Resistance of concrete with blast-furnace slag against chlorides, investigated by comparing chloride profiles after migration and diffusion

The effect of partial replacement of ordinary Portland cement (OPC) by blast-furnace slag (BFS) on concrete’s resistance to chlorides was examined by comparing chloride profiles and colour change boundaries after non-steady state migration and natural diffusion tests, cfr. NT Build 492 and NT Build 443, respectively. Chloride profiles were obtained by means of potentiometric titration and the colour change boundaries by spraying 0.1 M AgNO₃-solution. One OPC mixture and three BFS mixtures, with cement replacement levels of 50, 70 and 85 %, were tested. It was found that, compared to OPC concrete, migration coefficients of 50 and 70 % BFS concrete are decreased with 32 and 57 %, respectively. Diffusion coefficients are reduced by 66 and 50 %, respectively. Nevertheless, the chloride binding capacity of BFS concrete, determined from the difference between total and free chlorides, was lower than for OPC concrete. The results for 85 % BFS concrete differ from the results for 50 and 70 % BFS concrete, since higher migration and diffusion coefficients are found. Besides, free chloride concentrations at the colour change boundary (ccb), obtained after spraying with AgNO₃-solution, amounted to 0.30 mol/l on average. There was no clear relation between the free chloride content at ccb and the BFS content.     

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.     

Effect of silica fume addition and repeated loading on chloride diffusion coefficient of concrete

In this paper, the effect of silica fume (SF) on the chloride diffusion coefficient of concrete subjected to repeated loading was examined. Portland cement was replaced by 5 and 10 % SF. Five cycles repeated loadings were applied to concrete specimens, the maximum loadings were 40 and 80 % of the axial cylinder compressive strength ( $ f_{\text{c}}^{\prime } $ ), respectively. The diffusion coefficients were calculated from the steady state in the chloride migration test using the Nernst-Planck equation. The service life of concrete in chloride environment was predicted by Life-365 model. The results indicate that the diffusion coefficients of concrete containing 5 and 10 % SF replacements are lower than that of the control concrete at the age of 28 days. This trend increases with the increase of SF replacement. Five cycles repeated loading at 40 % $ f_{\text{c}}^{\prime } $ or 80 % $ f_{\text{c}}^{\prime } $ increase the diffusion coefficients (D ₂₈) for all mixes investigated in this study. However, the effect of 80 % $ f_{\text{c}}^{\prime } $ on D ₂₈ of concrete with 10 % SF is significantly lower than that of the control concrete without SF. Compared with the control concrete without SF, 10 % SF replacements increase the service life of concrete by more than 10 times.     

Restrained shrinkage cracking of recycled aggregate concrete

The increase in drying shrinkage and decrease in tensile properties of concrete proportioned with recycled concrete aggregate (RCA) can result in a high risk of cracking under restrained conditions. However, the reduction of the modulus of elasticity of such concrete, can lead to greater stress relaxation and reduction in cracking potential. An experimental program was undertaken to evaluate the effect of using RCA at high substitution rates of 50 and 100% (by vol.) on the cracking potential under restrained conditions. Four different types of coarse RCA, two binder types, and water-to-cementitious materials ratio (w/cm) of 0.37 and 0.40 were considered in the study. Mechanical properties, drying shrinkage, and cracking potential using the ring test were investigated. Test results indicated no cracking up to 35 days in the case of the reference mixture and the concrete prepared with 50% RCA replacement. The 28-day stress rate of such mixtures were limited to 0.12 MPa/day. Depending on the RCA type, the incorporation of 100% coarse RCA in a binary system made with 0.40 w/cm increased the 35-day cracking potential to up to 74%, with values of stress rate ranging from 0.25 to 0.34 MPa/day. The mixtures proportioned with 100% RCA developed tensile creep coefficient of 0.34–0.78 at the time of cracking compared to 0.34–0.36 for the reference concrete at the same age. However, greater elastic concrete strain and lower tensile strength resulted in reduced time to cracking at 100% RCA replacement, which was 9.0–11.0 days.     

