Early Concrete Research Articles

Numerical identification of the thermal properties of early age concrete using inverse heat transfer problem

The procedure of numerical identification of thermophysical properties of concrete during its hardening is presented. Heat of cement hydration, thermal conductivity and specific heat are determined for purpose of modelling temperature evolution in massive concrete elements. The developed method is based on point temperature measurements in a cylindrical mould and the numerical solution of the inverse heat transfer problem by means of direct search optimization algorithm. The determined thermal characteristics of concrete are not constant and depend on the maturity of concrete. The procedure was verified on set of concrete mixes designed with Portland cement CEM I 42.5R and Portland-slag cement CEM II/B-S 32.5 N. Calcareous fly ash was also used for partial replacement of cement in the mixtures. The obtained results have been compared with experimentally measured temperature in concrete and a fair agreement has been found.

Characterization of carbon fiber reinforced polymer strengthened concrete and gap detection with a piezoelectric-based sensory technique

In this article, a piezoelectric-based sensory technique is proposed for detection of the gap between surfaces of a carbon fiber reinforced polymer plate and a concrete specimen and characterization of shrinkage of early-age concrete. The proposed technique uses the propagation properties of the guided waves in the carbon fiber reinforced polymer plate excited and received by piezoelectric transducers attached to an external surface of the carbon fiber reinforced polymer–strengthened concrete specimen. Measurements are conducted with fresh and hardened early-age concrete specimens and two carbon fiber reinforced polymer plates at different gaps. A piezoelectric actuator is excited using a sine burst signal, and the generated wave is received by a sensor after propagation along the specimen. The received signal at different gap values is used to detect a gap. To quantify the gap, damage indices, including correlation coefficient, peak-to-peak amplitude of resultant signal, and root-mean-square deviation, are used. The shrinkage of concrete is detected and predicted by comparing the damage indices at different gaps with the indices at different stages of early-age concrete. The proposed technique is relatively simple method using small transducers. It is one-sided, non-destructive, and cost-effective solution for gap detection and concrete characterization.

A meso-scale analysis of the hygro-thermo-chemical characteristics of early-age concrete

Concrete has a high degree of heterogeneity and a complex porous structure, so to accurately study the performance evolution mechanism of concrete, multi-scale research should be carried out. A three-phase meso-model of concrete is constructed based on the method of randomly generating particles. A meso-macro connection framework is built, and the accuracy of the fixed hygro-thermo-chemical model at the meso-scale is verified through comparing its outcomes with experimental data and macroscopic results. The hygro-thermo-chemical behaviours of early-age concrete at the meso-scale are studied under adiabatic–sealed, radiating–sealed and adiabatic–drying conditions, by considering the cooling effect of the aggregate, low diffusivity of the aggregate, high diffusivity of the interface transition layer and concrete gradation. Additionally, distribution and evolution of several eigenvalues, such as the relative humidity, are obtained.

Triaxial mechanical behavior of early age concrete: Experimental and modelling research

Considering the triaxial behavior of concrete is highly related to its hydration process with curing temperature and time, the triaxial mechanical behavior of early age concrete under different curing conditions is investigated. Uniaxial/triaxial compression tests of a C30 concrete cured at representative temperature and time are performed while its pore characteristic is determined. Based on experimental analysis, a curing factor indicative of the effect of curing conditions is proposed and discussed by empirical functions. It is found that the uniaxial/triaxial failure strength and elastic modulus increase with curing factor, but the deformation decreases slightly. The pore structures evolve with curing factor. Furthermore, the influence of curing factor on the strength decreases with the confining pressure. Finally, a hydration dependent modelling is proposed by introducing curing factor. Numerical simulations show that the proposed modelling can well predict the triaxial mechanical behavior of early age concrete under different curing conditions.

Research on the Crack Risk of Early-Age Concrete under the Temperature Stress Test Machine.

A new temperature stress test machine (TSTM) was developed to improve temperature control accuracy and efficiency. As there is no uniform standard for the threshold value of TSTM, quite different threshold values are used in the test, which results in the influences of the threshold value on the evaluation of thermal stresses and crack risk of concrete not being determined. To illustrate the importance of an appropriate threshold value, different threshold values were used to evaluate the influences on the thermal stresses and cracking resistance of concrete specimens under a complete restraint test with semi-adiabatic temperature development. The results show that the maximum compressive stress of a concrete specimen with a threshold value 3 με was slightly larger than that of 1.5 με when the growth rate of tensile stress of the concrete specimen with the threshold value 3 με was slightly greater than that of concrete specimen with the threshold value 1.5 με. Based on the combination of crack risk coefficient and restraint degree, a new system for evaluating the crack resistance of concrete was proposed, in which different threshold values were used to estimate their influences on the crack risk of concrete. Thus, an appropriate range of threshold values could be determined.

