Early-age Cracking Of Concrete Research Articles

Mechanical properties of early-age concrete reinforced with multi-walled carbon nanotubes

The results of mechanical property tests on early-age concrete reinforced with industrial-grade carbon nanotubes (CNTs) are reported. The objective of the study was to validate the effectiveness of CNTs and establish the development of concrete properties with CNT dosage and concrete age. Ultrasonication, ultraviolet–visible spectrophotometry and scanning electron microscopy were employed to inspect or improve the dispersion quality. The test programme considered four CNT dosages and seven concrete ages, with three replicates. The results indicated that industrial-grade CNTs can lead to increases in cubic compressive strength, tensile splitting strength, prismatic compressive strength and ultimate strain of concrete (up to 34·2, 11·1, 19·4 and 8·3%, respectively) comparable to those obtained with the use of fine nanotube particles; no significant influence was found for the other tested concrete properties. The results of this study also indicated that increases in strengths and strain were most significant during the first 7 d, the period when concrete is most prone to cracking due to shrinkage and hydration heat. This implies that industrial-grade CNTs may have a great potential in reducing early-age concrete cracking with much less health risk and lower testing requirements compared with the use of fine nanotube powders.

Mitigation of Early-Age Cracking of Concrete Based on a New Gel-Type Superabsorbent Polymer

AbstractShrinkage cracking of concrete at an early age has notable and adverse effects on the durability and long-term performance of the material because it damages surface integrity or undermines...

Tensile creep and early-age concrete cracking due to restrained shrinkage

Concrete structures are prone to cracking due to restraint provided to early age autogenous and drying shrinkage. In addition, the risk of early-age thermal cracking is increased by increasing the degree of restraint to early-age thermal contraction. At this early-age, tensile creep plays a key role in relaxing shrinkage induced tensile stresses and delaying the time to cracking. However, limited data are available concerning tensile creep of concrete and the magnitude and rate of development of the early-age shrinkage of concrete. As a consequence, restraint to shrinkage is often poorly modelled in structural design. In order to accurately quantify the early-age shrinkage and tensile creep of concrete, a comprehensive experimental program is being conducted at the UNSW Centre for Infrastructure Engineering and Safety. Tensile creep is measured on dog-bone shaped specimens subjected to constant sustained tensile stress, while shrinkage is measured on identical unloaded specimens. Restrained ring tests were also performed to validate the tensile creep coefficients calculated from dog-bone specimens. A simple analytical procedure to accurately predict the degree of restraint and the tensile stresses in concrete induced by shrinkage is described for the restrained ring specimens.

Internal curing of concrete using lightweight aggregates

ABSTRACTInternal cured concrete (ICC) has been recently used in the local and international construction markets. ICC contains surplus amount of water to compensate the shrinkage of the mix and the volumetric changes which result in early-age cracking of concrete. Concrete cracking is a direct result of the shrinkage of the water–cement paste during early stages of the hydration process and continues for a significant amount of time during the life span of the concrete section. Early-stage shrinkage, prior to the concrete hardening, is associated with volumetric changes, until final setting is achieved. Afterward, the reduction in cement paste particle size results in increased voids within the concrete structure. These voids result in increased permeability, additional sulfate and chloride attacks on steel reinforcement, and internal tensile stresses in concrete, which result in significant cracking. ICC uses the additional water added to the mix in counteracting the reduced volume of the concrete. Several techniques are used for internal curing (IC). In this research, water-saturated lightweight aggregates (LWAs) are used in partial replacement of normal weight aggregate as a source of additional water. LWA is submerged in water prior to concrete mixing to absorb a significant amount of water, which is stored within the LWA particles. Once added to the mix, the water is gradually desorbed and compensates the water losses during hydration. Hence, it counteracts the shrinkage induced. Different ICC mixes are developed in this research using two different sizes of LWA, and supplementary binding materials are used to improve compressive strength. ICC compressive strength and reduced shrinkage attained are presented. ICC mixes developed in this research can be successfully used in pouring highway segments and bridge decks with lower cracks and reduced life cycle cost due to reduced maintenance.

Effect of internal curing with super absorbent polymers on residual stress development and stress relaxation in restrained concrete ring specimens

The early age cracking of concrete is a persistent problem for structures. Internal curing was developed to mitigate the shrinkage of low w/c concrete in recent decades by introducing additional water into concrete. Although shrinkage development and the early age cracking resistance of internally cured concrete under restraint were investigated, result on stress relaxation of internally cured concrete with super absorbent polymers (SAPs) by ring test is still lacking. Tests on the cracking resistance of concrete with different amounts of SAPs (0%, 0.05%, 0.16%, and 0.26% by weight of cement) were conducted using restrained ring test in present study. Test results showed that: (1) the strain in the restrained steel ring decreased with the increase of amount of SAPs; (2) the residual stress of concrete ring decreased with the increase of amount of SAPs; (3) the stress rate decreased with the increase of amount of SAPs; (4) the relaxed stress increased with the increase of amount of SAPs after the initiation of drying; and (5) the time to cracking for concrete increased with the increase of amount of SAPs.

