Steam Curing Process Research Articles

Optimization of steam curing process for Chinese Western and Northeast regions' high‐speed railway concrete prefabricated components

AbstractThis article aims to address the issues of high curing temperatures and thermal damage in the production of prefabricated concrete components for high‐speed railways in high‐altitude and high‐latitude cold regions of China. Various steam‐curing processes for concrete are designed to optimize the high‐quality preparation process of steam‐cured concrete prefabricated components in cold environments. With the goal of controlling the residual expansion deformation and considering the overall impact of curing process on the mechanics, durability, and interface transition zone of steam‐cured concrete, the main conclusions obtained in this study are as follows. Within a pre‐curing time of 3–6 h, when the curing temperature is maintained at 45–60°C, the final residual expansion deformation can be controlled below 300 με. The compressive strength, dynamic elastic modulus, peak stress, water absorption and Chloride ion resistance of steam‐cured concrete show the great improvement under above curing processes. Curing at 80°C should be actively avoided, and it is recommended to adopt a 6 h pre‐curing time with a maximum curing temperature of 45°C, especially for cold regions in China. This study can serve as a valuable reference and provide support for the preparation of prefabricated concrete components in Chinese high‐altitude and high‐latitude areas.

Investigation on the early proceeding of cement hydration containing dispersed nano Calcium Silicate Hydrated (CSH) seeds

Steam curing process is a traditional method for accelerating the progression of cement hydration to satisfy the rapid development of compressive strength in precast concrete industry. However, steam curing would release significant amount of CO2 into the atmosphere. In order to avoid the serious environmental pollution caused by the steam curing process, Calcium Silicate Hydrate (CSH) dispersed by comb-like polycarboxylate superplasticizer (PCE), shorted as dispersed nano CSH seed, was utilized to promote the early cement hydration at normal temperature. This investigation aims at disclosing the hydration process of cement and the microstructure evolution of cement paste in the presence of dispersed nano CSH seed during the early stage via setting time, ultrasonic pulse velocity (UPV), compressive strength, conductivity, pH, thermogravimetric (TG), 29Si NMR and nitrogen adsorption/desorption. The results demonstrated that with the dispersed nano CSH seed increasing, the setting process of cement paste was shortened dramatically, the compressive strength and UPV of cement mortar increased significantly especially at 12 h. The variation of conductivity and pH of cement suspension indicated that the ion dissolution process was promoted by dispersed nano CSH seed, which therefore conducive to the precipitation of cement hydration products. TG curves exhibited that the formation of cement hydration products, including CH (Calcium Hydrated), AFt and CSH gel were accelerated by dispersed nano CSH seed. The 29Si NMR (Nuclear Magnetic Resonance) and nitrogen adsorption/desorption results validated that the incorporating dispersed nano CSH seed promoted considerably the polymerization of CSH gel. The aforementioned results are correlated well with the fast formation of microstructure and in turn the rapid development of compressive strength at early age.

Mechanical characteristics of concrete under initial steam curing using solar energy

In this article, the study is based on the effect of initial steam curing using solar energy on the mechanical characteristics of concrete for precast concrete production. During the early hours of heating, the concrete reaches the minimum strength considered essential for the rotation of the molds, with which he is able to support loads without deform or loss of bearing capacity. The curing period and temperature, is an important parameter in the steam curing process. An experimental program was conducted to investigate the effect of hardening time by steam curing on the mechanical characteristics of concrete. The concrete specimens were cast with a water/cement ratio of 0.40 and were subjected to steam hardening according to two cycles of covering steam curing of two periods of the year hot and cold at 45 and 29 °C for each one 8 hours, then they are left in the open air cured for 3 and 7 days. The results show that a gain of time and shorter manufacturing lead times to reach the mechanical strengths (compressive strength and flexural) at 28 days in the free air after a one day steam curing and 3 days of hardening in the free air.

Early hydration heat temperature field of precast concrete T-beam under steam curing：Experiment and simulation