Comparison of Compressive, Tensile, and Flexural Creep of Early-Age Concretes under Sealed and Drying Conditions

The compressive creep of concrete is normally adopted to predict the stress or the time-dependent deformation of concrete members, without considering the difference of creep under different loading types, i.e., compressive, tensile, and flexural loads. This will inevitably lead to inaccurate prediction of stress and deformation, particularly at early ages when creep effect is pronounced. This study conducts a systematic creep comparison for early age concrete under different loading types and curing conditions. Three types of creep devices are developed for compressive, tensile, and flexural creep measurements of concrete under both sealed and drying conditions. Low-strength and high-strength concretes with water:cement (w/c) ratios of 0.3, 0.4, and 0.5 are tested at the age of 7 days. The measured creep is compared in terms of loading types and curing conditions (sealed versus drying). The ratios of different types of creep are quantified and compared with the literature. The rationality of using compressive creep to substitute tensile or flexural creep is justified. The results of this study provide quantitatively the creep difference under different conditions, which helps to gain confidence about the predicted stress and the strain when different creep models are used.

Modeling of Capillary Shrinkage and Cracking in Early-Age Concrete

. Scientific hypothesis on moistening shrinkage mechanism for cement stone and concrete has been assumed as a basis for the present paper. Physical ideas on a mechanism for cracks volume increment in a concrete model presented as two-level structure have been accepted as a theoretical basis for a calculation method of crack resistance during capillary shrinkage. These ideas are the following: a matrix of hardening cement stone with inclusions and emptiness of various forms (cracks) as result of influences that change an intense deformed state in a point and a volume. The following assumptions have been accepted while making a theoretical justification for a calculation method of shrinkable concrete crack resistance. Following this methodology approaches of fracture mechanics according to a generalized criterion have been applied in the paper. Concrete is considered as an elastic quasi-homogeneous two-component medium which consists of the following parts:a) constructive part: a matrix – a cement stone with structural elements of crushed stone, sand; b) destructive part: emptiness – capillaries cracks and pores (cavities with initial cracks in walls). Emptiness in a matrix and contact zones are presented by a coordinated five-level system in the form and sizes which are multiple to a diameter due to impacts while reaching critical sizes. These critical sizes make it possible to pass from one level into another one according to the following scheme: size stabilization – accumulation delocalization – critical concentration in single volume – transition to the following level. Process of cracks formation and their growth are considered as a result of non-power influences on the basis of crack theory principles from a condition that fields of deformation and tension creating schemes of a normal separation and shift occur in the top part of each crack at its level in the initial concrete volume. Ксij(t) parameter as algebraic amount of critical values Kij in the whole system of all levels of cracks filling canonical volume up to critical concentration has been accepted as a generalized constant of property for concrete crack resistance in time, its resistance to formation, accumulation in volumes of micro-cracks and formation of trunk cracks with critical values. External temperature, moistening long influences create fields of tension in the top parts of cracks. Concrete destruction processes due to cracks are considered as generalized deformedintensed state in some initial volume having physical features which are inherent to a composite with strength and deformative properties. It is possible to realize analytical calculations for assessment of tension and crack resistance of concrete at early age on the basis of a generalized criterion in terms of stress intensity factor due to modern experimental data on capillary pressure value (70 kPa in 180 min after concrete placing). The developed algorithm of calculation allows to consider factors influencing on capillary pressure: type of cement, modifiers and mineral additives, concrete curing conditions.

The Effects of Insulation Thickness on Temperature Field and Evaluating Cracking in the Mass Concrete

This paper is mainly to study the effects of insulation thickness (0 - 3) cm on temperature field and evaluating cracking in the mass concrete by using Finite Element Midas Civil 2011. The finite element method is developed for simulation analysis of the temperature field in the concrete in early age mass concrete. From the results temperature field and evaluating cracking for the mass concrete it is concluded that insulation with adequate thickness at the face of massive concrete should be used to reduce the temperature differentials and control cracking of early-age concrete. Finally, the results are applied to provide some references for the constructions in the mass concrete such as dams, bridges beams, bridge piers, foundations of bridges and buildings.

