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- The effect of multi-recycling on the mechanical performance of coarse recycled aggregates concretePublication . Abreu, Vilson; Evangelista, Luis; De Brito, JorgeThis paper presents the mechanical performance results obtained to evaluate the effect of incorporating coarse recycled aggregates from various recycling cycles. These aggregates were obtained from crushing elements of concrete of controlled origin, which were mixed to have the same performance in each cycle. The specific purpose was to study the mechanical performance of concrete designed with incorporation of coarse recycled aggregates from three successive recycling cycles at two replacement ratios, 25% and 100%, comparing it with that of a reference concrete, a mix with the same composition but where all aggregates are natural. An experimental campaign was carried out: to obtain all the necessary coarse recycled aggregates; to produce the concrete mixes that were the source of the recycled coarse aggregates; to produce the concrete mixes that were studied and; to perform all the necessary tests to evaluate the mechanical properties of these concrete mixes. Compressive strength, modulus of elasticity, tensile strength and abrasion resistance were tested. The results prove that, with the increase of the number of recycling cycles of the coarse aggregates, there is a decrease of its quality that affects the mechanical performance of concrete. That mechanical performance decreases asymptotically with the number of recycling cycles, tending towards a final value representative of the property’s stabilization, and linearly with the increase in the ratio of incorporation of recycled coarse aggregates. It is concluded that, by knowing the final value, concrete with incorporation of coarse recycled aggregates from any recycling cycle can be designed for the most diverse applications with a good safety level.
- Thermal performance of concrete with reactive magnesium oxide as an alternative binderPublication . Forero Valencia, Javier Andres; Bravo, Miguel; Pacheco, João; Brito, Jorge de; Evangelista, LuisThis study evaluates the thermal conductivity of concrete produced with reactive magnesium oxide (MgO) as a partial replacement for cement. MgO is a viable option for the concrete industry, mainly due to its benefits in sustainability and reducing CO2 emissions compared to cement emissions. Four different MgO's produced in Australia, Canada, and Spain were used in concrete mixes as a partial replacement of cement at 5%, 10%, and 20% by mass. The experimental results showed that the thermal conductivity is higher when MgO increases in mixes after 28 days of curing. With the incorporation of MgO, the thermal conductivity increased between 3.2% and 10.2%, and the mechanical properties declined: compressive strength between 12.7% to 26.2%, splitting tensile strength between 9.7% to 34.0%, and modulus of elasticity between -4.1% to 7.8%. Finally, it is important to highlight that the addition of different contents of MgO in the concrete mixes modified the microstructure of the cement matrix. As a result, there was an increase in porosity, which negatively influenced the mechanical properties and thermal conductivity. Therefore, the relationships between these properties were also analyzed.
- The influence of curing conditions on the mechanical performance of concrete made with recycled concrete wastePublication . Fonseca, N.; De Brito, Jorge; Evangelista, LuisResearch 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. (C) 2011 Elsevier Ltd. All rights reserved.
- Durability performance of concrete made with fine recycled concrete aggregatesPublication . Evangelista, Luis; De Brito, JorgeFine recycled aggregates are seen as the last choice in recycling for concrete production. Many references quote their detrimental influence on the most important characteristics of concrete: compressive and tensile strength; modulus of elasticity; water absorption; shrinkage: carbonation and chloride penetration. These two last characteristics are fundamental in terms of the long-term durability of reinforced or prestressed concrete. In the experimental research carried out at IST, part of which has already been published, different concrete mixes (with increasing rates of substitution of fine natural aggregates sand - with fine recycled aggregates from crushed concrete) were prepared and tested. The results were then compared with those for a reference concrete with exactly the same composition and grading curve, but with no recycled aggregates. This paper presents the main results of this research for water absorption by immersion and capillarity, chloride penetration (by means of the chloride migration coefficient), and carbonation resistance, drawing some conclusions on the feasibility of using this type of aggregate in structural concrete, while taking into account any ensuing obvious positive environmental impact.