Parameter Analysis of the Reinforcement for the Width and Spacing Control of the Early-Age Cracks in Concrete

Early-age volume changes in concrete induced by temperature change, hydration, autogenous and drying shrinkage can lead to concrete cracking and this can have lasting effects on serviceability, durability or aesthetics of the structure. The restraint to thermal movement is the product of the coefficient of the temperature fall from a peak level during cement hydration and a restraint factor. In most cases it is not necessary and also not economical to avoid cracks. In these cases, crack widths are limited due to water tightness, durability or aesthetic reasons. If early-age thermal cracking cannot be prevented, crack width can be controlled with reinforcement. The reinforcement distributes cracks and consequently reduces their widths and spacing. As a result, there forms a large number of smaller cracks instead of a few through-cracks. This means, that due to the formation of fine cracks, the strain capacity of a reinforced concrete element before the occurrence of through cracks can be increased with the help of skin reinforcement. This paper discusses the parameters of reinforcement affecting the width and spacing of early-age cracks in concrete. The effect of reinforcement on early-age cracking in concrete was investigated on numerical simulation and in full-scale experiments. The test variables were the reinforcement ratio and the cover thickness of the longitudinal reinforcing bars.

Acoustic Emission Behavior of Early Age Concrete Monitored by Embedded Sensors.

Acoustic emission (AE) is capable of monitoring the cracking activities inside materials. In this study, embedded sensors were employed to monitor the AE behavior of early age concrete. Type 1–3 cement-based piezoelectric composites, which had lower mechanical quality factor and acoustic impedance, were fabricated and used to make sensors. Sensors made of the composites illustrated broadband frequency response. In a laboratory, the cracking of early age concrete was monitored to recognize different hydration stages. The sensors were also embedded in a mass concrete foundation to localize the temperature gradient cracks.

Ring Test for the Measurement of Restrained Shrinkage of Concrete

Shrinkage of repair material is the major factor inducing cracking in concrete repairs. Induced cracks in repair materials are due to restrained shrinkage. The ring test is commonly used to assess the potential for restrained shrinkage induced cracking due to its simplicity and versatility. The restrained ring test is becoming widely used as a standard test method to assess the potential for early-age cracking in concrete. In this investigation, a ring test set up was developed to measure the restrained shrinkage of various repair materials available in the local market. Keeping in view the importance of the local environment, two environment conditions were used; Lab Environment (Temperature 23±2°C) and the open environment of Riyadh. These environmental conditions were used taking into account the local environmental conditions. The samples were outside for almost one year and encountered all weather conditions of Riyadh. It has been observed that some materials although manufactured by highly reputed companies didnt comply fully in restrained shrinkage tests.

Importance of Insulation at the Bottom of Mass Concrete Placed on Soil with High Groundwater

Frequently, when mass concrete is placed directly on top of a soil layer, an insulation layer is not used at the bottom of the concrete. The rationale for this practice is that the soil on which the concrete is placed is already an insulating material. This study investigated the question of whether the absence of an insulating layer between the mass concrete and the soil may cause a problem with cracking of the concrete at an early age. A three-dimensional finite element model was used for this investigation. The typical soil condition in Florida, where the ground-water level is high, was considered. The soil layer beneath the concrete was modeled to simulate realistic heat transfer between the concrete and the soil. To validate the developed model, temperature development in a bridge pier footing constructed in the field in Florida was compared with the computed temperature distribution from the finite element model. The results showed that the temperatures predicted by the model closely agreed with those measured in the field. A parametric analysis was also conducted to determine the effects of insulation on the temperature distribution, induced tensile stresses, and cracking risk in the concrete. From the thermal cracking analysis for the monitored footing, it is concluded that full insulation with adequate thickness (a 63-mm-thick blanket at the top, 63-mm plywood panels at the sides, and a 63-mm polystyrene foam board at the bottom) should be used to reduce the temperature differentials and prevent early-age cracking in concrete.