STRUCTURAL HEALTH MONITORING USING PZT A REVIEW

Due to the involution of damage occurring in the structure it is difficult to monitor structural health. Electromechanical Impedance technique (EMI) is used for the damage detection in the structural member experimental results are obtained by the applying piezoceramic patches surface bonded and embedded PZT (Lead Zirconate Titanate) patchs are used for the detection of structure failure. This papers reviews online strength gain monitoring of early age concrete is detected with the help of EMI sensing technique. By using this methods detection s well as location, severity estimation is also possible. The piezoceramic materials performs the both works sensor as well as actuator. The proposed method of damage detection are quit effective and sensitive to concrete.

COST TU1404 benchmark on macroscopic modelling of concrete and concrete structures at early age: Proof-of-concept stage

Modelling of early-age behaviour of cement-based materials is still a challenging task. The challenge is implied by the extent of the knowledge on the subject which results in a variety of different models used for simulation of cement-based materials. That is why a numerical benchmark program has been launched within the COST Action TU1404 aiming at improvement and harmonisation of computational prediction of early-age behaviour of cement-based materials as well as its behaviour on structural level. This paper presents the result of the proof-of-concept stage of the benchmark.The goal of this stage of benchmark was to compare the performance of currently used models for simulation of early-age behaviour of concrete. The participants were requested to simulate thermo-chemo-mechanical behaviour of simple concrete elements covering adiabatic and real evolution of temperature, shrinkage, stiffness and stresses accounting for early-age creep. The tasks were formulated based on the experimental measurements. This stage of benchmark allowed to evaluate the influence of different phenomena occurring in early-age concrete on the behaviour of early-age concrete structures, define the discrepancies between experimental results and numerical simulations, as well as to indicate the weak points in the models.

A Study on the Influence of Stage Load on Health Monitoring of Axial Concrete Members Using Piezoelectric Based Smart Aggregate

Piezoceramic based smart aggregate (SA) has been employed to monitor the strength development of early age concrete. The validity of SA-based active sensing method was tested and verified with loading and unloading conditions. However, the early age concrete in buildings is subjected to many load increments during the construction process. The influence of incremental load on the properties of the propagating waves is still unclear. This study aims to investigate the effects of axial stage loads on the signal response of the SA. The concrete specimens that are embedded with SA’s were loaded step by step, and the amplitude and wave velocity of the sensing signals were measured at each stress state. The experimental results indicated that the amplitude of the received signal decrease with the increase of the stress level. As for the velocity of the propagated stress wave, however, the velocity shows an increasing trend before a sharp decline at high stress level.

Early-age thermal analysis and strain monitoringof massive concrete structures

Hydration heat and thermal induced cracking have always been a fatal problem for massive concrete structures. In order to study a massive reinforced concrete wall of a storage tank for liquefied natural gas (LNG) during its construction, two mock-ups of 0.8 mx0.8 mx0.8 m without and with metal corrugated pipes were designed based on the actual wall construction plan. Temperature distribution and strain development of both mock-ups were measured and compared inside and on the surface of them. Meanwhile, time-dependent thermal and mechanical properties of the concrete were tested standardly and introduced into the finite-element (FE) software with a proposed hydration degree model. According to the comparison results, the FE simulation of temperature field agreed well with the measured data. Besides, the maximum temperature rise was slightly higher and the shrinkage was generally larger in the mock-up without pipes, indicating that corrugated pipes could reduce concrete temperature and decrease shrinkage of surrounding concrete. In addition, the cooling rate decreased approximately linearly with the reduction of heat transfer coefficient h, implying that a target cooling curve can be achieved by calculating a desired coefficient h. Moreover, the maximum cooling rate did not necessarily decrease with the extension of demoulding time. It is better to remove the formwork at least after 116 hours after concrete casting, which promises lower risk of thermal cracking of early-age concrete.

Numerical Modelling of Field Test for Crack Risk Assessment of Early Age Concrete Containing Fly Ash

The high‐strength/high‐performance concretes are prone to cracking at early age due to low water/binder ratio. The replacement of cement with mineral additives such as fly ash and blast‐furnace slag reduces the hydration heat during the hardening phase, but at the same time, it has significant influence on the development of mechanic and viscoelastic properties of early age concrete. Its potential benefit to minimize the cracking risk was investigated through a filed experiment carried out by the Norwegian Directorate of Roads. The temperature development and strain development of the early age concrete with/without the fly ash were measured for a “double‐wall” structure. Based on experimental data and well‐documented material models which were verified by calibration of restraint stress development in TSTM test, thermal‐structural analysis was performed by finite element program DIANA to assess the cracking risk for concrete structures during hardening. The calculated and measured temperature and strain in the structure had good agreement, and the analysis results showed that mineral additives such as flay ash are beneficial in reducing cracking risk for young concrete. Furthermore, parameter studies were performed to investigate the influence of the two major factors: creep and volume change (autogenous shrinkage and thermal dilation) during hardening, on the stress development in the structure.

