# Recycled Concrete as Aggregate for Structural Concrete Production

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## Abstract

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## 1. Introduction

## 2. Basic Properties of Concrete with Recycled Concrete Aggregate

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- decreased specific gravity [3],
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- increased crushability [3],
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- increased quantity of dust particles [3],
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- increased quantity of organic impurities if concrete is mixed with earth during building demolition [3], and
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- possible content of chemically harmful substances, depending on service conditions in building from which the demolition and crushing recycled aggregate is obtained [3].

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## 3. Experimental Investigation

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- same cement content,
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- same workability after 30 min,
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- same maximum grain size (32 mm),
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- same grain size distribution for aggregate mixture,
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- same type and quantity of fine aggregate,
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- variable type and quantity of coarse aggregate.

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- the first concrete mix had 100% of natural river coarse aggregate (R0), control mixture,
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- the second concrete mix had 50% of natural river coarse aggregate and 50% of recycled coarse aggregate (R50),
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- the third concrete mix had 100% of recycled coarse aggregate (R100).

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- workability (slump test) immediately after mixing and 30 minutes after mixing,
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- bulk density of fresh concrete,
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- air content,
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- bulk density of hardened concrete,
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- water absorption (at age of 28 days),
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- wear resistance (at age of 28 days),
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- compressive strength f
_{c}(at age of 2, 7 and 28 days), - -
- splitting tensile strength (at age of 28 days),
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- flexural strength (at age of 28 days),
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- modulus of elasticity (at age of 28 days),
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- drying shrinkage (at age of 3, 4, 7, 14, 21 and 28 days),
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- bond between ribbed and mild reinforcement and concrete.

#### 3.1. Component Materials

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- Portland-composite cement CEM II/A-M(S-L) 42.5R, (Lafarge-BFC),
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- fine aggregate (river aggregate, separation Luka Leget, grain size 0/4 mm),
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- two types of coarse aggregate: river aggregate, separation Luka Leget, and recycled concrete aggregate, grain sizes 4/8, 8/16 and 16/31.5 mm,
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- water.

**Figure 3.**Recycled concrete aggregate fractions. From left to right; 4–8 mm, 8–16 mm and 16–31.5 mm coarse aggregates.

Tested property | Measured value | Grain size | Quality requirement | |||

0/4 | 4/8 | 8/16 | 16/32 | |||

Crushing resistance
(in cylinder) | mass loss (%) | - | 14.0 | 18.6 | 23.8 | <30 |

Freezing resistance test | mass loss (%) | 1.8 | 1.6 | 1.4 | 1.5 | <12 |

Content of weak grains | (%) | - | 0 | 0 | 0 | <3 (4) |

Crushing resistance
(Los Angeles test) | mass loss (%) | - | 26.3 | 29.0 | 29.2 | <30 |

Water absorption after 30 minutes | (%) | 0.7 | 0.4 | 0.4 | 0.3 | - |

Fines content | (%) | 1.6 | 0.23 | 0.15 | 0.12 | <5 (<1) |

Specific gravity | kg/m^{3} | 2,655 | 2,666 | 2,669 | 2,671 | 2,000–3,000 |

Bulk density, uncompacted | kg/m^{3} | 1,611 | 1,490 | 1,470 | 1,460 | - |

Bulk density, compacted | kg/m^{3} | 1,729 | 1,590 | 1,570 | 1,560 | - |

Tested property | Measured value | Grain size | Quality requirement | ||

4/8 | 8/16 | 16/32 | |||

Crushing resistance
(in cylinder) | mass loss (%) | 18.3 | 26.7 | 30.7 | <30 |

Freezing resistance test | mass loss (%) | 2.0 | 1.4 | 1.0 | <12 |

Chemical testing (mortar part of recycled aggregate) | chloride content | 0 | 0 | 0 | <0.1 |

sulfate content | in traces | in traces | in traces | <1.0 | |

pH | 9.85 | 9.85 | 9.85 | - | |

Content of weak grains | (%) | 0 | 3.7 | 7.1 | <3 (4) |

Crushing resistance
(Los Angeles test) | mass loss (%) | 29.6 | 33.7 | 34.0 | <30 |

Water absorption after 30 minutes | (%) | 4.59 | 2.87 | 2.44 | - |

Fines content | (%) | 0.45 | 0.23 | 0.36 | <1.0 |

Specific gravity | kg/m^{3} | 2,346 | 2,458 | 2,489 | 2,000–3,000 |

Bulk density, uncompacted | kg/m^{3} | 1,275 | 1,239 | 1,236 | - |

Bulk density, compacted | kg/m^{3} | 1,388 | 1,323 | 1,325 | - |

#### 3.2. Mix Proportion Design

Concrete mixture | Cement (kg/m³) | Effective water (kg/m³) | Aggregate (kg/m³) | Additional water (kg/m³) | Effective water-cement ratio | Total water-cement ratio | Bulk density (kg/m³) |
---|---|---|---|---|---|---|---|

