Single-point magnetic resonance imaging (MRI) of cement based materials

Materials and Structures - The use of Magnetic Resonance Imaging (MRI) as a tool for non-destructive testing of moisture distribution and moisture migration in concrete and mortar is described. MRI...     

Numerical simulation of heat and mass transport during hydration of Portland cement mortar in semi-adiabatic and steam curing conditions

A model that describes hydration and heat-mass transport in Portland cement mortar during steam curing was developed. The hydration reactions are described by a maturity function that uses the equivalent age concept, coupled to a heat and mass balance. The thermal conductivity and specific heat of mortar with water-to-cement mass ratio of 0.30 was measured during hydration, using the Transient Plane Source method. The parameters for the maturity equation and the activation energy were obtained by isothermal calorimetry at 23 °C and 38 °C. Steam curing and semi-adiabatic experiments were carried out to obtain the temperature evolution and moisture profiles were assessed by magnetic resonance imaging. Three specimen geometries were simulated and the results were compared with experimental data. Comparisons of temperature had maximum residuals of 2.5 °C and 5 °C for semi-adiabatic and steam curing conditions, respectively. The model correctly predicts the evaporable water distribution obtained by magnetic resonance imaging.     

An improved procedure for obtaining and maintaining well characterized partial water saturation states on concrete samples to be used for mass transport tests

A conditioning procedure is proposed allowing to install into the concrete specimens any selected value of water saturation degree with homogeneous moisture distribution. This is achieved within the least time and the minimum alteration of the concrete specimens. The protocol has the following steps: obtaining basic drying data at 50 °C (water absorption capacity and drying curves); unidirectional drying of the specimens at 50 °C until reaching the target saturation degree values; redistribution phase in closed containers at 50 °C (with measurement of the quasi-equilibrium relative humidities); storage into controlled environment chambers until and during mass transport tests, if necessary. A water transport model is used to derive transport parameters of the tested materials from the drying data, i.e., relative permeabilities and apparent water diffusion coefficients. The model also allows calculating moisture profiles during isothermal drying and redistribution phases, thus allowing optimization of the redistribution times for obtaining homogeneous moisture distributions.     

Drying of saturated lightweight concrete: an experimental investigation

The changes of moisture content during drying are experimentally investigated in the present work. Particular emphasis is placed on the initial stage of drying of saturated concrete, when moisture contents are high, since the resistance of the material to several deterioration processes is reduced at high moisture content levels, and experimental data for this stage of drying is lacking. The experimental investigation is performed for concrete cylinders of different lengths with one end exposed to drying. In this way, moisture flow is forced to be unidirectional. The gravimetric method is employed to obtain moisture content distribution in the material at different times of drying. The cylinders are made of lightweight concrete with varying water-to-cement ratios and moist curing times, and the influence of these variables upon the drying process is assessed. Higher initial water content and more rapid changes of water content occur in lightweight concrete with a higher w/c ratio. An increased moist-curing period results in a decrease of drying rates throughout the drying process.     

Mechanism of early age shrinkage of concretes

Moist curing improves the properties of concrete. However, shrinkages at early ages are found to increase with increased curing. The reason for this phenomenon is studied with four binders and two types of curing. The binders are comprised of Portland cement/slag blends with 0, 35, 50 and 65% of slag. Initial moist curing times of 1 and 7 days were studied. The samples were then exposed to standard drying conditions (23°C and 50% RH). During drying, the moisture losses in 7-day cured concretes were about 50% less than in 1-day cured concretes; however, the early age shrinkages were significantly higher in 7-day cured concrete. Pore size distribution tests and analyses showed that the pore radius where meniscus forms during drying is smaller in 7-day cured concrete due to finer pores, as compared to 1-day cured concrete. Further, good correlation can be seen between the meniscus radius and shrinkage, regardless of the binder and curing types. This provides the explanation for the increased early age shrinkage with increased curing. Further, this study demonstrates that the capillary tensile force is the governing mechanism for early age shrinkage. The work presented in this paper was carried out when Tarek Aly was a research student at the Department of Civil Engineering, Monash University.     

Determination of Moisture Diffusivity as a Function of Both Moisture and Temperature

The effect of moisture and temperature on liquid water transport in porous media was studied. Specimens of autoclaved aerated concrete were subjected to one-sided water penetration in isothermal conditions at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C. After specified time intervals, moisture profiles were determined gravimetrically. The moisture diffusivity was calculated for a particular temperature as a function of moisture content, using an inverse analysis. The results demonstrate the dependence of the moisture diffusivity on the moisture content and the temperature of the samples. The moisture diffusivity for high moisture content can be as much as one order of magnitude greater than for the lowest moisture content studied. The moisture diffusivity was found to increase by as much as a factor of two when the temperature is increased from 20 °C to 80 °C.     