A novel electromechanical impedance–based model for strength development monitoring of cementitious materials

Strength monitoring of early age concrete improves the efficiency of construction as it provides information on the optimum time for shoring removal and pre-stress transferring. Electromechanical impedance technique has been proven to be a useful tool for strength monitoring of cementitious materials. One of the key limitations of this technique is the lack of physical models, which resulted in strong reliance on statistical analysis tools to quantify the strength of structure being monitored. In this proof-of-concept study, a novel electromechanical impedance–based model with the potential of monitoring the strength of cementitious materials using the concept of Smart Probe is proposed. Instead of directly bonding a lead zirconate titanate patch on the host structure, a lead zirconate titanate was first surface-bonded on a pre-fabricated aluminum beam, which is termed Smart Probe. The Smart Probe was then partially embedded into cementitious materials for strength monitoring. The structural resonant frequencies of the Smart Probe can be identified from the conductance signatures measured from the lead zirconate titanate patch throughout the curing process and serve as strength indicator. By modeling the cementitious material as an elastic foundation supporting the Smart Probe, an analytical model was developed to predict the dynamic modulus of elasticity of cementitious materials based on the resonance frequency of the Smart Probe. Experimental study was carried out on a mortar slab specimen to verify the model and to investigate the performance of the Smart Probe. It was found that the dynamic modulus of elasticity of the host structure could be predicted from the conductance signatures using the proposed model. Compressive strength assessment was achieved by establishing an empirical relation with the dynamic modulus. The proposed electromechanical impedance–based model with Smart Probe is physically parametric in nature and shows high repeatability, which renders its superiority over the conventional statistical method–based electromechanical impedance technique for strength monitoring of cementitious materials.

Influence of recycled coarse aggregates on permeability of fresh concrete

The following work is an experimental study of the behaviour of very early-age concrete. Six different concretes, four of them containing recycled coarse aggregates were studied for the first 2.5 h. The studies were carried out in a ventilated tunnel in order to imitate severe desiccation conditions. In order to indirectly obtain the permeability coefficient, settlement, capillary depression and evaporation were measured for all six concretes. The initial permeability coefficient of each concrete is determined starting from initial bleeding rate. The use of recycled coarse aggregates leads to a high bleeding rate for high water to cement ratios. Permeability coefficients at air entry are then determined starting from capillary depression gradients. Recycled coarse aggregates do not seem to influence the air entry value which is highly dependent on the paste quality. At air entry, the permeability coefficient of recycled coarse aggregates concrete mixes is higher than that of natural aggregates concrete mixes. At high evaporation rates, in severe desiccation conditions, recycled coarse aggregates seem to reduce bleeding for mixture with low water cement ratios. Permeability coefficient is a key physical parameter to understand drying of fresh concrete.

Evaluation of Early-Age Concrete Compressive Strength with Ultrasonic Sensors.

Surface wave velocity measurement of concrete using ultrasonic sensors requires testing on only one side of a member. Thus, it is applicable to concrete cast inside a form and is often used to detect flaws and evaluate the compressive strength of hardened concrete. Predicting the in situ concrete strength at a very early stage inside the form helps with determining the appropriate form removal time and reducing construction time and costs. In this paper, the feasibility of using surface wave velocities to predict the strength of in situ concrete inside the form at a very early stage was evaluated. Ultrasonic sensors were used to measure a series of surface waves for concrete inside a form in the first 24 h after placement. A continuous wavelet transform was used to compute the travel time of the propagating surface waves. The cylindrical compressive strength and penetration resistance tests were also performed during the test period. Four mixtures and five curing temperatures were used for the specimens. The surface wave velocity was confirmed to be applicable to estimating the concrete strength at a very early age in wall-like elements. An empirical formula is proposed for evaluating the early-age compressive strength of concrete considering the 95% prediction intervals.

