Production Of New Concrete Research Articles

Concrete Containing Recycled Concrete Aggregate with Modified Surface

This paper is focused to the recycling of one kind of construction & demolition waste (C&DW), namely concrete. After crushing and sorting, such recycled concrete aggregate (RCA) can be used for production of new concrete. The use of RCA may help to save natural aggregate sources and thus contributes to environmental protection.The most negative properties of such aggregate, comparing the natural one are roughness of its surface and high water absorptivity as well. Those features cause complications during concrete production technology mainly they have negative impact on the amounts of cement and mixing water and also on the final properties of concrete. Using laboratory techniques, the 4/8 fraction of RCA was prepared for experiments aimed on the enhancement of surface quality and performance of such aggregate. To modify the surface properties of RCA, geopolymer material based on fly ash was used in different ways. The first one is the coating of RCA by geopolymer slurry, then drying under ambient conditions and finally application into concrete mixture. The second one is coating directly during mixing of concrete, using so called “triple stage mixing”.In the paper, properties of both sets of concretes will be compared and analyzed in terms of workability, real density, total water absorption and compressive strength. Slight differences in those properties were found, comparing the reference samples with no any surface treatment of RCA, as well as comparing the ways of RCA modification each other; that is why the modification conducted directly during mixing of concrete can be recognized to be preferable due to better simplicity.

Use of coarse recycled aggregates from precast concrete rejects: Mechanical and durability performance

This paper presents the results of an experimental campaign developed with the aim of evaluating the effect of the incorporation in concrete of recycled aggregates from crushed elements produced by the precasting concrete industry. The specific purpose was to study the use of the coarse fraction of these aggregates. This was done by studying various replacement ratios of the coarse fraction of the aggregates. The possibility/need to correct the composition of concrete mixes with recycled aggregates was also investigated, by testing various contents of a superplasticizer, intending to replicate the characteristics of a reference concrete, i.e. a mix with an analogous composition but where all aggregates are natural. An extensive set of tests was performed to evaluate the mechanical and durability properties of all concrete mixes. The results prove that the concrete mixes with recycled aggregates have an equivalent performance to that of the reference concrete in most of the properties. In the others, where a performance loss was registered, that loss was lower than the one reported in the literature review performed. This was expected since the quality of the coarse aggregates recycled from precast elements is very good and totally fit for the production of new concrete, originating concrete compositions with good quality, namely in terms of hardened state properties. As expected the use of a superplasticizer is beneficial for concrete performance in all properties analysed.

MECHANICAL PROPERTIES OF CONCRETE CONTAINING RECYCLED CONCRETE AGGREGATES SUBJECTED TO DIFFERENT FIRE DURATIONS

The amount of construction and demolition waste has increasedconsiderably over the last few years. Crushing concrete to producecoarse aggregate for the production of new concrete is one commonmeans for achieving a more environment-friendly concrete. Thisexperimental study demonstrates the mechanical properties of concretecontaining recycled concrete aggregate (RCA) subjected to different firedurations. In the experimental work, the fresh concrete properties (slump test) and the hardened concrete properties, (compressive strength,splitting tensile strength, flexural strength and modulus of elasticity)were carried out. A total of thirty concrete mixes with different cementcontents (300, 350, and 400 Kg/m3) at w/c ratio (0.5) were made and thepercentage of substitution of natural concrete coarse aggregate by therecycled concrete coarse aggregate subjected to fire durations (1,2 and 3hours) at fire temperature (600oC) was (0%, 25%, 50% and 100%). Inthe process of mixing, equal consistence among all concrete mixes wasachieved. The obtained results indicate that the recycled coarseaggregate subjected to fire can successfully be used for makingstructural concrete.

The influence of curing conditions on the mechanical performance of concrete made with recycled concrete waste

Abstract Research on the use of Construction and Demolition Waste (CDW) as recycled aggregate (in particular crushed concrete) for the production of new concrete has by now established the feasibility of this environmentally-friendly use of otherwise harmful waste. However, contrary to conventional concrete (CC), no large applications of concrete made with recycled concrete have been made and there is still a lack of knowledge in some areas of production and performance of recycled aggregate concrete (RAC). One issue concerns curing conditions: these greatly affect the performance of concrete made on site and some potential users of RAC wonder how RAC is affected by far-from-ideal curing conditions. This paper shows the main results of experiments to determine the influence of different curing conditions on the mechanical performance of concrete made with coarse recycled aggregate from crushed concrete. The properties analyzed include compressive strength, splitting tensile strength, modulus of elasticity, and abrasion resistance. The general conclusion in terms of mechanical performance is that RAC is affected by curing conditions roughly in the same way as CC.

