Properties of recycled ceramic aggregate concretes: Water resistance

Water permeability is a durability indicator, for it quantifies concrete resistance to penetration by external agents, due to that water is one of the main carriers of aggressive substances. The present study explores whether substituting 20% and 25% recycled sanitary ware for gravel in coarse aggregate affects structural recycled concrete resistance to water. The findings reveal that the slightly higher porosity in the recycled concrete does not translate into greater permeability. These new recycled concretes, which prove to be as durable as the conventional material, will therefore perform well throughout their design service life.     

Effects of microstructure on restrained autogenous shrinkage behavior in high strength concretes at early ages

Fluorescence microscopic examinations were conducted to identify damages induced by restraining autogenous shrinkage. Characteristics of fluorescent areas and their correspondence to autogeneous shrinkage behavior of high strength concretes were discussed. Silica fume concrete exhibited a greater creep potential when loaded at very early ages. The microstructure in sealed concretes with an extremely low water/binder ratio was porous. The vicinity of aggregate grains was more porous and weaker than the bulk matrix in sealed concretes. In addition, sealed silica fume concretes contained many unhydrated cement particles which were profiled by thin gaps between the core cement particles and the surrounding cement paste matrix. These features of microstructure were not observed in water ponded concretes. The detected fluorescent areas may be defects caused by selfdesiccation and autogenous shrinkage. The flaws had little effects on the development of strength. However, the presence of thin gaps around remnant cement particles may increase creep deformation to relieve internal stresses.     

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.     

Effects on concrete durability of using recycled ceramic aggregates

Ceramic waste from ceramic and construction industries is one of the most important parts in the global volume of construction and demolition waste (CDW). Ceramic waste may have several uses, one of which as coarse aggregate for concrete artefacts. Within a research campaign in course at Instituto Superior Técnico (IST), concerning the reuse and recycling of CDW, the viability of replacing primary limestone aggregates with ceramic waste on the production of concrete pavement slabs has been studied. Compression and bending tests previously performed have shown the mechanical suitability of replacing, at least partially, limestone aggregates with ceramic recycled ones. In this paper, the results of the water absorption tests, either by capillarity or by immersion, and the results of the abrasion resistance tests are presented, all related to long-term concrete durability.     

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%.     

Mechanical behaviour of non-structural concrete made with recycled ceramic aggregates

In order to reduce the volume of ceramic waste from the construction industry, it is possible, among other applications, to use it as aggregates in the production of non-structural concrete artefacts. The main characteristics of such aggregates as well as those of the fresh and hardened concrete made with them (after a standardized pre-saturation procedure) are presented here and compared with those of conventional materials (primary stone aggregates and the concretes made exclusively with them), within a larger experimental investigation to maximize the reuse and reutilization of construction and demolition waste. The results show that there is a potential for the use of these ceramic aggregates in elements in which the primary requirement is not compressive strength but tensile strength and abrasion resistance, such as for concrete pavement slabs.     

An analysis of the shear strength of recycled aggregates

Increasing the use of recycled aggregates in civil engineering applications has the potential to reduce the current natural aggregate consumption requirements of the construction industry. Current levels of natural aggregate production are under threat due to tighter restrictions on planning issues related to quarrying. This paper presents a study on the shear strength of recycled aggregates with relevance to the use of such materials as backfill to structures. The tests were conducted on crushed concrete and demolition debris in a 300 mm×300 mm×179 mm shear box, and the results were compared with those of a limestone aggregate. The paper also includes an investigation into the possible application of an analysis of dilatancy (originally developed for use in soil mechanics) to aggregates and in particular to those aggregates of a soft-grain type such as recycled aggregates.

