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Recycled Concrete: Use of recycled brick coarse aggregates in making concrete

Updated on June 2, 2014

Introduction

In recent years, the demand of construction material, ‘Concrete’ has increased due to large scale boom in construction industry. Subsequently, it has indirectly affected coarse aggregate consumption. Concrete consists up to 80% of coarse aggregate. Normally crushed rocks are used as coarse aggregate to make concrete. But, it is not appropriate to solely rely on single source of aggregates as it may cause shortage of natural rock aggregates in the future. Researchers have established new alternatives to procure aggregates by the method of Recycling. Now days recycled concrete or recycled Bricks are incorporated as the aggregates to make concrete. This process of recycling has not only solved the problem of disposing Construction and Demolition waste (C&D) but also made availability of economical concrete.

In this research study crushing strength of concrete, made by incorporating crushed bricks as coarse aggregate, is determined. This concrete is generally known as Recycled Concrete and particularly Brick coarse aggregate concrete.

Cylinder with 60% replacement Mean crushing strength of Cylinder = 11.17 MPa (1621 Psi)
Cylinder with 60% replacement Mean crushing strength of Cylinder = 11.17 MPa (1621 Psi)

AIMS AND OBJECTIVES OF THE STUDY

  • The main aim of this study is to investigate the compressive strength of concrete incorporating recycled crushed brick pieces as the coarse aggregates.
  • To investigate the crushing strength of Cubes and Cylinders made with Brick aggregate concrete.
  • To design optimum mix of concrete by using recycled crushed brick aggregates.
  • To study the effect of quantity of recycled crushed brick aggregates on compressive strength.

RESEARCH METHODOLOGY

MATERIALS

Following are the materials used in making Brick aggregate concrete

Water

The water used in making concrete specimen was of good drinkable quality. As the quality of water effects the strength of concrete .The water cement ratio adopted was 0.50. The strength of concrete depends upon water cement ratio. Greater the water cement ratio, lesser will be the strength of concrete.

Cement

Ordinary Portland cement of brand, Lucky Cement Industry, available in local market was used. The quality of cement effects the strength of concrete.

Hill Sand

Hill sand was used in preparation of concrete specimen and is passed through the sieve # 16.

Coarse Aggregate

Two types of aggregates, natural (Virgin) aggregates and recycled brick aggregates were used by passing both aggregates through 19mm sieve. The old bricks were obtained from the dismantled brick masonry. After obtaining bricks, they were manually crushed and used as ¾ in. (19 mm) down-graded coarse aggregate.

Shape and Size of Specimen (Cube)

  • Cube of standard size 100mm x 100mm x 100mm were cast and tested.
  • Cylinder of dia 4 inch and height 8 inch

PROPORTIONS

Water Cement Ratio

A fixed water cement ratio of 0.50 has been used and kept constant through the study.

Concrete Ratio

A fixed ratio 1:1.5:3 by weight is used and kept constant through the study.

Variables

Natural Aggregate

The percentage of Natural aggregates varies according to the batch. In first batch 100% natural aggregates are used and this percentage continued to decrease by 20% in later batches thus no natural aggregates are used in 6th batch.

Brick Coarse Aggregate

The percentage of recycled crushed brick aggregates varies according to the batch. In first batch no brick aggregates are used but this percentage was increased by 20% in later batches thus 100% recycled crushed brick aggregates are used in 6th batch.

MIX DESIGN

Before having any concrete mixing, the selection of mix materials and their required materials proportion must done through a process called mix design. There are lots of methods to determine concrete mix design. In this study investigation, 6 basic mixes designated by A, B, C, D, E and F were made with the following percentages of recycled crushed brick aggregates.

Proportioning

S. NO
Batch/Mix Designation
Stone or Natural Aggregates (%)
Crushed Brick Aggregates (%)
1
A
100
0
2
B
80
20
3
C
60
40
4
D
40
60
5
E
20
80
6
F
0
100

MIXING OF CONCRETE

The performance of brick aggregate concrete was influenced by the process of mixing. This means that a proper and good practice of mixing can lead a better performance and quality of the brick aggregate concrete. The quality of the concrete can also be influenced by the homogeneity of the mix material during the mixing and after the placement of fresh concrete. A proper mix of concrete is encouraged to the strength of concrete and better bonding of cement with the aggregate.

Once the concrete mix design was calculated, the mixing of the concrete can be carried out. In this study investigation materials were manually mixed by maintaining the quantity of raw material properly.

PLACING, COMPACTION AND CASTING OF CONCRETE SPECIMENS

Before the placing of concrete, the concrete mould must be oiled for the ease of concrete specimens stripping. The oil used is a mixture of diesel and kerosene. Special care was taken during the oiling of the moulds, so that there no concrete stains and left on the moulds. Once the concrete mix is ready, the fresh concrete must be poured into the concrete moulds for hardened properties tests. Every batch of brick coarse aggregate concrete consisted 3 cubes and 3 cylinders.