Testing of Strength of Recycled Waste Concrete and Its Applicability

This paper gives results of studies undertaken to assess suitability of construction demolition as coarse aggregate in new concrete production. Three sets of concrete materials considered are fresh concrete (FCM), waste concrete (WCM), and waste concrete strengthened with admixtures (SWCM). Various mixes were prepared for carrying out the research by varying the proportions of cement, sand, and aggregates. All mixes were designed for characteristic strength (fck) of M20. The compressive strength of concretes was tested in laboratory after 3, 7, and 28 days. The specimens used for testing include cubes, cylinders, and flexural beams. The influence of admixtures on the strength of waste concrete was examined. The compressive strengths of FCM, WCM, and SWCM are compared and the results show that there is not much difference in the strengths of FCM and SWCM after 28 days. For this reason, this study recommends the recycling of waste concrete as an aggregate material in the production of new concrete.

순환골재 콘크리트의 염화물 확산성에 관한 연구

폐콘크리트를 굵은 골재의 대체 재료로 재활용하게 되면, 건설폐기물의 증가위기와 천연골재의 고갈문제를 해결해 줄 수 있다. 그러나 순환골재는 콘크리트용 골재의 품질보다 저하되는 경향이 있기 때문에, 이를 개선하여 콘크리트 재료로 활용해야 한다. 순환골재 콘크리트의 내구성에 관한 논문은 부족한 실정이므로, 본 연구에서는 염화물 확산계수에 의해 포졸란재를 혼입한 순환골재 콘크리트의 염화물 확산성에 대한 기초적인 자료를 제시하는데 그 목적이 있다. 주요 결과로는 다음과 같다. 1) 재령에 따라 포졸란재를 혼입한 순환골재 콘크리트의 압축강도가 증가하였으며, 염화물 확산성은 감소하였다. 2) 모든 재령에서, 순환굵은골재의 치환율이 30%인 경우 고로슬래그 및 메타카올린을 혼입한 순환골재 콘크리트의 염화물 확산계수는 플레인 콘크리트와 유사하거나 더 낮은 결과를 나타내었다. The recycling of demolished concrete as an alternative source of coarse aggregates for the production of new concrete can help to solve the growing waste disposal crisis and the problem of the depletion of natural aggregates. The purpose of this study is to investigate the chloride migration of recycled aggregate concrete containing pozzolanic materials by the chloride migration coefficient. The specimens were made with recycled coarse aggregate at various replacement ratios (10, 30, 50%) and metakaolin, blast furnace slag, and fly ash is replaced for recycled concrete with a mixing ratio of 20%. The major results are as follows. 1) The compressive strength of recycled aggregate concrete containing pozzolanic materials increases as the curing age and chloride diffusivity decreases. 2) When the replacement ratio of recycled coarse aggregate is 30%, the chloride migration coefficient of recycled concrete containing blast furnace slag or metakaolin that shows a value similar to or lower than that of plain concrete at all ages.

Effect of natural coarse aggregate type on the physical and mechanical properties of recycled coarse aggregates

Many environmental problems caused by the large volumes of construction and demolition waste (C&DW), the lack of adequate deposition sites and the shortage of natural resources have led to the use of C&DW as replacement of natural aggregates in the production of new concrete. As in the case of natural aggregates, when recycled aggregates are used to manufacture structural concrete, the assessment of their physical, mechanical and durable characteristics is a key issue. The different physical and mechanical properties of the recycled coarse aggregate (RCA) are evaluated. RCA was obtained by crushing conventional concretes with different strength levels (different w/c ratios) containing four different types of natural coarse aggregates (three crushed stones and a siliceous gravel), which differ in shape, composition and surface texture. There is a significant influence of the natural coarse aggregate (NCA) on the properties of RCA, which in many cases is greater than that of the w/c ratio of the source concrete.

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 Tecnico (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.

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.

Waste wash water recycling in ready-mixed concrete plants

Production of large amounts of waste wash water coming from ready-mixed concrete plants leads to problems of environmental impact. National laws usually prohibit the disposal of such types of water, due to their extremely high pH value and suspended matter amount, and require the water to be treated prior to discharge. prEN 1008 provides for recycling waste water in the production of new concrete, but gives some restrictions for its composition and use. In this paper, the use of waste wash water (coming from a medium-size ready-mixed concrete plant) in mixing water for concrete and mortars has been investigated: the effects on physical–mechanical properties and microstructure are investigated as a function of the characteristics of waste water used. The results have shown that mortar and concrete prepared with recycled water exhibit 28-day mechanical strength in no way lower than 96% of the reference materials (90% is the minimum allowed in prEN 1008) and, in some cases, even better. Moreover, the use of wash water in concrete leads to a reduction of the concrete capillary water absorption and mortar microporosity, which surely improves the durability of the material. This effect can be ascribed to the filling action of the fines present in the wash water and to the slight reduction of the actual water/cement ratio.

Recycled concrete and silica fume make calcium silicate bricks

When old concrete is crushed to coarse aggregate for production of new concrete, approximately 20 weight percent of crusher fines are generated. It has been shown that calcium hydroxide from hydrated cement of such fines reacts in the autoclave with siliceous particles in the fines to give calcium silicate products. Such products are similar in nature to sand-lime bricks. When silica are greatly improved. It has further been shown that crusher fines from recycled concrete do not qualify as hydraulic cements even when ground to cement fineness.