3.1 Introduction
Two types of recycled coarse aggregates have been used in this research work.
Recycled brick aggregate is obtained from the demolished concrete waste of an old residential building and recycled stone aggregate is obtained from the waste of crushed cylinders of Concrete and Materials laboratory of Civil Engineering, BUET.
Natural stone aggregates have been replaced at various percentages with recycled brick and stone aggregates to prepare brick recycled aggregate concrete and stone recycled aggregate concrete. Superplasticizer was used in half of the prepared specimens. The water/cement ratio was kept constant during the whole experimental program for comparative purpose. Then various tests were conducted to evaluate the mechanical and durability properties of the recycled aggregate concrete and the effect of superplasticizer on it.
3.2 Materials Used
The type and properties of materials have direct influence on the properties of concrete. So, materials selection and identification of properties is one of the most important part of the research work. Various tests were conducted on materials to evaluate properties like fineness modulus, specific gravity, absorption capacity, unit weight etc. Fig 3.1 shows the different materials used during preparation of concrete specimens.
3.2.1 Cement
Cement is one of the important constituents of concrete as it binds everything together. Mechanical and durability properties of concrete depend on the type of cement used. Here, in this research Portland Composite Cement (PCC) conforming to BDS EN 197-1:2003 CEM II/B-M (S-V-L) 42.5 N has been used as binding material.
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(a) Natural Stone Aggregate (b) Recycled Stone Aggregate
(c) Recycled Brick Aggregate (d) Fine Aggregate (Sylhet Sand)
(e) Portland Composite Cement (f) Superplasticizer
Fig 3.1: Different materials used in preparation of concrete specimens
3.2.2 Water
Water is the most sensitive and important raw material used to provide the required workability to the concrete and to ensure cement hydration. The reason for being sensitive and important is the fact that the amount of water can affect all properties of
37
fresh and hardened concrete. Here, in this research tap water has been used for concrete mixing.
3.2.3 Fine Aggregate
Fine aggregate is another major constituent of concrete which can influence concrete mix design substantially. Various factors such as fineness modulus, moisture content, specific gravity and silt content affect the mix proportions of concrete. Fine aggregates are the particles that pass through 4.75mm sieve and retain on 0.075mm sieve. Sand, surki, stone screenings, burnt clays, cinders, fly ash etc. can be used as fine aggregates in concrete. In this research, Sylhet sand has been used as fine aggregate.
3.2.4 Coarse Aggregate
Coarse aggregate is one of the essential components of concrete and occupies the largest volume in the mix. That is why, it greatly affects the concrete mix design. Its properties such as strength, maximum size, shape and water absorption influence the quantity of water, cement and fine aggregate in a concrete mixture. Coarse aggregates are particulates that are greater than 4.75mm. Brick chips, stone chips, gravels, pebbles etc. are used as coarse aggregate in concrete. In this research, three types of coarse aggregates have been used. They are natural stone (black Indian stone), recycled stone aggregate and recycled brick aggregate.
3.2.5 Superplasticizer
Superplasticizers, also known as high range water reducers, are additives used in making high strength concrete. They are chemical admixtures that allow reduction in water content by 30% or more as per the manufacturer. Superplasticizers result in substantial enhancement in workability at a given water cement ratio. There are various types of superplasticizers being used in concrete mixture. Here, a third generation superplasticizer made of polycarboxylic ether has been used in concrete mixture. It conforms to ASTM C494, Type F and G. Technical properties of the superplasticizer as provided by the manufacturer is given in Table 3.1.
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Table 3.1: Technical properties of the superplasticizer (as per manufacturer)
Form Light Yellow Liquid
Specific Gravity 1.07 ± 0.02 (at 270 C ± 30 C) Setting and
Hardening
Sets very early with moderate dosage.
There may be retarding effect on the setting time with high dosage.
Chloride Content Nil
Air Entrainment May increase very marginally
pH 7 ± 1
Compatibility
Can be used with all types of Portland and Slag cements including microsilica,
fly ash, GGBFS.
3.3 Properties of Aggregate
Aggregates occupy most of the volume of concrete. They are used in concrete to make it cheaper and economic. Aggregates act as filler materials only. These do not react with cement and water. But the properties of aggregates greatly influence the overall characteristics of fresh and hardened concrete. Various properties of aggregate are needed to be known for concrete mix design. Following tests were conducted for fine and coarse aggregates to evaluate their physical properties:
1. Sieve analysis of fine and coarse aggregate as per ASTM C 136.
2. Specific gravity (relative density) and absorption capacity of fine aggregate as per ASTM C 128.
3. Specific gravity (relative density) and absorption capacity of coarse aggregate as per ASTM C 127.
4. Unit weight and voids in aggregate as per ASTM C 29.
5. Moisture content as per ASTM C 566.
3.3.1 Properties of Fine Aggregate
Locally available Sylhet sand was used as fine aggregate. It was yellowish in color which is shown in Fig 3.1 (d). The granules of the fine aggregate were slightly coarser in size. Sieve analysis was done to evaluate the grain size distribution of the fine
39
aggregates. The fineness modulus (FM) of the fine aggregate was found to be 2.94.
The gradation curve is shown in Fig 3.2. The curve is S-shaped and indicates well distribution of the particle sizes. Unit weight of the fine aggregate was 1543 kg/m3 and the sample contained 40.38% voids. The water absorption capacity of the fine aggregate was 0.73%. Bulk specific gravity (OD) was 2.59 and bulk specific gravity (SSD) was 2.61.