Precast concrete T-beam steam curing can improve production efficiency, realize large-scale production of T-beams, and achieve better economic benefits. We performed tests and numerical simulations on a 30-meter-long full-size precast concrete T-beam to study the economical and reasonable steam curing scheme. The study found that in the early stages of steam curing, the temperature at the mid-span section increases faster than that at the end-span section. The highest temperatures of the tested beams were found in end-span section., and the core-surface temperature difference increased and then decreased, with the maximum temperature difference occurring at the junction of the top and web slabs of the concrete T-beam. To meet initial prestress tensioning requirements, the steam curing of increasing the temperature for 2 h to 50 ℃, keeping the temperature constant for 7 h, and cooling the temperature for 1 h is the most economical scheme, as seen from the comparison and analysis of the development law of concrete hydration heat temperature and the results of concrete test blocks under the same curing. Under this steaming scheme, the end-span section of the concrete T-beam reached a peak temperature of 68.5 °C at 14 h, and the core-surface temperature difference reached a maximum of 8.6 °C at 4.5 h; the peak temperature of the mid-span section was 60.2 °C, with a maximum temperature difference of 8.3 °C. In order to accurately predict the law of T-beam concrete temperature development under steam curing, two subprograms, HETVAL and UMATHT, were compiled on the ABAQUS secondary development platform. When calculating by natural age t, the maximum differences between the measured peak values and the peak values calculated by HETVAL and UMATHT were 30.0% and 27.7%, respectively. After considering the equivalent age te of concrete, the maximum differences between the measured peak values and the peak values calculated by HETVAL and UMATHT were 8.2% and 1.9%, respectively. Therefore, the UMATHT subprogram, considering equivalent age and hydration degree, can accurately predict the development of the concrete hydration heat temperature field under the steam curing process.

THE EFFECT OF STEAM CURING METHOD TO THE COMPRESSIVE STRENGTH OF GEOPOLYMER CONCRETE WITH DIFFERENT MOLARITY

Cement, an important part of concrete mixture, determine the mechanical properties of concrete. The mixture of cement and water will stimulate the hydration reaction which will produce CSH gel and heat of hydration. The high level of CO2 pollution produced in the cement manufacturing process requires alternative efforts to replace cement with other pozzolanic materials. One of the materials that can replace the role of cement in concrete mixture is fly ash. The replacement of cement 100% with fly ash will produce a geopolymer concrete, which requires an alkaline solution as a reagent from fly ash. This research was conducted with the aim to explore the effect of steam curing to the compressive strength of geopolymer concrete with different molarities. As the test object in this study was a geopolymer concrete cylinder with a standard size i.e. 150 mm × 300 mm. The molarity of NaOH solution varying from 6, 8, 10, 12, 14 to 16 M. The temperature of steam curing was carried out varied from 60oC, 80oC, up to 90oC, with duration from 1 until 4 hours. The compressive strength test conducted for 28 days old of concrete cylinder specimens. The results show an indication of an increase in the compressive strength of the concrete along with the increase in the molarity of the NaOH solution. In addition, it can also be shown that the steam curing process at a temperature of 60oC with 4 hours in duration shows optimal compressive strength results.

Effect of steam‒curing on the glass fiber reinforced concrete

Due to the increased need to use precast concrete to reduce construction duration and to accelerate the cement reaction process to achieve the required concrete resistance that enables the elements to gain the required strength to handle the loads generated by the transportation process, many companies use steam curing methods to expedite the hydration process. The steam curing process negatively affects the concrete strength, especially in the long term. Fibers of different types are used to improve the interior composition of concrete and increase its crack resistance. The purpose of the current study was to determine the effect of the glass fiber on the behavior of steam-cured concrete. In this study 90 concrete cubes were used with 15cm dimensions, three different weight ratios of glass fibers (0%, 0.12%, and 0.24%) with two curing methods standard curing in the water tank (water curing - WC) for 3, 7, and 28 days, and steam curing (SC) for 4 and 8 hours. Nine specimens of each mix were cast in 12 mm, and 24 mm fibers length and tested for each curing duration and method. The results of this study indicate that fiber glass addition to the steam-cured concrete has a positive effect on the concrete unit weight and the ultrasonic pulse velocity. Moreover, the result showed that the tensile and compressive strength of the concrete has been positively affected by the length of the fiber more than the fiber weight percentage.

A precise internal-stress calculation algorithm for the evolution of cracking risk of the steam-curing box girder

• The refined numerical model considers the temporal-spatial evolutionary difference. • The algorithm is established to reveal the thermal behavior of box girder. • The thermal stress evolution processes are monitored by the field test. • The results of numerical simulation and field tests are compared. • Parametric analysis is performed to investigate the influence of key factors. Thermal cracking caused by the combination of heat liberation and thermal boundaries becomes a more and more serious problem during the entire steam-curing process. To understand the rules of cracking risk and find the key factors determining the distribution of thermal stress, the numerical simulation algorithm is established and the verification field tests are deployed. Firstly, based on the equivalent maturity concept, a refined mathematic model of the thermo–mechanical coupling field is established, which fully considers the temporal-spatial evolutionary difference of material properties and hydration heat release. Secondly, the numerical algorithm is further set up for the evolution law of the thermal stress and cracking risk based on the finite element method. Thirdly, triaxial temperature strains of key points are monitored using Fiber Bragg Grating (FBG) sensors in the field. Effectiveness and accuracy of the model are verified by the agreement between numerical simulation and experimental results. Finally, based on the verified numerical model, parametric analysis is performed to investigate the influence of various key factors affecting the distribution law of the thermal stress field and the evolution law of the cracking risk. The results show that the curing temperature ( T h ) and parameters of the hydration exothermic model ( θ u and m ) are crucial to influencing the thermal behavior of the steam-curing box girder. The research provides a refined numerical algorithm of the thermo-mechanical coupling field and contributes to the high quality of huge box-girder during the entire steam-curing process.

