dispersed AR-glass fibers on cracking control.

When standard concrete and SCC results were compared, it was observed that, although SCC drying shrinkage was larger, standard concrete with a similar performance in the hardened state produced equivalent cracking area.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar

In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.     

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).     

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.     

A new testing configuration for shrinkage cracking of shotcrete and fiber reinforced shotcrete

Shotcrete and fiber reinforced shotcrete are commonly employed to produce layers or linings with large surface area versus volume ratios. Restrained shrinkage cracking is hence an important concern. The common test set-up used for shrinkage cracking of concrete, with a ring specimen cast around a stiff steel form, is not applicable to shotcrete. A new testing configuration, consisting of a shotcrete specimen bonded to a steel I-section and angles, is therefore proposed. In this investigation, a finite element analysis was first performed to identify member sizes that provide a good compromise between the effectiveness of constraint and weight of steel members. Restrained shrinkage tests using this new configuration were performed for plain and fiber reinforced shotcrete. Despite the simplifying assumptions in the finite element analysis, the predicted degree of restraint is in reasonable agreement with test results. From the results, the proposed set-up is shown to be a practical and viable approach for investigating the shrinkage cracking behavior of shotcrete and fiber reinforced shotcrete.     

Prediction of restrained shrinkage based on restraint factors in patching repair mortar

Shrinkage of repair material, especially in patching repairs, is the major factor inducing cracking in concrete repairs. Induced cracks in repair materials are due to restrained shrinkage. Although in usual practice, the free shrinkage of the repair mortar is measured, in reality, cracking is not due to free shrinkage. It is well known that cracking is due to restrained shrinkage. It is very hard to measure the restrained shrinkage; therefore, to overcome this problem a restraint factor (R) is used to modify the free shrinkage and come up with the restrained shrinkage. The restraint factor is influenced by the surface and boundary condition.In this study, the restraint factor for patching repair with different boundary conditions (with eaves and without eaves) and surface condition (rough and smooth) of the substrate concrete is investigated.The results show that the restraint factor R lies between 0.1 and 0.94; with an increase of restraint, the restraint factor is increased. In situations with a high level of restraint (eaves at the perimeter and a rough surface of substrate), the average R is 0.83. while with a low level of restraint (without eaves at the perimeter and a smooth surface of the substrate), the average R is 0.22.     

Desiccation shrinkage of cementitious materials as an aging, poroviscoelastic response

This paper describes and evaluates a new model that utilizes aging poroviscoelasticity for predicting the shrinkage of cementitious materials induced by loss of moisture from the pore structure (i.e. desiccation). The new model incorporates well-accepted mechanisms for desiccation shrinkage and accounts for the effect of changing concentrations of dissolved species in the pore fluid. Additionally, the model is used to interpret viscoelastic behavior during the drying process via comparisons of model predictions with measured shrinkage of hardened portland cement paste. It was found that while a poroelastic model under predicts the measured shrinkage, the poroviscoelastic model significantly over predicts the shrinkage unless intrinsic aging of the C–S–H gel is included in the model.     

Development of a shrinkage performance specifications and prediction model analysis for supplemental cementitious material concrete mixtures

The unrestrained shrinkage along with restrained cracking tendency of concrete mixtures typically used by the Virginia Department of Transportation (VDOT) were assessed to establish an appropriate limit on drying shrinkage for use in a performance specification. Five existing shrinkage prediction models were assessed to determine the accuracy and precision of each model as it pertains to the VDOT mixtures used in this study. The five models assessed were the ACI 209 Code Model, Bazant B3 Model, CEB90 Code Model, Gardner/Lockman Model, and the Sakata Model. The Gardner/Lockman Model performed best for the supplemental cementitious material (SCM) mixtures.Based on a comparison of the unrestrained drying shrinkage and restrained cracking tendency, it was determined that the potential for cracking could be minimized by limiting the unrestrained shrinkage of the concrete mixtures. The recommended percentage length change specification limits for the supplemental cementitious material mixtures are 0.0400 at 28 days and 0.0500 at 90 days.     

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.     

