Influence of recycled aggregate on slump and bleeding of fresh concrete

The recycling of construction and demolition (C&;D) waste as a source of aggregates for the production of new concrete has attracted increasing interests from the construction industry. While the environmental benefits of using recycled aggregates are well accepted, some unsolved problems prevent this type of material from wide application in structural concrete. One of the major problems with the use of recycled aggregates in structural concrete is their high water absorption capacity which leads to difficulties in controlling the properties of fresh concrete and consequently influences the strength and durability of hardened concrete. This paper presents an experimental study on the properties of fresh concrete prepared with recycled aggregates. Concrete mixes with a target compressive strength of 35 MPa are prepared with the use of recycled aggregates at the levels from 0 to 100% of the total coarse aggregate. The influence of recycled aggregate on the slump and bleeding are investigated. The effect of delaying the starting time of bleeding tests and the effect of using fly ash on the bleeding of concrete are explored.     

Tensile characteristics and fracture energy of fiber reinforced and non-reinforced ultra high performance concrete (UHPC)

In the framework of this study, various mixtures of fiber reinforced and non-reinforced ultra high performance concrete (UHPFRC and UHPC) were produced and tested with focus on the determination of the fracture energy and its comparison to standard mechanical material parameters. For some mixtures a compressive strength of more than 300 MPa was reached still retaining good fresh characteristics of the UHPC. These mixtures were examined for properties of fresh and hardened concrete, focusing on tensile strength properties and fracture energy. The fracture energy was determined to describe the work capacity, i.e. the potential energy intake until the failure of the material. Thereby, a significant increase of the work capacity could be achieved by the addition of steel fibers. Furthermore, the impact of a vacuum treatment of the freshly mixed concrete in regard to fresh and hardened concrete characteristics as well as the influence of aftertreatment (heat treatment and water storage) on compressive and tensile properties of the UHPC was investigated.     

The behavior of self-compacting concrete containing micro-encapsulated Phase Change Materials

In order to come to a sustainable built environment the construction industry requires new energy saving concepts. One concept is to use Phase Change Materials (PCM), which have the ability to absorb and to release thermal energy at a specific temperature. This paper presents a set of experiments using different amounts of PCM in self-compacting concrete mixes. The study focuses on the direct mixing of micro-encapsulated PCM with concrete and its influence on the material properties. Therefore, the fresh concrete properties and the hardened properties are investigated. The hardened properties comprise strength tests and a thorough assessment of the thermal properties. It will be shown that increasing PCM amounts lead to lower thermal conductivity and increased heat capacity, which both significantly improve the thermal performance of concrete and therefore save energy. On the other hand a significant loss in strength and micro-structural analysis both indicate that a large part of the capsules is destroyed during the mixing process and releases its paraffin wax filling into the surrounding matrix. However, the compressive strength of our specimens still satisfies the demands of most structural applications.     

Self compacting concrete from uncontrolled burning of rice husk and blended fine aggregate

This paper presents an experimental study on the development of normal strength Self compacting concrete (SCC) from uncontrolled burning of rice husk ash (RHA) as a partial replacement to cement and blended fine aggregate whilst maintaining satisfactory properties of SCC. Experiments on the fresh and hardened state properties have been carried out on RHA based SCC from uncontrolled burning. The dosages of RHA are limited to 0%, 20%, 30% and 40% by mass of the total cementitious material in the concrete. The experiments on fresh state properties investigate the filling ability, the passing ability and the segregation resistance of concrete. The experiments on hardened state properties investigate the compressive and the splitting tensile strengths. The water absorption level of the concrete with changing RHA levels has also been monitored. The experimental studies indicate that RHA based SCC developed from uncontrolled burning has a significant potential for use when normal strength is desired.     

The behaviour of concrete in hot climates

The paper, after very briefly discussing the classification of world climates, considers hot climates before taking up a review of papers dealing with the effects of hot climates on the properties of both fresh and hardened concrete. Raw materials for concrete are discussed, where the adverse and positive effects of C₃A and gypsum contents of Portland cement, thermal movement of concrete and sulphate and chloride contents of aggregate and mixing water in hot climates are pointed out. The effect of elevated temperature, ambient and initial paste temperature, ambient relative humidity, solar radiation, and water/cement ratio on the rate of Portland cement hydration and hydration product structure are discussed. The behaviours of both fresh and hardened concrete are taken up. The effect of evaporation from a fresh concrete surface on the behaviour of hardened concrete is discussed. The paper concludes that the presently published works on the properties of concrete in hot climates are fragmentary, uncoordinated and at times contradictory. It points out that systematic studies of the properties of concrete cast and continuously exposed to either hot-humid or hot-dry climates remain to be done.     