How do recycled concrete aggregates modify the shrinkage and self-healing properties?

This paper presents the main results of a research carried out to analyze the mechanical properties, intrinsic permeability, drying shrinkage, carbonation, and the self-healing potential of concrete incorporating recycled concrete aggregates. The recycled concrete mixtures were designed by replacing natural aggregates with 0%, 30%, and 100% of recycled concrete gravel (RG) and 30% of recycled concrete sand (RS). The water to equivalent binder ratio was kept constant and recycled concrete aggregates were initially at saturated surface dried (SSD) state. The contribution of the porosity of natural and recycled aggregates to the porosity of concrete was estimated to understand the evolution of the intrinsic permeability and the open porosity. At long term, the maximum variation of drying shrinkage magnitude due to recycled concrete gravels did not exceed 15%. The correlation between drying shrinkage and mass-loss through “drying depth” concept showed that recycled concrete aggregates are affected by drying as soon as concrete is exposed to desiccation. A good correlation between 1-day compressive strength and 18-month carbonation depth was observed. The recycled concrete aggregates presented a good potential for self-healing as the relative recovery of cracks reached up to 60%.     

A comparison of chloride ion diffusion coefficients derived from concentration gradients and non-steady state accelerated ionic migration

A method for electrical indication of the ability of concrete to resist chloride ion penetration was first developed in the USA in the late 1970's. The test method (AASHTO T 277) related chloride ion diffusion to the measurement of electrical charge passed through a test specimen in a relatively short period of time. In this paper, chloride concentration profiles obtained from specimens used in the initial development of the method are used to compute apparent chloride diffusion coefficients, D_app, by application of ionic migration principles. These coefficients are compared to D_app calculated from fitting data obtained from 90-day ponding tests carried out on companion specimens not subjected to application of electrical currents. It is shown that there is a strong linear relationship for conventional concretes between coefficients calculated from chloride concentration gradients produced either by long-term ponding or by accelerated ingress of chloride into the test specimens. Also included in the comparative study are effective diffusion coefficients, D_eff, based on the measurement of specimen resistance and calculated using Einstein's equation. Results indicate that values calculated by any of the techniques yield lower values for diffusion coefficients than those obtained from longer-term ponding. It is shown, however, that rankings of concretes on the basis of diffusion coefficient are similar for both calculated and measured values. Large errors occur in calculated coefficients in cases where anomalies exist in measured chloride gradients. These errors are related to the dependence of calculated diffusion coefficients on chloride concentration ratios. For specialty polymer concretes, mechanisms of penetration of chloride into concrete may be different in the presence of an applied electrical field. These differences may be the result of the short test time in the AASHTO T 277 procedure as well as of the relatively high voltage used. These effects prevent the concrete from reaching the same chloride surface concentration, C_s, as in longer-term (90-day) ponding. Anomalously high D_app valves are computed when the concrete is too resistive or has been surface treated. In these cases, the C_s valves are far greater than those obtained during 90-day ponding.     

Evaluation of a carbonation model for existing concrete facades and balconies by consecutive field measurements

The square root model is widely used to predict the initiation phase of reinforcement corrosion induced by carbonation of the concrete cover. The model is based on diffusion laws which makes its validity arguable. The model has been accused of not being able to model accurately carbonation in structures exposed to drying/wetting cycles. The model was evaluated by field measurements on 18 existing concrete buildings conducted twice at an average interval of 8 years. Data from individual parallel samples as well as averaged measurement data were produced. The propagation of carbonation over a certain period of time and variation in the carbonation coefficient were studied using the data. Individual measurements indicated high variation and even inconsistency in carbonation depth. Thus, the carbonation coefficient calculated for the square root model also varied widely. Despite the high scatter, the averaged carbonation of many buildings was found to be closely in line with the prediction of the square root model.     

