Effect of supplementary cementitious materials on shrinkage and crack development in concrete

The autogenous and drying shrinkage of Portland cement concrete, and binary and ternary binder concretes, were measured and compared. The binary and ternary binder concretes were formed by replacing part of the cement with fly ash, very fine fly ash and/or silica fume. Restrained shrinkage test was also performed to evaluate the effect of binder type on early age cracking. After the cracking of the restrained ring samples, crack widths were measured and compared with the results of an R-curve based model, which takes post-peak elastic and creep strains into account.The incorporation of fly ash and very fine fly ash decreased the autogenous shrinkage strain but increased the drying shrinkage strain. Since the total shrinkage strains of both the ternary and the binary concrete mixtures were similar, the strength development became an important factor in the cracking. The lower strength of the concrete with ternary binders led to earlier cracking compared to the binary binder concrete. Portland cement concrete cracked the earliest and had the greatest crack width. Measured crack widths were in accordance with the crack widths calculated with the R-curve model.     

Evaluating the use of supplementary cementitious materials to mitigate damage in cementitious materials exposed to calcium chloride deicing salt

This paper discusses the role of supplementary cementitious materials (SCM) in reducing damage caused by calcium oxychloride formation. Calcium oxychloride is a destructive product of a reaction between calcium hydroxide (CH) that exists in a cementitious matrix and CaCl₂ that can enter the pores of the matrix when it is used as a deicing salt. Paste samples were prepared where a percentage of ordinary portland cement was replaced with various types of SCM (including fly ash, slag, and silica fume). This paper examined the amount of calcium oxychloride that formed using low-temperature differential scanning calorimetry, and damage development detected using acoustic emission. Thermogravimetric analysis was also performed to determine the relationship between the amount of CH in cementitious materials and the amount of calcium oxychloride formation. The results show that the use of SCM is effective in reducing the calcium oxychloride formation and resulting damage when cementitious materials are exposed to various compositions of solution containing CaCl₂. The explanation of the benefit of using SCM is that it can reduce the calcium oxychloride formation due to a reduction in the amount of CH in the cementitious materials through pozzolanic reaction and dilution of cement. As a result, cementitious materials with SCM exposed to CaCl₂ may experience less damage and have a longer service life.     

Thixotropy and structural breakdown properties of self consolidating concrete containing various supplementary cementitious materials

In this study, thixotropy and structural breakdown of 57 self-consolidating concrete (SCC) mixtures containing various supplementary cementitious materials (SCM) were investigated by different approaches. The effects of SCM type and content on high range water reducer demand and plastic viscosity were also studied. For these purposes, various amounts of silica fume (SF), metakaolin (MK), Class F fly ash (FAF), Class C fly ash (FAC) and granulated blast-furnace slag (BFS) were utilized in binary, ternary, and quaternary cementitious blends in three water/binder (w/b) ratios. Results showed that except BFS, use of SCM in SCC mixtures increased thixotropy values in comparison with the mixtures containing only portland cement (PC). Good correlations were established between structural breakdown area and drop in apparent viscosity values for all w/b ratios. The different methods used to evaluate the thixotropy and structural breakdown got more consistent with each other as w/b decreased.     

Effects of deicers on the performance of concrete pavements containing air-cooled blast furnace slag and supplementary cementitious materials

This study investigates the effects of continuous deicer exposure on the performance of pavement concretes. For this purpose, the differences in the compressive strength, the changes in the dynamic modulus of elasticity (DME) and the depth of chloride ingress were evaluated during and after the exposure period. Eight different concrete mixtures containing two types of coarse aggregates (i.e. air-cooled blast furnace slag (ACBFS) and natural dolomite) and four types of binder systems (i.e. plain Type I ordinary portland cement (OPC) and three combinations of OPC with fly ash (FA) and/or slag cement (SC)) were examined. These mixtures were exposed to three types of deicers (i.e. MgCl₂, CaCl₂, and NaCl) combined with two different exposure conditions (i.e. freezing-thawing (FT) and wetting-drying (WD)). In cold climates, these exposure conditions are the primary durability challenges that promote the physical deterioration of concrete pavements. The results indicated that among the studied deicers, CaCl₂ had the most destructive effect on the tested concretes while NaCl was found to promote the deepest level of chloride ingress yet was shown to have the least damaging impact on concretes. The microstructure evaluation revealed that the mechanism of concrete deterioration due to the deicer exposure involved chemical reactions between the deicers and concrete hydration products. The use of FA or SC as partial replacements for OPC can offset the detrimental effects of both deicers and FT/WD cycles.     

