Relative Humidity Of Concrete Research Articles

Mesoscopic analysis of heat and moisture coupled transfer in concrete considering phase change under frost action

Freeze-thaw damage in cold areas often decreases the performance and service life of concrete structures. This work presents a mesoscopic numerical method to investigate the heat and moisture coupled transfer of concrete subjected to frost action at the meso-scale while accounting for phase change kinetics. In this method, a new multiphase hydrothermal coupled model is established to describe the coupling effects of heat exchange, moisture migration, ice formation, and vapor condensation in concrete exposed to freeze-thaw cycles. To consider its meso-structural characteristics, a meso-structure of concrete consisting of mortar, interfacial transition zones (ITZs) and aggregates is generated and employed to conduct some numerical simulations. Results indicate that the proposed mesoscopic method is capable of predicting hydrothermal behaviors of concrete at the meso-scale. The hysteresis of the evolution of relative humidity with temperature becomes more pronounced as aggregate content increases from 20% to 50%, owing to the role of phase change. In particular, the relative humidity of concrete containing 50% aggregates rises to 91% and then swiftly reduces to 85% as the temperature decreases. Furthermore, when the permeability ratio of the ITZ to mortar is reduced from 10 to 1, the ITZ diffusivity only affects the relative humidity during the thawing process.

Experimental Investigation on Correlation between Autogenous Shrinkage and Internal Relative Humidity of Superabsorbent Polymer–Modified Concrete

This research investigated the effect of superabsorbent polymers (SAPs) (0%, 0.17%, 0.35%, and 0.49% of cement content by weight) on the correlation between autogenous shrinkage (AS) and internal relative humidity (IRH) of high strength concrete under sealing conditions. An integrated device was designed to measure the IRH and AS of concrete simultaneously. Experimental results and analysis indicated that (1) the IRH of the SAP-modified concrete at 28 days and the critical time of the IRH increased as dosages of SAPs increased; (2) the expansion of the SAP-modified concrete at an early age was obvious within the first day, and the expansion peak increased as the dosages of SAPs increased; (3) the AS at 28 days and the AS rate of the SAP-modified concrete decreased as the dosages of SAPs increased; and (4) a prediction model for the AS of SAP-modified concrete was proposed considering concrete relative humidity and the quantity of internal curing water.

Exploring the causes and repair measures of early cracks in prefabricated bridges

The study aims to explore the causes of early cracks in prefabricated bridges and propose repair measures. The stude designs a test first. Analysis of the mix proportion and uneven shrinkage of concrete shows that, in concrete, the ratio of air content to water-binder is a quadratic parabola, the fly ash and water binder ratio have a linear relationship, and slurry collection ratio and the water-binder ratio is inversely proportional. The test results suggest that the cracks of bridges are caused by the decrease of relative humidity in concrete. Afterward, some corresponding repair measures are put forward to safeguard the prefabricated bridges.

Mechanical properties and autogenous deformation behavior of early-age concrete containing pre-wetted ceramsite and CaO-based expansive agent

Autogenous shrinkage (AS) of high-strength and high-performance concrete (HSHPC) is large, which easily leads to early-age cracks and then severely deteriorates the concrete durability. CaO-based expansive agent (CEA) is extensively applied to mitigate the AS of HSHPC, but CEA could not fully achieve its expansion efficacy in HSHPC because of its low water to binder ratio and water amount, causing that CEA hydrates incompletely and the concrete contracts sharply and substantially after the initial expansion. Pre-wetted ceramsite (PWC), as an internal curing agent, could decrease effectively the AS of HSHPC. Based on the mechanism of AS reduction of CEA and PWC, the combination use of CEA and PWC could reach a synergistic effect on AS in HSHPC. However, there still lacks a synthetic study on the effects of the compound use of PWC and CEA on the mechanical properties and autogenous deformation of concrete. Thus, this study carried out the experiments on mechanical behaviors and autogenous deformation of concrete with PWC or/and CEA addition. The test results claimed that the PWC addition enhanced the compressive and splitting tensile strength while the opposite results were obtained by CEA addition. In addition, both PWC and CEA addition weakened the dynamic elastic modulus of concrete. Meanwhile, the PWC and CEA incorporation had a significant improving effect on the internal relative humidity in concrete. The AS of concrete was greatly mitigated by PWC addition. The IC water of PWC could improve the expansion efficacy of CEA as well as greatly decrease the contraction at the later stage. A remarkable synergistic mechanism on AS mitigation was observed with the combined use of PWC and CEA, which is significantly beneficial to control the AS-induced cracking.

