1. INTRODUCTION
⌅In
the modern era, sustainable development has become a central concern,
shaping discussions across various industries, especially in the
construction sector (11.
Lokeshwari M, Swamy CN. 2011. Sustainable development through recycling
of construction and demolition wastes in India. Nat. Environ. Pollut.
Technol. An. Int. Q. Sci. J. 10(01):27–32. Retrieved from www.neptjournal.com.
).
This field faces significant challenges related to resource depletion
and environmental impact, making the integration of recycled materials a
focal point (22.
Marinković S, Josa I, Braymand S, Tošić N. 2023. Sustainability
assessment of recycled aggregate concrete structures: A critical view on
the current state-of-knowledge and practice. Struct. Concr.
24(2):1956–1979. https://doi.org/10.1002/suco.202201245.
).
By 2030, construction and demolition (C&D) waste are projected to
increase by 2.59 billion tonnes, reaching 3.40 billion tonnes by 2050 (33.
Kaza S, Yao L, Bhada-Tata P, Van Woerden F. 2018. What a waste 2.0
introduction -snapshot of solid waste management to 2050. Overview
booklet. Urban Dev. Ser. 1–38. https://doi.org/10.1596/978-1-4648-1329-0_ch1.
).
Dumping this waste in landfills not only depletes disposal sites but
also leads to adverse environmental effects. One viable solution to this
problem is utilizing C&D waste as aggregates for producing new
concrete, given that 70-80% of concrete volume comprises aggregates (44.
Ahmed SFU. 2013. Properties of concrete containing construction and
demolition wastes and fly ash. J. Mater. Civ. Eng. 25(12):1864–1870. https://doi.org/10.1061/(ASCE)MT.1943-5533.0000763.
).
With concrete production demands increasing at a rate of 7.7% annually,
employing C&D waste as aggregates could be both economically and
environmentally beneficial (55.
Taffese WZ. 2018. Suitability investigation of recycled concrete
aggregates for concrete production: an experimental case study. Adv.
Civ. Eng. 2018:8368351. https://doi.org/10.1155/2018/8368351.
).
However, to ensure the feasibility of C&D waste-produced concrete
in various civil engineering projects, it must meet specific standards,
necessitating thorough examination (66. Asif H, Assas MM. 2013. Utilization of demolished concrete waste for new construction. Int. J. Civil. Environ. Struct. Constr. Archit. Eng. 07(01):37–42. Retrieved from: https://www.researchgate.net/publication/292770154.
). Structural health monitoring (SHM) is crucial for maintaining the long-term performance of concrete made from C&D waste (77.
Pedro D, de Brito J, Evangelista L. 2017. Structural concrete with
simultaneous incorporation of fine and coarse recycled concrete
aggregates: Mechanical durability and long-term properties. Constr. Build. Mater. 154:294–309. http://doi.org/10.1016/j.conbuildmat.2017.07.215.
).
Concrete structures often face stress due to drying-related shrinkage,
leading to fractures. Analyzing shrinkage strain is essential for
evaluating the long-term performance of fresh concrete (88.
Yu Y, Wang P, Yu Z, Yue G, Wang L, Guo Y. 2021. Study on the effect of
recycled coarse aggregate on the shrinkage performance of green recycled
concrete. Sustain. 13(23):13200. https://doi.org/10.3390/su132313200.
).
Moreover, surface electrical resistivity serves as a corrosion risk
indicator for concrete structures, guiding structural health monitoring
efforts (99.
Kurda R, de Brito J, Silvestre JD. 2018. Water absorption and
electrical resistivity of concrete with recycled concrete aggregates and
fly ash. Cem. Concr. Compos. 95:169-182. https://doi.org/10.1016/j.cemconcomp.2018.10.004.
).
Assessing chloride ion penetration in concrete is vital for corrosion
management, particularly when C&D waste is used as aggregates (1010. Whiting D, Mitchell TM. 1992. History of the rapid chloride permeability test. Transp. Res. Rec. 1335(1):55–62.
).
The microstructural properties of concrete are significantly influenced
by the interfacial transition zone (ITZ) between aggregates and cement
paste (1111.
Ke Y, Ortola S, Beaucour AL, Dumontet H. 2010. Identification of
microstructural characteristics in lightweight aggregate concretes by
micromechanical modelling including the Interfacial Transition Zone
(ITZ). Cem. Concr. Res. 40(11):1590–1600. https://doi.org/10.1016/j.cemconres.2010.07.001.
).
A dense and well-accounted ITZ fosters a strong bond, thereby enhancing
concrete performance. Considering these factors, the integration of
C&D waste into concrete production demands comprehensive analysis
and monitoring to ensure both economic viability and environmental
sustainability.
Concrete shrinkage poses a significant challenge
in the construction industry, prompting extensive studies to understand
its properties and effects. Well-designed laboratory experiments are
essential for comprehending concrete shrinkage characteristics (1212.
Tremper B, Spellman DC. 1963. Shrinkage of concrete-comparison of
laboratory and field performance. Highw. Res. Rec. 3:30–61. Retrieved
from http://onlinepubs.trb.org/Onlinepubs/hrr/1963/3/3-003.pdf.
).
In one investigation, the electrical resistivity of four different
cement types was analyzed using alternating current (A.C.) and direct
current (D.C.) electric fields, demonstrating that resistivity values
moderately fluctuate based on the size distribution of coarse particles (1313.
Hansson ILH, Hansson CM. 1983. Electrical resistivity measurements of
Portland cement based materials. Cem. Concr. Res. 13(5):675–683. https://doi.org/10.1016/0008-8846(83)90057-1.
, 1414.
Morris W, Moreno EI, Sagües AA. 1996. Practical evaluation of
resistivity of concrete in test cylinders using a wenner array probe.
Cem. Concr. Res. 26(12): 1779-1787. https://doi.org/10.1016/S0008-8846(96)00175-5.
).
Recycled aggregates have a notable impact on the strength of hardened
concrete. Studies indicate that recycled aggregates perform
satisfactorily at replacement ratios of 50% and 100% (1515.
Malešev M, Radonjanin V, Marinković S. 2011. Recycled concrete as
aggregate for structural concrete production. Sustainability.
3(2):465-468. https://doi.org/10.3390/su2051204.
).
For instance, a study on M20 concrete mix made with crushed demolition
and construction waste revealed that the compressive strength of
recycled aggregate concrete is typically 87% of natural aggregate
concrete on average. Although the slump of recycled aggregates is low,
improvements are possible (1616.
Madan Mohan Reddy K, Bhavani R, Ajitha B. 2012. Local construction and
demolition waste used as coarse aggregates in concrete. Int. J. Eng. Res. Appl. (IJERA). 2(5):1236-1238. Retrieved from https://www.ijera.com/papers/Vol2_issue5/GT2512361238.pdf.
).
When analyzing the drying shrinkage of recycled aggregate-based
concrete with or without fly ash, it was found that drying shrinkage can
be equivalent to conventional concrete. Without fly ash, there is a 25%
increase in drying shrinkage, which reduces to 7% with fly ash (1717.
