Journal May 2021

Feasibility of using recycled concrete aggregates to produce ultra high performance concrete: A preliminary study Zeger Sierens, Miquel Joseph, Jiabin Li

This paper presents a preliminary feasibility study on the utilisation of a high quality recycled concrete aggregates (RCAs) to manufacture ultra high performance concrete (UHPC), which represents a new type of cementitious material having superior workability, mechanical performance and durability properties in comparison to conventional concrete. The particle size of the RCA used in this paper ranges from 4 to 8 mm. A total of 4 mixtures are made in the test program, which include a reference mixture with natural coarse grains and 3 mixtures with RCA as coarse aggregates. Since the RCA has high water absorption, the incorporation of this material in producing concrete requires a compensation of the water demand to achieve the desired workability. In the 3 concrete mixtures with RCA, different compensation water amounts are added. Laboratory tests on the workability, compressive strength, modulus of elasticity and four point bending behavior of the concrete mixtures are carried out. The test results indicate that it is possible to use RCA to produce UHPC despite that its incorporation results in some reductions in the mechanical properties of the concrete mixture in comparison to the reference UHPC mixture with natural coarse aggregates.

Effect of aggregate grading on the fresh and mechanical performance of recycled aggregate self-compacting concrete Uma Shankar Biswal, Pasla Dinakar

Due to the substantial boom in infrastructure growth in developing countries such as India, the supplies of natural aggregates (NAs) are declining at a high rate and thereby causing an ecological imbalance. Contrary to that enormous volume of recycled aggregates (RAs) created from the waste of building and demolition (C&D). Therefore, in terms of preservation, the use of RA in the construction of reinforced concrete can be a great source of aggregate. It is well known that aggregates occupy nearly 70 percent of the volume in concrete, and they help in optimizing the cement and water and thus enabling higher strengths while lowering the shrinkage, creep, and temperature effects in concrete. The shape, size, grading, and texture (of natural, artificial, and recycled types) affects the water needed for certain workability considerably. The grading and proportions of the individual coarse and fine aggregates (either in all-in aggregate grading or otherwise) affects workability and this influence is more pronounced when self-compacting concrete is used. In the present investigation, self-compacting concrete (SCC) was developed with complete substitution of coarse NAs with coarse RAs by employing the allin aggregate grading curves of DIN standards. Supplementary cementing materials (SCMs) such as coal fly ash (CFA), ground granulated blast furnace slag (GGBS), and metakaolin (MK) have also been used as cement substitute materials to make the SCC more sustainable. Finally, based on fresh properties such as slump flow, T500, V-funnel, and L-box test, and mechanical properties through compressive strength test, a comparison is made for concrete with the use of DIN combined grading against the all-in grading curve defined in the BIS code. It is concluded that, especially in the presence of SCMs, the DIN all-in aggregate grading provides better workability and mechanical performance for SCCs compared to BIS all-in aggregate grading method.

An experimental study in developing self-compacting concrete as per IS: 10262 (2019) by utilising the marginal materials as fines Ajay N., Ashwin M. Joshi, Girish S., Namratha Bharadwaj

Self-compacting concrete (SCC) is a flowable mix, which can spread and fill the form work completely encapsulating the reinforcement and consolidating on its own self-weight while maintaining the homogeneity. In the present study an experimental work was carried out to develop M30 grade SCC using IS: 10262 (2019). Five different SCC mixes were developed by utilizing fly-ash and Ground Granulated Blast Slag as filler materials along with particle size less than 125 microns choosing from natural river sand, marble dust, fly-ash, dried ready-mix concrete sludge, and granite sludge as fine materials. The powder content, cement content, and water content of the developed mixes were maintained at 550 kg/m3, 334 kg/m3 and 200 lt/m3 respectively. The maximum size of the coarse aggregates was 20 mm. The developed SCC mixes satisfied the requirements of fresh properties. The morphology of the microstructure of developed SCC mixes was also analyzed through Scanning Electron Microscopy (SEM). The results indicate that the compressive strength of SCC increases with powder content for the same w/c ratio. Also, the results indicate that the SCC mixes were achieved at powder content above 520 kg/m3 with w/p ratios of 0.90 to 1.10. The fine materials may be utilized in developing SCC with granite sludge showing better performance as compared to other fines.

Sustainability assessment of M25 grade recycled aggregate concrete Tiju Susan Thomas, V. Sankaran, K. V. Jisha, B. S. Dhanya

Construction activities can enhance both economic as well as environmental burden owing to the high cost of building materials, consumption of natural resources and release of emissions to the environment. Conversion of Construction and Demolition (C&D) waste into aggregates that can be used in concrete has emerged as one of the solutions to the above mentioned problems. Through this paper, an attempt is made to assess the sustainability of M25 grade of concrete with different replacement levels of recycled coarse aggregate following a cradle-to-gate life cycle assessment approach. In order to achieve the strength requirement, the mix design parameters were suitably modified with increase in replacement levels of recycled aggregates. Impact assessment was done using CML 2001 baseline method. Results showed that, with optimum usage of coarse recycled

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