Effects of porous shale waste brick lightweight aggregate on mechanical properties and autogenous deformation of early-age concrete

High-strength and high-performance concrete (HSHPC) easily generates early-age cracking owing to the large autogenous shrinkage (AS), which severely affects the durability of concrete. Internal curing (IC) by lightweight aggregate is one of the effective methods to mitigate early-age AS of HSHPC. To reduce carbon emissions and increase waste resource re-utilization efficiency, crushed porous shale waste brick (PSWB) is introduced as IC agent. However, a comprehensive understanding on the effects of PSWB addition on the performances of concrete is still lacking. Therefore, the influence of PSWB on the setting time, mechanical properties and autogenous deformation of concrete was investigated experimentally in this study. The test results showed that the setting time and mechanical properties were strongly affected by PSWB. PSWB addition retarded the initial and final setting times. The compressive and splitting tensile strengths first increased and subsequently decreased with PSWB content, but the 28-day decrease rate of compressive strength was no more than 5.0% compared with the reference group. In addition, the dynamic elastic modulus decreased with PSWB content. The internal relative humidity in concrete was significantly improved with PSWB incorporation. For PSWB content ≤ 15%, the autogenous deformation showed rapid expansion first and then shrinkage, but the development rate of retraction was remarkably inhibited by PSWB. The interesting feature of the concrete with PSWB content >15% was that the concrete first expanded followed by a slight retraction, and then expanded continually until the age of 28 days, which was of great benefit in AS-induced cracking control. The mechanism of the initial expansion of PSWB concrete was mainly due to the C-S-H gel swell with the IC water ingress.

Effects of pre-soaked zeolite and CaO-based expansive agent on mechanical properties and autogenous deformation of early-age concrete

High-performance concrete (HPC) is prone to cracking at an early age due to the large autogenous shrinkage (AS), which severely weakens the durability of the concrete structure. CaO-based expansive agent (CEA) is widely utilized to compensate for the AS of HPC, but CEA cannot achieve its potential expansion in HPC due to a low water to binder ratio, resulting in an incomplete hydration, and a sharp and substantial retraction after initial expansion. Pre-soaked zeolite can be introduced as internal curing (IC) agent to reduce the AS of HPC owing to the IC water released from zeolite. According to the AS-reduction mechanism of CEA and zeolite, a synergistic mechanism can be achieved by the combination use of CEA and zeolite in HPC. However, a comprehensive understanding of the effects of the combination use of zeolite and CEA on the mechanical properties and autogenous deformation of concrete is still lacking. Therefore, the influence of zeolite and CEA addition on mechanical properties and autogenous deformation were investigated experimentally. The investigation results showed that the zeolite addition weakened the mechanical properties of concrete while the CEA addition had the opposite effect. The internal relative humidity in concrete was significantly improved with zeolite and CEA incorporation. The IC water released from zeolite could not only enhance the expansion efficiency at an early age but also greatly reduce the retraction at the later age. The results exhibited an obvious synergistic mechanism with the combination use of zeolite and CEA on AS mitigation, which is of great benefit in AS-induced cracking control.