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Résumé Augmenter l'utilisation des granulats recyclés dans des applications de génie civil pourrait réduire la demande actuelle par l'industrie de construction pour des granulats naturels. Le niveau existant de production des granulats naturels est menacé par des restrictions accrues sur l'exploitation des carrières. On présente ici une étude sur la résistance au cisaillement des granulats recyclés dans le cadre de l'utilisation des ces matériaux comme remblayage d'ouvrages. Les essais ont été effectués sur du béton concassé et des débris de démolition dans une éprouvette pour essai de cisaillement (300 mm×300 mm×179 mm). Les résultats ont été comparés avec ceux obtenus pour un granulat calcaire. On présente également une investigation sur la possibilité de l'appliquer une analyse de dilatance (élaborée à l'origine pour étudier la mécanique des sols) aux granulats, et en particulier, aux granulats à grains tendres tels que les granulats recyclés.

     

Colemanite ore waste concrete with low shrinkage and high split tensile strength

A laboratory study was undertaken to assess the compressive and cylinder splitting tensile strength and drying shrinkage properties of concrete mixtures containing colemanite ore waste (CW). Possibility of using Kütahya–Emet CW in cement based materials as a shrinkage reducing ingredients was also discussed. Five concrete mixtures including Portland cement and CW concrete mixtures were prepared. The compressive strengths of concrete mixtures were measured at 7, 28, 56 and 90 days. The splitting tensile strength was measured at 28 days. The strength results showed that concrete mixtures containing 3 and 5% CW developed higher strength values than control concrete. The test results also showed that Kütahya–Emet CW reduced drying shrinkage of the mortar by 37% when compared to control concrete. Based on these results, it was concluded that Kütahya–Emet CW can be used as a cost-effective shrinkage-reducing agent.     

Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates: durability performance

The growing difficulty in obtaining natural coarse aggregates (NCA) for the production of concrete, associated to the environmental issues and social costs that the uncontrolled extraction of natural aggregates creates, led to a search for feasible alternatives. One of the possible paths is to reuse construction and demolition waste (CDW) as aggregates to incorporate into the production of new concrete. Therefore, a vast and detailed experimental campaign was implemented at Instituto Superior Técnico (IST), which aimed at determining the viability of incorporating coarse aggregates from concrete and ceramic brick wall debris, in the production of a new concrete, with properties acceptable for its use in new reinforced and pre-stressed structures. In the experimental campaign different compositions were studied by incorporating pre-determined percentages of recycled coarse concrete aggregates and recycled coarse ceramic plus mortar particles, and the main mechanical, deformability and durability properties were quantified, by comparison with a conventional reference concrete (RC). In this article, these results are presented in terms of the durability performance of concrete, namely water absorption, carbonation and chlorides penetration resistance.     

Tensile creep behavior of high strength concretes at early ages

Early age tensile creep of high strength concrete loaded during the first day after casting was studied. The effect of silica fume was evaluated, using two types of loading scheme: (i) creep which occurs in a restrained autogenous shrinkage test and (ii) creep measured in a conventional test under constant load. The two methods showed that the addition of silica fume increased the early age tensile creep of high strength concrete, when compared to concrete without silica fume but with similar water/binder ratio. This enhanced creep is of practical significance as it provides a mechanism to relieve some of the restraining stress that develops due to autogenous shrinkage.

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Résumé Dans cet article, on a étudié le fluage en traction d'un béton de haute résistance, pendant le premier jour suivant sa fabrication. On a caractérisé l'effet de la fumée de silice, à partir de deux modes de chargement: (1) fluage dans le cas d'un essai en retrait intrinsèque empêché et (2) fluage mesuré dans un essai conventionnel sous charge constante. Les deux méthodes indiquent que l'addition de fumée de silice augmente le fluage initial d'un béton de haute résistance, par rapport à celui d'un béton de même rapport eau/liant sans fumée de silice. Cette augmentation du fluage est d'une signification pratique, en fournissant un mécanisme qui compense en partie les contraintes générées par le retrait intrinsèque.