Evaluation of Recycled Aggregate Concrete Cracking through Ring Test

Cracks can reduce the service life of a concrete structure by allowing aggressive agents to penetrate through it in easy ways. Free shrinkage evaluation alone is not enough to determine if cracking can be expected in a structure since concrete creep behaviour, stiffness and toughness also influence the potential for cracking. Consequently, it is rather interesting to perform restrained shrinkage tests, such as the ring test according to ASTM C 1581–04. The testing procedure involves concrete ring specimens restrained by an inner steel ring on which strain gauges are placed to determine the age of cracking, since abrupt changes in the steel strain occur when concrete is cracked. Both the ring test and free shrinkage test should be carried out in the same exposure conditions, 21°C and 50% relative humidity. Moreover, compressive and tensile strengths of concrete were evaluated on cubic specimens at the time of its cracking and up to 28 days of curing. By means of analytical and numerical models of the ring specimen, some useful information on the stress induced in the material and on the tensile creep behaviour of concrete can be extrapolated thus allowing to better interpret the experimental results. This experimental procedure enables to study the influence of concrete mixture composition on the potential for early-age cracking of concrete. In particular, in this work the influence on early-age cracking of recycled-concrete aggregate partially replacing virgin sand was tested.

Simulation analysis of crack cause of concrete overflow dam for Hadashan Hydro Project by 3-D FEM

This paper is mainly to study the cracking reasons for concrete overflow dam of Hadashan Hydro Project. The three-dimensional finite element method (3-D FEM) is developed to simulation analysis the temperature and thermal stress distribution in the concrete overflow dam during the construction period. The results show that the crack of the concrete overflow dam is temperature crack, mainly due to the combined action of the internal thermal gradient and the external restraints; and dramatic changes in ambient temperature exacerbate cracking of early-age concrete. Finally, the results are applied to provide some references for the construction in the related fields.

Experimental Study on Thermal Expansion Properties and Micro-Pore Texture of High Strength Concrete in Early Age

The early age cracking of concrete is concerned with its thermal expansion properties, which is mainly reflected by the thermal expansion coefficient. Reasonably controlling the coefficient is an effective way of reducing cracks in the early age of concrete. While thermal expansion properties are related to the micro-pore texture characteristics of the concrete. Micro-pore textures of concretes of different mixing ratios and curing time were measured via mercury intrusion porosimetry (MIP), and the thermal expansion coefficient was determined by the comparator. The analysis of test results indicates the correlation between the parameters of micro-pore texture and thermal expansion properties, and also shows a highly positive correlation between the pore area and the thermal expansion coefficient. The results provide a solution for reducing the thermal expansion coefficient, thus controlling the early age cracking.

Influence of Environmental Conditions on Temperature Field of Concrete

The temperature cracking of early-aged concrete is often met by constructors in recent years, because of large temperature difference of core and surface of concrete. Experimental study on temperature field of concrete under different environmental conditions is conducted. Different environmental conditions such as wind speed, formwork removal, sun radiation and conservation are involved. The influence of environmental conditions on the peak value of rising temperature, temperature difference of core and surface and descending speed of temperature of concrete is studied. Result obtained from this study can be used in controlling of early-age cracking of concrete.

Analysis of Crack Propagation due to Thermal Stress in Concrete Considering Solidified Constitutive Model

In recent years, interest in early age concrete cracking has increased due to its effects on the durability and performance of concrete structures. A time-dependent material model and a structural analysis method have been developed to evaluate thermal cracking behavior. To simulate such behavior at early ages, a solidified constitutive model is proposed, which is based on the solidification concept with dependence on time and strain histories. The unified numerical model consists of a Rigid-Body-Spring Network, representing the structural behavior, combined with a truss model to represent heat transfer. Wall concrete structures are analyzed to verify the solidified constitutive model and the overall approach. The proposed model results and the experimental results show reasonable agreement in terms of cracking behavior, stress distributions and structural deformations.

A multiscale micromechanics model for the autogenous-shrinkage deformation of early-age cement-based materials

Autogenous shrinkage is the term for the bulk deformation of a closed, isothermal, cement-based material system not subjected to external forces [Jensen OM, Freiesleben Hansen P. Autogenous deformation and RH-change in perspective. Cement and Concrete Research 2001;31:1859–65]. It is associated with the internal volume reduction of the cement/water mixture in the course of the hydration process. The question arising from engineering practice is the magnitude of the autogenous deformations, increasing the compliance of concrete support structures and inducing tensile stresses which may lead to cracking of early-age concrete. In this paper, a multiscale model for cement-based materials focusing on the prediction of autogenous-shrinkage deformations is presented. In addition to the formulation of the underlying multiscale framework, including the identification of observation scales and the development of appropriate upscaling schemes, shrinkage experiments on cement pastes were conducted and used for the validation of the developed multiscale model. Finally, the multiscale model is used to determine the early-age elastic and shrinkage properties of shotcrete. These properties are employed in the context of hybrid analyses of shotcrete tunnel linings, providing access to the level of loading of the tunnel support structure during NATM tunneling.

