Internal curing of concrete using lightweight aggregates

Internal cured concrete (ICC) has been recently used in the local and international construction markets. ICC contains surplus amount of water to compensate the shrinkage of the mix and the volumetric changes which result in early-age cracking of concrete. Concrete cracking is a direct result of the shrinkage of the water–cement paste during early stages of the hydration process and continues for a significant amount of time during the life span of the concrete section. Early-stage shrinkage, prior to the concrete hardening, is associated with volumetric changes, until final setting is achieved. Afterward, the reduction in cement paste particle size results in increased voids within the concrete structure. These voids result in increased permeability, additional sulfate and chloride attacks on steel reinforcement, and internal tensile stresses in concrete, which result in significant cracking. ICC uses the additional water added to the mix in counteracting the reduced volume of the concrete. Several techniques are used for internal curing (IC). In this research, water-saturated lightweight aggregates (LWAs) are used in partial replacement of normal weight aggregate as a source of additional water. LWA is submerged in water prior to concrete mixing to absorb a significant amount of water, which is stored within the LWA particles. Once added to the mix, the water is gradually desorbed and compensates the water losses during hydration. Hence, it counteracts the shrinkage induced. Different ICC mixes are developed in this research using two different sizes of LWA, and supplementary binding materials are used to improve compressive strength. ICC compressive strength and reduced shrinkage attained are presented. ICC mixes developed in this research can be successfully used in pouring highway segments and bridge decks with lower cracks and reduced life cycle cost due to reduced maintenance.     

Effects of distribution of lightweight aggregates on internal curing of concrete

This study investigates the effects of spatial distribution of lightweight aggregates (LWAs) on internal curing of concrete. As replacements for normal aggregates, different sizes and amounts of natural pumice LWAs were used as water reservoirs to provide internal curing in mitigating autogenous deformation. Water in the pre-soaked LWAs flows into cement paste during hydration and provides internal curing to counteract the RH loss due to self-desiccation of binding paste. The results show that variations in the autogenous strain of concrete can be evaluated in terms of LWA–LWA proximity. The protected paste volume approach, previously used for air-entrained concrete, is applied to calculate the internally-cured volume of paste. The results show that the experimental rate of mitigation of autogenous strain for different series of concrete specimens, with respect to the reference concrete, gave the best-fitted values at water flow distance of 1 mm. The results indicate that the protected paste volume in internal curing can be determined by calculating the water-entrained volume using image analysis.     

NMR relaxometry during internal curing of Portland cements by lightweight aggregates

The hydration of an ordinary Portland cement of low water-to-cement ratio under the influence of internal curing by addition of water-saturated lightweight aggregates (LWA) is investigated by non-destructive low-field NMR relaxometry. The methodology, which was recently developed to investigate internal curing by addition of a water-saturated polyelectrolyte gel, is applied to follow the transition of water from the mineral aggregates into hydrating cement paste and to detect the time dependence of the water consumption by the hydration reaction. By the changes of the transverse relaxation times of the physically bound water, the compaction of the pore space of the hardening cement is estimated qualitatively. The water retention potential of two types of LWA and the temporal moisture requirement of the cement during internal curing are determined without and with superplasticizer.     

Efficiency of lightweight aggregates for internal curing of high strength concrete to eliminate autogenous shrinkage

High Strength Concretes (HSC) with extremely low water to binder (w/b) ratios are characterized by high cracking sensitivity which is a consequence of increased autogenous shrinkage. The major reason for autogenous shrinkage—self-desiccation—cannot be eliminated by traditional curing methods. The application of the concept of internal curing by means of saturated lightweight aggregate was applied and shown to be effective in eliminating autogenous shrinkage. The present paper describes an approach to optimize the size and porosity of the light-weight aggregate to obtain effective internal curing with a minimum content of such aggregate.

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Résumé Les bétons de haute résistance qui ont un rapport eau/liant extrêmement bas, sont caractérisés par une haute sensibilité à la fissuration, qui est l'une des conséquences d'un retrait endogène plus important. La raison principale du retrait endogène—l'auto-dessiccation—ne peut être éliminée par les méthodes conventionnelles de cure. L'application du concept de la cure interne au moyen de granulats légers saturés a été pratiquée et s'est avérée effective pour éliminer le retrait endogène. Cet article décrit une approche visant à améliorer la taille et la porosité du granulat léger afin d'obtenir une cure interne effective avec un volume minimum d'un tel granulat.

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Editorial Note Prof. Arnon Bentur is a RILEM Senior Member. He is the Chairman of RILEM TC 181-EAS: ‘Early age shrinkage induced stresses and cracking in cementitious systems’. He is also the Chairman of the RILEM Technical Activities Committee (TAC). Dr. Konstantin Kovler is also a RILEM Senior Member, He participates in RILEM TCs 162-TDF ‘Test and design methods for steel fibre reinforced concrete’ and 181-EAS (above-mentioned). The Israeli National Building Research Institute is a RILEM Titular Member.     