Plastic viscosity of fresh concrete – A critical review of predictions methods

Rheological properties of fresh concrete, namely plastic viscosity and yield stress, are critical for the concrete industry because they affect placement and workability. Moreover, these rheological properties influence the productivity and quality of concrete, including mechanical properties and durability. Therefore proper characterization of these properties is needed to control the quality of fresh concrete and ensure sustainability of concrete structures.Fundamental and phenomenological rheological models have been proposed in the literature for characterizing the behaviour of fresh concrete. Establishing a model for predicting the plastic viscosity of concrete based on its composition will be extremely valuable for the concrete industry. This paper provides a critical review of the most prevailing models in concrete technology as well as models proposed in the literature for predicting the plastic viscosity of dense suspensions to a total of eight models. Review has revealed that Mahmoodzadeh and Chidiac models based on the cell method provides a higher degree of correlation to the experimental data as well as a more consistent and reliable predictions in comparison to the models currently proposed in the literature for concrete and/or dense suspensions.     

Properties of controlled low-strength materials incorporating cement kiln dust and slag

This research evaluated the potential use of cement kiln dust (CKD) together with slag to replace the use of cement in the production of controlled low-strength material (CLSM). The low strength requirements of CLSM compared to conventional concrete enable the use of industrial by-products for the production of CLSM. In this study, the workability-related fresh properties of CLSM mixtures were observed through slump flow diameter, V-funnel flow time and filling capacity. Setting times, temperature rise, air content and unit weight of CLSM mixtures were also determined as part of fresh properties. The hardened properties that were monitored for 28 days included the unconfined compressive strength. The test results presented herein show that a combination of less than 50 kg/m³ slag and up to 300 kg/m³ CKD provides a good mix that satisfies the requirements of a CLSM with similar or better properties to that of CKD-based CLSM mix containing Portland cement. Suitable CLSM mixtures with reasonable fresh and hardened properties could also be developed by using CKD alone. However, reduced strength in such CLSM mixtures may limit their field application. The slag significantly assisted in increasing compressive strength of CKD-based CLSM mixtures. A CLSM mix containing a combination of slag and CKD was shown to have excellent characteristics for flowable backfill and excavatable base material. Therefore, producing CKD/slag based CLSM through the use of co-generated products from the cement and iron manufacturing processes can provide leadership for the construction industry in the transition for sustainable development.     

Effect of prewetting methods on some fresh and hardened properties of concrete with pumice aggregate

One of the major problems in lightweight aggregate concrete production is the high water absorption characteristic of the aggregates due to their porous structure. This problem is usually overcome by prewetting the lightweight aggregates or increasing the amount of mixing water. Since aggregate prewetting methods significantly affect fresh and hardened lightweight concrete properties, it is important to take this into account before the concrete production process.This study is focused on the effects of three prewetting methods on some fresh and hardened properties of pumice lightweight concrete. Pre-soaking, water-soaking and vacuum-soaking methods were applied to pumice lightweight aggregate prior to mixing. Test results showed that fresh and hardened properties of concretes with vacuum-soaked and water-soaked lightweight aggregate were significantly better than that of concretes with pre-soaked lightweight aggregate. Vacuum-soaking and water-soaking of pumice aggregate improved workability, compressive strength and drying shrinkage of pumice lightweight concrete.     

Linking fresh and durability properties of paste to SCC mortar

In the last years many approaches to design SCC have been developed, but it remains a very complex process since it is necessary to manipulate several variables and understand their effects on concrete behaviour (fresh and hardened state). The prediction of concrete or mortar behaviour based on paste properties will be a significant contribution to simplify SCC design. With this purpose, two statistical experimental designs were carried out, one at paste level and the other at mortar level, to mathematically model the influence of mixture parameters on fresh and durability properties. The derived numerical models were used to define an area, labelled by self-compacting zone at paste level (SCZ), where fresh properties of the paste enable the design of SCC mortar. Furthermore, in order to extend this link to durability properties, the effect of including aggregate in cement paste was evaluated by means of the electrical resistivity test.     

A comprehensive investigation into the effect of aging and coarse aggregate size and volume on mechanical properties of self-compacting concrete

The popularity of self-compacting concrete (SCC), as an innovative construction materials in concrete industry, has increased all over the world in recent decades. SCC offers a safer construction process and durable concrete structure due to its typical fresh concrete behavior which is achieved by SCC’s significantly different mixture composition. This modification of mix composition may have significant effect on the hardened mechanical properties of SCC as compared to normal vibrated concrete (NVC). Therefore, it is necessary to know whether the use of all rules and relations that have been formulated for NVC in current design codes based on years of experience are also valid for SCC. Furthermore, this study represents an extensive evaluation and comparison between mechanical properties of SCC using current international codes and prediction equations proposed by other researchers. Thus, in this experimental study, major mechanical properties of SCC are investigated for twelve SCC mixes with wide spectrum of different variables i.e. maximum coarse aggregate size, coarse aggregate volume and aging. In the present study, an extensive body of data reported by many researchers for SCC and NVC has been used to validate the obtained results.     