Sulfate resistance of Portland-limestone cements in combination with supplementary cementitious materials

In this study, the sulfate resistance of five different high-C₃A Portland and Portland-limestone cements and their combinations with 30–50 % slag were examined at both 5 and 23 °C according to CSA A3004-C8 (similar to ASTM C1012). Also, XRD was used to identify the phases formed after sulfate attack. It was found that in 23 °C exposure, while 100 % cement mixes deteriorated due to conventional ettringite-based sulfate attack, partially replacing the cements with 30 or 50 % slag was effective in making the mixes highly sulfate-resistant. At 5 °C, all of the 100 % cement mortar bars expanded more than the test limits and eventually completely disintegrated due to the formation of thaumasite. Partially replacing cement with 30 % slag was only effective in controlling the deterioration for Portland cements but not Portland-limestone cements. However, all the Portland-limestone cements with 50 % slag were resistant to the thaumasite form of sulfate attack after 2 years.     

Comparison of ground waste glass with other supplementary cementitious materials

Finely ground glass has pozzolanic properties that make attractive its recycling as supplementary cementitious material. This paper compares the behaviour of waste glass powders of different fineness with that of natural pozzolana, coal fly ash and silica fume. Chemical analysis, compressive strength measurements and durability tests were carried out to investigate the effect of ground glass on strength and durability performances of mortars. Blended both with Portland cement and lime, ground glass improved strength, resistance to chloride penetration and resistance to sulphate attack of mortars more than natural pozzolana and similarly to fly ash. Mortars with ground glass immersed in water for seven years did not show any sign of degradation and increased their compressive strength. The ranking of ground glass with respect to the other mineral additions was not affected by fineness.     

Mechanical threshold of cementitious materials at early age

At early age, the mechanical characteristics of concrete, such as Young's modulus, follow a rapid rate of change. If strains are restricted or in the event of strain gradients, tensile stresses are generated and there is a risk of cracks occurring. Besides relaxation, change in Young's modulus as a function of the degree of hydration is a major parameter for the modeling of this phenomenon. In this evolution, a threshold of the degree of hydration has to be taken into account, below which concrete displays negligible stiffness. For cement pastes, a simplified hydration model shows that percolation of the solid phases depends on the w/c ratio, which is in accordance with experimental results. On the other hand, for mortar or concrete, the presence of aggregates means that the solid volumetric fraction is such that percolation is observed before hydration occurs. Therefore another parameter is introduced: cohesion due to hydration products. By coupling our model with a finite-element code (CAST3M), it is shown that the threshold for Young's modulus in mortar is almost independent of the w/c ratio, which is in accordance with experimental results.     

The effect of zinc oxide additions on the performance of calcined sodium montmorillonite and illite shale supplementary cementitious materials

The objectives of this study were to use activation treatments on sodium montmorillonite and illite shale, to alter early hydration or later pozzolanic reactivity when used as supplementary cementitious materials (SCMs). For comparison purposes, treatment methods were also applied to the highly reactive pozzolan, metakaolin, and the inert filler, quartz. Activation treatment strategies included the addition of 0.15 wt% zinc oxide and the use of thermal treatments to the SCMs at temperatures of 650 °C, 830 °C and 930 °C. The use of zinc oxide additions increased the early hydration rate of SCM-containing pastes, yet introduced a chemical retardation and negatively impacted early compressive strengths. Moreover, the results suggest that retardation was inversely correlated with the pozzolanic reactivity of the SCM used. Thermal treatment methods were effective at influencing the SCM pozzolanic reactivity, with montmorillonite calcined at 830 °C and illite calcined at 930 °C behaving as late-reacting pozzolans. SCMs calcined at these temperatures resulted in higher 90 day compressive strengths compared to mortars containing the quartz filler. Overall, this study provides insight into different strategies that maybe used to enhance the reactivity of impure calcined clays in order to facilitate their acceptance into the concrete industry.     