R0 | 350 | 180 | 1857 | 0 | 0.514 | 0.514 | 2,387 |

R50 | 350 | 180 | 1816 | 19 | 0.514 | 0.569 | 2,365 |

R100 | 350 | 180 | 1776 | 37 | 0.514 | 0.620 | 2,343 |

Concrete type | Natural river aggregate | Recycled concrete aggregate | |||||
---|---|---|---|---|---|---|---|

0/4 | 4/8 | 8/16 | 16/32 | 4/8 | 8/16 | 16/32 | |

R0 | 33 | 16 | 21 | 30 | 0 | 0 | 0 |

R50 | 33 | 8 | 10.5 | 15 | 6.5 | 7.5 | 19.5 |

R100 | 33 | 0 | 0 | 0 | 13 | 15 | 39 |

Concrete mixture | Content of natural river aggregate (kg/m³) | Content of recycled aggregate (kg/m³) | |||||

0/4 | 4/8 | 8/16 | 16/32 | 4/8 | 8/16 | 16/32 | |

R0 | 612 | 298 | 390 | 556 | 0 | 0 | 0 |

R50 | 600 | 145 | 191 | 272 | 118 | 136 | 354 |

R100 | 586 | 0 | 0 | 0 | 231 | 266 | 693 |

#### 3.3. Results of Fresh Concrete Testing

Concrete mixture | Cement (kg/m³) | Total water (kg/m³) | Aggregate (kg/m³) | Water/cement ratio^{1} | Aggregate/cement ratio | Slump^{2}(cm) | Slump^{3}(cm) | Air content (%) | Bulk density (kg/m³) |
---|---|---|---|---|---|---|---|---|---|

R0 | 352 | 181 | 1866 | 0.514 | 5.306 | 16 | 10 | 1.5 | 2,399 |

R50 | 352 | 200 | 1826 | 0.568 | 5.188 | 14.5 | 8.5 | 1.4 | 2,378 |

R100 | 348 | 216 | 1765 | 0.620 | 5.074 | 11 | 9 | 1.3 | 2,329 |

^{1}total water to cement ratio, including additional water content for workability.

^{2}measured slump immediately after mixing.

^{3}measured slump after 30 minutes.

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- Approximately the same workability after 30 minutes was achieved for all three concrete types using the additional water for concrete R50 and R100 (Figure 6b).
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- Concrete mixture R50 requires about 10% more total water quantity in comparison to mixture R0, and the corresponding value for concrete mixture R100 is about 20%.
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- Differences in air content (Δp) are insignificant. Air content in fresh concrete was determined by standard test method that is based on Boyle-Mariotte’s Law. In [26] was concluded that the air content of the RAC is higher than concrete made with NA at 100% replacement. However, the author used a gravimetric method for calculation of total air content, including aggregate porosity.
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- Bulk density of concrete depends on aggregate type and quantity. The highest bulk density has concrete with natural aggregate (R0) and the lowest concrete with maximum content of recycled aggregate (R100). The bulk density decrease is about 3%.

#### 3.4. Results of Hardened Concrete Testing

Concrete type | Concrete age (days) | Standard deviation (MPa) | ||
---|---|---|---|---|

2 | 7 | 28 | ||

R0 (MPa) | 27.55 | 35.23 | 43.44 | 1.5769 |

R50 (MPa) | 25.74 | 37.14 | 45.22 | 1.2089 |

R100 (MPa) | 25.48 | 37.05 | 45.66 | 3.5016 |

R50/R0 (%) | 93 | 105 | 104 | |

R100/R0 (%) | 92 | 105 | 105 |

Concrete type | 4 days(mm/m) | 7 days(mm/m) | 14 days(mm/m) | 21 days(mm/m) | 28 days(mm/m) | Relative drying shrinkage*, % |

R0 | 0.017 | 0.124 | 0.203 | 0.277 | 0.339 | 100 |

R50 | 0.036 | 0.086 | 0.176 | 0.254 | 0.306 | 90 |

R100 | 0.091 | 0.204 | 0.251 | 0.335 | 0.407 | 120 |

*****shrinkage value at the age of 28 days in relation to shrinkage of referent concrete R0.

Concrete type | R0 | R50 | R100 |
---|---|---|---|

Water absorption, (%) | 5.61 | 6.87 | 8.05 |

Splitting tensile strength, (MPa) | 2.66 | 3.20 | 2.78 |

Flexural strength, (MPa) | 5.4 | 5.7 | 5.2 |

Wear resistance, (cm³/50 cm) | 13.40 | 15.58 | 17.18 |

Modulus of elasticity (GPa) | 35.55 | 32.25 | 29.10 |

Bond between mild reinforcement and concrete, MPa | 6.48 | 5.87 | 6.76 |

Bond between ribbed reinforcement and concrete, MPa | 8.22 | 7.50 | 7.75 |

#### 3.5. Discussion of Hardened Concrete Properties

_{c}with time (t), a fraction Function (1) was adopted:

**Table 10.**Parameters of functional relationship between the compressive strength and age of the concrete.

Concrete type | a | b | r |
---|---|---|---|

R0 | 44.242 | 1.320 | 0.976 |

R50 | 47.556 | 1.761 | 0.997 |

R100 | 48.116 | 1.856 | 0.996 |

_{c}(t) for concrete R0, R50 and R100 are illustrated in Figure 9.