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Editorial Note Dr. K. Kovler and Prof. A. Bentur are RILEM Senior Members. Technion is a RILEM Titular Member. Prof. Amon Bentur is a Fellow of RILEM and a member of the RILEM Coordinating Committee. He is the Chaiman of TC 159-ETC: ‘Engineering of the interfacial transition zone in cementitious composites’ (co-chaired with Prof. Alxander) and of TC EAS: ‘Early age shrinkage induced stresses and cracking in cementitious systems’. Dr. K. Kovler is a Member of the TC EAS and of TC 162-TDF: ‘Test and design methods for steel fiber reinforced concrete’.     

Influence of mix proportions and curing conditions on tensile splitting strength of high strength concretes

The effect of the composition of high strength concretes with low water to binder ratio and silica fume on the development of splitting tensile strength was studied. A statistical approach was employed to develop formulation which could adequately describe the relations between splitting tensile strength and the concrete composition, when cured in two different regimes: water curing at 20°C and sealed curing at 30°C. Autogenous shrinkage was induced in the second type of curing but was largely eliminated in the first one. The relations were presented as nomograms which could be used as a basis for mix design. The correlation between tensile splitting strength and compressive strength could not be described in terms of a simple linear relation with a characteristic constant. For the range of variables studied, the ratio between tensile and compressive strength varied over a large range of 0.08 to 0.12. As a result, the relations developed here for tensile strength are quite different in nature than those for compressive strength in a previous study. Analysis of the data suggest that tensile strength is sensitive to effects which induce autogenous shrinkage to a much greater extent than compressive strength. It is proposed that this may be the main reason for the different trends observed for the relations between the composition of the low water/binder ratio concretes and their compressive and tensile strength.     

Pore characterization of manufactured aggregates: recycled concrete aggregates and lightweight aggregates

Accurate characterization of aggregates plays an important role in mixture proportioning of concrete mixes. Decisions made during the concrete design phase in terms of characterization techniques adopted for quantification of specific gravity or water absorption affect the development of fresh properties during the construction phase as well as impacting the long term performance of concrete. Manufactured aggregates such as recycled concrete aggregates (RCA) and lightweight aggregate (LWA) are more absorptive than natural aggregate. Due to the thrust on construction of sustainable structures usage of LWA and RCA has increased significantly in the last decade. In this study, standard ASTM techniques adopted for porosity and specific gravity measurement of aggregates were compared with automated testing equipment such as the helium pycnometer and the envelope density analyzer. Porosity of different aggregates obtained using different test methods was compared with image analysis. Pore diameter characterization of different RCA was conducted using image analysis.     

Practical prediction of creep and shrinkage of high strength concrete

Model for practical prediction of creep and shrinkage of normal strength concrete, developed previously, is extended to high strength concrete. It is found that only a minor adjustment for the concrete strength effect is needed in the formulas for drying creep. The formulas for basic creep and shrinkage need no adjustment. The prediction model is compared with test data for creep and shrinkage obtained recently by Ngab, Nilson and Slate, and by Collepardi, Corradi and Valente, and a satisfactory agreement is demonstrated. The coefficient of variation of the deviations from test data is not larger than that for the normal cient of variation of the deviations from test data is not larger than that for the normal strength range. However, the existing data are rather limited and further testing is desirable.     

Effect of polypropylene fibers on shrinkage and cracking of concretes

The effects of admixed polypropylene (PP) fibers on the drying shrinkage of hardened concrete are presented in this paper. Concrete mixtures made with Ordinary Portland cement (OPC) and OPC/Slag blended cements containing various volume fractions of PP fiber were tested. The results show small but consistently higher drying shrinkages in concretes incorporating PP fibers than that without fiber. The effect is more pronounced in slag concretes and in concretes cured for only 1 day. An attempt to explain this phenomenon was made by water loss, nitrogen adsorption, sorptivity and scanning electron microscopy tests on the same concretes. Additional moisture loss and porosity are proposed as possible reasons. The results of early-age restrained shrinkage tests on slag concretes show that PP fiber concrete had higher cracking tendency than the concrete without fiber. This was found to be due to higher shrinkage and elastic modulus of PP fiber concrete.     