Mix design for self-compacting palm oil clinker concrete based on particle packing

The consumption of waste materials in self-compacting concrete (SCC) in the construction industry will not only help to conserve the natural resources but also promote sustainability in preserving the environment. Palm oil clinker (POC) is a waste by-product from the incineration process of oil palm shells and fibres. They are porous and lightweight in nature, which makes them suitable for use as a lightweight aggregate (LWA). In this study, a new procedure was employed to obtain the mix design based on the particle packing (PP) concept to ensure the fresh and hardened properties of SCC are achieved. The actual packing level of aggregate and paste volume is integrated into the proportioning method to obtain the final mix design. The proposed procedure was verified by evaluating the SCC formed for self-compactability and mechanical properties. Based on the overall performance of fresh and hardened properties, it can be deduced that the procedure satisfied the requirements for SCC. The satisfactory results indicate that the mix design can be employed not only for POC but also for a variety of combinations of aggregate.     

Research on the influence of the fly ash on the concrete carbonation

The influence of fly ash on the fresh properties, mechanical properties and carbonation properties were studied in this paper. The performance of a kind of curing agent which was applied to the hardened concrete surface was evaluated. Incorporating large volume of fly ash will risk the concrete carbonation. And the curing agent could prevent the concrete carbonation. And the mechanism was explained.     

An empirical approach for the optimisation of aggregate combinations for self-compacting concrete

The fresh and hardened properties of self-compacting concrete (SCC) depend on number of factors such as paste composition, paste content, aggregate content, aggregate gradation etc. In the present investigation, the influence of the packing density of aggregates on the properties of SCC was evaluated. Experiments were conducted to measure the packing density for different combinations of aggregates precisely. A ternary packing diagram (TPD) was developed based on the packing density of measured and interpolated data. Considering the limitations in generalising the TPD and the difficulty involved in adopting mathematical models for aggregates, an attempt was made to establish a simple method for the selection of the combination of aggregates resulting in maximum packing density from the particle size distribution of aggregates (represented by the Coefficient of uniformity??C _u). Further, studies were extended to investigate the effect of aggregate packing density on fresh and hardened SCC properties. The results indicate that for a constant paste volume and paste composition, with increase in packing density of aggregates, the fresh properties and the compressive strength of SCC were improved positively. An attempt was also made to identify the influence of 10 different proportions of aggregates having the same packing density on the properties of SCC. The results indicate that at the same aggregate packing density, the fresh concrete properties were influenced significantly by the choice of the aggregate combination, while there was little or no influence on the hardened properties. Furthermore, the experimental data obtained was used for supplementary validation of the existing model (compressible packing model) for predicting the packing density and the fresh behaviour of SCC.     

Statistical models to predict fresh and hardened properties of self-consolidating concrete

Several material properties and mix design parameters affect the performance of self-consolidating concrete (SCC) and need to be taken into consideration to enhance the fresh and hardened properties of the concrete. A factorial design was conducted to model the effect of mixture parameters and material properties on workability, mechanical properties, and visco-elastic properties of SCC used for the construction of precast/prestressed structural elements. The modeled mixture parameters included the binder content, binder type, water-to-cementitious materials ratio, sand-to-total aggregate ratio (S/A), and dosage of thickening-type viscosity-modifying admixture. In total, 16 SCC mixtures were investigated to establish a factorial design with five main factors. Three replicate SCC mixtures were prepared to estimate the degree of the experimental error for the modeled responses. The mixtures were evaluated to determine several key responses that affect the performance of precast, prestressed concrete, including the filling ability, passing ability, filling capacity, stability, compressive strength, modulus of elasticity, flexural strength, autogenous shrinkage, drying shrinkage, and creep. The derived statistical models enable to quantify the level of significance of each of the five investigated parameters on fresh and hardened properties of SCC, which can simplify the test protocol needed to optimize SCC. Based on the results derived from the factorial design, recommendations for the proportioning of SCC in terms of workability, mechanical properties, and visco-elastic properties are given.     

Compressing fresh concrete technique and the effect of excess water content on physical–mechanical properties of compressed concrete

This paper offers an innovative practical technique for applications in which high workability concrete is needed. In this technique, concrete is produced by compressing the fresh concrete through a fabricated pressure apparatus without incorporating additives and no need for external vibration and workability control. Applying this technique, excess water is completely expelled out from the fresh concrete and porosity is remarkably decreased. In this study, several mixes having different excess water contents with the same cement and aggregates were prepared to attain different workability levels. To evaluate the effect of excess water content on properties of hardened concrete, the physical and mechanical properties of both compressed and uncompressed concrete were determined, including compressive strength, modulus of elasticity, strain at peak stress, stress–strain curve, failure mode, water absorption, density and ultrasonic pulse velocity. The results obtained from this study showed that the excess water content added to the fresh concrete does not influence the physical–mechanical properties of the compressed concrete while those of the uncompressed concrete are significantly degraded. Moreover, compressing the fresh concrete dramatically improves the properties of the compressed concrete, as compared to the corresponding uncompressed concrete.     