Evolution on fracture properties of concrete during steam curing

This paper investigated the fracture properties of concrete after steam curing ranging from 4 to 48 h at a typical constant temperature of 60 °C. The evolution pattern of fracture properties for concrete during steam curing was revealed, and the equivalent maturity was adopted to estimate the development of fracture parameters. The results indicated similar to the mechanical parameters, the initial fracture toughness KIcini exhibited an increase-decrease tendency as increasing steam curing hold (SCH) time. However, the unstable fracture toughness KIcun and fracture energy GF were found to increase monotonously with SCH time, leveling off approximately between 16 h and 48 h. The equivalent maturity approach was proved to be available to evaluate the fracture parameters of steam-cured concrete with a relative error of less than 10%. In addition, the crack extension resistance during whole fracture process was also analyzed, and it was found the SCH time should be kept within 16 h at 60 °C for the development rate of fracture properties. The test results can provide a basis for the control of cracks of concrete in steam curing process.

Evolution of temperature stress and tensile properties of concrete during steam-curing process

Nowadays, steam curing is widely used in the production of prefabricated components. However, studies have shown that steam curing can lead to micro or macro-cracks in concrete caused by the curing regime which can affect the performance of the concrete in all aspects. Herein, the internal stress and tensile strength development of concrete during the steam-curing process was studied, so as to evaluate the crack risk of concrete during steam curing. To evaluate the internal stress during the steam curing, the internal temperature and thermal deformation of the concrete were tested. Considering the difficulty of measuring the tensile properties of early-age cement-based materials, a mortar tensile test device combined with digital image correlation (DIC) was designed in the paper. Meanwhile, thermal analysis tests were done to analyse the CH content of the concrete during steam curing. The results indicated that the mortar tensile test device in combination with the DIC designed in the paper was an effective method for testing the early tensile properties of cement-based materials. In the four stages of steam curing, micro-cracks in the concrete were most likely to occur in the cooling process. The addition of supplementary cementitious material is an effective method for restraining surface cracking in steam-cured concrete, and to ensure the tensile strength, sufficient water vapour is needed in the steam-curing process for a concrete mixture with large mineral admixtures, otherwise, a reduction in the tensile strength of more than 50% will occur.

Zero Cement Concrete with Self-Curing Technology: An Overview

Geopolymer concrete is the concept of environmental friendly construction material which helps in reducing the greenhouse gas emission, however it cannot be applied directly on field due to its steam curing process. To overcome this defect various researcher have found self-curing or ambient curing process to achieve same strength as in steam curing process for geopolymer. This paper presents an over review about self-cured or ambient cured geopolymer concrete produced by various methods.

Volume Deformation of Steam-Cured Concrete with Slag during and after Steam Curing.

In order to better predict the development of shrinkage deformation of steam-cured concrete mixed with slag, a deformation-temperature-humidity integrated model test, a hydration heat test, and an elastic modulus test were performed. The effects of the steam-curing process and the content of slag on shrinkage deformation, hydration degree and elastic modulus of concrete were studied. The results indicate that during the steam-curing process, the concrete has an “expansion-shrinkage” pattern. After the steam curing, the deformation of concrete is dominated by drying shrinkage. After the addition of slag, the shrinkage deformation of steam-cured concrete is increased. The autogenous shrinkage increases by 0.5–12%, and the total shrinkage increases by 1.5–8% at 60 days. At the same time, slag reduces the hydration degree of steam-cured concrete and modulus of elasticity. A prediction model for the hydration degree of steam-cured concrete is established, which can be used to calculate the degree of hydration at any curing age. Based on the capillary tension generated by the capillary pores in concrete, an integrated model of autogenous shrinkage and total shrinkage is established with the relative humidity directly related to the water loss in the concrete as the driving parameter. Whether the shrinkage deformation is caused by hydration reaction or the external environment, this model can better predict the shrinkage deformation of steam-cured concrete.