Release of internal curing water from lightweight aggregates in cement paste investigated by neutron and X-ray tomography

A sealed sample of cement paste containing a pre-wetted and a dry lightweight aggregate (LWA) particle was investigated in the period between 0.5 and 20.3 h after mixing. Changes in the local water distribution in the sample during hydration were evaluated using the subtraction of 3D images obtained by subsequent neutron tomographies (NT). As both water retention in the LWA and its release to the cement paste are influenced by the pore structure of the aggregate, a high-resolution image of the sample was subsequently captured by X-ray tomography. The internal curing water released from the LWA traveled at least 3 mm from the LWA into the cement paste in the first day. Hardly any gradient in the water content of the cement paste against the distance from the LWA was observed. This suggests that the release of water for internal curing (IC) is relatively fast and the water is distributed fairly homogeneously from the LWA for at least 3 mm within the hydrating cement paste.     

Mitigation of autogenous shrinkage in alkali activated slag mortars by internal curing

Alkali activated slag shows considerable promise as an environmentally friendly alternative to binders produced from ordinary portland cement. The shrinkage behavior of alkali activated slags, however, is not well understood, and is a hurdle to widespread adoption. The use of pre-wetted lightweight aggregate-based internal curing to mitigate shrinkage in slags activated by Na₂CO₃ solution or waterglass/NaOH solution has been investigated. Chemical shrinkage measurements were used to determine the amount of additional curing water needed by the mixtures, and autogenous and total shrinkage measurements used to determine the effects of internal curing on the overall shrinkage of the systems. Internal curing can completely mitigate autogenous shrinkage; however, in the systems examined here, drying shrinkage was the dominant shrinkage factor. In such a case, the benefits of internal curing are most clearly observed during the first 7 days.     

Influence of surface charge on ingress of chloride ion in hardened pastes

The amount of free chloride content in concrete is one of major factors in initiating the corrosion process. The material and environmental factors play a key role in diffusing the chloride ion through the cover concrete to reinforcement. Thus, the electrochemical study is indispensable to understand the mechanism of chloride ingress into concrete. Determination of surface charge and its influence on diffusion of chloride ion into cement matrix of concrete are researched for Ordinary Portland Cement (OPC) paste and cement paste containing Ground Granulated Blastfurnace Slag (GGBS). Different kinds of experiments such as measurement of membrane potential, determination of porosity and pore size distribution, determination of pore solution concentration, and steady state diffusion coefficient of chloride and sodium ions are employed to understand the mechanism of chloride ingress. The obtained results show that the positive surface charge on the pore walls of hardened paste regardless of GGBS’s presents. The surface charge of hardened paste mainly depends on pore solution concentration and cement composition. The physiochemical characteristics of the pores are affecting on transporting ions through it. Hardened paste has greater resistance to diffusing sodium ions than chloride ions. Moreover, there is a strong interaction between transport of chloride ion and surface charge in matured hardened paste.     

Efficiency of internal curing by superabsorbent polymers (SAP) in PC-GGBS mortars

This paper evaluates the effect of superabsorbent polymers (SAP) on hydration and microstructure of PC-GGBS mortars. Development of autogenous shrinkage, microstructural characteristics (MIP/SEM) and compressive strength were analysed during the first 90 days. Four levels of Portland cement (PC) replacement by GGBS (0%, 25%, 50% and 75%) and two types of SAP with different water absorption capacities were considered. The results proved the efficiency of internal curing by SAPs in PC-GGBS systems due to significant reduction in autogenous shrinkage, especially for higher contents of GGBS. SAP facilitates GGBS hydration activated by portlandite; its products can be deposited into the nano pores leading to a small relative expansion of the hardened bulk volume. This process is initiated during the second week and it lasts until the sixth week. Despite increased total porosity, compressive strength of SAPs modified mortars is comparable to the reference samples for low GGBS contents in advanced ages.     

Properties of alkali-activated mortar and concrete using lightweight aggregates

This study reports the testing of 12 alkali-activated (AA) mortars and six AA concretes using lightweight aggregates. These tests aimed to explore the significance and limitations of the development of lightweight AA mortar and concrete. Ground granulated blast-furnace slag, which was used as source material, was activated by sodium silicate powder. The main parameter investigated was the replacement level of lightweight fine aggregates to the natural sand. The effect of the water–binder ratio on the compressive strength development was also studied in AA mortars. Initial flow and development of compressive strength were recorded for the lightweight AA mortar. For the lightweight AA concrete, many factors were measured: the variation of slump with elapsed time, the development of compressive strength, splitting tensile strength, moduli of rupture and elasticity, stress–strain relationship, bond strength and shrinkage strain. Test results showed that the compressive strength of AA mortar decreased linearly with the increase of the replacement level of lightweight fine aggregates, regardless of the water–binder ratio. The compressive strength of AA concrete, however, sharply decreased when the replacement level of lightweight fine aggregates exceeded 30%. In particular, the increase in the discontinuous grading of lightweight aggregate resulted in the deterioration of the mechanical properties of AA concrete.     