Diagnosis of concrete structures: the influence of sampling parameters on the accuracy of chloride profiles

For determining the chloride threshold level of steel in concrete, the withdrawal of concrete samples is not a trivial task. The elected sampling method, the associated step size of depth intervals and the depth position of the steel with regard to the interval limits can exert a significant influence on the accuracy of the results. Also local variations of chloride ingress may lead to significant deviations from the effective chloride concentration in the depth of interest. In this paper, the chloride profiles from grinding powder samples with small interval sizes of 2 mm are compared to drilling powder samples with interval sizes of 10, 15 and 20 mm. Regression curves according to Fick’s second law were generated and the corresponding chloride diffusion coefficients were determined. The deviations of the curves and the resulting diffusion coefficients were illustrated. The assumption of considering the determined chloride concentration as constant and valid for the entire interval depth was found to lead to huge deviations from the effective profiles which are not acceptable. The implementation of regression approaches from drilling powder samples lead to an extreme reduction of deviations, thus resulting profiles may be suitable for certain inspection tasks, even though most profiles examined do not exhibit a sufficient number of data points from depth ranges completely controlled by diffusion. Additionally, a theoretical modeling of chloride penetration curves was performed. A quantitative approach for the election of depth intervals in practice was developed based on the input parameters diffusion coefficient and exposure time.     

Compressive strength and durability properties of ceramic wastes based concrete

This paper presents an experimental study on the properties and on the durability of concrete containing ceramic wastes. Several concrete mixes possessing a target mean compressive strength of 30 MPa were prepared with 20% cement replacement by ceramic powder (W/B = 0.6). A concrete mix with ceramic sand and granite aggregates were also prepared as well as a concrete mix with natural sand and coarse ceramic aggregates (W/B = 0.5). The mechanical and durability performance of ceramic waste based concrete are assessed by means of mechanical tests, water performance, permeability, chloride diffusion and also accelerated aging tests. Results show that concrete with partial cement replacement by ceramic powder although it has minor strength loss possess increase durability performance. Results also shows that concrete mixtures with ceramic aggregates perform better than the control concrete mixtures concerning compressive strength, capillarity water absorption, oxygen permeability and chloride diffusion. The replacement of cement and aggregates in concrete by ceramic wastes will have major environmental benefits.     

Anti-vibration characteristics of rubberised reinforced concrete beams

The flexural and vibration properties were examined in order to evaluate the anti-vibration characteristics of rubber modified reinforced concrete beam. The rubberised mixtures were produced by replacing 5, 7.5, and 10 % by mass of the fine aggregate with 1–4 mm scrap truck tyre crumb rubber particles. A series of reinforced concrete beam (1,200 × 135 × 90 mm³) was tested in a free vibration mode and then subsequently in a four point flexural tests. The input and output signals from vibration tests were utilised to calculate various dynamic parameters such as natural frequencies, frequency response function, dynamic modulus of elasticity and damping ratio. The results showed that compared to control mixture, gradual reductions of natural frequencies in first six modes of all rubberised beams with the highest being in the mixture with 10 % rubber contents. In addition, despite the reduction in overall strength, rubberised mixtures showed flexibility under loading due to the higher energy absorption capacity of rubber particles. Compared to control mixture, the results also showed a uniform global decrease in the dynamic modulus over the span. The reduction was found as high as 26 % in the mixture with 10 % rubber content. The results indicated that the rubberised concrete exhibits better anti-vibration properties and could be a viable alternative to use as vibration attenuation material where resistance to impact or blast is required such as in railway buffers, jersey barriers (a protective concrete barrier used as a highway divider and a means of preventing access to a prohibited area) and bunkers.     