Determinant factor for dynamic equilibrium of transformation between vapor in atmosphere and water in concrete

Under certain conditions, liquid water in concrete pores can evaporate, forming vapor to the surrounding atmosphere; vapor in the surrounding atmosphere can condense, forming liquid water in concrete pores. Therefore, there exists a water-to-vapor transformation causing change of liquid water content in concrete. This study is aimed at analyzing the determinant factor for the water-to-vapor transformation based on the responses of relative humidity in concrete to different relative humidity and temperature of the surrounding atmosphere. Relative humidity in concrete was measured with two temperature and humidity (T&RH) probes (One was embedded into concrete and the other was placed in the cavity formed by embedding a PVC pipe) under two conditions: (1) constant temperature and variable relative humidity of the surrounding atmosphere (2) constant relative humidity and variable temperature of the surrounding atmosphere. Moreover, a specially designed device was adopted to verify the determinant factor for the water-to-vapor transformation. The results showed that relative humidity recorded by the embedded T&RH probe was always 100%, while that recorded by the placed T&RH probe had responses to the surrounding atmosphere under conditions (1) and (2). According to the results, the determinant factor for the water-to-vapor transformation is vapor partial pressure, namely absolute humidity, of the surrounding atmosphere. Furthermore, as long as there is liquid water in concrete, vapor in concrete pores is in saturation state, indicating the internal relative humidity of concrete should be a constant of 100%. Consequently, relative humidity in the cavity is inaccurate to represent moist condition of concrete.

Chloride Penetration into Concrete under the Coupling Effects of Internal and External Relative Humidity

Chloride penetration frequently happens in an unsaturated concrete; in this case, the chloride migration is under the coupling effects of internal moisture and external relative humidity (RH). Various chloride attack tests were designed in this paper to investigate the effects of internal moisture and external RH (50–100%) on the chloride penetration into an unsaturated concrete, and the chloride content was determined to quantify the chloride migration in concrete. The results highlighted that there was a good correlation between the chloride penetration into concrete and its internal RH. The chloride ion was difficult to penetrate into concrete when its internal RH was below 50%; however, the chloride penetration rate increased when the internal RH of concrete was above 75%, and the chloride migration increased with increasing RH and exposure duration. Water evaporation in concrete increased with decreasing external RH, and an obvious increase in the surface chloride content may be observed when external RH was 50%; however, when external RH was above 75%, the inner chloride content increased and surface chloride content decreased with increasing exposure duration. Simulating the chloride penetration under wet‐dry cycling, the water migration resulted in an increase in the surface chloride content and a decrease in the inner chloride content, and the chloride crystal precipitated on the surface of concrete with a high w/c ratio.

The Measurement of the Internal Relative Humidity of Concrete at Early Ages

Abstract Measurement of the internal relative humidity (RH) in concrete can be used to describe the availability of water, which has implications on several properties including shrinkage and hydration rate. However, accurate measurement of the internal RH of concrete is challenging, especially at early ages. This article examines different methods of using capacitive sensors for measuring the internal RH of concrete. The capacitive RH sensors are typically placed inside a void that enables the RH of the air within the void to be measured. Condensation of water vapor (caused by variation in temperature of the sample) has been observed to result in unreliable measurements, calibration issues, sensor malfunction, and sensor damage. Sensors may be heated cyclically to evaporate the condensed water vapor; however, many of these sensors failed to operate properly. Continuously heated sensors maintain the sensor at a higher temperature than the ambient concrete to reduce the potential for the formation of condensation. The continuously heated sensor provided the best results and is a good candidate for future testing. Suggestions for the measurement of internal concrete RH and calibration of the sensors are provided.

Microstructure-Based Relative Humidity in Cementitious System Due to Self-Desiccation.

The internal relative humidity (RH) plays a crucial role in most of the concrete properties. Self-desiccation caused by continuous cement hydration is a major factor affecting the RH of concrete. This paper investigates the relationship between RH and microstructure for cementitious systems in the case of self-desiccation. A series of paste specimens prepared with different binder and water-binder-ratio (w/b) were cured under sealed conditions from 1 day to 1.5 years. The RH and microstructure of the paste specimens were measured. The microstructure characteristics under study include porosity, pore size, evaporable and non-evaporable water content. The results reveal that the RH of cementitious system drops to a great extent in the first 105 days’ hydration and decreases slowly afterwards. The blended materials such as fly ash, slag or limestone powder have different influences on the RH. A mathematical model between RH and the average pore diameter is proposed for cementitious systems under self-desiccation, regardless of age, w/b or cement type.