Improving the structure of the interfacial zone and its influence on the long-term behavior of mortars

The structure of the interfacial zone between cement paste and aggregate was improved by using a pretreated quartz aggregate on whose surface the hydraulic mineral β?C₂S was directly formed. The composition and morphology of the structure of the interfacial zone was examined, and its influence was studied by comparisons between mortars made with pretreated quartz aggregate and with normal quartz aggregate. The results showed that the composition and morphology of the interfacial zone were substantially improved by the hydration of the mineral β?C₂S of the pretreated quartz aggregate. The influence of the interfacial structure on the strength of mortars was related to structural development. It appears to be slight at early hydration, but significant at late hydration.     

Influence of silane coupling agent on quality of interfacial transition zone between concrete substrate and repair materials

Silane coupling agent (SCA) was introduced as a modifying material to significantly improve the bond quality of the repaired interfacial transition zone. SCA aqueous solutions with various concentrations were used to coat the surfaces of a granite and of old concrete substrates before applying the repair materials. Both pull-off bond strength test and microstructure observation of the different repair interfacial layers were performed. The test results show that coating a concrete substrate with a SCA aqueous solution with an appropriate concentration can noticeably modify the microstructure of the interfacial transition zone, and therefore, significantly increase the bond strength.     

预湿轻集料对混凝土内部相对湿度特性的影响

为缓解混凝土的早期自干燥,减小自收缩,用预湿轻集料对其内部相对湿度进行调控.采用湿度测试仪和混凝土收缩测试仪,研究了水胶比、矿物掺合料对混凝土内部相对湿度与自收缩的影响规律,及预湿轻集料对自收缩的抑制效果.研究表明:混凝土内部相对湿度早期下降快,后期下降慢;降低水胶比、掺加硅灰会加快内部相对湿度的下降速度,而粉煤灰的加入可延缓低水胶比混凝土早期内部相对湿度的下降过程.预湿轻集料的引入可对混凝土内部相对湿度起到补偿作用,减小自收缩,作用效果随轻集料引入水量的增大而增强.     

Prediction of chloride ingress and binding in cement paste

This paper summarizes recent work on an analytical model for predicting the ingress rate of chlorides in cement-based materials. An integral part of this is a thermodynamic model for predicting the phase equilibria in hydrated Portland cement. The model’s ability to predict chloride binding in Portland cement pastes at any content of chloride, alkalis, sulfates and carbonate was verified experimentally and found to be equally valid when applied to other data in the literature. The thermodynamic model for predicting the phase equilibria in hydrated Portland cement was introduced into an existing Finite Difference Model for the ingress of chlorides into concrete which takes into account its multi-component nature. The “composite theory” was then used to predict the diffusivity of each ion based on the phase assemblage present in the hydrated Portland cement paste. Agreement was found between profiles for the Cl/Ca ratio predicted by the model and those determined experimentally on 0.45 water/powder ratio Portland cement pastes exposed to 650 mM NaCl for 70 days. This confirms the assumption of essentially instantaneous binding where quasi-equilibrium is established locally. This does not imply steady state diffusion however. It simply implies that incremental increases in the concentration of diffusing ions in the pore solution will rapidly re-equilibrate with the hydrates present locally, where, the greater the ratio of bound to free ions, the greater the buffering effect which slows down the rate of ingress. In the case of chlorides, this buffering effect is greatest at high contents of AFm (AFm is aluminium ferrite compounds with a single (mono) formula unit CaX.) phase and low alkali metal contents.     

Influence of temporal resolution and processing of exposure data on modeling of chloride ingress and reinforcement corrosion in concrete

The impacts of temporal resolution and processing of exposure data on the long-term chloride ingress and reinforcement corrosion in concrete were studied. Exposure data from one simulated and two real climates was processed to create boundary conditions for a one-dimensional geometry studied using a numerical heat and mass transport model that includes full coupling of heat, moisture and ion transport. Heat, moisture, and chloride concentration distributions were passed to a simplified reinforcement corrosion initiation and propagation model. The numerical study indicates that processing and temporal resolution of the exposure data has a considerable impact on long-term hygrothermal distribution, chloride ingress, and reinforcement section loss results. Use of time-averaged exposure data in the heat and mass transport model reduces the rate of chloride ingress in concrete and affects prediction of reinforcement corrosion initiation and propagation. Randomly sampled exposure data at daily, weekly, or monthly resolution yields prediction of reinforcement corrosion initiation and propagation closer to original resolution results than time-averaged exposure data.     