Whiting B, McCarthy T, Lume E. 2013. Drying shrinkage of concrete made
from recycled concrete aggregate. From Mater. Struct. Adv. through
Innov. Proc 22nd Australas Conf. Mech. Struct. Mater. ACMSM 2012.
2013:1199–204.
). In terms of workability and
compressive strength, substituting 30% of natural aggregates with
recycled coarse aggregates yielded comparable results to conventional
concrete (1818. Monish M, Srivastava V, Agarwal VC, Mehta PK, Kumar R. 2013. Demolished waste as coarse aggregate in concrete. J. Acad. Indus. Res. 1(9):540.
).
Concrete resistance to chloride ion permeability, assessed using RCPT
(ASTM C1202), exhibited high reliability and accuracy, with a 10% lower
performance cost compared to other equipment. However, this tested
concrete proved less durable in marine environments (1919.
Iffat S, Emon AB, Manzur T, Ahmad SI. 2014. An experiment on durability
test RCPT. of concrete according to ASTM standard method using low-cost
equipments. Adv. Mater. Res. 974:335–340. https://doi.org/10.4028/www.scientific.net/AMR.974.335.
).
Moreover, experiments showed that the percentage of recycled aggregates
impacts concrete permeability, emphasizing its potential in C&D
waste-based concrete production (2020.
Özalp F, Yilmaz HD, Kara M, Kaya Ö, Şahin A. 2016. Effects of recycled
aggregates from construction and demolition wastes on mechanical and
permeability properties of paving stone kerb and concrete pipes. Constr. Build. Mater. 110:17–23. https://doi.org/10.1016/j.conbuildmat.2016.01.030.
).
Concrete with recycled aggregates and cementing agents demonstrated
lower permeability compared to conventional concrete, making it less
sensitive to stress (2121.
Wang H, Sun X, Wang J, Monteiro PJM. 2016. Permeability of concrete
with recycled concrete aggregate and pozzolanic materials under stress.
Materials. 9(4):252. https://doi.org/10.3390/ma9040252.
).
Studies exploring the partial replacement of cement with demolished
waste concrete powder over 10-20 years observed a 20% increase in
compressive strength. Surprisingly, the age of the concrete powder from
demolished waste had no discernible impact (2222.
Raihan R, Vajid A, Sabik M. 2017. Demolished waste concrete powder as
partial replacement of cement. Int. J. Res. Eng. Technol. 6(4):56-59. https://doi.org/10.15623/ijret.2017.0604013.
).
Electrical
resistivity emerges as a valuable tool for investigating concrete
strength and maturity in relation to curing time, binder type, and
aggregates, without causing damage (2323.
Mishra RK, Tripathi RK. 2017. Early age strength and electrical
resistivity of concrete as durability indicator through maturity. Int.
J. Earth. Sci. Eng. 10(3):677–82.
). Another study found that a mixture of 30% clay and 10% fly ash in ternary mixed concrete minimized drying shrinkage (2424.
Babu MS, Kumar TS. 2018. Study on drying shrinkage of ternary blended
concrete by partial replacement of cement with china clay and fly ash.
Int. J. Eng. Technol. 7(4.28):559-562. Retrieved from https://www.sciencepubco.com/index.php/ijet/article/view/25387/12952.
).
Concrete made from demolished waste aggregates can be equivalent to
conventional concrete with a 30% replacement for fine aggregates and 20%
for coarse aggregates (2525.
Gupta V, Patel A, Dubey G, Choudhary J, Gupta R, Dhawade S. 2018.
Experimental investigation of concrete on replacement of aggregates with
demolished. Int. Adv. Res. J. Sci. Eng. Technol. 5(3):190–197.
Retrieved from https://iarjset.com/wp-content/uploads/2018/07/ICACE-18-101.pdf.
).
Research revealed that replacing natural aggregates with demolition
project waste and clay brick aggregates at a 25% ratio leads to superior
concrete properties (2626.
Zheng C, Lou C, Du G, Li X, Liu Z, Li L. 2018. Mechanical properties of
recycled concrete with demolished waste concrete aggregate and clay
brick aggregate. Results Phys. 9:1317–1322. https://doi.org/10.1016/j.rinp.2018.04.061.
).
Utilizing C&D waste for concrete production enhances compressive
strength, especially when waste concrete materials (WCM) with admixtures
(WCA) are used at a 90% aggregate replacement percentage compared to
fresh concrete materials (FCM) (2727.
Dhapekar NK, Mishra SP. 2018. Efficient utilization of construction and
demolition waste in concrete. urban challenges emerg econimics.
Proceedings. ASCE India Conference 2017. Urbanization challenges in
emerging economies. 216–226. https://doi.org/10.1061/9780784482032.023.
).
Furthermore, a study investigating the drying shrinkage properties of
concrete from recycled heavy-weight glass and steel slag aggregates
found that increasing the proportion of steel slag replacements reduces
modulus and drying shrinkage, indicating enhanced durability (2828.
Choi SY, Kim IS, Yang EI. 2020. Comparison of drying shrinkage of
concrete specimens recycled heavyweight waste glass and steel slag as
aggregate. Materials. 13(22):5084. https://doi.org/10.3390/ma13225084.
).
Lastly, the electrical resistance of concrete made from recycled brick
aggregates decreases as the percentage of recycled brick aggregates
(RBA%) rises (2929.
Azba AH, Alnuman BS. 2021. Strength and electrical resistivity of
recycled concrete made of aggregates from waste bricks. Proc. 7th Int.
Eng. Conf. Research Innov. Amid. Glob. Pandemic IEC 2021. 96–100. https://doi.org/10.1109/IEC52205.2021.9476139.
).
These findings collectively emphasize the potential and versatility of
recycled materials in improving various aspects of concrete production.
Recycling
construction and demolition (C&D) waste into recycled aggregates
(RCA) has become increasingly popular for sustainable concrete
production, contributing to reduced landfill waste, conservation of
natural resources, and the promotion of the circular economy (3030.
Panghal H, Kumar A. 2023. Effects of surface modified recycled coarse
aggregates on concrete’s mechanical characteristics. Mater. Res.
Express. 10(9):095506. https://doi.org/10.1088/2053-1591/acf915.
).
However, ensuring the quality and performance of recycled aggregates
meet industry standards is of paramount importance. Researchers have
conducted various studies by replacing coarse aggregates in different
proportions. Yet, no specific technique has been proposed to determine
the optimal percentage of RCA replacement for achieving the highest
strength in concrete. This study addresses this gap by focusing on these
parameters, alongside an in-depth analysis of mechanical properties and
other essential characteristics.
The notable aspects of this research are as follows:
-
This study delves into the efficient utilization of RCA for sustainable concrete production. While RCA is often incorporated into concrete at limited proportions, this research examines five different ratios (0%, 25%, 50%, 75%, and 100%) to comprehensively assess their impact.
-
The study bridges existing research gaps by employing standard compacting methods to establish the order of compressive strength. Additionally, it verifies the influence of RCA proportions on various aspects, including workability (fresh), and properties such as compressive strength, flexural strength, split tensile strength, drying shrinkage, electrical resistivity, and rapid chloride penetration (hardened). Microstructural characteristics are also examined through techniques like XRD, SEM, and EDAX. By comparing the results with the control mix, this study identifies the most significant differences.