Properties of high performance concrete containing fine-ground ceramics as supplementary cementitious material

This paper presents experimental work regarding the basic physical characteristics, mechanical and fracture-mechanics properties, durability characteristics, hydric and thermal properties of high performance concrete (HPC) with up to 60% of Portland cement replaced by fine-ground ceramics. Experimental results show that the amount of the ceramics in the mix is limited mainly by the resistance against de-icing salts which is found satisfactory only up to the cement replacement level of 10%. The mechanical and water transport properties are not significantly impaired by ceramic additions of up to 20%, whereas the effective fracture toughness, specific fracture energy, and chemical resistance (to MgCl₂, NH₄Cl, Na₂SO₄, HCl) are effectively maintained up to 40%. The frost resistance, water vapor transport and storage parameters and thermal properties are not significantly impaired even up to a 60% replacement level.     

Examining the pozzolanicity of supplementary cementitious materials using isothermal calorimetry and thermogravimetric analysis

It has recently been proposed that the pozzolanicity of supplementary cementitious materials can be determined by monitoring the heat released when supplementary cementitious materials are mixed with calcium hydroxide at high temperature and high pH. In this study, the heat release is measured using this procedure for a variety of different supplementary cementitious materials. In addition, thermogravimetric analysis is performed on the reacted material to determine the amount of calcium hydroxide consumed. The heat release and calcium hydroxide consumption can be used in conjunction to compare supplementary cementitious materials. Calcium hydroxide consumption can be used to determine the extent of reaction of supplementary cementitious materials in pastes where supplementary cementitious materials are used to replace a portion of cement.     

Greek supplementary cementing materials and their incorporation in concrete

Sustainable development of the building sector can be achieved by a significant incorporation of cementitious and pozzolanic by-products, such as fly ash and slag, as well as some natural pozzolanic materials (supplementary cementing materials––SCM). Millions of tons of SCM, especially in Greece, are dumped due to overproduction or non-conformity with the existing standards. In this work, various types of SCM produced in Greece are investigated for a potential use in concrete. Their behavior as regards strength and durability is approached by a practical efficiency factor (k-value). The work is further focused on fly ashes, as they constitute the vast and more active majority of Greek SCM. The effectiveness of the Greek fly ashes in concrete improvement is widely proved through strength and durability measurements at laboratory and pilot-plant scale. However, they contain high amounts of free lime and sulphates, which may cause durability problems. In addition, they exhibit inadequate fineness and composition variations. Thus, an appropriate treatment is required and proposed by the present study in order to eliminate or reduce any harmful factors. Finally, a mix design strategy ensuring optimum strength, durability and ecological profile is proposed.     

Flexural behavior of reinforced concrete beams with cementitious repair materials

There is an urgent need to develop efficient methods for the repairs and rehabilitation of currently existing structures, they are being deteriorated over time, and also the magnitude of loadings keeps rapidly increasing for such structures. Possibly one of the most challenging taks in the rehabilitation processes is to upgrade the overall capacity of deteriorated concrete structures. Recently, considerable efforts are being directed toward developing new construction materials. This paper presents the experimental study for the flexural behavior of reinforced concrete beams repaired by Polymer Cement Mortar (PCM) and Ordinary Portland Cement Mortar (OPCM) in the tension region. Tests were performed for eight reinforced concrete beams with varying reinforcement ratios, repair materials and repair lengths. Emphasis is given to overall bending capacity, deflection, ductility index, failure mode and crack development of repaired beams. The results are compared with those from the control beam.

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Résumé Il est urgent de développer une méthode efficace pour le renforcement et la réparation afin de remettre en état la structure dont le fonctionnement est dégradé en raison de l'accroissement de la charge ou de l'écoulement du temps. Au cours du processus de renforcement, un des paramètres les plus importants est d'améliorer la performance globale des structures en béton endommagées. La tendance d'aujourd'hui est de développer un nouveau matériau. Dans cet article sont menées des études expérimentales sur la flexion de poutres en béton armé réparées ou renforcées avec du mortier de ciment polymère (MCP) et du mortier de ciment portland ordinaire (MCPO) repectivement. Pour des essais, 8 échantillons d'essais ont été fabriqués en variant le rapport d'armature, le matériau de réparation, ainsi que la longueur à réparer, avant de faire les essais. Les résultats confirment: la capacité de la flexion, une flèche, un indice de la ductilité, une rupture apparente, la forme de la fissure des poutres réparées. Et ces résultats ont été comparés avec celui de l'échantillon de référence.