Early shrinkage of cement pastes,mortars, and concretes

In the contribution the authors present measurements of volume changes for cement pastes, mortars, and concretes in an early stage of their hardening. The proposed method makes it possible to measure volume changes from the first setting phase of the measured sample. A contact measurement is made with the aid of inductance transducers. The values of volume changes of the mixture are transmitted by the aid of built-in probes. The temperature of the mixture and of the ambient air was measured by resistance thermometers. All measured values were investigated in set up time intervals by the aid of a measuring station. The investigated mixtures were made of type SC 70 cements for concrete pavements. The composition of the various mixtures is given in the paper. Measurements were carried out at a temperature of 20±1°C and a relative humidity of 50±3%. Most investigations lasted 24 to 48 hours. The measurement results are given in the enclosed illustrations.     

The effects of fibres on the plastic shrinkage cracking of high strength concrete

Plastic shrinkage cracking in concrete usually occurs during the first 5 hours after placing and therefore the mechanics of fibre reinforcement were studied during this period. Two types of polypropylene fibres were mixed at 0.1% by volume. The development of bond strengths and the stresses which the fibres could sustain across cracks were measured by uniaxial tensile tests during the first 5 hours after mixing. Fibre stresses up to 130 MPa at 5 hours were achieved which were equivalent to a composite post-crack strength of 65 kPa. Restrained ring tests were used to assess the amount of cracking which occurred during the first 24 hours in a different high strength mix and, although the results were very variable, the fibres were found to reduce the crack area by between 40% and 85% compared with plain concrete, depending on fibre type.     

Influence of recycled aggregates and high contents of fly ash on concrete fresh properties

This study presents the fresh properties of concrete with supplementary cementitious materials (SCM) and recycled concrete aggregates (RCA), with emphasis on the feasibility of using high volumes of fly ash (FA) in RCA concrete. For this purpose, two mix families (0% coarse RCA and 100% coarse RCA) were produced, both with and without superplasticizers (SP). The coarse natural aggregates (NA) were replaced with coarse RCA at 0% and 100%, respectively. For each of the mentioned families, three incorporation levels (0%, 50% and 100%) of fine RCA were used with 0%, 30% and 60% of FA, resulting in 28 compositions. Each mix was tested in the fresh state by means of slump, density and air content. The results of this study show that RCA decreased the slump of concrete mixes, but the required water content can be minimized by incorporation FA. Regardless of the water absorption of the aggregates, for a given fine RCA incorporation ratio and the same ratio of FA, no increase in water content is required to obtain the same target slump as in the reference concrete. On the other hand, for a given coarse RCA incorporation ratio, a five times lower FA ratio is enough to obtain the same target slump as in the reference concrete. Air voids in concrete mixes were more affected by the shape of the aggregates than by their water absorption. The air content of concrete mixes increased as the incorporation levels of FA and RCA increased. However, in comparison with the individual effects, the air content decreased by combining the incorporation of both FA and RCA. Moreover, the rate of reduction in fresh density by increasing the incorporation of RCA and FA was similar in concrete mixes with and without SP.     

基于SLS的高强度低密石墨陶瓷复合隔热材料快速制备

本文率先利用选择性激光烧结技术快速制备了高强度石墨陶瓷复合隔热材料,重点研究了二次固化、真空压力浸渍、碳化和高温烧结等后处理工艺以及材料配方组成对其密度、抗压强度和导热系数的影响。研究发现加入适量的硅粉和可膨胀石墨可以对石墨陶瓷复合隔热材料的密度、抗压强度和导热系数进行调控,采取合适后处理工艺路线可以改变石墨陶瓷复合隔热材料的综合性能。最终实现了低密度(<1.2 g/cm^3)、高抗压强度(>10 MPa)、低的导热系数(<2 W/(m·K))和耐高温(>1650℃)等多个性能指标的统一,满足了工业应用需求。