Calculation of shrinkage stress in concrete structures with impact of internal curing

This paper presents recent advances in the calculation of shrinkage stress in concrete structures, especially focus on the effect of internal curing. In the modeling, an integrative model for autogenous and drying shrinkage predictions of concrete under drying environments is introduced first. Second, a model taking both cement hydration and moisture diffusion into account synchronously is used to calculate the distribution of internal relative humidity in concrete. Using the above two models, along with the shrinkage stress model for concrete pavement under nonlinear strain along the slab thickness, the development of shrinkage stress in concrete pavements since casting made by low, middle and high strength concrete respectively is calculated. In particular, the effects of internal curing with pre-wetted lightweight aggregates on the progress of shrinkage stress are investigated and discussed. Based on the present study, the following conclusions can be drawn: (1) the progress of shrinkage stress in concrete pavements obeys two-stage mode since concrete casting, a small stress stage (close to zero) in the moisture saturated period and a gradual increasing stage with the relative humidity gradually decreasing in the concrete; (2) the closer to the drying face, the higher the shrinkage stresses; (3) the shrinkage stress is greater in high strength slab than that in low or middle strength slab; (4) internal curing with pre-wetted lightweight aggregate can greatly reduce shrinkage stress in concrete pavement, either in low strength slab or high strength slab.

A Mesoscale Three-Dimensional Moisture Transport Model in Concrete

Concrete was considered as a three-component composite material consisting of mortar matrix, coarse aggregates and interfacial transition zones (ITZ) at the mesoscale level. Based on the random accumulation model of spherical aggregates, a mesoscale geometric model was developed after discretizing the mortar matrix and ITZ into mesh elements using Voronoi diagram method. Combined with the third boundary condition, a mesoscale three-dimensional model to simulate the moisture transport process in concrete exposed to atmosphere environment was then developed using finite difference numerical method, where the transport of liquid water and water vapor were considered as permeation and diffusion respectively. Moreover, the model was verified and then applied to investigate the influence of ITZ on the distribution of relative humidity in concrete. The results indicated that the moisture transport process was overall accelerated since blocking effect of coarse aggregates was partially counteracted by the existence of ITZ.

Effect of limestone powder on creep of high-strength concrete

Effect of limestone powder on creep characteristics and internal relative humidity of high-strength concrete has been investigated. The results indicate that limestone powder can reduce creep of concrete, with a greater reduction in creep at higher replacement levels. Limestone powder delays the variation of internal relative humidity of concrete, and there exists a good linear relationship between creep and internal relative humidity of concrete, so creep can be predicted according to internal relative humidity. The creep prediction data of CEB-FIP 90 model obviously overestimate the test data of high-strength concrete with limestone powder always.

The effect of external load on internal relative humidity in concrete

In poromechanical modeling of unsaturated concrete, the effect of external load on internal relative humidity (RH) is usually neglected. In this study we present experimental measurements of RH changes in a low w/c mortar during uniaxial compressive loading. We observed an instantaneous RH increase of almost 2% RH inside sealed mortar samples at loading with a stress/strength ratio of 30%. The initial RH increase is further recovered over time while the sample is under constant load. Upon unloading, an instantaneous RH decrease of roughly the same magnitude takes place. These results confirm that the hygro–mechanical coupling is relevant for understanding concrete deformations, both quasi-immediate and possibly also volumetric creep and drying creep. The observed coupling supports the theory of local water redistribution from load-bearing water layers in hindered adsorption areas toward larger pores as a result of applied compressive stress.

