Compressive Strength of Concrete Using Recycled Concrete Aggregate as Complete Replacement of Natural Aggregate

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1 Compressive Strength of Concrete Using Recycled Concrete Aggregate as Complete Replacement of Natural Aggregate Daniel Yaw Osei, Lecturer, Department of Civil Engineering, Cape Coast Polytechnic, Cape Coast, Ghana ABSTRACT This paper presents a report of an experimental investigation on the effect of complete replacement of natural aggregate by recycled concrete aggregate in the production of concrete on the compressive strength of concrete. Two sets of concrete mixtures of ratios 1:3:6, 1:2:4, 1:1 1 / 2 :3, 1:1:2 by mass were cast using natural aggregates and recycled aggregates concrete respectively. The 28-day compressive strengths of 1:3:6, 1:2:4, 1:1 1 / 2 :3, 1:1:2 concrete using recycled concrete aggregates were Nmm -2, Nmm -2, Nmm -2 and 25.81Nmm -2 respectively corresponding to 33%, 20%, 11% and 20% reduction in strength compared to concrete using natural aggregate.. The densities and compressive strengths of natural aggregate concrete were higher than that of corresponding recycled aggregate concrete. The results of the study showed that recycled concrete aggregate can potentially replace completely natural aggregate in the production of both non-structural and structural concrete. Keywords: compressive strength, coarse aggregate, concrete, natural aggregate, recycled concrete aggregate. Introduction The growing need for infrastructural development has placed a huge demand on coarse aggregates, which make up about three-quarters of concrete, the most used man-made material in construction [1]. This demand exerts pressures on aggregate resources and creates ecological imbalance which impacts negatively on the environment; rendering the production of concrete using natural aggregates unsustainable. In addition to ecological imbalances created by over-exploitation of aggregates, the open disposal of commercial, industrial and agricultural wastes has exacerbated environmental conditions. Due to the scarcity and increasing prices of construction materials, there is the need to investigate and utilize alternative materials in construction. The increasing demand and interest in aggregates from non-traditional sources such as from industrial by-products and recycled construction and demolition wastes [2] provides opportunities to make use of wastes which would otherwise have negative consequences on the environment. Recycling of such wastes for use in construction therefore provides a means to address both environmental and infrastructural development challenges. A report by [3] mentioned that the use of recycled concrete aggregate provides significant benefits towards sustainable development by reducing the need of landfilling while conserving the use of increasingly scarce good quality virgin aggregate; potentially leading to an annual savings of three hundred million dollars ($300,000,000.00) in operator costs by US ready-mixed concrete industry. Concrete made of recycled aggregate is less workable than that made of conventional aggregate as a result of its higher water absorption capacity [4]. The compressive strength and elastic modulus of concrete containing RA is lower than that of the control concrete [3]. [5] recommended that up to 50% recycled aggregates may be used in structural elements such as single-storey houses in Ghana. [6] investigated the effect of superplasticisers on the compressive strength and workability of concrete and found that concrete incorporating recycled aggregate performed poorer relative to control concrete but improved with the addition of superplasticisers. The use of plasticisers can improve the performance of concrete made with fine recycled aggregates to a level at par with conventional concrete [7]. In an investigation on the effect of replacing natural aggregate with mixed recycled aggregate, [8] found out that the compressive, tensile and flexural strength properties decreased as the proportion of mixed recycled aggregate increased. According to [9], recycled aggregate concrete is just as durable as ordinary concrete. However, [10] recommends that prior to the use of RA concrete in aggressive environments, appropriate testing should be conducted due to opposing conclusions about durability-related properties of RA concrete in existing literature. The objectives of this study are to investigate the effect of complete replacement of natural aggregate (NA) with RA on the compressive strength of concrete and to evaluate the possibility of using RA as complete replacement of natural aggregate in concrete. Blue Ocean Research Journals 26