- Performance of concrete made with aggregates recycled from precasting industry waste: influence of the crushing processPublication . Pedro, Diogo; De Brito, Jorge; Evangelista, LuisThe aim of this paper is to evaluate the influence of the crushing process used to obtain recycled concrete aggregates on the performance of concrete made with those aggregates. Two crushing methods were considered: primary crushing, using a jaw crusher, and primary plus secondary crushing (PSC), using a jaw crusher followed by a hammer mill. Besides natural aggregates (NA), these two processes were also used to crush three types of concrete made in laboratory (L20, L45 e L65) and three more others from the precast industry (P20, P45 e P65). The coarse natural aggregates were totally replaced by coarse recycled concrete aggregates. The recycled aggregates concrete mixes were compared with reference concrete mixes made using only NA, and the following properties related to the mechanical and durability performance were tested: compressive strength; splitting tensile strength; modulus of elasticity; carbonation resistance; chloride penetration resistance; water absorption by capillarity; water absorption by immersion; and shrinkage. The results show that the PSC process leads to better performances, especially in the durability properties.
- Mechanical performance of concrete made with aggregates from construction and demolition waste recycling plantsPublication . Bravo, Miguel; De Brito, Jorge; Pontes, Jorge; Evangelista, LuisThis research aims at analysing the mechanical performance of concrete with recycled aggregates (RA) from construction and demolition waste (CDW) from various locations in Portugal. First the characteristics of the various aggregates (natural and recycled) used in the production of concrete were thoroughly analysed. The composition of the RA was determined and several physical and chemical tests of the aggregates were performed. In order to evaluate the mechanical performance of concrete, compressive strength (in cubes and cylinders), splitting tensile strength, modulus of elasticity and abrasion resistance tests were performed. Concrete mixes with RA from CDW from several recycling plants were evaluated, in order to understand the influence that the RA's collection point, and consequently their composition, has on the characteristics of the mixes produced. The analysis of the mechanical performance allowed concluding that the use of RA worsens most of the properties tested, especially when fine RA are used. On the other hand, there was an increase in abrasion resistance when coarse RA were used. In global terms, the use of this type of aggregates, in limited contents, is viable from a mechanical viewpoint. (C) 2015 Elsevier Ltd. All rights reserved.
- Can we truly predict the compressive strength of concrete without knowing the properties of aggregates?Publication . De Brito, Jorge; Kurda, Rawaz; Raposeiro Da Silva, PedroThis paper is focused on the influence of the geological nature and quality of the aggregates on the compressive strength of concrete and explains why it is important not to ignore the characteristics of aggregates in the estimation of the strength of concrete, even for virgin aggregates. For this purpose, three original (Abrams, American Concrete Institute Manual of concrete practice and Slater) and two modified (Bolomey and Feret) models were used to calculate the strength of concrete by considering results of various publications. The results show that the models do not properly predict the strength of concrete when the characteristics of aggregates are neglected. The scatter between the calculated and experimental compressive strength of concrete, even when made with natural aggregates (NAs) only, was significant. For the same mix composition (with similar cement paste quality), there was a significant difference between the results when NAs of various geological nature (e.g., limestone, basalt, granite, sandstone) were used in concrete. The same was true when different qualities (namely in terms of density, water absorption and Los Angles abrasion) of aggregates were used. The scatters significantly decreased when the mixes were classified based on the geological nature of the aggregates. The same occurred when the mixes were classified based on their quality. For both modified models, the calculated strength of mixes made with basalt was higher than that of the mixes containing other types of the aggregates, followed by mixes containing limestone, quartz and granite. In terms of the quality of the aggregates, the calculated strength of concrete increased (was overestimated) as the quality of the aggregates decreased. The influence of the aggregates on the compressive strength of concrete became much more discernible when recycled aggregates were used mainly due to their more heterogeneous characteristics.