-
The research investigates the effects of RCA on the morphology and composition, including chemical and mineralogical aspects, of RCA concrete. By analyzing the generation of microstructure, this study sheds light on the influence of RCA on the concrete’s microstructure and emphasizes the importance of determining the optimum RCA content for sustainable concrete development.
In summary, this study not only explores the efficient use of RCA for concrete production but also addresses crucial research gaps by employing rigorous methodologies to assess a wide range of properties. Furthermore, it delves into the intricate details of microstructural changes, emphasizing the significance of finding the optimal RCA content to advance the development of sustainable concrete.
2. EXPERIMENTAL PROGRAMME
⌅2.1. Materials
⌅The study employs Ordinary Portland cement grade 43, compliant with IS 269-2015 (3131. IS:269 IS: 269-2015. 2017. Ordinary portland c ement indian standard New Delhi. 41(December 2015).
). Table 1 presents the physical test results for the cement.
| S.N. | Types | Value Measured | As per IS 269-2015 (31) |
|---|---|---|---|
| 1 | Consistency | 31% | - |
| 2 | Initial Setting Time | 58 | ˃ 30 Minutes |
| 3 | Final setting Time | 435 | ˂ 10 Hours |
| 4 | Specific Gravity | 3.11 | 3.0 to 3.15 |
For fine aggregate, natural sand with a particle size less than 4.75 mm, confirming to IS 383-2016 (Reaffirmed 2021) (3232. IS:383; 2016. 2016. Coarse and fine aggregate for concrete -— Specification. Bur Indian Stand New Delhi India. 19(January).
),
is used. The coarse aggregates consist of crushed stone ranging from
4.75 mm to 20 mm, confirming to IS 383-2016 (Reaffirmed 2021) (3232. IS:383; 2016. 2016. Coarse and fine aggregate for concrete -— Specification. Bur Indian Stand New Delhi India. 19(January).
).
This aggregate, passing through 40 mm sieves and retained on 4.75 mm
sieves, has a passing percentage of 90.76% for 20 mm sieves and 4.52%
for 10 mm sieves. Recycled coarse aggregates (RCA) from crushed concrete
C&D waste are used after reducing them to the necessary size using
an impact crusher. These RCA, also passing through 40 mm sieves and
retained on 4.75 mm sieves, exhibit passing percentages of 92.10% for 20
mm sieves and 0.38% for 10 mm sieves. Table 2 provides a detailed overview of particle packing density within
specific cylindrical volumes, achieved through standard compaction
techniques.
| S. No | Coarse Aggregates Mixtures | Weight |
|---|---|---|
| 1 | NCA | 14.68 |
| 2 | RCA 25 + NCA 75 | 15.25 |
| 3 | RCA 50 + NCA 50 | 14.62 |
| 4 | RCA 75 + NCA 25 | 13.73 |
| 5 | RCA 100 | 13.15 |
This analysis involves various mixtures of coarse aggregates, revealing crucial insights into their compactness and structural viability. Firstly, the table outlines the results for NCA (Natural Coarse Aggregates), where the weight attained within the specific cylindrical volume stands at 14.68 units. Subsequently, the combination of RCA 25 + NCA 75 showcases a notably higher packing density, registering a weight of 15.25 units. Another blend, RCA 50 + NCA 50, which comprises equal parts of Recycled Coarse Aggregates (RCA) and Natural Coarse Aggregates (NCA), exhibits a weight of 14.62 units within the specified volume. A mixture with RCA 75 + NCA 25 composition, consisting of 75% RCA and 25% NCA, yields a slightly lower weight of 13.73 units. Lastly, using RCA 100, indicating the complete substitution of Natural Coarse Aggregates with 100% Recycled Coarse Aggregates, results in a weight of 13.15 units within the specific cylindrical volume. Significantly, the data highlights that the blend of RCA 25 + NCA 75boasts the densest packing density among these combinations. This finding is essential, indicating superior structural integrity. This critical insight, complemented by Figure 1 showcasing particle size distribution, informs the selection of optimal concrete compositions for construction applications, ensuring the durability and stability of structures.
Table 3 presents the physical and mechanical properties of both natural and recycled coarse aggregates, alongside natural fine aggregates (NFA), natural coarse aggregates (NCA), and recycled coarse aggregates (RCA).
| Property | NFA | NCA | RCA | Standard Limits |
|---|---|---|---|---|
| Bulk Density (kg/m3) | 1625 | 1740 | 1660 | 1200-1750 (3333.
IS 2386- Part III 1963. 2021. Methods of test for aggregates for
concrete. Bur Indian Stand New Delhi India. Reaffirmed 2021. ) |
| Specific gravity | 2.675 | 2.754 | 2.681 | 2.30-2.90 (3333.
IS 2386- Part III 1963. 2021. Methods of test for aggregates for
concrete. Bur Indian Stand New Delhi India. Reaffirmed 2021. ) |
| Water Absorption (%) | 0.51 | 1.12 | 2.35 | ≤ 2.0 (IS 2386 Part 3) (3333.
IS 2386- Part III 1963. 2021. Methods of test for aggregates for
concrete. Bur Indian Stand New Delhi India. Reaffirmed 2021. ) |
| Abrasion Loss (%) | 15.52 | 25.43 | 28.76 | ˂ 30 (IS 2386 Part 4) (3434.
IS: 2386; Part IV. 1963. 2021. Methods of test for aggregates for
concrete. Bur Indian Stand New Delhi. 2366 (Reaffirmed 2021). ) |
| Crushing Value (%) | 16.21 | 26.24 | 27.71 | ˂ 30 (IS 2386 Part 4) (3434.
IS: 2386; Part IV. 1963. 2021. Methods of test for aggregates for
concrete. Bur Indian Stand New Delhi. 2366 (Reaffirmed 2021). ) |
| Impact Value (%) | 15.31 | 17.31 | 20.68 | ˂ 30 (IS 2386 Part 4) (3434.
IS: 2386; Part IV. 1963. 2021. Methods of test for aggregates for
concrete. Bur Indian Stand New Delhi. 2366 (Reaffirmed 2021). ) |
Furthermore, a chemical admixture in the form of super-plasticizers (C-MAX), conforming to IS 9103-1999 (3535. IS 9103. 1999. Specification for concrete admixtures. Bur Indian Stand Dehli. 1–22.
),
is used at 1% by weight of cement. Potable water from the laboratory is
employed for mixing and curing purposes, ensuring consistent and
reliable experimental conditions.