Test and simulation on moisture flow in earlyage concrete under drying

ABSTRACTIn this paper, moisture flow in two concrete with and without internal curing using presoaked lightweight aggregate (PSLWA) under surface drying is experimentally investigated by measuring the weight of the concrete samples. Mathematical modeling on moisture flow, especially moisture transfer coefficient between concrete and surrounding air, normally called surface factor is performed. The results show that moisture flow through the drying surface of concrete can be characterized by a constant moisture loss stage (I) followed by a gradually reducing moisture loss stage (II). For the given environmental condition, the length of stage I increases with a decrease in water-to-cement ratio. Internal curing with PSLWA will prolong the length of Stage I. Surface factor is a function of location along air flow direction, air flow speed, and temperature of air and concrete surface, while it is independent of mix proportions of concrete. Higher air flow rate, higher temperature of air and/or concrete will result in a larger surface factor. Element size along air flow direction significantly influences the value of surface factor. Smaller element size along the air flow direction will result in a larger surface factor. The comparison on surface factor between experimental determination and theoretical calculation is performed and a good agreement between them is obtained.

Quantitative characterization and elastic properties of interfacial transition zone around coarse aggregate in concrete

Backscattered electron images (BSE) obtained by scanning electron microscope was used to quantitatively characterize the microstructure of interfacial transition zone (ITZ) in concrete. Influences of aggregate size (5, 10, 20, and 30 mm), water to cement ratio (0.23, 0.35 and 0.53) and curing time (from 3d to 90d) on the microstructure of interfacial transition zone between coarse aggregate and bulk cement matrix were investigated. The volume percentage of detectable porosity and unhydrated cement in ITZ was quantitatively analyzed and compared with that in the matrix of various concretes. Nanoindentation technology was applied to obtain the elastic properties of ITZ and matrix, and the elastic modulus of concrete was then calculated based on the Lu & Torquato model and self-consistence scheme by using the ITZ thickness and elastic modulus obtained from this investigation. The experimental results demonstrated that the microstructure and thickness of ITZ in concrete vary with a variety of factors, like aggregate size, water to cement ratio and curing time. The relative low elastic properties of ITZ should be paid attention to, especially for early age concrete.

Artificial Neural Network-Based Early-Age Concrete Strength Monitoring Using Dynamic Response Signals.

Concrete is one of the most common materials used to construct a variety of civil infrastructures. However, since concrete might be susceptible to brittle fracture, it is essential to confirm the strength of concrete at the early-age stage of the curing process to prevent unexpected collapse. To address this issue, this study proposes a novel method to estimate the early-age strength of concrete, by integrating an artificial neural network algorithm with a dynamic response measurement of the concrete material. The dynamic response signals of the concrete, including both electromechanical impedances and guided ultrasonic waves, are obtained from an embedded piezoelectric sensor module. The cross-correlation coefficient of the electromechanical impedance signals and the amplitude of the guided ultrasonic wave signals are selected to quantify the variation in dynamic responses according to the strength of the concrete. Furthermore, an artificial neural network algorithm is used to verify a relationship between the variation in dynamic response signals and concrete strength. The results of an experimental study confirm that the proposed approach can be effectively applied to estimate the strength of concrete material from the early-age stage of the curing process.

Improving the strength properties of recycled asphalt aggregate concrete

The querying of aggregate, for use in the manufacture of concrete, can have adverse environmental and ecological effects. The replacement of gravel aggregate with recycled (reclaimed) asphalt would help in the reduction of these effects. It can also help in reducing the quantity of recycled asphalt obtained from road re-surfacing that would otherwise be disposed off in landfill sites. The applications of concrete containing recycled asphalt have been very limited due to its low strength. This study investigates the feasibility of improving the strength of recycled asphalt concretes. This would increase the potential for using this type of concrete as an alternative to normal concrete, especially where medium strengths might be required. Concrete strength improvements would be achieved by changing the surface characteristics of recycled asphalt aggregate. This has been sought by using two methods; mechanical roughening and chemical solvent etching. It has been possible to improve the strength of concrete containing recycled asphalt to values similar to that of normal (gravel) concrete, by applying the mechanical roughening technique. Chemical etching has no effect on strength improvement of the concrete. The concretes have also been tested using non-destructive methods in the form of ultrasonic pulse velocity and rebound number. The effects of replacing gravel with different percentages of recycled asphalt on the strength, pulse velocity, and surface hardness of concrete are considered initially. The inclusion of recycled asphalt at 25% has resulted in the reduction of strength. The strength decreases further with an increase in the percentage (up to 100%) of recycled asphalt. All investigations were performed on early age concrete (1–28days).