Hydration and multiscale pore structure characterization of steam-cured cement paste investigated by X-ray CT

Abstract Cement pastes subjected to initial steam-curing cycle have a poor later strength and durability. This adverse effect is originated from the steam-curing process. In order to investigate the microstructure changes of cement paste during steam-curing process, the XCTs with different spatial resolutions were combined used to detect the pore structure and the hydration development of cement pastes subjected to different curing regimes. The hydration heterogeneity and spatial distribution of pore network of cement pastes at various steam-curing ages were obtained on both mesoscale and microscale. The results indicate that the XCT is competent for figuring out the hydration development and of which the steam-cured cement paste is much higher than the standard-cured one at early age but has slight difference at later age. Besides, the steam-cured cement paste has a higher porosity and pore connectivity in the studied scales than the standard-cured one with the same hydration volume. Moreover, the pore structure of steam-cured cement paste has a less fractal characteristic, resulting in a microstructural damage and thus a lower fracture toughness and higher brittleness under elevated temperature steam-curing condition.

Effect of steam curing regimes on temperature and humidity gradient, permeability and microstructure of concrete

Steam curing has an adverse effect on the long-term performance of concrete, which is inseparable from the early hygro-thermal coupling action. The temperature-humidity field and performance of steam-cured concrete with different surface covering methods were characterized under the treatment temperature ranging from 45 to 75 ℃ by water permeability along with hydration characteristics, porosity, microhardness, and microstructure. The results show that steam-curing regimes directly affect the long-term performance of steam-cured concrete by changing internal temperature-humidity field. During the steam curing process, a higher treatment temperature often corresponds to a more complex temperature-humidity gradient, so that the steam-cured concrete exhibits low compactness and high permeability. In addition, the surface covering methods can reduce the surface porosity and interface transition zone width of steam-cured concrete. Meanwhile, with the increase of treatment temperature, the improvement effect of the surface covering methods on the impermeability of steam-cured concrete is more obvious.

Experimental investigation on the influence of phase change materials on properties and pore structure of steam-cured mortar

Steam-cured concrete with high early strength has been widely applied in high-speed railway. However, heat damage caused by the steam curing process brings adverse effects on the pore structure and durability of steam-cured concrete. It has been found that phase change materials (PCMs) help improve the durability of cement-based materials. Therefore, the influences of two PCMs on the strength, water absorption, and microstructure of steam-cured mortar are discussed in this study. Two PCMs used include pure paraffin and a composite phase change material composed of porous diatomite and paraffin (D/P). The results show that the addition of appropriate PCMs does not significantly reduce the strength of steam-cured mortar. The addition of a small amount of PCMs shows little influence on the pore structure of steam-cured mortar but effective in improving the water-penetration resistance. This is partially attributed to the improvement of hydrophobic characteristics in the pores, resulting from the adhesion of hydrophobic PCMs on the pores during the steam curing. Compared with specimens containing pure paraffin, the pore structure of specimens containing D/P is refined, resulting in higher compressive strength. This may be due to the uniform distribution of small particles D/P in the specimen and the pozzolanic reaction of diatomite in D/P. The use of suitable phase change materials will be helpful to improve the durability of steam-cured concrete.

Water evolution and hydration kinetics of cement paste under steam-curing condition based on low-field NMR method

The presented study focuses on the water evolution and hydration characteristics of Portland cement paste under steam-curing condition. The detailed information concerning water consumption and micro-pore network as well as the corresponding hydration kinetics of hydrating cement paste (HCP) during steam-curing process were discussed based on the low-field nuclear magnetic resonance measurement. The results indicate that, within steam-curing period, most of the free water in HCP are distributed in smaller pores less than 500 nm. The water evolution rate of paste system can be divided into three stages, i.e. acceleration stage within heating period and the first 2 h of constant-temperature period, rapid low-down stages in later constant-temperature period and diffusion stage in rest period. The curing temperature predominantly controls the water evolution rate. Finally, the hydration kinetics model of paste within steam-curing period is established and verified by experiment.

Thermal damage and its controlling methods of high‐speed railway steam‐cured concrete: A review

AbstractThe steam curing technology is worldwide applied in the production of concrete prefabricated components, including the production of track slab and girder used for the high‐speed railway in China. However, the steam curing process at elevated temperature will often generate the heat damage on concrete and thus degrade the long‐term performance of the steam‐cured concrete prefabricated components. It is very important to deeply understand the mechanism of heat damage and thus to find effective method reducing or avoiding the damage of steam curing on concrete. The main purpose of this paper is to systematically classify the types of thermal damages on concrete during the steam curing and its mechanism. The influences of main factors on three thermal damages such as the unrecoverable expansion deformation, the surface layer defect, and the embrittlement of the steam‐cured concrete are discussed in detail. The controlling methods of thermal damages including optimization of the steam curing regime and mixing proportions, the addition of supplementary additives, the self‐healing technology, and other curing technology are compared and summarized based on the literatures. Finally, suggestions for future research are proposed in order to effectively reduce or even avoid the thermal damage of the steam‐cured concrete. And also, some new technology for developing the green and high performance early strength concrete with low CO2 emission are presented.