Evaluating ion diffusivity of cracked cement paste using electrical impedance spectroscopy

Cracking can significantly accelerate mass transport in concrete and as such, impact its durability. This paper is aimed at quantifying the effect of saturated cracks on ion diffusion. Electrical conductivity, measured by electrical impedance spectroscopy (EIS), was used to characterize the diffusion coefficient of fiber-reinforced cement paste disks that contained one or two through-thickness cracks. Crack widths in the range 20–100 μm were generated by controlled indirect tension test. Crack profiles were digitized and quantified by image analysis to determine crack volume fraction and average crack width. Crack connectivity (e.g., inverse of tortuosity) was calculated from the conductivity results measured by EIS. The results suggest that the diffusion coefficient of cracked samples is strongly and linearly related to the crack volume fraction; but is not directly dependent on crack width. Crack tortuosity does reduce the ion diffusion through cracks, but its impact is not very significant. Overall, the most important parameter governing ion diffusion in saturated cracked concrete is the volume fraction of cracks. Theoretical justifications of these observations are also provided.     

Shrinkage-cracking behavior of OPC-fiber concrete at early-age

The current paper presents the results of early-age restrained shrinkage (RS) tests on Ordinary Portland Cement (OPC) concretes incorporating admixed polypropylene fiber (PP). Four concrete mixtures made with OPC containing various volume fractions of PP fiber were tested. Two identical specimens of each mixture were tested: one subjected to fully restrained conditions and the other allowed to shrink freely, both under the drying conditions of 23°C and 50% relative humidity at the age of 24 h. Direct and indirect tensile tests were also performed in the same concretes to monitor the tensile strength development. With increasing fiber contents in mixture, the tensile strength, creep and elastic modulus characteristics have not significantly changed during the first week of age. Increasing the volume fractions of PP fiber significantly delayed the time of cracking owing to the delayed onset of RS, which is beneficial to crack resistance.     

A comparison of transport properties for concrete using the ponding test and the accelerated chloride migration test

In order to develop a better understanding of the relationship between 90-day salt ponding test and accelerated chloride migration test (ACMT; the electrochemical technique is applied to accelerate chloride ion migration), the transport properties for concrete obtained from ACMT are compared to the diffusion coefficient from ponding test. The plain cement concrete, fly ash concrete, and slag concrete with different w/b ratios (0.35, 0.45, 0.55, and 0.65) were used. In this study, the total chloride content and penetration depth of concretes were measured after the ponding test, and the Fick's second law of diffusion was fitted to the data from experiment to determine the diffusion coefficient. The non-steady-state diffusion coefficient, the migration coefficient, and the current corresponding to the 30000 coulomb, charge passed obtained from ACMT in the previous works were compared with the diffusion coefficients obtained from ponding test. Parallel tests show that the diffusion coefficients obtained from ponding test correspond well with the non-steady-state diffusion coefficient, the migration coefficient, and the current corresponding to the 30000 coulomb charge passed obtained from ACMT, although the diffusion coefficient measured by ponding test is different from that measured by the ACMT in non-steady state and steady state.

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Résumé Pour mieux comprendre la relation entre l'essai de trempage dans l'eau salée pendant 90 jours et l'essai accéléré de migration de chlorure (ACMT; la technique électro-chimique est employée pour accélérer lamigration des ions chlorure), les propriétés de transport du béton obtenues par l'ACMT sont comparées au coefficient de diffusion de l'essai de trempage. Du ciment simple, des cendres volantes ainsi que des scories avec des rapports eau/liant différents (0.35, 0.45, 0.55 et 0.65) ont été utilisés. Dans cette étude, la teneur totale en chlorure et la profondeur de pénétration des ciments ont été mesurées après l'essai de trempage, et la seconde loi de diffusion de Fick correspond aux données tirées, d'expériences visant à déterminer le coefficient de diffusion. Le coefficient de diffusion de l'état non stationnaire, le coefficient de migration, et le courant de charge passé correspondant à 30000 coulombs obtenu à partir de l'ACMT d'expériences précédentes ont l'eté comparés aves les coefficients obtenus à partir de l'essai de trempage. Des essais parallèles montrent que les coefficients obtenus à partir de l'essai de trempage correspondent bien au coefficient de diffusion de l'état non stationnaire, le coefficient de migration, et le courant de charge passé correspondant à 30000 coulombs après l'ACMT, bien que le coefficient de diffusion mesuré par l'essai de trempage soit différent, dans les états non stationnaire et stationnaire de celui mesuré par l'ACMT.