2.2. Mix proportions
⌅To investigate the viability and impact of recycled coarse aggregates on the mechanical properties of concrete, five distinct concrete mixtures were formulated to achieve target strength of 27MPa. The compositions of these mixtures are detailed in Table 4. The reference mixture (RC) was crafted using natural aggregates. Additionally, four other mixtures, namely RCA-25, RCA-50, RCA-75, and RCA-100, were created by substituting natural coarse aggregates with recycled coarse aggregates at replacement percentages of 25%, 50%, 75%, and 100%, respectively. All these concrete blends were meticulously prepared through the weight batching method, maintaining a consistent water-cement ratio of 0.50. This methodical approach allowed for a systematic exploration of the mechanical behavior of concrete when integrating various proportions of recycled coarse aggregates, offering valuable insights into their feasibility and effectiveness in enhancing the concrete’s overall performance.
| S. No. | Mixture ID | NFA | NCA | RCA | Cement | W/C Ratio | Admixture |
|---|---|---|---|---|---|---|---|
| 1 | RC | 444.48 | 1511 | 0 | 400 | 0.5 | 4 |
| 2 | RCA 25 | 444.48 | 1133.25 | 377.75 | 400 | 0.5 | 4 |
| 3 | RCA 50 | 444.48 | 755.5 | 755.5 | 400 | 0.5 | 4 |
| 4 | RCA 75 | 444.48 | 377.75 | 1133.25 | 400 | 0.5 | 4 |
| 5 | RCA 100 | 444.48 | 0 | 1511 | 400 | 0.5 | 4 |
2.3. Testing programs
⌅Various
tests were conducted to assess different aspects of structural
concrete. Workability, following the IS 1199-1959 standard (3636. IS 1199. 1959. Methods of sampling and analysis of concrete. Bur Indian Stand. 1–49.
),
was measured to gauge the freshness of the concrete. Additionally,
mechanical characteristics were evaluated through tests for compressive
strength, flexural strength, and split tensile strength. To analyze the
long-term performance of concrete, tests were performed for drying
shrinkage (in accordance with IS 516 Part 6, 2020 (3737.
IS 516 Part 6. 2020. Determination of drying shrinkage and moisture
movement of concrete samples. Burau Indian Stand. 516(June). Retrieved
from: www.standardsbis.in.
)), electrical resistivity (following RILEM TC 154-EMC standards (3838. Polder R. 2000. RILEM TC 154 EMC: Electrochemical technique for measuring metallic corrosion. Mater. Struct. 33:603–611. https://doi.org/10.1007/BF02480599.
)), and rapid chloride penetration (as per ASTM C1202 guidelines (3939.
ASTM C1202. 2012. Standard test method for electrical indication of
concrete’s ability to resist chloride ion penetration. Am. Soc. Test
Mater. 2012(C):1–8.
)). Moreover, the microstructural
features of concrete samples from different mixtures were examined using
scanning electron microscopy (SEM), energy-dispersive X-ray
spectroscopy (EDAX), and X-ray diffraction (XRD) analyses. These
comprehensive assessments provided valuable insights into the various
properties and performance aspects of the concrete structures under
study.
3. RESULTS AND DISCUSSION
⌅3.1. Workability
⌅A slump test, in accordance with IS 1199-1959 (3636. IS 1199. 1959. Methods of sampling and analysis of concrete. Bur Indian Stand. 1–49.
),
was conducted to assess the workability of concrete mixes containing
varying proportions of Construction and Demolition (C&D) waste in
the form of recycled coarse aggregates. The results of these tests are
summarized in Table 5.
| Concrete Mix | Slump (mm) |
|---|---|
| RC | 111 |
| RCA 25 | 101 |
| RCA 50 | 95 |
| RCA 75 | 83 |
| RCA 100 | 70 |
The variations in slump for the different concrete mixtures are graphically represented in Figure 2.
The figure illustrates a consistent decrease in concrete slump with the
increase in recycled coarse aggregates. The workability of concrete
incorporating Recycled Coarse Aggregates (RCA) falls within the medium
range of 50 to 100 mm, as indicated by the slump values across all
mixtures. This reduction in slump is attributed to RCA absorbing more
water than natural aggregates, primarily due to the presence of old
adhered mortar on the RCA surface, which absorbs additional water. The
trend clearly demonstrates that an escalation in RCA content leads to a
decrease in the concrete slump. This pattern aligns with findings from a
previous study (4040.
Shahidan S, Azmi MAM, Kupusamy K, Zuki SSM, Ali N. 2017. Utilizing
construction and demolition (C&D) waste as recycled aggregates (RA)
in concrete. Procedia Engineering. 174:1028–1035. https://doi.org/10.1016/j.proeng.2017.01.255.
), confirming the consistent impact of increasing RCA content on reducing concrete workability.
3.2. Compressive strength
⌅A total of 30 specimens, moulded in steel cubes with dimensions 15×15×15 cm, were cast, cured, and subjected to compressive strength testing across various concrete mixtures. Using a compression testing machine with a capacity of 2000 KN, the samples were analyzed at 7 and 28 days, and the results are detailed in Table 6. Figure 3 visually represents the variations in compressive strength at 7 and 28 days for different replacement percentages of Recycled Coarse Aggregates (RCA) concerning the reference mixture. The results clearly indicate that concrete mixtures with higher RCA replacements exhibit lower compressive strength compared to the reference mixture. Notably, the optimum replacement percentage of recycled coarse aggregates is 25%.
| Concrete Mix | Compressive Strength (MPa) | |||
|---|---|---|---|---|
| 7 Days | % Variation With Respect to RC | 28 Days | % Variation With Respect to RC | |
| RC | 22.71 | - | 31.99 | - |
| RCA 25 | 25.44 | +12.02 | 35.69 | +11.56 |
| RCA 50 | 22.12 | -02.59 | 31.45 | -01.68 |
| RCA 75 | 19.02 | -16.24 | 27.74 | -13.28 |
| RCA 100 | 16.31 | -28.18 | 22.99 | -28.13 |
+ sign represents increase in strength and - sign represents decrease in strength.
This
observed trend can be explained by examining the voids and particle
packing within the mixtures. At higher percentage of RCA, especially at
25% replacement, results in fewer voids and optimal particle packing,
leading to higher strength. The study further emphasizes the importance
of fine aggregates’ particle size, specifically particles smaller than
600 microns, in forming a sufficient paste to fill voids in larger
particles. A considerable gap between these particle sizes results in
enhanced strength within specified limits. Conversely, smaller gaps lead
to reduced strength. At higher replacement percentages, increased void
content and the presence of old adhered mortar weaken the bonding with
aggregates, causing a decline in strength. Among the mixtures, RCA 25
exhibits the highest compressive strength (35.69 MPa), while RCA 100
displays the lowest (22.92 MPa). These findings align with previous
studies, such as those by Kessal et al. (4141.
Kessal O, Belagraa L, Noui A, Maafi N. 2020. Performance study of
eco-concrete based on waste demolition as recycled aggregates. Mater.
Int. 2(2):123–130. https://doi.org/10.33263/Materials22.123130.
), and support the results of Ankesh et al. (4242.
Ankesh SB, Venugopal ML, Sumanth S, Vinodkumar, Abhishek BS. 2020.
Experimental study on concrete blocks from recycled aggregates. Int.
Res. J. Eng. Technol. 7(8):277–288. Retrieved from https://www.irjet.net/archives/V7/i8/IRJET-V7I847.pdf.
), study, indicating that a 30% replacement of recycled coarse aggregates achieves strength exceeding the target levels.
The
strategic incorporation of Recycled Concrete Aggregate (RCA) at a 25%
replacement rate has showcased its significant potential, prompting a
detailed exploration of its multifaceted applications (4343.