Chemoplastic material model for the simulation of early-age cracking: From the constitutive law to numerical analyses of massive concrete structures

This paper deals with the development and application of a three-dimensional material model for the simulation of early-age cracking of concrete. The starting point is the determination of the intrinsic material function for the fracture energy of early-age concrete. For this purpose, results of beam bending tests reported in [Proceedings of the SEM/RILEM International Conference on Fracture of Concrete and Rock. Houston, Texas, USA: 1987. p. 409] are employed. The intrinsic material function serves as input for the calibration of the Rankine fracture criterion formulated in the framework of chemoplasticity. Finally, the developed 3D material model is employed for a chemomechanical analysis of a roller-compacted-concrete dam. The temperature fields and the field of the degree of hydration required for this analysis are obtained from a preceding thermochemical analysis of the dam.

Mitigation strategies for autogenous shrinkage cracking

Abstract As the use of high-performance concrete has increased, problems with early-age cracking have become prominent. The reduction in water-to-cement ratio, the incorporation of silica fume, and the increase in binder content of high-performance concretes all contribute to this problem. In this paper, the fundamental parameters contributing to the autogenous shrinkage and resultant early-age cracking of concrete are presented. Basic characteristics of the cement paste that contribute to or control the autogenous shrinkage response include the surface tension of the pore solution, the geometry of the pore network, the visco-elastic response of the developing solid framework, and the kinetics of the cementitious reactions. While the complexity of this phenomenon may hinder a quantitative interpretation of a specific cement-based system, it also offers a wide variety of possible solutions to the problem of early-age cracking due to autogenous shrinkage. Mitigation strategies discussed in this paper include: the addition of shrinkage-reducing admixtures more commonly used to control drying shrinkage, control of the cement particle size distribution, modification of the mineralogical composition of the cement, the addition of saturated lightweight fine aggregates, the use of controlled permeability formwork, and the new concept of “water-entrained” concrete. As with any remedy, new problems may be created by the application of each of these strategies. But, with careful attention to detail in the field, it should be possible to minimize cracking due to autogenous shrinkage via some combination of the presented approaches.

A multiscale micromechanics-hydration model for the early-age elastic properties of cement-based materials

The E-modulus of early age cement-based materials, and more importantly, its evolution in time, is one of the most critical material-to-structural design parameters affecting the likelihood of early-age concrete cracking. This paper addresses the problem by means of a multistep micromechanics approach that starts at the nanolevel of the C–S–H matrix, where two types of C–S–H develop in the course of hydration. For the purpose of homogenization, the volume fractions of the different phases are required, which are determined by means of an advanced kinetics model of the four main hydration reactions of ordinary portland cement (OPC). The proposed model predicts with high accuracy the aging elasticity of cement-based materials, with a minimum intrinsic material properties (same for all cement-based materials), and 11 mix-design specific model parameters that can be easily obtained from the cement and concrete suppliers. By way of application, it is shown that the model provides a quantitative means to determine (1) the solid percolation threshold from micromechanics theory, (2) the effect of inclusions on the elastic stiffening curve, and (3) the development of the Poisson's ratio at early ages. The model also suggests the existence of a critical water-to-cement ratio below which the solid phase percolates at the onset of hydration. The development of Poisson's ratio at early ages is found to be characterized by a water-dominated material response as long as the water phase is continuous, and then by a solid-dominated material response beyond the solid percolation threshold. These model-based results are consistent with experimental values for cement paste, mortar, and concrete found in the open literature.

What Is a “Massive” Concrete Structure at Early Ages? Some Dimensional Arguments

The risk of early-age concrete cracking depends on the capacity of hardening concrete to support the thermal stresses caused by the exothermic nature of the hydration process. This has been recognized for ''massive'' concrete structures. However, with the increasing use of high performance concretes, it is apparent that this problem also concerns traditionally ''thin'' structural members (columns, beams). The definition of a ''massive'' concrete structure, and how the structural dimension affects intensity and occurrence of chemically- induced strucctural degradation is the main focus of this paper. Based on dimensional analysis of the governing equations, a characteristic length scale, the hydration heat diffusion length, is derived; beyond this length the structure needs to be considered as ''massive,'' and latent hydration heat effects affect the long-term structural integrity. From experimental data of normal strength concrete and high performance concrete, it is shown that this hydration heat diffusion length of high performance concrete is of the order of < h = 0.2 m, and lh = 0.3 m for normal strength concrete. Through a number of case studies, the relevant similarity parameters of the risk of early-age concrete cracking are identified, which allow's the monitoring of the structural performance of early-age concrete structures.

Influence of temperature on autogenous deformation and relative humidity change in hardening cement paste

This paper deals with autogenous deformation and autogenous relative humidity change (RH change) in hardening cement paste. Theoretical considerations and experimental data are presented, which elucidate the influence of temperature on these properties. This is an important subject in the control of early age cracking of concrete. It is demonstrated that the traditional maturity concept generally is not applicable to autogenous deformation and autogenous RH change.