     

Effect of pore structure and moisture content on gas diffusion and permeability in porous building stones

Hazardous gases in buildings are a concern for public health and security. These gases can be released from the building materials to indoor air and their concentration may become critical where ventilation is hindered, as such in hypogean or more energetically efficient airtight constructions. Furthermore, the gas ventilation and the indoor gas concentration can considerably increase by the vapour condensation on the ceiling and walls of buildings. In this paper, we characterise the CO₂ gas diffusion for a representative range of building porous stones with the aim of establishing the effect of the water content in the gaseous diffusion coefficient. We propose a new methodology to measure gas diffusion with a laboratory device that works under different hygrometric conditions. Results reveal water pore condensation reduces both connected porosity and pore size and therefore, the CO₂ diffusion coefficient. This variation occurs in all the studied porous building stones although it is especially important in stones with small pores. Thus, the reduction of CO₂ diffusion coefficient for the stone with thinnest pores is by 50% when relative humidity varies from 20 to 90%. Permeability and gas diffusion coefficients present similar trends. Porous stones with larger pores and higher porosity values present the highest CO₂ diffusion, water and gas permeability coefficients. Pore size is the conclusive parameter within the transport coefficients. It greatly affects both the tortuosity factor of the CO₂ gaseous diffusion and the slip parameter of the Klinkenberg’s model for gas permeability coefficient. Finally, for studied samples, we establish a power regression, which correlates thoroughly both coefficients.     

Chloride diffusion coefficient: A comparison between impedance spectroscopy and electrokinetic tests

This paper proposes to compare several experimental ways of obtaining the chloride diffusion coefficient through saturated porous materials. The first method is based on the application of the Nernst-Einstein equation for which the conductivity of the saturated sample is measured by impedance spectroscopy for two kinds of materials: inert samples of TiO₂, and concretes based on type I and type V cements. The second method is a migration test in which the flux of chloride measured upstream allows calculating the diffusion coefficient by means of the Nernst-Planck equation. In the third case, the diffusion coefficient is calculated by current measurements in an equivalent configuration as the second method. It is shown that the formation factor does not vary neither with the ionic strength of the saturation solution, nor with the change in the pore solution constituents when the material is without mineral additions. The CEM-V concrete exhibits a specific behavior with a strong influence of the addition of chloride in its pore solution on the formation factor. The diffusion coefficients calculated with the three methods are in good agreement provided the metrology of the experiments is carefully controlled.     

Comparison of in situ properties of wall elements cast using self-consolidating concrete

An experimental program was undertaken to investigate the homogeneity of in situ properties of wall elements cast with various commercially available self-consolidating concrete (SCC) mixtures. Wall elements measuring 5?m in length, 1.6?m in height, and 0.35?m in width were cast. The SCC mixtures were proportioned with various binders and admixtures and had different levels of slump flow consistency and design compressive strengths. The SCC mixtures exhibited adequate passing ability with L-box blocking ratio higher than 0.70 and filling capacity greater than 80%. All cast walls exhibited homogeneous in situ mechanical properties, regardless of the core location. The coefficient of variation (COV) of in situ compressive strength values ranged from 3 to 6%. Conventional vibrated concrete used to cast a reference wall element led to relatively large spread in in situ rapid chloride-ion permeability values and high COV of spacing factor compared to those of the various SCC wall elements (17% vs. 6?C12%). Good correlation was established between surface settlement and top-bar factor of reinforcing bars embedded along the various heights of the 1.6-m high walls. On average, concrete with 0.50% surface settlement was shown to exhibit a maximum top-bar factor of 1.4. The mean top-bar factor values near the top of walls cast with SCC were limited to 1.4, which is considered a low value given the high slump flow of the concrete and the depth of cast elements.     

