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- Experimental investigation on the variability of the main mechanical properties of concrete produced with coarse recycled concrete aggregatesPublication . Pacheco, João; De Brito, Jorge; Chastre, Carlos; Evangelista, LuisResearch on the variability of the properties of recycled aggregate concrete is lacking and is necessary for the development of reliability analyses and code calibration procedures. This paper presents an experimental programme on the within-batch variability of the compressive strength, Young's modulus, and splitting tensile strength of several recycled and natural aggregate concrete mixes. The influence of the recycled concrete aggregates on the mechanical properties and variability of concrete is analysed and discussed and benchmarks with standard predictions for the variability of natural aggregate concrete are made. It was found that full recycled aggregate concrete incorporation did not increase the variability of any of the properties tested, but intermediate ratios of recycled aggregate incorporation did. The properties of high-strength concrete mixes were more variable than that of all other mixes, irrespective of recycled aggregate incorporation. All properties of all compositions were suitably modelled by normal distributions. The coarse recycled aggregates were sourced from concrete waste.
- Statistical analysis of Portuguese ready-mixed concrete productionPublication . Pacheco, João; De Brito, Jorge; Chastre, Carlos; Evangelista, LuisThis paper evaluates and compares the statistics of compressive strength data from three Portuguese ready-mixed concrete plants. A hierarchical model showed that different groups of concrete strength records are not statistically equivalent, even if they were produced in the same plant and using the same concrete composition. This finding is related to autocorrelation. For the same specified strength class, compositions produced less often result in higher average compressive strength and variability. The statistics of one of the plants were quite different from those of the others, even though the concrete of this plant also complied with the specifications. It was found that the average compressive strength of a mix may be quite dependent on the plant that produced it, even if the compressive strength complies with quality control specifications. Conformity with the target slump and strength class was checked following the conformity criteria of EN 206-1 for continuous production. Nonconformity with slump is more frequent than failure to comply with the strength class. A bias factor for reliability analyses was proposed.
- 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.
- Eurocode shear design of coarse recycled aggregate concrete: reliability analysis and partial factor calibrationPublication . Pacheco, João; Brito, Jorge de; Chastre, Carlos; Evangelista, LuisThis paper contributes to the definition of design clauses for coarse recycled aggregate concrete. One of the main reasons for scepticism towards recycled aggregate concrete is the perceived notion that the heterogeneity of recycled aggregates may increase the uncertainty of the behaviour of concrete. Therefore, the paper uses structural reliability concepts to propose partial factors for recycled aggregate concrete's design for shear failure. The paper builds upon a previous publication by the authors, in which the model uncertainty of recycled aggregate concrete elements designed for shear, with and without shear reinforcement, was compared with that of natural aggregate concrete elements. In that paper, the statistics of the model uncertainty for recycled aggregate concrete shear design were indeed found to be less favourable than those of natural aggregate concrete. Therefore, a partial factor for recycled aggregate concrete design is needed to ensure safety. This paper presents partial factors calibrated with explicit reliability analyses for different cases of design concerning beams (in the case of shear design of elements with shear reinforcement) and slabs (for the design of elements without shear reinforcement). For full incorporation of coarse recycled concrete aggregates and the design of elements without shear reinforcement, the calibrated partial factor reduces the design value of shear resistance by 10% (design with EN1992) or 15% (design with prEN1992) in comparison to natural aggregate concrete's design. For the shear design of elements with shear reinforcement, the partial factor decreases resistance by 5% but a sensitivity analysis showed that the reduction might be, under pessimistic expectations, of up to 20%.
- Scatter of constitutive models of the mechanical properties of concrete: comparison of major international codesPublication . Pacheco, João; De Brito, Jorge; Chastre, Carlos; Evangelista, LuisAn investigation on the scatter of code-type constitutive models that relate compressive strength (f(c)) with tensile strength (f(ct)) and Young's modulus (E-c) of standard concrete specimens is presented. The influence of the mix design on the accuracy of the f(c) vs. f(ct) and f(c) vs. E-c relationships is discussed, with emphasis on the lithological type and morphology of the coarse aggregates. The uncertainty of the constitutive models is analysed in probabilistic terms and random variables that model the uncertainty of the f(c) vs. f(ct) and f(c) vs. E-c relationships are proposed for reliability analyses of serviceability limit states. The suitability of the models proposed is assessed through preliminary conservative estimates of their design values.