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Size effect and continuous damage in cementitious materials

The dependence between the mechanical properties of concrete structures and their sizes is a problem addressed in this paper. Two types of size effect are distinguished: the first one is probabilistic and is related to random distributions of defects in a volume of material; the second one is purely deterministic and is related to fracture propagation in brittle heterogeneous media. These two aspects are combined in a continuous damage model. The initiation of damage is probabilistic and the evolution of damage is nonlocal. Comparisons with experiments on notched and unnotched bending beams show that the model provides a realistic prediction of the various size effects.     

Decoupling the physical and chemical effects of supplementary cementitious materials on strength and permeability: A multi-level approach

Supplementary cementitious materials (SCMs) represent an alternative for the industry to achieve sustainability by reducing cement contents without significant compromises of the mechanical properties and enhancing durability. SCMs play a dual role during hydration: a physical effect promoting nucleation and cement hydration and a chemical effect through pozzolanic activity. Rice husk ash (RHA) and natural pozzolans (NP) were evaluated using compressive strength and durability tests in a multi-level experimental program. RHA increased the strength more than NP, which is well explained by its prominent chemical effect (78%) assessed by isothermal calorimetry and its high amorphous silica content. Both RHA and NP produced significant reductions in the permeability of the concrete, which is mostly explained by the chemical effect. Decoupling the physical and chemical effects of a SCM allows for optimisation of its manufacturing process.     

Micro-structural development of gap-graded blended cement pastes containing a high amount of supplementary cementitious materials

To clarify the strength improvement mechanism of gap-graded blended cements with a high amount of supplementary cementitious materials, phase composition of hardened gap-graded blended cement pastes was quantified, and compared with those of Portland cement paste and reference blended cement (prepared by co-grinding) paste. The results show that the gap-graded blended cement pastes containing only 25% cement clinker by mass have comparable amount of gel products and porosity with Portland cement paste at all tested ages. For gap-graded blended cement pastes, about 40% of the total gel products can be attributed to the hydration of fine blast furnace slag, and the main un-hydrated component is coarse fly ash, corresponding to un-hydrated cement clinker in Portland cement paste. Further, pore size refinement is much more pronounced in gap-graded blended cement pastes, attributing to high initial packing density of cement paste (grain size refinement) and significant hydration of BFS.     

Applicability of rheological models to high-performance grouts containing supplementary cementitious materials and viscosity enhancing admixture

The knowledge of yield stress and plastic viscosity of cement-based materials is of special interest in various applications, including consolidation grouting, post-tensioning systems, and for numerical simulations. These rheological parameters are generally estimated from the shear stress-shear rate data using an empirical model. Highly pseudoplastic systems may not be adequately modeled using flow models typically used for conventional grouts. This paper summarizes the results of a study undertaken to evaluate the applicability of a number of analytical models to fit experimental data obtained on cement grout. In total, 44 grouts containing silica fume replacements of 1.5%, 3%, and 5% by mass of cementitious materials, blast furnace slag substitutions of 20% and 40%, and various combinations of high-range water-reducer and viscosity-enhancing admixture were evaluated. All mixtures were prepared with 0.40 water-cementitious material ratio.Test results highlight the difficulties encountered when using conventional models to fit flow data of highly pseudoplastic mixtures of low yield stress. This is true for mixtures incorporating a viscosity-enhancing admixture, especially when combined with low dosage of high-range water-reducer. In general, the Herschel-Bulkley, Robertson, De Kee, and Casson models were found to be adequate for use with highly pseudoplastic grouts. A new model is proposed to provide better fitting of rheological profiles of highly flowable, yet stable, pseudoplastic mixtures that exhibit particularly low yield stress values.