Hydration heat evolution of Portland cement paste during unsteady steam curing process: Modelling and optimization

The hydration process under unsteady steam curing condition is difficult to monitor in real time by experimental method. This study developed a numerical method to describe the hydration heat release characteristics of Portland cement under atmospheric steam curing condition. Instantaneous activation energy and an iterative algorithm were employed in this method to improve the accuracy. It was found that prolonging the precuring time to over 7 h or heating time to over 3 h would both decrease the peak value of hydration heat release rate, while a treatment time over 8 h would slightly affect the total heat release during steam curing. The validity of the method was confirmed by the experimental results. This proposed method can rapidly analyze the effect of steam curing regimes on hydration heat release behavior of Portland cement and thus optimize the hydration process of Portland cement under steam curing process in order to mitigate or avoid heat damage of steam curing concrete.

Evolution of mechanical properties and permeability of concrete during steam curing process

Steam curing involves a complex moisture-thermal coupling process, which greatly affects the performance evolution of cement-based materials. The effects of mineral admixtures and curing ages on the hydration process, mechanical properties and permeability of concrete during the steam curing process had been studied, and the pore structure, micromorphology and interfacial transition zone (ITZ) were also determined by multi-scale image processing and scanning electron microscopy. The results show that the concrete performances are significant changed during the steam curing process, which can be divided into three stages. In the early stage, hydration products are less, the porosity is larger, and the ITZ is weaker. With the increase of the curing temperature and age, the hydration products are continuously deposited, the microstructure is denser, the mechanical properties are enhanced, and the permeability is decreased significantly. As a result, an empirical model is developed to describe the relationship between effective pore structure (multi-scale pores structure and ITZ) and gas permeability.

Properties evolution of high-early-strength cement paste and interfacial transition zone during steam curing process

At present, concrete prefabricated elements commonly used in railway engineering in China were produced by steam curing. Steam curing is a complex hygro-thermal coupling process that has a significant impact on the hydration process and microstructure formation of cement-based materials. This paper investigated the macroscopic mechanical properties, hydration process, pore structure, microstructure formation and micromechanical properties of cement-based materials during steam curing process. It is discovered that the evolution of cement-based materials under steam curing is a dynamic process that can be divided into three stages. The fastest evolution stage of the performance is the rising stage and the first 2 h of treatment stage. At this time, the microstructure of the pastes is rapidly formed, and the mechanical properties are significantly improved. Porosity and interfacial transition zone widths also rapidly decrease. The reaction activity of cementitious material is one of the critical factors affecting the properties evolution of steam-cured paste. In addition, an evolution model of the pore structure was established based on the change in the hydration degree during steam curing, which is proved to be well consistent with the measured results.

Mechanical properties and water absorption of steam-cured mortar containing phase change composites

Steam curing is a common method used in the production of precast concrete components. However, the formation of microcracks and large pores during steam curing process are very detrimental to the long-term performance of cement-based materials. The use of phase change materials may be a potential method for improving disadvantages caused by steam curing. Phase change composites are prepared by implanting paraffin into diatomite, expanded graphite, and mixture of diatomite and expanded graphite. They are represented by D/P, EG/P and D/EG/P in turn. The thermal properties and static water contact angle of phase change composite (CPCM) are tested. The mechanical and water absorption properties of steam-cured mortars with CPCM are investigated. Results indicate that the latent heat values of EG/P, D/P and D/EG/P are 80%, 30% and 36% of the values for pure paraffin, respectively. The CPCMs exhibit hydrophobic properties due to the roughness of solid surface. The presence of CPCM greatly improve the water resistance of steam-cured mortar without sacrificing mechanical properties, and steam-cured mortar with 1 vol% D/P has the best performance. The possible reason is that D/P reduces the porosity and improves porous microstructure of steam-cured mortar. The cracks and holes in the mortar matrix can be covered and filled with paraffin from D/P by SEM analysis. There is no interfacial gap between D/P and the mortar matrix through SEM analysis, which promotes the formation of a dense and strong mortar matrix. Therefore, the addition of an appropriate CPCM is helpful to improve the durability of the steam-cured cementitious materials.