Avindana J, Kumar Mittal S, Dhapekar NK. 2017. Applicability of
construction and demolition waste concrete in construction
sector-review. Int. J. Civ. Eng. Res. 8(2): 131-138. Retrieved from: https://www.ripublication.com/ijcer17/ijcerv8n2_05.pdf.
).
This optimized mixture has proven highly advantageous across diverse
construction sectors. Non-structural elements like decorative facades
and interior designs can now benefit from an eco-friendly and
cost-effective solution. In the domain of pavements, where durability
and load-bearing capacity are critical, this blend aligns seamlessly
with sustainable infrastructure goals. Furthermore, its adaptability
extends to specialized projects, particularly in eco-friendly
constructions and landscaping efforts, where minimizing the carbon
footprint is imperative. In these contexts, RCA-integrated concrete
emerges as a sustainable choice, combining aesthetic appeal with
environmental consciousness. While higher RCA percentages do impact
concrete properties, the construction industry’s commitment to
sustainability leads to exploration beyond traditional structural
applications. In scenarios where sheer structural strength isn’t the
sole focus, such as in landscaping or road base layers prioritizing
stability, RCA-integrated concrete proves invaluable. In environmentally
conscious construction projects emphasizing innovation and
eco-friendliness, this approach gains significant traction. The
versatility of RCA-integrated concrete expands beyond conventional
boundaries, finding utility in diverse construction contexts. By
exploring these alternative paths and carefully evaluating different
construction scenarios, the industry can harness the adaptability and
effectiveness of this optimized blend. This method not only tackles
challenges associated with higher RCA percentages but also pioneers a
future where sustainability and ecological responsibility drive
construction practices. Through these strategic integrations, the
industry edges closer to its sustainability objectives while enriching
the landscape of eco-conscious construction methodologies.
3.3. Flexural strength
⌅Thirty specimens, cast in steel rectangular moulds measuring 50×10×10 cm, underwent curing and flexural strength testing across various concrete formulations. These samples were meticulously evaluated using a flexural testing machine with a capacity of 2000 KN. The average flexural strength values were derived from three specimens for each mix, and the tests were conducted at both 7 and 28 days, with the results documented in Table 7.
| Concrete Mix | Flexural Strength (MPa) | |||
|---|---|---|---|---|
| 7 Days | % Variation With Respect to RC | 28 Days | % Variation With Respect to RC | |
| RC | 3.18 | - | 3.92 | - |
| RCA 25 | 3.28 | +03.14 | 4.04 | +03.06 |
| RCA 50 | 3.09 | -02.83 | 3.87 | -01.27 |
| RCA 75 | 2.90 | -08.80 | 3.62 | -07.65 |
| RCA 100 | 2.68 | -15.72 | 3.34 | -14.79 |
+ sign represents increase in strength and - sign represents decrease in strength.
Figure 4 graphically illustrates the variations in flexural strength at 7 and 28
days for different replacement percentages of Recycled Coarse
Aggregates (RCA) concerning the reference mixture. Notably, the results
mirrored a pattern similar to compressive strength, demonstrating that
concrete mixtures with higher RCA replacement percentages exhibit lower
flexural strength than the reference mixture. Among the mixtures, RCA 25
showcased the highest flexural strength (4.04 MPa), while RCA 100
displayed the lowest flexural strength (3.34 MPa). These findings are
consistent with the observations made by other researchers (4040.
Shahidan S, Azmi MAM, Kupusamy K, Zuki SSM, Ali N. 2017. Utilizing
construction and demolition (C&D) waste as recycled aggregates (RA)
in concrete. Procedia Engineering. 174:1028–1035. https://doi.org/10.1016/j.proeng.2017.01.255.
, 4444.
Surya M, Rao VVLK, Lakshmy P. 2013. Recycled aggregate concrete for
transportation infrastructure. Procedia Soc. Behav. Sci. 104:1158–1167. https://doi.org/10.1016/j.sbspro.2013.11.212
), indicating a parallel trend between compressive and flexural strength concerning varying RCA replacement percentages.
3.4. Split tensile strength
⌅A comprehensive evaluation was conducted on 30 specimens cast in steel cylindrical moulds with a diameter (ϕ) of 15 cm and height (H) of 30 cm. These specimens were meticulously cured and subjected to split tensile strength testing using a compression testing machine with a capacity of 2000 KN. The average split tensile strength values were derived from three specimens for each mix, and the tests were conducted at both 7 and 28 days, with the results detailed in Table 8.
| Concrete Mix | Split Tensile Strength (MPa) | |||
|---|---|---|---|---|
| 7 Days | % Variation With Respect to RC | 28 Days | % Variation With Respect to RC | |
| RC | 2.19 | - | 2.51 | - |
| RCA 25 | 2.36 | +07.76 | 2.64 | +05.17 |
| RCA 50 | 2.12 | -03.19 | 2.47 | -01.59 |
| RCA 75 | 1.91 | -12.78 | 2.25 | -10.35 |
| RCA 100 | 1.76 | -19.63 | 2.11 | -15.93 |
+ sign represents increase in strength and - sign represents decrease in strength.
The
variation in split tensile strength for different replacement
percentages of Recycled Concrete Aggregate (RCA) concerning the
reference mixture is depicted in Figure 5.
The trend observed in the results parallels that of compressive
strength. It was discerned that the split tensile strength of concrete
mixtures decreased concerning the reference mixture with higher
proportions of RCA replacement. Among the mixtures, RCA 25 exhibited the
highest split tensile strength (2.64 MPa), while RCA 100 displayed the
lowest (2.11 MPa) in the group. These findings align with the work
reported by Kessal et al. (4141.
Kessal O, Belagraa L, Noui A, Maafi N. 2020. Performance study of
eco-concrete based on waste demolition as recycled aggregates. Mater.
Int. 2(2):123–130. https://doi.org/10.33263/Materials22.123130.
).
3.5. Drying shrinkage
⌅The
assessment of concrete mixes’ drying shrinkage involved measuring their
length variations over time, adhering to IS 516 (Part 6) 2020 (3737.
IS 516 Part 6. 2020. Determination of drying shrinkage and moisture
movement of concrete samples. Burau Indian Stand. 516(June). Retrieved
from: www.standardsbis.in.
) standards, as depicted in Figure 6.
Each of the 5 different mixtures was represented by three specimens of
size 75×75×285 mm, which were stored for 28 days. Initial readings were
recorded using length comparator tools with a precision count of 0.005
mm. Subsequent readings were taken at 28, 56, and 90 days using dial
gauges, with each specimen’s length change calculated based on the
difference between the final and initial values. The outcomes of the
drying shrinkage tests are summarized in Table 9.
| Concrete Mix | Drying Shrinkage (10-6) | |||||
|---|---|---|---|---|---|---|
| 28 Days | % Variation With Respect to RC | 56 Days | % Variation With Respect to RC | 90 Days | % Variation With Respect to RC | |
| RC | 344 | - | 422 | - | 461 | - |
| RCA 25 | 362 | +05.23 | 457 | +08.29 | 496 | +07.59 |
| RCA 50 | 415 | +20.63 | 501 | +18.72 | 549 | +19.08 |
| RCA 75 | 464 | +34.88 | 562 | +33.17 | 595 | +29.06 |
| RCA 100 | 499 | +45.05 | 624 | +47.86 | 641 | +39.04 |
+ sign represents increase in drying shrinkage and - sign represents decrease in drying shrinkage.