During the placing of fresh concrete into the moulds, vibrations were done using an immersion vibration. The vibration of concrete allows full compaction of the fresh concrete to release any entrained air voids contained in the fresh concrete. If the concrete specimens are not compacted in a proper manner, the maximum strength of the concrete cannot be achieved. The vibration was done every sufficient one third layer of the fresh concrete was poured into the moulds. It is found that the placing and compacting of concrete is getting difficult when the percentage of brick coarse aggregate increased. This shows that the workability of brick aggregate concrete used is less than regular concrete.

The leveling of concrete was done on the surface of the concrete. Leveling is the initial operation carried out after the concrete has been placed and compacted. After the leveling of the fresh concrete specimen was done, the concrete in the mould was left overnight to allow the fresh concrete to set.

STRIPPING AND CURING OF CONCRETE SPECIMENS

After leaving the fresh concrete in the moulds to set overnight, the concrete specimens in the moulds were stripped. The identification of concrete specimens was done and the moulds were cleaned and oiled for the next batch of concrete mix. All concrete specimens were placed into the curing tank in normal room temperature for 28 days for the hardened properties test of brick aggregate concrete.

Curing is an important process to prevent the concrete specimens from losing of moisture while it is gaining its required strength. Lack of curing tends to lead the concrete specimen to perform less well in its strength required. Inadequate curing will cause the unexpected cracks appear on the concrete specimens. After 28 days of curing, the concrete specimens were removed from the curing room to conduct hardened properties test of brick aggregate concrete.

Workability

The surface of the brick is rougher than natural stone so the aggregates obtained by crushing the brick will also have the rough texture which will require more work and energy to manipulate a freshly mixed quantity of brick aggregate concrete, means the workability, flowability, compactability, stability, pumpability of brick aggregate concrete is lesser than that of virgin (natural) aggregate concrete.

Also the absorption of the brick aggregate is drastically more than natural aggregates, so much of the water in the mix will be absorbed by brick aggregates which could be used for increasing its workability, so for this sake, it is necessary to determine the absorption of brick aggregates in order to make compensation of water content by enhancing water content in the mix excess of what is needed in natural aggregate concrete. For this reason the crushed brick coarse aggregates were soaked in water before using them in the concrete.

Workability of the brick aggregate concrete and regular concrete, in this research study, was solely observed and not any physical test was performed.

Cube with 80% replacement MEAN CRUSHING STRENGTH OF CUBE = 15.41 MPa (2236 Psi)
Cube with 80% replacement MEAN CRUSHING STRENGTH OF CUBE = 15.41 MPa (2236 Psi)

Testing of hardened concrete speciemens

1. The testing for the specimens should be carried out as soon as possible after the removal of specimens from curing tank. The specimens need to be measured before the testing.

2. The dimension of the specimens were measured and recorded. The weight of each specimen was measured and recorded too.

3. The platens of the testing machine were cleaned with a clean rag.

4. Cleaned the uncapped surface of the specimen and place the specimen in the testing machine.

5. The platen was lowered to the surface of specimen until the uniform bearing was obtained.4

6. The force was applied and increased continuously until the specimen failed.

7. Maximum force was recorded from the testing machine

TESTING RESULTS

All samples were tested in the UTM and ultimate compressive strength of all specimens produced by designated batches (i.e. A, B, C, D, E and F) was calculated.

COMPRESSIVE STRENGTH (CUBE)

S. No.
Batch Designation
Percentage of Brick aggregates
Average Crushing Strength (MPa)
1
A
0%
28.27
2
B
20%
25.72
3
C
40%
21.79
4
D
60%
17.92
5
E
80%
15.41
6
F
100%
13.19
GRAPH: Crushing Strength of Cubes in all Brick Aggregate Percentages
GRAPH: Crushing Strength of Cubes in all Brick Aggregate Percentages

COMPRESSIVE STRENGTH (CYLINDER)

S. No.
Batch Designation
Percentage of Brick aggregates
Average Crushing Strength (MPa)
1
A
0%
19.30
2
B
20%
16.76
3
C
40%
13.77
4
D
60%
11.17
5
E
80%
9.45
6
F
100%
8.71
GRAPH: Crushing Strength of Cylinders in all Brick Aggregate Percentages
GRAPH: Crushing Strength of Cylinders in all Brick Aggregate Percentages
GRAPH: Comparison Between Crushing Strength of Cylinders and Cubes
GRAPH: Comparison Between Crushing Strength of Cylinders and Cubes

CRUSHING STRENGTH OF CUBES AND CYLINDERS (Psi)