Use of tomography to understand the influence of preconditioning on carbonation tests in cement-based materials

Recognition of the rising amount of atmospheric CO₂ has brought renewed interest in understanding the effects of carbonation on reinforced concrete performance. In laboratory testing, the specimens must be preconditioned to effectively study carbonation. This paper studied the influence of several preconditioning schemes on the carbonation profiles of cement paste specimens subjected to accelerated carbonation tests. The evolution of microstructure and moisture during carbonation were investigated accordingly. Bulk of the work was based on an extended X-ray attenuation method (XRAM), which relied on X-ray computed tomography (CT). A novel method was introduced to evaluate the extent of damage due to drying. Based on extent of damage, the paper recommends standard-cured specimens for carbonation tests as compared to water-cured specimens. Also, when comparing between oven drying and mass balancing, the latter was shown to be more suitable, as the inner moisture distribution becomes more uniform after this drying protocol, and less fluctuation of humidity will occur during carbonation.     

Porosity of cement paste cured at 45 °C as a function of location relative to casting position

The influence of location relative to the casting position, on porosity and pore size distribution of cement pastes, was investigated. Three different pastes were prepared at a constant water/binder ratio of 0.45. The pastes were the control paste (CP) in which Portland cement was used and no cement replacement materials were added, pastes with 22% and 9% replacement (by mass) of cement with fly ash (FA) and silica fume (SF), respectively. Paste specimens were cast in cube moulds and were either cured in air at a temperature of 45 °C and relative humidity of 25% for 28 days or moist cured for 14 days after casting at 45 °C, followed by air curing at 45 °C and 25% relative humidity for further 14 days. Samples were taken from various locations of the cube specimens. Porosity and pore size distribution were conducted on the paste samples using the mercury intrusion porosimetry technique.The results show that large differences in porosity and pore size distribution exist between samples taken from different locations relative to casting positions. These differences are larger in pastes subjected to dry curing as compared to pastes subjected to some initial moist curing. The influence of sample location relative to casting position on porosity and pore size distribution of paste is compared with absorption of concrete performed in a previous investigation. The correlation between pore volume of paste and water absorption of concrete is also conducted.     

Effect of cementitious permanent formwork on moisture field of internal-cured concrete under drying

Drying shrinkage of concrete may still be the main source of cracking in concrete structures, even though the autogenous shrinkage of concrete can be effectively reduced by using internal curing. In the present paper, the effect of internal curing with pre-soaked lightweight aggregate and engineered cementitious composite permanent formwork (ECC-PF) on a moisture distribution in three kinds of concrete in a drying environment are investigated from both aspects of experiments and theoretical modeling. The test results show that the combination use of ECC-PF and internal curing can well maintain the humidity at a relatively high level not only at a place far from drying surface, but also at a place close to the drying surfaces. The developed model can well catch the characteristics of the moisture distribution in concrete under drying and the impacts of internal curing and ECC-PF can well be reflected as well. The model can be used for the design of concrete structures with combination use of internal curing and permanent formwork.     

Experimental Analysis and Simultaneous Heat and Moisture Transfer with Coupled CFD Model for Convective Drying of Moist Object

Convective drying of rectangular-shaped moist object has been analyzed both experimentally and numerically. Transient mass of the potato sample is measured experimentally. Moisture content, diffusivity, and density of the object are calculated at different drying air temperatures from 40°C to 70°C with an air velocity of 2 m/s. A three-dimensional (3D) finite volume method (FVM) based numerical model is developed to predict the temperature and moisture distribution. A computational fluid dynamics (CFD) code is used for predicting heat and mass transfer coefficients required in the boundary conditions of the heat and mass transfer model. The experimental and numerical data are compared and good agreement is observed.     

Moisture conditioning and transport properties of concrete test specimens

An experimental investigation is reported of drying and conditioning concrete at 50°C to obtain a uniform moisture distribution, prior to testing for air permeability and water absorption rate. The use of 100 mm cubes of concrete with a cast-in cylindrical cavity facilitated assessment of moisture distribution and measurement of air permeation through the surface layers of concrete: the moisture distribution was assessed by comparing the relative humidities measured within the cavity and at the surface of the test specimen. Partial drying followed by sealed storage at 50°C for a few days provided a rapid and convenient method of obtaining a uniform moisture distribution. The test results for a range of concretes indicated that air permeability and water absorption rate were very sensitive to the moisture content of the concrete, particularly at relative humidities above 60% and which were common for field exposure. The transport properties of the empty capillary pore system could be assessed using test specimens preconditioned at 60% relative humidity, but preconditioning at 85% relative humidity might be more appropriate for assessing field performance if there is a risk of carbonation induced corrosion.     