Figure 7 illustrates the variations in shrinkage strain concerning different concrete mixtures at different drying ages. The graph demonstrates how concrete contracts more as it dries, indicated by the relationship between the two factors. Notably, the slope of the drying shrinkage curve changes from steep to flat as drying time increases, indicating a decreasing rate of change in drying shrinkage over time.
Additionally,
it was observed that the drying shrinkage of concrete steadily
increases with the rise in the percentage of coarse aggregate
replacement at all drying ages. This increase can be attributed to the
presence of additional cement around the surface of Recycled Concrete
Aggregate (RCA), absorbing more water and shrinking significantly during
the drying process. While the addition of RCA led to increased drying
shrinkage due to its higher water absorption, all mixtures remained
within the typical range specified by Indian Standards, indicating that
RCA can be used within specific limits without adversely affecting its
shrinkage properties. These findings align with previous studies, such
as Kioumarsi et al. (4545.
Kioumarsi M, Azarhomayun F, Haji M, Shekarchi M. 2020. Effect of
shrinkage reducing admixture on drying shrinkage of concrete with
different W/C ratios. Materials. 13(24):5721. https://doi.org/10.3390/ma13245721
),
and support the systematic increase in concrete’s drying shrinkage with
higher proportions of recycled coarse aggregates, as observed by Yu et
al. (88.
Yu Y, Wang P, Yu Z, Yue G, Wang L, Guo Y. 2021. Study on the effect of
recycled coarse aggregate on the shrinkage performance of green recycled
concrete. Sustain. 13(23):13200. https://doi.org/10.3390/su132313200.
).
3.6. Electrical resistivity
⌅The
electrical resistivity of concrete is a critical factor in assessing
the risk of corrosion for the steel reinforcement bars within the
concrete structure. The method employed measures the surface electrical
resistivity of fresh concrete using a four-electrode setup. Utilizing
the Resipod Resistivity Meter, illustrated in Figure 8, which features a 4-point Wenner Probe following the RILEM TC 154-EMC (3838. Polder R. 2000. RILEM TC 154 EMC: Electrochemical technique for measuring metallic corrosion. Mater. Struct. 33:603–611. https://doi.org/10.1007/BF02480599.
)
standard, concrete samples are prepared in cylindrical molds with a
diameter of 100 mm and a height of 200 mm. Tests are conducted at 28 and
56 days, and the results are presented in Table 10.
| Concrete Mix | Electrical Resistivity (KΩ) | |||
|---|---|---|---|---|
| 28 Days | % Variation With Respect to RC | 56 Days | % Variation With Respect to RC | |
| RC | 48.54 | - | 59.19 | - |
| RCA 25 | 52.81 | +08.79 | 63.45 | +07.19 |
| RCA 50 | 46.69 | -03.81 | 57.28 | -03.22 |
| RCA 75 | 37.33 | -23.09 | 47.76 | -19.31 |
| RCA 100 | 16.01 | -67.01 | 26.29 | -55.58 |
+ sign represents increase in electrical resistivity and - sign represents decrease in electrical resistivity.
The variation for the 28 and 56 days old electrical resistivity for different replacement percentages of C&D waste aggregates concerning the reference mixture is shown in Figure 9.
The
results reveal that concrete mixtures with higher percentages of RCA
replacement exhibit lower electrical resistivity compared to the
reference mixture, indicating reduced corrosion risks. The high porosity
of RCA traps water with dissolved ions, creating a low-resistance route
for electric current, thereby lowering electrical resistivity.
Specifically, RCA 25 demonstrates the highest electrical resistivity,
while RC and RCA 50 are comparable. In descending order, RCA 75 and RCA
100 exhibit the least electrical resistivity. These findings suggest
that concrete produced with up to 75% RCA replacement poses a low risk
of corrosion, whereas concrete with 100% RCA replacement presents a
moderate risk. This aligns with prior studies (4545.
Kioumarsi M, Azarhomayun F, Haji M, Shekarchi M. 2020. Effect of
shrinkage reducing admixture on drying shrinkage of concrete with
different W/C ratios. Materials. 13(24):5721. https://doi.org/10.3390/ma13245721
). and resonates with Arredondo-Rea et al.’s research (4646.
Arredondo-Rea SP, Corral-Higuera R, Gómez-Soberón JM, Gámez-García DC,
Bernal-Camacho JM, Rosas-Casarez CA. 2019. Durability parameters of
reinforced recycled aggregate concrete: case study. Appl. Sci. 9(4):617. https://doi.org/10.3390/app9040617
),
which concluded that electrical resistivity in concrete is minimally
impacted when replacing up to 30% of coarse aggregates and 20% of fine
aggregates with recycled counterparts. Consequently, concrete derived
from C&D waste indicates a moderate risk of corrosion for the
embedded steel bars.
3.7. Rapid chloride penetration
⌅The RCPT unit, depicted in Figure 10, follows the ASTM C1202 (3939.
ASTM C1202. 2012. Standard test method for electrical indication of
concrete’s ability to resist chloride ion penetration. Am. Soc. Test
Mater. 2012(C):1–8.
) standard, featuring a mold
measuring 100 mm in diameter and 50 mm in height. Thirty specimens were
prepared for the study. The unit comprises two chambers filled with
sodium chloride (NaCl) at a concentration of 0.24M and sodium hydroxide
(NaOH) at 0.3M. Specimens and test cells are meticulously sealed. A 60V
current source connects the negative end to the NaCl cell and the
positive end to the NaOH cell. Readings are taken at 30-minute intervals
for up to 6 hours. Tests at 28 and 56 days gauge the charge passed,
indicating resistance to chloride ion penetration. Results are presented
in Table 11.
| Concrete Mix | Charge Passed (Coulomb) | |||
|---|---|---|---|---|
| 28 Days | % Variation With Respect to RC | 56 Days | % Variation With Respect to RC | |
| RC | 704 | - | 683 | - |
| RCA 25 | 671 | -04.68 | 636 | -06.88 |
| RCA 50 | 824 | +17.04 | 782 | +14.49 |
| RCA 75 | 953 | +35.36 | 950 | +39.09 |
| RCA 100 | 1053 | +49.57 | 1050 | +53.73 |
+ sign represents increase in charge passed and - sign represents decrease in charge passed.
The variation in chloride permeability for different replacement percentages of C&D waste aggregates concerning the reference mixture is demonstrated in Figure 11.
Chloride
permeability tests reveal that concrete with higher percentages of
C&D waste aggregate replacement has increased chloride permeability
compared to the reference mixture, indicating reduced durability. This
rise is attributed to the high porosity of C&D waste aggregates,
trapping water with ions, creating a low-resistance path for electric
current. The highest chloride permeability occurs at 100% RCA
replacement. Concrete with up to 50% RCA replacement exhibits high
durability, whereas concrete with over 75% RCA replacement shows
moderate durability. Increasing RCA replacement decreases concrete
durability and increases chloride permeability. The study aligns with
Yang and Lee’s findings (4747.