A classification of studies on properties of foam concrete

Though foam concrete was initially envisaged as a void filling and insulation material, there have been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilization of wastes like fly ash. The focus of this paper is to classify literature on foam concrete in terms of constituent materials (foaming agent, cement and other fillers used), mix proportioning, production methods, fresh and hardened properties of foam concrete. Based on the review, the following research needs have been identified: (i) developing affordable foaming agent and foam generator, (ii) investigation on compatibility between foaming agent and chemical admixtures, use of lightweight coarse aggregate and reinforcement including fibers, (iii) durability studies, and (iv) factors influencing foam concrete production viz., mixing, transporting and pumping.     

Effects of mineral admixtures on fresh and hardened properties of self-compacting concretes: binary,ternary and quaternary systems

The paper presented herein investigates the effects of using supplementary cementitious materials in binary, ternary, and quaternary blends on the fresh and hardened properties of self-compacting concretes (SCCs). A total of 22 concrete mixtures were designed having a constant water/binder ratio of 0.32 and total binder content of 550 kg/m³. The control mixture contained only portland cement (PC) as the binder while the remaining mixtures incorporated binary, ternary, and quaternary cementitious blends of PC, fly ash (FA), ground granulated blast furnace slag (GGBFS), and silica fume (SF). After mixing, the fresh properties of the concretes were tested for slump flow time, L-box height ratio, V-funnel flow time, setting time, and viscosity. Moreover, compressive strength, ultrasonic pulse velocity, and electrical resistivity of the hardened concretes were measured. Test results have revealed that incorporating the mineral admixtures improved the fresh properties and rheology of the concrete mixtures. The compressive strength and electrical resistivity of the concretes with SF and GGBFS were much higher than those of the control concrete.     

Effects of limestone fillers on surface free energy and electrical conductivity of the interstitial solution of cement mixes

Most of concrete formulations actually contain mineral additives that are used as an inert filler or supplementary cementitious material (as type II addition). These materials are generally used in order to increase concrete performances both in its fresh and hardened states. The mineral additives usually improve the consistency and workability of fresh concrete and reduce the water demand for a required concrete slump because of an additional volume of fines. The way of action depends on the physical and chemical characteristics of fillers. The study of surface free energies and electric conductivities of interstitial solutions of cement mixes modified by blast furnace slags and limestone fillers contributes to a better knowledge of hardening and interaction process.     

Influence of mixing procedure and mixer type on fresh and hardened properties of concrete: a review

Mixing concrete is not yet a fully understood issue, with many parameters having an influence on the resulting fresh and hardened concrete properties. Even for the same composition, a somewhat different microstructure can be obtained by changing the mixing procedure and the mixer type. A mixing procedure can differ in mixing time, mixing speed, air pressure in the mixing pan, addition time of the superplasticizer, temperature, etc. The concrete industry shows a great interest in controlling these influences in order to produce a concrete of which the mechanical, rheological and durability properties are well known. In this overview, different concrete mixers, mixing times, mixing speeds, different addition times of the superplasticizer and a different air pressure in the mixing pan will be examined. A review of existing literature is presented, as well as some new experimental results.     

Feasibility analysis of the reuse of waste filler of bituminous mixtures for the production of self-compacting concrete

The emergence of self-compacting concrete is a significant development in the construction industry since it greatly reduces execution time and labour costs, auxiliary resources and energy consumption. However, this type of material must continue to evolve to provide more efficient management of resources and a contribution to sustainable development. On the other hand, the possibility of revaluing the residues from one industry by reusing them as raw materials for another is nowadays seen as one of the most efficient solutions to the environmental problems caused by the construction industry. This paper conducts a feasibility analysis of the reuse of waste filler of bituminous mixtures for the production of self-compacting concrete. For that purpose, a comparative study of a reference self-compacting concrete, made with commercial filler, with another, made with recovered filler extracted from production plants of bituminous mixtures, was carried out. This article shows the results obtained after studying the behaviour of these two types of concrete both in fresh state (flowability and blocking resistance) and in hardened state (porosity, compressive strength and the evolution of carbonation depth in an accelerated carbonation process). According to the results obtained, the reuse of this waste material for the production of self-compacting concrete is not only perfectly possible but also provides the same guarantees as the usual commercial filler.