Decrease Of Internal Relative Humidity Research Articles

Effect of water‐to‐cement ratio on internal relative humidity and autogenous shrinkage of early‐age concrete internally cured by superabsorbent polymers

AbstractSelf‐desiccation is one common phenomenon of concrete with low water‐to‐cement (w/c) ratios, which may lead to the decrease of internal relative humidity (IRH), and therefore causing autogenous shrinkage (AS). Superabsorbent polymers (SAPs) are used as a kind of internal curing material to mitigate AS of concrete. The IRH and AS are significantly affected by the w/c ratio, and several attempts have been made to investigate the IRH or AS of concrete with different w/c ratios. However, the prediction model of AS considering IRH and w/c ratio of internally cured (IC) concrete is limited. In the present study, the effect of w/c ratio on IRH and AS of IC concrete was investigated, and a two‐stage prediction model of AS based on the results of IRH for IC concrete considering the effect of w/c ratio was proposed. The experiment results and analysis indicated that IRH of IC concrete increased as increase of w/c ratio, and development of IRH experienced two stages: a water‐vapor saturated stage with 100% RH (stage I), and a gradually reducing stage in which IRH decreased gradually (stage II); critical time of IC concrete increased as increase of w/c ratio; expansion peak of IC concrete increased as increase of w/c ratio; AS and AS rate of IC concrete decreased as increase of w/c ratio; IRH, critical time, and expansion peak of IC concrete were higher than that of ordinary concrete; AS and AS rate of IC concrete were lower than that of ordinary concrete.

Prediction model for internal relative humidity in early-age concrete under different curing humidity conditions

The decrease of internal relative humidity (IRH) in early-age high performance concrete (HPC) may increase the autogenous shrinkage under the combined effects of self-desiccation and moisture diffusion. Correspondingly, the tensile stress induced by restrained shrinkage may cause premature cracking of HPC. Evaluating the cracking potential of concrete needs to know the development of autogenous shrinkage. The prediction of autogenous shrinkage requires quantitative analysis of the variations of IRH in HPC. Although the variations of IRH and prediction models for IRH in concrete have been studied, a prediction model for the IRH in HPC at early age based on experimental results considering the effect of curing humidity remains lacking. Thus, the critical time of IRH, IRH, IRH decrease rate, and the moisture diffusion coefficient of HPC under different curing humidity conditions were studied in this research. Experimental results and corresponding analysis demonstrated that: (1) the critical time of IRH in sealed HPC was longer than that in unsealed HPC under different curing humidity conditions, and the critical time of IRH in unsealed HPC increased as the curing humidity increased; (2) the IRH in unsealed HPC increased as the curing humidity increased at the same curing age; (3) the IRH decrease rate of sealed or unsealed HPC gradually decreased with the growth of age, and the IRH decrease rate of unsealed HPC decreased as the curing humidity increased at the same curing age; (4) the moisture diffusion coefficient of unsealed HPC was calculated, and the prediction model for IRH in HPC at early age under the combined effects of self-desiccation and moisture diffusion considering the effect of curing humidity was proposed.

Capillary suction induced water absorption and chloride transport in non-saturated concrete: The influence of humidity, mineral admixtures and sulfate ions

Concrete structures in marine environments are gravely jeopardized by chloride attack caused by capillary suction. To investigate the erosion of concrete induced by capillary suction under various factors, seawater and NaCl-Na2SO4 solution were used as corrosive solution to obtain the evolution law of water absorbed amount and chloride content in concrete with different water to binder ratio, fly ash and slag dosages and internal relative humidity. The results show that the decrease of internal relative humidity leads to the increase of seawater absorbed by capillary suction in a brief period, and boost rapid growth of chloride content in surface of concrete. For early age (28 d) concrete specimens, when the fly ash dosage is 0–30% or the slag dosage is 0–50%, the water absorption rate and chloride content of concrete are more likely to drop with the dosage increasing. Furthermore, under short-time capillary suction condition, SO42− has a minor impact on water absorption of concrete, while slightly increases the chloride content. However, the chloride content does not evolve coincidently with the change of SO42− concentration. Moreover, within a brief exposure period, the chloride content linearly develops with absorbed sweater amount, and it is inappropriate to use chloride diffusion coefficient to reflect the chloride transport rate of concrete under capillary absorption effect.

Predicting relative humidity of early-age concrete under sealed and unsealed conditions

The decrease of internal relative humidity (IRH) in early-age concrete induces autogenous shrinkage. Autogenous shrinkage usually increases the risk of cracking if the concrete is restrained from shrinking freely at an early age. Predicting autogenous shrinkage of early-age concrete requires knowledge of IRH. Therefore, investigation of IRH is fundamental to evaluate cracking resistance. Although investigations of IRH in hardened concrete have been conducted, a prediction model for IRH in early-age concrete that considers the water/cement ratio (w/c) is still lacking. Therefore, experimental studies and analysis of IRH in early-age concrete with different values of w/c were conducted. Results showed that: (i) the IRH in concrete at 28 d after casting increased with increasing w/c under sealed and unsealed conditions; (ii) the average rate of IRH decrease in early-age concrete decreased with increasing w/c; (iii) the critical time of IRH (the length of the water-vapour saturated stage with 100% IRH) in early-age concrete increased with increasing w/c. Models for early-age IRH are proposed in consideration of w/c, critical time and age of concrete under sealed and unsealed conditions.