Hygrometric Assessment of Internal Relative Humidity in Concrete: Practical Application Issues

The use of embedded relative humidity (RH) sensors for assessing the internal humidity in concrete is widely spread, dully backed by existing standards. Even though the approaches adopted in the literature seem to have several differences between each other, few or none research works were found to focus on the comparison of performance of sensors and methods for RH measurement. In view of this, several sets of experiments comparing the performances of different sensors and monitoring procedures will be presented in this paper, discussing the main findings and providing recommendations for the strategies to be adopted in what concerns the measurement technique. The main points addressed in this work are: (i) comparisons between readily available systems for RH measurement in concrete, as well as custom measurement strategies reported in the literature; (ii) issues related to calibration procedures and re-calibration necessity; (iii) relevance of the existence of an interface porous material between the embedded sensor and the measurement spot in concrete; (iv) importance of the size of the embedment body into which the RH sensor is inserted; (v) equivalence of results obtained when the probe is constantly inserted into the embedment body, or placed inside it at discrete instants.

Discussion: Relationship of moisture content with temperature and relative humidity in concrete

Discussion: Relationship of moisture content with temperature and relative humidity in concrete

Relationship of moisture content with temperature and relative humidity in concrete

Variations of temperature and moisture content in concrete arise as the effects of climate load in a natural environment; because it can be easily measured, relative humidity is often used as a transition index reflecting moisture content. Determination of the relationship between moisture content and temperature/humidity in concrete is of great significance in accurate prediction of the durability of reinforced concrete structures. Under constant temperature conditions, measurements of both relative humidity and moisture content in concrete are carried out synchronously. Results show that pore water saturation, considered as the characterisation of moisture content, changes non-linearly with relative humidity in concrete; the evolution is significantly influenced by the water–cement ratio and temperature. Finally, based on experimental results and theoretical analysis, a model of the relationship between pore water saturation, temperature and relative humidity in concrete is established, and its feasibility is verified through an independent test.

Moisture levels and drying potential of the concrete in Swedish reactor containments

The climatic conditions inside nuclear reactor containments are continuously monitored by the plant owners in order to e.g. maintain a suitable climate for the mechanical and electrical equipment. Due to the high temperatures, during operation, the concrete within the containment may act as a moisture source contributing to the humidity. To evaluate if, and to what extent, the concrete inside the containment influences the climatic conditions, the moisture distribution has to be determined. In order to do this, a measurement setup has been developed and tested. The results showed that the measurement setup was suitable for long term measurements of internal relative humidity in concrete. However, measurements close to the concrete surfaces showed tendencies of leakage, increasing the uncertainties of those measurements. The measurement setup was installed at three reactor containments in Sweden during the summer of 2012. Results from the monitoring campaign on one of the three reactor containments are presented in this paper. The results showed that the concrete was still drying after 30-35 years of exposure inside the containment.

Effect of Ceramsite Type on the Internal Relative Humidity of Ceramsite Concrete

The internal relative humidities of concrete using different types of ceramsites with different prewetting degrees were monitored by humidity sensors. The study reveals that the internal relative humidity of concrete using low water absorption rate ceramsites declines more slowly than that of concrete using high water absorption rate ceramsites when not prewetting; the internal relative humidity of concrete using high water absorption rate ceramsites is higher than that of concrete using low water absorption rate ceramsites when their prewetting degrees (1h or 24h) are the same; the internal relative humidity of concrete using ceramsites is higher than that of ordinary concrete, whether the ceramsites are prewetted or not.

Internal relative humidity distribution in concrete considering self-desiccation at early ages

The internal relative humidity (RH) and humidity gradients in concrete at early ages have significant influence upon the properties of concrete, where exists, great discrepancy among the test results under different methods. By comparing and analyzing the common measuring methods of the RH in concrete, a newly developed measuring method will be introduced in this study, which could measure the internal RH in cement-based materials accurately, conveniently and digitally. The changing laws of the internal RH in concrete at early ages were discussed. And the numerical simulation of the internal RH was carried out, using the calculated parameters appropriately. The results indicate that the measuring results under the newly developed system was more accurate than that by the predrilled-hole method, while the external environment does not have any effect on the measured results obtained by using the new measurement system. There exist obvious humidity gradients in concrete. In concrete with low water/cement ratio, self-desiccation considerably influence on internal RH distribution. The finite element analysis results from moisture diffusion theory were in good agreement with experimental results. Key words: Concrete, low water-cement ratio, admixtures, early ages, internal relative humidity, a new measurement system, self-desiccation, finite element analysis.