S. No.
Batch Designation
Percentage of Brick Aggregates
Averages Crushing Strength of Cubes (Psi)
Averages Crushing Strength of Cylinders (Psi)
1
A
0%
4100
2800
2
B
20%
3730
2432
3
C
40%
3160
1998
4
D
60%
2600
1621
5
E
80%
2236
1371
6
F
100%
1914
1264

REDUCTION IN WEIGHT OF CONCRETE MADE WITH CRUSHED BRICK AGGREGATES AS THE COARSE AGGREGATES

Brick aggregate concrete is light weight concrete; this considerable reduction in weight is caused due to reduced weight of brick aggregates as compared to the virgin aggregates. The more the percentage of virgin aggregates are replaced by crushed brick aggregates, the more will be the reduction in its unit weight

The following tables show the reductions in weight of concrete as the percentage of crushed brick aggregates are increased. Weight details of all the concretes (i.e. A, B, C, D, E and F) made during experimental investigation are mentioned in the following tables.

AVERAGE WEIGHT OF CUBES AND CYLINDERS AFTER CURING

S. No.
Batch Designation
Percentage of Brick Aggregates
Average Weight Of Cube (kg)
Average Weight Of Cylinder
1
A
0%
2.6 kg
3.75
2
B
20%
2.5 kg
3.65
3
C
40%
2.4 kg
3.55
4
D
60%
2.3 kg
3.45
5
E
80%
2.2 kg
3.35
6
F
100%
2.1 kg
3.25
GRAPH
GRAPH

Unit Weight of Concrete

Considering the above experimental investigations of the research study, reduction in density of concrete can be computed. The tables below show the reduction in density of Brick aggregate concrete, as the gradual increase in the percentage of brick aggregates is made.

UNIT WEIGHT OF CONCRETE IN KG/CUBIC METER

S. No.
Batch Designation
Unit weight (kg/Cubic Meter)
1
A
2475
2
B
2380
3
C
2285
4
D
2190
5
E
2095
6
F
2000
GRAPH
GRAPH
Cylinder with 100% replacement Mean crushing strength of Cylinder = 8.71 MPa (1264 Psi)
Cylinder with 100% replacement Mean crushing strength of Cylinder = 8.71 MPa (1264 Psi)

CONCLUSION AND RECOMMENDATIONS

CONCLUSIONS

  • Compressive strength of cubes reduces with increase in crushed brick aggregate. The maximum strength is 28.27 MPa when no brick aggregates are added. It reduces to 25.72, 21.79, 17.92, 15.41, and 13.19 when 20%, 40%, 60%, 80% and 100% crushed brick aggregates are added respectively.
  • Compressive strength of cylinders reduces with increase in crushed brick aggregate. The maximum strength is 19.30 MPa when no brick aggregates are added. It reduces to 16.76, 13.77, 11.17, 9.45, and 8.71 when 20%, 40%, 60%, 80% and 100% crushed brick aggregates are added respectively.
  • A reduction of small range is observed in unit weight of concrete with addition of brick pieces as coarse aggregates. The maximum unit weight is 2475 Kg/m3 in case of concrete with no brick aggregates followed by 2380 Kg/m3, 2285 Kg/m3, 2190 Kg/m3, 2095 Kg/m3 and 2000 Kg/m3 for concrete with 20%, 40%, 60%, 80% and 100% crushed brick aggregates respectively.
  • Looking at the results and discussions of this experimental study it is concluded that the strength is not major parameter. The brick aggregate up to 80% of total coarse aggregate can be used to produce a concrete of adequate strength i.e. 15 MPa (approximate) with ratio 1:1.5:3.

RECOMMENDATIONS

Further testing and studies on the brick aggregate concrete is highly recommended to indicate the strength characteristics of recycled crushed brick aggregates for application in high strength concrete. Below are some of the recommendations for further studies:

  • Although with 0.50 water/cement ratio, brick aggregate concrete can achieve high strength up to 13 MPa. But the workability will be very low. Therefore, it is recommended that adding admixtures such as super plasticizer and silica fume into the mixing so that the workability will be improved.
  • More investigations and laboratory tests should be done on the strength characteristics of recycled crushed brick aggregate. It is recommended that suitable admixtures be added into the mixing so that the strength of brick aggregate concrete may increase and testing can be done. It is also recommended that testing can be done on concrete slabs, beams and walls. Some mechanical properties such as creeping, poison ratio and abrasion are also recommended
  • More trials with different particle sizes of brick coarse aggregate and percentage of replacement of recycled crushed brick aggregate are recommended to get different outcomes and higher strength characteristics in the brick aggregate concrete.

Note:

This research study was conducted by the Hubber and his group in final 
year thesis at Quaid-e-Awam University OF Engineering, Science & 
Technology, Nawabsahah, Pakistan.

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