- Fracture behaviour of concrete with reactive magnesium oxide as alternative binderPublication . Forero, J. A.; Bravo, M.; Pacheco, João; De Brito, Jorge; Evangelista, LuisThis research evaluates the fracture behavior of concrete with reactive magnesium oxide (MgO). Replacing cement with MgO is an attractive option for the concrete industry, mainly due to sustainability benefits and reduction of shrinkage. Four different MgO's from Australia, Canada, and Spain were used in the concrete mixes, as a partial substitute of cement, at 5%, 10%, and 20% (by weight). The fracture toughness (K-I) intensity factor and the stress-strain softening parameters of the wedge split test were evaluated after 28 days. The experimental results showed that the replacement of cement with MgO reduced the fracture energy between 13% and 53%. Moreover, the fracture energy was found to be correlated with both compressive strength and modulus of elasticity. A well-defined relationship between these properties is important for an adequate prediction of the non-linear behavior of reinforced concrete structures made with partial replacement of cement with MgO.
- Durability and shrinkage performance of concrete made with coarse multi-recycled concrete aggregatesPublication . Silva, Stefano; Evangelista, Luis; De Brito, JorgeSince construction is one of the most environmental impacting activities in existence, it is important to study and develop solutions to make its processes more efficient. Thus, multiple-recycling of concrete presents itself as a possible alternative for the reutilization of construction and demolition waste (CDW) over a large period of time. This paper presents the results and conclusions obtained from an extensive experimental campaign intended to study the durability performance of concrete made with aggregates resulting from multiple recycling cycles. Concrete mixes with 25% and 100% of substitution of natural coarse aggregates with recycled coarse concrete aggregates obtained from one, two and three recycling cycles were produced. Tests such as sieve analysis, water absorption, particle density, shape index, particle bulk density and Los Angeles abrasion were made to study the recycled coarse aggregates properties. Additionally, the workability and bulk density of fresh concrete were measured, and water absorption by immersion and capillarity, carbonation and chloride penetration were assessed to evaluate the durability performance of hardened concrete, as well as shrinkage. The results obtained in the various tests show that, with the increase of recycling cycles, the recycled coarse aggregates demonstrate a quality decrease in their properties, resulting in a worse durability and shrinkage performance of the resulting concrete. Furthermore, it is shown that the decrease in performance tends to slow down with the increase of the recycling cycles, thus presenting an asymptotic behaviour. However, in most cases, it was not possible to establish that three recycling cycles were enough to stabilize the properties.
- Use of bottom ash additions in the production of concrete with recycled aggregatesPublication . Raposeiro Da Silva, Pedro; Silva, Rui V.; Brito, Jorge deThis paper provides a literature review on the use of bottom ashes in the production of concrete with recycled aggregates. Three types of bottom ash were studied, namely: biomass bottom ashes, coal bottom ashes and sewage sludge bottom ash. The characterization of these ashes focused on the analysis of their physical, chemical, and mineralogical properties. The effect of these ashes was subsequently studied on the fresh, mechanical, and durability-related performances of concrete. Bottom ashes generally present lower pozzolanicity than that typically observed, for example, in coal fly ashes. Their use as partial cement replacement normally leads to some loss in performance of the resulting cementitious composites. Also, using them as aggregates or in combination with recycled aggregates of other sources similarly causes an overall loss in performance. Nevertheless, such decline is still acceptable and often within manageable limits for the production of concrete under specific conditions including some structural applications. The use of these by-products including recycled aggregates may assist in solving a two-fold problem. Firstly, it reduces the consumption of cement and, consequently, the extraction of natural resources, also including the decrease of the consumption of natural aggregates to produce concrete. Furthermore, it solves the problem of the final destination for the significant quantities of bottom ashes produced by different industrial processes. In general, it is possible to conclude that, in moderate contents and when adequately processed, bottom ashes can be considered as viable substitutes of cement with manageable losses in terms of mechanical and durability-related performances. The use of coal bottom ashes was also found to significantly reduce the drying shrinkage strain of concrete.