Application of Time-Domain Reflectometry for Measurement of Moisture Profiles in a Drying Experiment

The time-domain reflectometry (TDR) method is used for the measurement of moisture profiles in calcium silicate during a drying experiment. The specimens are saturated at first by water, and their lateral sides and one of the face sides are water- and vapor-proof insulated to ensure one-dimensional (1-D) water transport. Then, the drying process is started in an environment with a relative humidity of 20%. Moisture profiles are measured at specified time intervals using the TDR method. The experiment is stopped when the moisture content along the whole length of the sample is lower than the maximum hygroscopic moisture content. The obtained results can be used for the determination of moisture diffusivity in the drying phase of moisture transport.     

Pore water distribution in mortar during drying as determined by NMR

The drying of a saturated mortar sample was measured using Nuclear Magnetic Resonance. NMR is a technique which gives the total moisture content of a specific volume at a certain time. By looking also at the relaxation of behaviour of the NMR signal, more information can be obtained. For every time and every position, the pore size distribution of the mortar was measured. Other techniques show that NMR is a reliable method to measure pore size distributions. Two distinctive pore sizes can be seen in the pore size distribution. This corresponds to the microstructure of mortar, which has small (< 10nm) gel pores and bigger) ≈ 10 to 1000 nm) capillary pores. A one-dimensional drying experiment was carried out by blowing dry air (relative humidity =0+5%; temperature=20±1°C) ove the top of a cylindrical mortar sample, while all the other surfaces were sealed to prevent drying in other directions. The changing moisture content in the pores was followed with time during drying for about 3 days. This leads to the conclusion that the water cannot be extracted from the gel pores, while the capillary pores dry within about 20 hours under the drying conditions and time adopted.     

Long-term hygrothermal performance of nuclear reactor concrete containments – Laboratory evaluations of measurement setup,in situ sampling,and moisture flux calculations

A measurement setup for in situ sampling of internal relative humidity and temperature in concrete structures was developed. The setup was used to evaluate the moisture conditions and to determine whether drying of the concrete components within a nuclear reactor containment contributes to the moisture conditions. The measurement setup was tested for accuracy and thereafter installed in Swedish nuclear reactor containments for in situ monitoring. Results from the measurements confirmed that the setup is suitable, especially for long-term measurements at depths of 50 mm or more. Complementary moisture transport calculations showed that the moisture flux from the concrete to the interior of the reactor containment have a noticeable effect on the environmental conditions in the containments. The calculations of the moisture condition in the concrete show that 15–30% of the evaporable water in the concrete has been dried out during the 30 years of operation.     

Effect of gypsum on free and restrained shrinkage behaviour of slag-concretes subjected to various curing conditions

The effect of gypsum in slag-blended cement on free and restrained shrinkage of concrete subjected to various curing conditions is presented in this paper. Added gypsum in slag-blended cements was found to increase the autogenous shrinkage of concrete up to 56 days. However, added gypsum caused small reduction in the long-term shrinkage when the concrete was exposed to drying. Slag concretes with 3% added gypsum content, when exposed to drying at the age of 24 h, exhibited more cracking tendency than comparable concrete with 0% added gypsum. This is attributed to the increased shrinkage evolution of slag concrete with 3% gypsum content at early ages. However, if moist cured for 7 days, increasing the amount of gypsum from 3 to 5% in slag-blended cement reduced the cracking tendency. It is concluded that the beneficial effect of increasing gypsum in reducing cracking tendency in slag concrete is only favourable if moist cured for 7 days.     

Effects of super plasticizer and curing conditions on properties of concrete with and without fiber

In this study, effects of super plasticizer (SP) and curing conditions on properties of concrete with and without fiber were investigated. In the concrete mixtures, Portland cement, artificial aggregate, SP and steel fibers were used. SP in concrete mixtures was used with ratios of 1.0%, 1.5%, and 2.0% by weight of cement and so C25 concrete was produced with and without fiber. Specimens were cured under two different curing conditions being continuous moist curing and open-air curing. Produced concrete with and without fiber were compared with each other as well as with Portland cement concrete. The highest compressive and flexural strength were obtained with 1.0% and 1.5% SP fiber reinforced concrete, respectively.     