Influence of internal curing and viscosity modifiers on resistance to sulfate attack

Sulfate attack is one of the common degradation mechanisms for concrete in severe environments. While various strategies for minimizing sulfate attack are well recognized, including using an ASTM C150 Type V cement, employing supplementary cementitious materials, and/or reducing water-to-cementitious materials ratio, this paper explores two new approaches for increasing a mortar’s resistance to sulfate attack. In internal curing, fine lightweight aggregates (LWAs) are pre-wetted to provide additional curing water to maximize cement hydration and enhance the microstructure of the interfacial transition zone. The concurrent reductions in connected porosity should contribute to a reduction in the transport rates of sulfate from the environment into the concrete, while the isolated pores present in the LWA may help to accommodate the formation of expansive degradation products, such as ettringite, without creating substantial stresses and subsequent cracking. In the second approach, previously verified for its efficacy to reduce chloride ingress, a viscosity modifier is added to the concrete mixture to increase the viscosity of the pore solution and thus slow down the ingress of sulfates from the environment. While each approach is observed to significantly reduce the measured expansion of mortar bars in standard ASTM C1012 testing, the best performance is observed when the two are combined together by pre-wetting the LWA with a 50:50 solution of the viscosity modifier in water. With the combined approach, the time for the mortar bars to reach a critical expansion level of 0.05 % was over 80 % longer than that measured for the control mortar specimens. The expansion measurements are supported by accompanying measurements of mortar bar mass and surface resistivity throughout their exposure to the sulfate solution, along with micro X-ray fluorescence imaging and X-ray microtomography analysis of specimens extracted from the mortar bars after 9 months of exposure to the sulfate solution.     

偏高岭土-矿渣基地聚物与花岗岩骨料界面的分布特性及影响因素

以碱激发偏高岭土-矿渣作为胶凝材料、花岗岩为骨料制备地聚物混凝土,通过扫描电镜SEM-EDS及显微硬度分析研究地聚物与骨料的界面粘结区的微观结构、分布,以及液固比和骨料尺寸对地聚物-骨料界面的影响。研究结果表明,在地聚物与骨料的界面区域存在界面过渡区(ITZ),包含了以N-A-S-H凝胶为主的固相和收缩裂缝,化学组分与地聚物凝胶有较大不同。界面过渡区沿骨料周围不同位置表现出明显的分布不均匀特性,骨料下缘处的界面过渡区的微观结构和硬度都显著更差。随着液固比及骨料半径的增大,其分布的不均匀性增加:骨料下界面ITZ中的裂缝宽度增大,N-A-S-H凝胶厚度减小且强度降低;但配比及骨料尺寸对骨料上界面及侧界面的ITZ影响并不显著。骨料下界面ITZ应是偏高岭土-矿渣基地聚物混凝土的薄弱区域。     

Modification effects of nanosilica on the interfacial transition zone in concrete: A multiscale approach

The effects of colloidal nanosilica on the interfacial transition zone (ITZ) in concrete at three days are studied. Mechanical properties are investigated at macro-scale, followed by nanoindentation characterization at micro-scale. A top-down and a bottom-up modelling are carried out, respectively, at macro- and micro-scales. Macro-mechanical results show that nanosilica addition is especially beneficial for the improvement of ITZ performance. Estimates from statistical nanoindentation provide evidence, suggesting that the hydration acceleration effect of nanosilica dominates in the modification of ITZ in an early age. It is revealed by modelling at both scale levels that the ratio of the Young's modulus of ITZ to that of bulk paste increases from around 50%–80% if nanosilica is incorporated. This work further confirms that a substantial improvement on ITZ can be obtained by ultra-fine nanosilica modification.     

The role of aggregates on the structure and properties of lime mortars

Lime mortars have been used for centuries in civil engineering construction. Considering ancient monuments and historical buildings it seems that these mortars have proved to be durable and reliable materials although they are of low strength in comparison with cement mortars. Nowadays, they are used for the repair of monuments and for the manufacture of renderings and plasters.

In the present paper the role of aggregates on the structure and behaviour of lime mortars is examined by studying the influence of the aggregate content and the grain size on strength, porosity and volume stability of the mortars. Capillary porosity by suction was also measured as an indicator of resistance to weathering. It is shown that coarse aggregates contribute to the volume stability of lime mortars independent of strength enhancement when adequate compaction reduces the capillary pores. The highest strength values, and consequently, the low porosity, were attained by lime mortars of low binder/aggregate ratio which contained aggregates of maximum size 0–4 mm.