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- all three concrete types have approximately the same compressive strength development with time,
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- all three concrete types have 28-day compressive strength that is larger than 40 MPa,
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- differences between compressive strengths of concrete R0, R50 and R100 are negligible for the same concrete age.

_{0 }< t

_{α}

- t
_{0 }= quintile of Student distribution for number of degree of freedom ν = n_{1}+ n_{2}− 2 - x
_{av,1 }= average value (set I) - x
_{av,2 }= average value (set II) - n
_{1 }= number of test results (set I) - n
_{2 }= number of test results (set II) - t
_{α }= critical value of Student distribution for number of degree of freedom ν = n_{1 }+ n_{2}− 2 - σ
_{1 }= standard deviation (set I) - σ
_{2 }= standard deviation (set II)

Test pairs | n_{1} | n_{2} | s | t_{0} | t_{α}, for α = 0.05 |
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(R0 and R50) | 6 | 6 | 1.406523 | 2.189924 | 2.2281 |

(R0 and R100) | 6 | 6 | 2.7163 | 1.417718 | |

(R50 and R100) | 6 | 6 | 2.61943 | 0.29425 |

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- the lowest shrinkage rate was for concrete R50 (0.3 mm/m), and the highest for R100 (0.4 mm/m),
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- drying shrinkage of concrete R100 is 20% higher than shrinkage of concrete R0,
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- difference between 28-day shrinkage of concrete R0 and R50 is less than 10%.

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- the lowest water absorption was registered in concrete R0 and the highest in R100,
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- concrete R50 has 22% higher absorption, while concrete R100 has 44% higher absorption than control concrete R0.

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- difference between lowest and highest bond for both reinforcement types is about 10%,
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- bond between tested concretes and ribbed reinforcement is higher at least 15% than bond between tested concretes and mild reinforcement.

#### 3.6. Load Testing of Reinforced Concrete (RC) Beams

Phase | Load (kN) | Beam edge | Stress in concrete (MPa) | Stress in reinforcement (MPa) | Deflection (mm) | Crack width (mm) |
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I | 5 | upper | 4.075 | 0.46 | 0.017 | |

lower | 65.569 | |||||

II | 10 | upper | 7.278 | 1.54 | 0.062 | |

lower | 117.092 | |||||

III | 20 | upper | 13.683 | 3.87 | 0.137 | |

lower | 220.139 | |||||

IV | 30 | upper | 20.087 | 6.10 | 0.207 | |

lower | 323.186 | |||||

V | 40 | upper | 26.492 | 8.33 | 0.276 | |

lower | 426.233 | |||||

VI | 50 | upper | 32.897 | failure | ||

lower | 529.279 |

**Figure 15.**Arrangement of measuring spots throughout the beam. (U—deflection; T—strain in reinforcement; D—strain in concrete).

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- First crack appears in the middle of the span in the third load phase (P = 20 kN).
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- The maximum width of cracks after collapse is between 2.0 and 2.7 mm.
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- Similar disposition and width of cracks was registered on all tested RC beams.

Phase | Load kN | Deflection, (mm) | Concrete compressive stress, (MPa) | ||||

R0 | R50 | R100 | R0 | R50 | R100 | ||

I | 5 | 0.55 | 0.67 | 0.73 | - | - | - |

II | 10 | 0.89 | 1.21 | 1.37 | 1.32 | 2.64 | 3.04 |

III | 20 | 2.68 | 2.78 | 2.94 | 7.00 | 8.05 | 8.71 |

IV | 30 | 4.66 | 5.97 | 6.89 | 12.80 | 10.96 | 14.78 |

V | 40 | 7.43 | 10.52 | 11.78 | 20.20 | 21.12 | 24.02 |

VI | 50 | failure |

## 4. Conclusions

## Acknowledgements

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## Share and Cite

**MDPI and ACS Style**

Malešev, M.; Radonjanin, V.; Marinković, S.
Recycled Concrete as Aggregate for Structural Concrete Production. *Sustainability* **2010**, *2*, 1204-1225.
https://doi.org/10.3390/su2051204

**AMA Style**

Malešev M, Radonjanin V, Marinković S.
Recycled Concrete as Aggregate for Structural Concrete Production. *Sustainability*. 2010; 2(5):1204-1225.
https://doi.org/10.3390/su2051204

**Chicago/Turabian Style**

Malešev, Mirjana, Vlastimir Radonjanin, and Snežana Marinković.
2010. "Recycled Concrete as Aggregate for Structural Concrete Production" *Sustainability* 2, no. 5: 1204-1225.
https://doi.org/10.3390/su2051204