Interactions between shrinkage reducing admixtures (SRA) and cement paste's pore solution

Shrinkage reducing admixtures (SRA) have been developed to combat shrinkage cracking in concrete elements. While SRA has been shown to have significant benefits in reducing the magnitude of drying and autogenous shrinkage, it has been reported that SRA may cause a negative side effect as it reduces the rate of cement hydration and strength development in concrete. To examine the influence of SRA on cement hydration, this study explores the interactions between SRA and cement paste's pore solution. It is described that SRA is mainly composed of amphiphilic (i.e., surfactant) molecules that when added to an aqueous solution, accumulate at the solution-air interface and can significantly reduce the interfacial tension. However, these surfactants can also self-aggregate in the bulk solution (i.e., micellation) and this may limit the surface tension reduction capacity of SRA. In synthetic pore solutions, SRA is observed to form an oil-water-surfactant emulsion that may or may not be stable. Specifically, at concentrations above a critical threshold, the mixture of SRA and pore fluid is unstable and can separate into two distinct phases (an SRA-rich phase and an SRA-dilute phase). Further, chemical analysis of extracted pore solutions shows that addition of SRA to the mixing water depresses the dissolution of alkalis in the pore fluid. This results in a pore fluid with lower alkalinity which causes a reduction in the rate of cement hydration. This may explain why concrete containing SRA shows a delayed setting and a slower strength development.     

Influence of alkali on restrained shrinkage behavior of cement-based materials

Experiments have been conducted to study effects of high alkalinity on restrained shrinkage behavior and cracking sensitivity of cement-based materials at early ages. The restrained shrinkage test has been conducted with an ellipse ring setup and the initial cracking time was monitored with a continuous conductive strip. Alkali content and alkali type as well as the shrinkage-hydration relationship have been studied. The experimental results have shown that the cracking sensitivity of a cement-based material is increased with an increase in alkali content. The influence of the excess alkali on the cracking sensitivity is more obvious for cement paste with a low water-to-cement ratio (w/c) than that with a high w/c. The hydration processes and microstructure development of cement paste have been investigated using heat of hydration measurement and electrical resistivity measurement. The superimposed resistivity curve and heat evolution curve provide more comprehensive understanding on factors influencing shrinkage development.     

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.     

Properties of fresh and hardened concrete

The present paper reviews the literature related to the properties of fresh and hardened concrete published after the previous (12th) International Congress on the Chemistry of Cement held in Montreal in 2007.Workability and fundamental rheological properties, reversible and non-reversible evolution, thixotropy, slump loss, setting time, bleeding, segregation and practical issues related to formwork filling and pressure, are addressed among the properties of fresh concrete.Among hardened concrete properties compressive strength and other mechanical and physical properties of hardened concrete, such as tensile strength, elastic properties, shrinkage, creep, cracking resistance, electrical, thermal, transport and other properties are covered. Testing, interpretation, modeling and prediction of properties are addressed, as well as correlation with properties of fresh concrete and durability, effects of special binders, recycled and natural aggregates, fiber reinforcement, mineral and chemical admixtures. Special attention is given to the properties of hardened lightweight and self-compacting concrete.     

Early-age behavior of materials with a cement matrix

Materials with a cement matrix classically present early-age volume variations (shrinkage and/or swelling). This intrinsic early-age behavior strongly influences the length of time the buildings and structures will last because of the micro-cracking and cracking that results from it. One explanation for the macroscopic shrinkage is the presence of pore pressure in the porous medium. In this study, fine modeling of the coupling mechanism behind these internal strains is proposed. The chemical reaction associated with hydration is considered as the main force behind the hydric and mechanical evolutions in an endogenous configuration. Thus, the influence of chemical contraction, porosity, pore-size distribution and pore pressure are central to the study in the light of the numerical and experimental results obtained. A self-leveling layer of mortar of sulfo-aluminous concrete base was used.     

Evaluation of a basic creep model with respect to autogenous shrinkage

This paper shows the results obtained from an experimental study to evaluate a basic creep model. In this study, four different mixture proportions were placed, and tests on the specimens for autogenous shrinkage and basic creep were conducted with respect to age and stress level. The primary test variable was the water/cement ratio (w/c).From this research, it was found that for low w/c concrete, as well as at an early age of normal-strength concrete, a significant difference exists between apparent basic creep (including autogenous shrinkage) and real basic creep (excluding autogenous shrinkage). Furthermore, creep strain was not directly proportional to the applied stress level after one day. It was also discovered that when the current basic creep model that includes autogenous shrinkage is used in creep analysis, considerable errors as well as some computational problems may result. We therefore recommend modifying the equations of the current basic creep model with respect to autogenous shrinkage.