Setting Characteristics, Mechanical Properties and Microstructure of Cement Pastes Containing Accelerators Mixed with Superabsorbent Polymers (SAPs): An NMR Study Combined with Additional Methods.

In this study, the setting property and mechanical strength of cement pastes containing accelerators (CPCA) with or without superabsorbent polymers (SAPs) were first studied. The early microstructure evolution and water distribution at 7 and 28 days were probed by 1D (T2) and 2D (T1-T2 maps) H1 nuclear magnetic resonance (NMR) relaxometry, and the microstructure was systematically investigated by using mercury intrusion porosimetry (MIP), back-scattered electron (BSE) image and energy-dispersive X-ray spectroscopy (EDX) analysis. Results showed that the SAPs in the cement paste containing accelerators had various influences on setting time and compressive strength depending on the type of accelerators. The presence of SAPs in the cement paste containing alkaline free accelerators could alleviate the decrease of internal relative humidity, promote hydration and help to modify the pore structure. Moreover, it was observed that the SAP cavities could be nicely filled with calcium hydroxide (CH) in the cement paste with alkaline free accelerators.

Prediction of internal relative humidity in concrete modified with super absorbent polymers at early age

Shrinkage of high-performance concrete (HPC) mixtures may ultimately lead to early-age cracking and a reduction of service life of concrete structures. Autogenous and drying shrinkage are direct consequences of the decrease of internal relative humidity (IRH) due to self-desiccation and external drying. Therefore prediction of shrinkage of HPC requires the knowledge of IRH variations. Internal curing (IC) with super absorbent polymers (SAPs) is utilized to mitigate the shrinkage of HPC. Although investigations on IRH in concrete have been conducted, the prediction model for IRH in early-age concrete internally cured with SAPs in consideration of both water-to-cement (w/c) ratio and critical time is still lacking. Therefore investigations on predicting the IRH in early-age concrete internally cured with SAPs were conducted in present study and results showed that: (1) a prediction model for the working w/c ratio in early-age concrete internally cured with SAPs was proposed in consideration of age, effective and IC w/c ratios of concrete; (2) a prediction model for the critical time of IRH was proposed in consideration of both effective and IC w/c ratios; (3) a prediction model for the IRH in early-age concrete internally cured with SAPs was proposed in consideration of working w/c ratio, critical time, and age. These models showed the good accuracy of the test results.

Prediction model for relative humidity of early-age internally cured concrete with pre-wetted lightweight aggregates

The decrease of internal relative humidity (IRH) in high-performance concrete (HPC) induces autogenous shrinkage, which is fundamental to evaluate the cracking resistance. Internal curing (IC) with pre-wetted lightweight aggregates (LWAs) is used to mitigate the shrinkage of HPC, and prediction of shrinkage of HPC requires knowledge of the IRH variations. Although investigations on IRH in concrete have been conducted, the prediction model for IRH in internally cured concrete with LWAs considering IC remains lacking. Experimental research and analysis on IRH in early-age concrete with pre-wetted LWAs were conducted. Test results show that: (1) the average value of IRH decrease rate in internally cured concrete with pre-wetted LWAs decreased nonlinearly with the increase of IC water-to-cement (w/c) ratio; (2) the prediction models for critical time and working w/c ratio were proposed; (3) a prediction model for the IRH in early-age internally cured concrete was proposed considering the working w/c ratio, critical time, and age. These models showed the good accuracy of the experimental results.

Internal Relative Humidity and Drying Shrinkage of Lightweight Aggregate Concrete

In this paper, the influence of water cement ratio, prewetting degree of lightweight aggregate, flyash content and air content on the internal relative humidity (IRH) and drying shrinkage of lightweight aggregate concrete (LAC) were studied using a relative humidity sensor and drying shrinkage apparatus. Results showed that the IRH of concrete decreased rapidly at early age and slowed down at late age, and prewetting lightweight aggregate could compensate the loss of IRH. The effect of compensation could be enhanced with the increasing of prewetting degree of lightweight aggregate, and the addition of flyash retarded the decrease of IRH at early age. The drying shrinkage of LAC increased quickly at early stage and it slowed down to the even after 60 days or longer age. Furthermore, the drying shrinkage of LAC increased with increasing of water cement ratio. The effect of air content on the drying shrinkage of LAC was limited at early stage. However, at the late stage it grew at different level with increasing of air content. By raising the prewetting degree of lightweight aggregate, mixing with proper dosage of flyash, the shrinkage rate of LAC can be reduced markedly. Key words: lightweight aggregate concrete, internal relative humidity, drying shrinkage, prewetting degree, flyash.