2 Experimental Programme Materials Rapid hardening cement was used as the main binder in producing concrete. It conformed to the requirements of [11]. Potable water supplied by Ghana Water Company was used in mixing materials for production of concrete. It appeared clean and free from any deleterious material and conformed to the requirements of [12]. Sand, of bulk density 1550kgm -3 was used as fine aggregate. It was sourced from a local supplier in Cape Coast. Crushed granite (Figure 1) and recycled concrete aggregates (RA) were used as coarse aggregates. Crushed granite was obtained from Sarobi quarry near Elmina in the Central Region of Ghana while RA was obtained by manually crushing demolished concrete (Figure 2) and sieving through 5mm BS sieve. The sieve analyses of sand and coarse aggregates used are shown in Table 1 and Table 2 respectively. BS sieve 5mm mm mm µm µm µm 0.00 Percentage passing Table 2 Sieve analysis of coarse aggregates BS Percentage passing sieve (mm) Crushed granite Recycled concrete Figure 1 Natural aggregate Mixing and specimen preparation Two sets of concrete mixtures of ratios 1:3:6, 1:2:4 1:1½:3 and 1:1:2 by mass were produced using a water-cement ratio of 0.5; one set utilizing crushed granite as coarse aggregate and the other utilizing RA. Each aggregate was used in the twelve specimens at each mix ratio. A total of ninety six (96) cubes were produced. Concrete was mixed in an electric mixer (Figure 3). Concrete was cast in cast iron moulds measuring 150mm 150mm 150mm internally (Figure 4). The specimens were made in accordance with [13]. After casting, the moulds were covered with a plastic sheet to prevent water loss. After twenty four hours, the specimens were demoulded and placed in a curing tank to cure for strength gain and improvement of properties in the hardened state. Figure 2 Demolished concrete Table 1 Sieve analysis of sand Blue Ocean Research Journals 27

3 1:2: :1½: :1: Figure 3 Concrete mixer As expected, the compressive strength of each mixture increased with age. As the quantity of coarse aggregate increased, the strength of concrete reduced. Since the water cement ratio remained constant, an increase in the quantity of aggregate required more cement for adequate bond in order to increase strength. At each age of testing, concrete mixture of ratio 1:1:2 attained the highest compressive strength, followed by mixtures of ratios 1:1½:3, 1:2:4 and 1:3:6. The results of compressive strength testing RA concrete are presented in Table 4. Table 4 Compressive strength of RA concrete(nmm -2 ) Figure 4 Concrete specimens Testing The compressive strengths of the specimens were determined by crushing at 7, 14, 21 and 28 days of curing using a 1500kN capacity Matest compression tester. On the day off testing, the cubes were removed from the curing tank and placed in the laboratory environment for about two hours, after which their densities were determined prior to crushing. The results presented are the average of three tests. Results and Discussion Compressive Strength The results of the compressive testing of concrete using NA are shown in Table 3. Table 3 Compressive strength of NA concrete (Nmm -2 ) Mix ratio 1:3: Table 5 Relative strength (%) The 28-day strengths of RA concrete mixtures of ratios 1:3:6, 1:2:4, 1:1 1 / 2 :3, 1:1:2 were MPa, MPa, MPa, and MPa Blue Ocean Research Journals 28