Comparison of curing compounds to reduce volume change from differential drying in concrete pavement

Curing compounds are used to retain moisture which promotes hydration to develop a tight microstructure. Because of the large surface-to-volume ratio of these structures, differential moisture loss can cause unwanted deflections, reduction in ride quality, and cracking. This paper quantitatively compares the effectiveness of different curing methods with an emphasis on curing compounds to resist moisture loss and subsequent volume changes caused by differential shrinkage. This work provides a quantitative comparison between different curing compounds, wet, sealed and no curing not made in previous publications. A performance-based cost analysis over the different curing compounds is also included. The result shows that the poly-alphamethylstyrene curing compound causes the lowest mass loss and subsequent deflections compared to the water–resin and water–wax-based curing compounds at equal coverage rates at equal costs. The work also shows that a double application of curing compound shows greater benefit than a single layer with the same volume for water–wax-based curing compounds. The results show that if adequate amounts of curing compounds are used then they are a useful curing method for members sensitive to differential drying such as concrete pavements.     

Some effects of cement and curing upon carbonation and reinforcement corrosion in concrete

Experimental data are presented to illustrate the effects of cement type and curing upon the depth of carbonation and reinforcement corrosion in cover concrete after exposure for 18 months at 20°C and 60% relative humidity. Three curing periods (1, 3 and 28-days) and 17 cements, with various proportions of granulated blastfurnace slag or limestone, were used to make concretes, at 0.59 water/cement ratio, with 28 day strengths in the range 26 to 46 MPa. The depth of carbonation after 18 months was 64% greater than after 6 months and was affected more by cement type than by curing. The depth of carbonation increased when Portland cement clinker was replaced by 19% or more of limestone or granulated blastfurnace slag. The depth of carbonation after 18 months correlated better with the air permeability of cover concrete, initial weight loss (an indicator of moisture diffusion rate in cover concrete) or the cube strength 8 days after the end of curing than it did with 28-day cube strength. The rate of reinforcement corrosion increased steeply when the carbonation front approached the reinforcing steel, and it was still increasing after the carbonation front had completely passed the reinforcement. For a given unneutralised remainder (i.e. cover depth minus the depth of carbonation), curing had little effect upon the rate of corrosion but higher rates were observed when the cement contained granulated blastfurnace slag. The results were broadly consistent with a simple engineering strategy in which the rate of carbonation was related to the air permeability of cover concrete, and the rate of any subsequent reinforcement corrosion was largely dependent upon moisture conditions, without any obvious influence of the cover depth or the permeability of the cover concrete. The results also suggested that estimation of the rate of reinforcement corrosion could be improved by taking account of the cement type and treating the unneutralised remainder as a variable.     

Accelerated carbonation and performance of concrete made with steel slag as binding materials and aggregates

Steel slag has been used as supplementary cementitious materials or aggregates in concrete. However, the substitution levels of steel slag for Portland cement or natural aggregates were limited due to its low hydraulic property or latent volume instability. In this study, 60% of steel slag powders containing high free-CaO content, 20% of Portland cement and up to 20% of reactive magnesia and lime were mixed to prepare the binding blends. The binding blends were then used to cast concrete, in which up to 100% of natural aggregates (limestone and river sands) were replaced with steel slag aggregates. The concrete was exposed to carbonation curing with a concentration of 99.9% CO₂ and a pressure of 0.10 MPa for different durations (1d, 3d, and 14d). The carbonation front, carbonate products, compressive strength, microstructure, and volume stability of the concrete were investigated. Results show that the compressive strength of the steel slag concrete after CO₂ curing was significantly increased. The compressive strengths of concrete subjected to CO₂ curing for 14d were up to five-fold greater than that of the corresponding concrete under conventional moist curing for 28d. This is attributed to the formation of calcium carbonates, leading to a microstructure densification of the concrete. Replacement of limestone and sand aggregates with steel slag aggregates also increased the compressive strengths of the concrete subjected to CO₂ curing. In addition, the concrete pre-exposed to CO₂ curing produced less expansion than the concrete pre-exposed to moist curing during the subsequent accelerated curing in 60 °C water. This study provides a potential approach to prepare concrete with low-carbon emissions via the accelerated carbonation of steel slag.     

The impact of wet curing on curling in concrete caused by drying shrinkage

This paper examines the impact of wet curing with different durations on differential drying shrinkage of concrete beams. The work shows that in severe drying conditions the magnitude of curling increased with the duration of wet curing of concrete specimens. Testing results are presented along with an explanation for the mechanisms observed. These findings provide guidance for wet curing durations for concrete slabs that must be resistant to volume change from drying.