The Prediction of the Emissivity and Thermal Conductivity of Powder Beds

The present study attempts to understand drying characteristics of rubber wood sawdust in a tray dryer as it is the simplest and oldest of the dryers known commercially. An increase in temperature, flow rate of the heating medium, and initial moisture content was found to increase the drying rate. However, an increase in the particle diameter and bed height was found to reduce the drying rate. The increase in drying rate with temperature and moisture content was attributed to increase in the diffusion coefficient, while the increase due to the flow rate is attributed to reduction in the external mass transfer resistance during early stages of drying while the drying rate was high. An increase in bed height as well as particle size increases the diffusion path length for moisture, which contributes to the reduction in drying rate. The experimental data were modeled using Fick's diffusion equation, and the effective diffusivity coefficient was evaluated by minimizing the error between the experimental data and the prediction using the model equation. The effective diffusion coefficient was found to increase with increase in temperature, initial moisture content, and the flow rate of the heating medium, while it was found to decrease with increase in particle size. The diffusion coefficient was not found to vary with the bed height/solid loading. The effective diffusion coefficient was found to vary within 9.1 × 10^?9to 22 × 10^?9 m²/min. The standard deviation of error between the experimental data and prediction using the model, using the estimated effective diffusivity coefficient, was found to be less than 0.07 for the entire set of data, indicating the appropriateness of the model in predicting drying kinetics.     

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.     

Taylor dispersion analysis of mixtures

Taylor dispersion analysis (TDA) is a fast and simple method for determining hydrodynamic radii. In the case of sample mixtures, TDA, as the other nonseparative methods, leads to an average diffusion coefficient on the different molecules constituting the mixture. We set in this work the equations giving, on a consistent basis, the average values obtained by TDA with detectors with linear response functions. These equations confronted TDA experiments of sample mixtures containing different proportions of a small molecule and a polymer standard. Very good agreement between theory and experiment was obtained. In a second part of this work, on the basis of monomodal or bimodal molar mass distributions of polymers, the different average diffusion coefficients corresponding to TDA were compared to the z-average diffusion coefficient (D(z)) obtained from dynamic light scattering (DLS) experiments and to the weight average diffusion coefficient (D(w)). This latter value is sometimes considered as the most representative of the sample mixture. From these results, it appears that, for monomodal distribution and relatively low polydispersity (I = 1.15), the average diffusion coefficient generally derived from TDA is very close to Dw. However, for highly polydisperse samples (e.g., bimodal polydisperse distributions), important differences could be obtained (up to 35% between TDA and D(w)). In all the cases, the average diffusion coefficient obtained by TDA for a mass concentration detector was closer to the Dw value than the z-average obtained by DLS.     

Mock-up experiments on permeability measurement of concrete and construction joints for air tightness assessment

Concrete permeability is subject to various test conditions, and on-site measurement at in situ structural scale is much preferable. This paper presents an experimental study to measure the permeability of concrete linings and their construction (placing) joints between old and new concretes using a novel in situ permeability testing system. Using the developed system, we performed in situ scale permeability tests of rectangular concrete specimens with dimensions of 500 × 500 × 2800 mm³, within which the construction joints were artificially placed. From this model experiment, we verified an effective applicability of the system to both low permeability concrete matrices and highly permeable construction joints thanks to its selective capability of gas or water permeability test, depending on the air/water tightness of tested materials. The experimental results presented in this paper also showed that the intrinsic permeability of the construction joint could be higher than that of the concrete matrices by orders of magnitude (10¹–10⁴ times), but it could be reduced to as low as those of the concrete matrix by pasting a bonding agent on the interfacing surfaces. As a result of geomechanical monitoring during the experiment, the opening displacement of construction joints with relatively higher stiffness values showed a reversible deformation when the gas injection pressure was unloaded, which is much preferable in a storage performance perspective of underground lined rock caverns.