4 respectively. At all ages, 1:1:2 RA concrete attained the highest compressive strength, followed by 1:1 1 / 2 :3, 1:2:4 and 1:3:6 mixes. Table 5 shows the relative strength of RA concrete. The value is the compressive strength of RA concrete expressed as a percentage of the compressive strength of NA concrete. It can be seen that all ages, NA concrete performed better than RA concrete. At the same mix ratio, the compressive strength of NA concrete was higher than RA concrete. The 28-day compressive strengths of 1:3:6, 1:2:4, 1:1 1 / 2 :3, 1:1:2 NA concrete were 67%, 80%, 89%, and 80% of the corresponding RA concrete. Previous research by [3], [6], [8] and [10] yielded similar results. Table 6 shows the recommended grades of concrete according to [14] Table 6 Recommended Grades of Concrete [14] Grade Characteristics Concrete Class Compressive Strength (MPa) C7 7.0 Plain concrete C C Reinforced concrete with lightweight C20 C C aggregate Reinforced concrete with dense aggregate Concrete with post-tensioned tendons Mix ratio 1:3: :2: :1½: :1: It can be seen from Table 7 that, densities of NA concrete mix of ratio 1:1:2 has the highest 28-day density, followed by concrete mix of ratios 1½:3, 1:2:4, 1:3:6. The densities of the various mixes fell within the range ( kgm -3 ), regarded as density of normal weight concrete [15]. The results of density determination of RA concrete are shown in Table 8. RA concrete mixture of 1:1:2 has the highest 28-day density, followed by concrete mix of ratios 1½:3, 1:2:4, 1:3:6. The densities of the various mixtures fell within the range ( kgm -3 ), regarded as density of normal weight concrete [15]. Table 8 Density of RAC (kgm -3 ) Mix ratio 1:3: C45 C50 C Concrete with pre-tensioned tendons 1:2: :1½: Based on the results presented in Table 4, it can be seen that 1:2:4 and 1:1 1 / 2 :3 NA concrete can potentially be used in the production of reinforced concrete while 1:1:2 NA concrete is potentially suitable for prestressed concrete. Similarly, 1:1 1 / 2 :3 and 1:1:2 RA concrete can be used in producing grade 20 and grade 25 concrete respectively, recommended for use in reinforced concrete construction. Density The densities of NA concrete are shown in Table 7. Table 7 Density of NA concrete (kgm -3 ) 1:1: The 28-day density of 1:3:6, 1:2:4, 1:1 1 / 2 :3, 1:1:2 RCA concrete is 98%, 95%, 95%, and 95% of density of the corresponding NAC. The density of RAC concrete is marginally lower than NAC concrete of the same mix ratio. Conclusion In this study, the compressive strength properties of concrete were investigated by completely replacing natural aggregate with recycled concrete aggregate. The following conclusions are drawn: 1. Densities of both RA concrete and NA concrete were within the range of normal weight concrete. Blue Ocean Research Journals 29

5 2. Both RA concrete and NA concrete showed the similar trends in the variation of strength and density with time. 3. Reduction in the 28-day compressive strength of concrete due to complete replacement of natural aggregates with recycled concrete aggregate range from 11% to 33%. 4. RA can replace NA in the production of both nonstructural and structural concrete. References [1] Naik T R., Sustainability of Concrete Construction. Practice Periodical on Structural Design and Construction 13(2), 2008, pp [2] Cement Concrete & Aggregates Australia, Use of Recycled Aggregates in Construction, May [3] Karthik Obla, Haejin Kim, and Colin Lobo, Crushed Returned Concrete as Aggregates for New Concrete, RMC REF Report: September [4] Etxeberia M., Vázquez E., Marí A. and Barra M., Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. Cement and Concrete Research, Vol.37 (5), 2007, pp [5] Adom-Asamoah, M., Wiafe Ampofo, J. and Owusu A.R., Flexural and shear behaviour of reinforced concrete beams made from recycled materials, Journal of Ghana Institution of Engineers, Vol.6&7,No. 1, 2009, pp [6] Pereira P., Evangelista L. and de Brito J. The effect of superplasticisers on the workability and compressive strength of concrete made with fine recycled concrete aggregates, Construction and Building Materials,28(1), 2012, pp [7] Pereira P., Evangelista L. and de Brito J. (2012)The effect of superplasticizers on the mechanical performance of concrete made with fine recycled concrete aggregates, Cement and Concrete Composites,34(9),pp [8] Mas B., Cladera A., del Olmo T., and Pitarch, F., Influence of the amount of mixed recycled aggregates on the properties of concrete for nonstructural use, Construction and Building Materials, 27(1), 2012, pp [9] Fung W.K., Durability of Concrete using Recycled Aggregates, SCCT Annual Concrete Seminar, February [10] Mirjana Malešev, Vlastimir Radonjanin, and Snežana Marinković, Recycled Concrete as Aggregate for Structural Concrete Production, Sustainability, 2, 2010, pp [11] Ghana Standards Authority, GS 22: Specifications for ordinary and rapid hardening portland cement, 2004, Accra. [12] British Standard Institution, BS 1348: Part 2: 1980, Test of Water for Making Concrete, BSI, London. [13] British Standard Institution. BS 1881: Part 108:1983, Method for making test cubes from fresh concrete, BSI, London. [14] British Standard Institution, BS 8110-Part I, 1997, The Structural Use of Concrete, BSI, London. [15] Neville A. M., Properties of concrete ( 4th Edition), Longman Group Ltd, 1996, London. Blue Ocean Research Journals 30