- Bond of recycled coarse aggregate concrete: model uncertainty and reliability-based calibration of design equationsPublication . Pacheco, João; Brito, Jorge de; Chastre, Carlos; Evangelista, LuisThis paper concerns the design of lap splice lengths for ribbed steel reinforcement bars embedded in concrete produced with coarse recycled concrete aggregates. Recycled aggregates are weaker and typically lead to concrete with lower tensile strength. Both aspects change the model uncertainty of bond strength formulae and a major topic of the paper is the influence of recycled aggregates on the model uncertainty of the bond strength model of fib Bulletin 72. A stochastic model for this model uncertainty is developed from a meta-analysis. The model uncertainty, estimated from analogue specimens made with either natural aggregate concrete or recycled aggregate concrete, is compared and the incorporation of recycled aggregates was indeed found to have a detrimental influence on the model uncertainty. A partial factor for lap splice length design is calibrated through reliability analyses so that the probability of failure of the bond length design of recycled aggregate concrete is equivalent to that of natural aggregate concrete. Two design equations were studied: that of the fib Bulletin 72 and that of the D6 draft of the second generation of Eurocode 2.
- Durability and shrinkage of concrete with CDW as recycled aggregates: Benefits from superplasticizer’s incorporation and influence of CDW compositionPublication . Bravo, Miguel; De Brito, Jorge; Evangelista, Luis; Pacheco, JoãoThe shrinkage and durability properties of a total of 34 concrete mixes with recycled aggregates produced from different untreated construction and demolition waste (CDW) were tested. The effect of a polycarboxylic superplasticizer on the enhancement of these properties is presented, discussed, and compared with the findings of studies on concrete whose recycled aggregates are sourced from concrete. All properties were significantly affected by recycled aggregate incorporation and this effect was strongly dependent on the properties of the recycled aggregates of each specific source. The superplasticizer was less efficient in mixes with CDW than when the recycled aggregates were produced from concrete, the most common source of recycled aggregates in experimental works, despite untreated CDW being more practical and desirable from an industrial and environmental perspective.
- 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.
- Probabilistic conversion of the compressive strength of cubes to cylinders of natural and recycled aggregate concrete specimensPublication . Pacheco, João; De Brito, Jorge; Chastre, Carlos; Evangelista, LuisThis paper investigates the effect of recycled coarse aggregate incorporation on the relationship between 150 mm cubic and F 150 mm cylindrical compressive strength (the reference strength of standards) by comparing data from recycled and natural aggregate concrete compositions in which both cubes and cylinders were tested. A conversion factor from cubic to cylindrical strength is proposed in two versions: A deterministic and a probabilistic one. Such factor has not been studied before and researchers have been converting cubic data as if natural aggregate concrete were tested. The probabilistic factor is intended for reliability analyses on the structural behaviour of recycled aggregate concrete using data from laboratory cube tests. It was found that the incorporation of recycled coarse aggregates sourced from concrete waste significantly decreases the expected value of the factor but the factor’s scatter is relatively unaffected.
- Uncertainty of shear resistance models: Influence of recycled concrete aggregate on beams with and without shear reinforcementPublication . Pacheco, João; De Brito, Jorge; Chastre, Carlos; Evangelista, LuisThe model uncertainty of the shear resistance equations of three design codes {the current Eurocode 2 (2004), Model Code (2010), and the final draft of the Eurocode 2 (2020)} was investigated for coarse natural and recycled aggregate concrete beams. Databases of beams with and without shear reinforcement were made with clearly defined criteria. The statistics of the model uncertainty of natural and recycled concrete beams were compared and it was found that recycled aggregate incorporation has detrimental effects on the model uncertainty of shear design. Surprisingly, recycled concrete aggregate beams designed following the current version of Eurocode 2 (2004) are safer than those designed using the other codes. This is due to the shear resistance equations of the latter overestimating the aggregate interlock of recycled aggregate. A preliminary partial factor was proposed, offsetting the influence of recycled aggregate on the safety of beam designs. The database of beams with shear reinforcement lacks representativeness but hinted that recycled aggregate incorporation also reduces the safety of this type of shear design. The paper finishes presenting suggestions of experiments that would complement the current knowledge on this topic.