Yang S, Lee H. 2021. Drying shrinkage and rapid chloride penetration
resistance of recycled aggregate concretes using cement paste
dissociation agent. Materials. 14(6):1478. https://doi.org/10.3390/ma14061478
),
indicating decreased chloride ion penetration with concrete age
regardless of RCA content. This research aligns with Arredondo-Rea et
al.’s study (4646.
Arredondo-Rea SP, Corral-Higuera R, Gómez-Soberón JM, Gámez-García DC,
Bernal-Camacho JM, Rosas-Casarez CA. 2019. Durability parameters of
reinforced recycled aggregate concrete: case study. Appl. Sci. 9(4):617. https://doi.org/10.3390/app9040617
), underscoring that C&D waste concrete exhibits moderate chloride permeability.
3.8. X-Ray Diffraction (XRD)
⌅X-ray Diffraction (XRD) is a powerful analytical technique employed to explore the crystalline characteristics and mineralogical composition of concrete samples, especially when incorporating recycled coarse aggregates. In this study, the Bruker D-8 advanced diffractometer system was utilized, scanning samples at an angle of 2 degrees within the range of 3-70 degrees. The scanning speed was set at 2 degrees per minute with a sampling interval of 0.005 degrees. The obtained data were analyzed using Jade 7 X-ray diffraction software. The XRD analysis revealed intriguing insights. As the replacement percentage of Recycled Coarse Aggregates (RCA) increased, the net intensity of minerals such as Calcium Silicate Hydrate (CSH), Calcium Hydroxide (CH), and Ettringite decreased. This decrease indicated a reduction in the density of total CSH in the concrete mixtures. Figure 12 vividly displays the XRD patterns for different concrete mixtures, including RC, RCA 25, RCA 50, RCA 75, and RCA 100. To provide a comprehensive understanding, the diffraction peak angles of various compounds are presented in Table 12.
| Concrete Mix | CSH (2Ɵ) | ETTRINGITE (2Ɵ) | CH (2Ɵ) |
|---|---|---|---|
| RC | 26.65 | 37.84 | 50.00 |
| RCA 25 | 26.66 | 40.48 | 50.22 |
| RCA 50 | 27.86 | 39.24 | 48.32 |
| RCA 75 | 21.24 | 29.50 | 48.48 |
| RCA 100 | 27.58 | 39.40 | 50.12 |
CSH- Calcium Silicate Hydrate, CH- Calcium Hydroxide, Ettringite- Hydrated Calcium Aluminum Sulfate.
The
XRD analysis clearly illustrates that the incorporation of recycled
coarse aggregates significantly influences the phase composition of
minerals in concrete. The table presents detailed information regarding
the diffraction peak angles of three essential compounds - Calcium
Silicate Hydrate (CSH), ettringite, and Calcium Hydroxide (CH) - in
various concrete mixtures. These mixtures are denoted as RC, RCA 25, RCA
50, RCA 75, and RCA 100. CSH, a vital mineral phase in concrete
responsible for its strength and durability, is represented by a
diffraction peak angle of 26.65° in regular concrete (RC). Ettringite, a
hydrated calcium aluminum sulfate compound, exhibits a peak at 37.84°,
while CH, also known as hydrated lime, shows a peak at 50.00° in this
standard mixture. It is important to note that the intensity of these
peaks indicates the quantity of these compounds; however, the
non-homogeneity of the concrete samples at the microscopic level was
considered during the analysis. The analysis extends to concrete
mixtures incorporating recycled concrete aggregate (RCA) at different
percentages. Specifically, RCA 25, RCA 50, RCA 75, and RCA 100 represent
concrete mixes with 25%, 50%, 75%, and 100% recycled aggregate content,
respectively. As the proportion of recycled aggregate increases, the
diffraction peak angles for CSH, ettringite, and CH vary. For instance,
in the RCA 75 mixture, the diffraction peak angles for CSH, Ettringite,
and CH shift to 21.24°, 29.50°, and 48.48°, respectively. This
alteration in peak angles suggests potential changes in the crystal
structure of CSH and ettringite due to the introduction of 75% recycled
concrete aggregate. This study aligns with previous research conducted
by Joseph et al. (4848.
Joseph HS, Pachiappan T, Avudaiappan S, Flores EIS. 2022. A study on
mechanical and microstructural characteristics of concrete using
recycled aggregate. Materials. 15(21):7535. https://doi.org/10.3390/ma15217535
),
highlighting the consistency of these findings in the scientific
community. In summary, the table’s data offers insights into the
composition of concrete mixtures, focusing on changes in key compounds
(CSH, ettringite and CH) as recycled concrete aggregate percentages
vary. These changes significantly impact concrete properties and
performance in construction, emphasizing the need for effective material
engineering and construction practices.
3.9. Scanning electron microscopy (SEM)
⌅The analysis of Scanning Electron Microscopy (SEM) plays a pivotal role in understanding the intricate microstructure and surface morphology of concrete samples, especially when incorporating Recycled Coarse Aggregates (RCA). In this study, the JSM 6610V scanning electron microscope, operating at 30KV, was utilized at the University Science Instrumentation Center (USIC) in Delhi. Figure 13 showcases detailed micrographs obtained at 28 days for different concrete mixtures, shedding light on the formation of crucial hydration products at the microstructural level, which significantly influence concrete strength. In these micrographs, distinct features reveal the presence of key compounds formed during the hydration process. Hexagonal crystal formations signify the presence of calcium hydroxide (CH), while flower-shaped structures indicate calcium silicate hydrate (CSH) gel, and needle-like structures are evidence of ettringite. These compounds are essential contributors to concrete strength and durability.
The
SEM analysis yielded crucial insights. Incorporating 25% RCA led to a
significant enhancement of the concrete microstructure, producing a
dense cement paste. This dense paste fostered a robust bond with
aggregates, ultimately enhancing the compressive strength of the
concrete. However, when the RCA replacement percentage was increased to
50%, the resulting concrete exhibited lower density, leading to a
reduction in compressive strength. A further increase to 75% RCA content
resulted in the formation of voids and loose structures, leading to
decreased compressive strength. Remarkably, an RCA replacement of 100%
yielded a porous microstructure, where incomplete hydration reactions
occurred around the recycled coarse aggregates, resulting in fewer
hydration products and, consequently, reduced strength. The SEM findings
highlighted a crucial pattern: lower proportions of RCA in the mixtures
promoted the formation of more calcium silicate hydrate (CSH) gel,
which densified the cement paste matrix and consequently increased the
concrete’s strength. However, excessive RCA incorporation led to fragile
mixtures with inadequate CSH production, resulting in insufficient
strength. These observations align with similar interpretations made in
previous studies (4848.
Joseph HS, Pachiappan T, Avudaiappan S, Flores EIS. 2022. A study on
mechanical and microstructural characteristics of concrete using
recycled aggregate. Materials. 15(21):7535. https://doi.org/10.3390/ma15217535
), reinforcing the consistency and reliability of these findings in the scientific community.
3.10. Energy dispersive X-ray spectroscopy (EDAX)
⌅Energy-Dispersive X-ray Spectroscopy (EDAX) serves as a powerful tool in dissecting the intricate chemical composition of concrete. This study delves into qualitative and quantitative analyses of diverse elements within different concrete mixtures. Figures 14 meticulously delineate these analyses, offering a comprehensive view of the elemental content.
The EDAX analysis reveals the presence of pivotal elements: Calcium (Ca), Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Potassium (K), Sodium (Na), and Magnesium (Mg). In-depth scrutiny illustrates intriguing patterns. In mixture RCA 25, calcium content surges, fostering the creation of hydration products like Calcium Hydroxide (CH), Calcium Silicate Hydrate (CSH), and Calcium Aluminosilicate Hydrate (CASH). Contrastingly, higher RCA replacement percentages witness a decline in calcium content, leading to diminished hydration products. Simultaneously, silica content diminishes at lower RCA replacement percentages but exhibits a gradual increase with higher RCA replacements. The lower alumina content in these mixtures curtails ettringite formation. SEM results corroborate these findings, evidencing robust CSH formation up to a 25% RCA replacement. The chemical compositions highlighted by EDAX validate this observation. Notably, higher RCA replacements inhibit calcium compound formation, directly correlating with decreased concrete strength. Table 13 encapsulates the quantitative chemical composition, providing a succinct overview. It is essential to note that all EDAX graphs affirm the presence of varied elements like calcium, aluminum, iron, etc. However, due to microscopic heterogeneity, specific peaks lack visual representation. This comprehensive EDAX analysis not only dissects the elemental intricacies but also substantiates the influence of RCA replacement percentages on concrete composition and, consequently, its mechanical properties.
| Mixture ID | (%) | Ca-K | O-K | Si-K | Al-K | Fe-K | K-K | Na-K | Mg-K |
|---|---|---|---|---|---|---|---|---|---|
| RC | Weight | 28 | 48.21 | 10.49 | 5.27 | 4.12 | 1.43 | 1.04 | 1.46 |
| Atomic | 15.54 | 67.03 | 8.3 | 4.34 | 1.64 | 0.81 | 1 | 1.33 | |
| RCA-25 | Weight | 46.76 | 48.8 | 1.6 | 0.58 | 0.64 | 0.42 | 0.92 | 0.29 |
| Atomic | 26.71 | 69.81 | 1.31 | 0.49 | 0.26 | 0.24 | 0.91 | 0.27 | |
| RCA-50 | Weight | 38.7 | 41.3 | 9.22 | 4.26 | 4.59 | 0.87 | 0..64 | 0.43 |
| Atomic | 23.08 | 61.71 | 7.84 | 3.77 | 1.97 | 0.53 | 0.67 | 0.42 | |
| RCA-75 | Weight | 36.12 | 52.23 | 4.08 | 2.25 | 1.57 | 0.92 | 1.88 | 0.94 |
| Atomic | 19.74 | 71.48 | 3.18 | 1.82 | 0.62 | 0.52 | 1.79 | 0.85 | |
| RCA-100 | Weight | 30.81 | 52.78 | 6.99 | 3.25 | 1.97 | 0.75 | 1.82 | 1.64 |
| Atomic | 16.57 | 71.13 | 5.36 | 2.6 | 0.76 | 0.41 | 1.71 | 1.45 |
Ca-Calcium, O-Oxygen, Si-Silicon, Al-Aluminum, Fe-Iron, K-Potassium, Na-Sodium, and Mg-Magnesium.
4. ANALYTICAL INVESTIGATION
⌅In the realm of concrete technology, understanding the interplay between different properties is paramount. Employing the sophisticated technique of linear regression analysis, this study meticulously delves into the properties of concrete infused with varying proportions of recycled coarse aggregates (RCA). Through empirical analysis, a linear relationship was established between key properties of fresh concrete, correlating them linearly with the RCA replacement ratio. This correlation, based on extensive experimental data collected from various properties of recycled coarse aggregate concrete after 28 days of water curing, was graphically represented using Origin Software, resulting in well-fitted curves reflecting the intricate dynamics at play.
The linear correlations between the RCA replacement level and different mechanical properties were expressed mathematically. The equations derived for compressive strength (Fc), flexural strength (Ff), split tensile strength (Fs), drying shrinkage (Fd), electrical resistivity (FE), and rapid chloride permeability (FR) are represented respectively as Equations [1], [2], [3], [4], [5] and [6]:
Here, Fc represents compressive strength, Ff denotes flexural strength, Fs signifies split tensile strength, Fd illustrates drying shrinkage, FE quantifies electrical resistivity, and FR characterizes rapid chloride permeability. The parameter Rr corresponds to the percentage of the RCA replacement level, encapsulating the essence of the study. This comprehensive analysis, yielding coefficients of determination (R²) such as 0.64073, 0.56775, 0.40756, 0.96623, 0.6101 and 0.94818 for compressive, flexural, and split tensile strength, drying shrinkage, electrical resistivity, and rapid chloride permeability respectively, signifies the robustness of the correlation. These values affirm the efficacy of the linear regression model in predicting concrete properties accurately, ensuring a reliable bridge between theoretical predictions and experimental results.
5. CONCLUSIONS
⌅This study marks a significant stride in promoting sustainable construction practices by harnessing Construction and Demolition (C&D) waste as Recycled Coarse Aggregates (RCA), thereby championing circular economy principles and waste reduction. The exploration into the influence of RCA on hardened concrete unveils pivotal insights crucial for eco-conscious construction practices.
Key Findings and Recommendations:
-
The study reveals a nuanced relationship between RCA content and concrete properties. Concrete exhibits enhanced mechanical properties up to an optimum replacement threshold of 25% RCA. Beyond this point, there’s a decline in performance. The findings underline the significance of adhering to this 25% replacement ratio for optimal concrete strength.
-
Considering the notable 11.56% increase in compressive strength, 3.06% improvement in flexural strength, and 5.17% enhancement in split tensile strength compared to natural coarse aggregates, it’s recommended that a mandatory 25% inclusion of RCA be established. This ratio ensures concrete strength surpasses that of traditional mixes.
-
RCA-optimized concrete at 25% replacement demonstrates exceptional strength, making it ideal for non-structural elements, pavements, and specific projects. However, higher RCA percentages compromise structural integrity, making them suitable for landscaping and road base layers, avoiding critical applications.
-
RCA inclusion increases drying shrinkage due to higher water absorption. RCA 25 exhibited a 5.23% higher shrinkage than conventional concrete, emphasizing the need for meticulous curing practices. Additionally, higher RCA percentages lower electrical resistivity, increasing corrosion risk, and reduce chloride permeability, impacting durability. RCA 25 and 50 exhibit high durability, whereas RCA 75 and 100 demonstrate moderate durability.
-
Robust linear correlations were established, offering predictions for concrete properties based on RCA content. These models can serve as valuable tools in sustainable construction projects, ensuring informed decision-making regarding material selection.
This research not only mitigates construction industry waste but also conserves natural resources. Sustainable concrete, exhibiting comparable mechanical properties to conventional counterparts, emerges as a viable construction alternative. Future studies can delve deeper, optimizing this process for wider industry adoption, and exploring innovative avenues to further enhance the sustainability quotient of concrete structures. As construction endeavors move forward, the integration of RCA promises a greener, more responsible future for the industry.