Fig 3.2: Gradation curve of fine aggregate (Sylhet sand)
3.3.2 Properties of Coarse Aggregate
Coarse aggregate occupies the maximum volume of concrete and here three different types of coarse aggregate have been used for concrete mixing. Also different types of coarse aggregate have been mixed in different proportions as per research requirement. Properties of coarse aggregate both in individual and mixed forms have been evaluated. The fineness modulus (FM) of the natural stone aggregate was found to be 6.97. The gradation curve is shown in Fig 3.3. The curve is S-shaped and indicates well distribution of the particle sizes. Unit weight was 1536 kg/m3 and the sample contained 44.71% voids. The water absorption capacity of the natural stone aggregate was 1.86%. Bulk specific gravity (OD) was 2.78 and bulk specific gravity (SSD) was 2.83.
0 20 40 60 80 100 120
0.1 1 10
% Finer
Seive Size (mm)
40
Fig 3.3: Gradation curve of natural stone aggregate
The fineness modulus (FM) of the recycled stone aggregate was found to be 6.47. The gradation curve is shown in Fig 3.4. The curve is S-shaped and indicates well distribution of the particle sizes. Unit weight was 1286 kg/m3 and the sample contained 42.3% voids. The water absorption capacity of the recycled stone aggregate was 7.53%. Bulk specific gravity (OD) was 2.23 and bulk specific gravity (SSD) was 2.4.
Fig 3.4: Gradation curve of recycled stone aggregate
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
41
The fineness modulus (FM) of the recycled brick aggregate was found to be 7.51. The gradation curve is shown in Fig 3.5. The curve is S-shaped and indicates well distribution of the particle sizes. Unit weight was 1039 kg/m3 and the sample contained 44.99% voids. The water absorption capacity of the natural stone aggregate was 11.55%. Bulk specific gravity (OD) was 1.9 and bulk specific gravity (SSD) was 2.12.
Fig 3.5: Gradation curve of recycled brick aggregate
Natural stone aggregates are replaced by recycled aggregates at different percentages and mixed thoroughly. Properties of mixed aggregates were then evaluated. The fineness modulus of coarse aggregate when 25% recycled stones were mixed with 75% natural stones was 6.94. The gradation curve is shown in Fig 3.6. The curve is S- shaped and indicates well distribution of the particle sizes. Unit weight was 1493 kg/m3 and the sample contained 42.97% voids. The water absorption capacity of the mixed aggregate was 3.45%. Bulk specific gravity (OD) was 2.62 and bulk specific gravity (SSD) was 2.71.
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
42
Fig 3.6: Gradation curve of mixed coarse aggregate (75% NA+25% RSA)
The fineness modulus of coarse aggregate when 50% recycled stones were mixed with 50% natural stones was 6.87. The gradation curve is shown in Fig 3.7. The curve is S-shaped and indicates well distribution of the particle sizes. Unit weight was 1457 kg/m3 and the sample contained 41.9% voids. The water absorption capacity of the mixed aggregate was 4.24%. Bulk specific gravity (OD) was 2.51 and bulk specific gravity (SSD) was 2.61.
Fig 3.7: Gradation curve of mixed coarse aggregate (50% NA+50% RSA)
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
43
The fineness modulus of coarse aggregate when 25% recycled bricks were mixed with 75% natural stones was 7.22. The gradation curve is shown in Fig 3.8. The curve is S- shaped and indicates well distribution of the particle sizes. Unit weight was 1439 kg/m3 and the sample contained 42.84% voids. The water absorption capacity of the mixed aggregate was 4.21%. Bulk specific gravity (OD) was 2.52 and bulk specific gravity (SSD) was 2.63.
Fig 3.8: Gradation curve of mixed coarse aggregate (75% NA+25% RBA)
The fineness modulus of coarse aggregate when 50% recycled bricks were mixed with 50% natural stones was 7.41. The gradation curve is shown in Fig 3.9. The curve is S- shaped and indicates well distribution of the particle sizes. Unit weight was 1300 kg/m3 and the sample contained 42.93% voids. The water absorption capacity of the mixed aggregate was 6.31%. Bulk specific gravity (OD) was 2.28 and bulk specific gravity (SSD) was 2.43.
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
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Fig 3.9: Gradation curve of mixed coarse aggregate (50% NA+50% RBA)
For better understanding of the obtained results and properties, comparison between different types of aggregates and mixed proportions have been done. Fig 3.10 to 3.15 shows the comparative values of fineness modulus (FM), unit weight, percentage voids, water absorption capacity and bulk specific capacity.
Fig 3.10: Comparative gradation curve of coarse aggregates
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
0 20 40 60 80 100 120
1 10 100
% Finer
Seive Size (mm)
RSA NA RBA 75% NA+25% RSA
50% NA+50% RSA 75% NA+25% RBA 50% NA+50% RBA
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Fig 3.11: Comparative values of fineness modulus (FM) of aggregates
Fig 3.12: Comparative values of unit weight of aggregates
2.94
6.97
6.47
7.51
6.94 6.87 7.22 7.41
0 1 2 3 4 5 6 7 8
FA NA RSA RBA 75% NA +
25% RSA
50% NA + 50% RSA
75% NA + 25% RBA
50% NA + 50% RBA
Fineness Modulus (FM)
0 200 400 600 800 1000 1200 1400 1600 1800
FA NA RSA RBA 75% NA +
25% RSA
50% NA + 50% RSA
75% NA + 25% RBA
50% NA + 50% RBA Unit Weight (kg/m3)
46
Fig 3.13: Comparative values of percentage voids in aggregates
Fig 3.14: Comparative values of absorption capacity in aggregates
38 39 40 41 42 43 44 45 46
FA NA RSA RBA 75% NA +
25% RSA
50% NA + 50% RSA
75% NA + 25% RBA
50% NA + 50% RBA
% Voids
0 2 4 6 8 10 12 14
FA NA RSA RBA 75% NA +
25% RSA
50% NA + 50% RSA
75% NA + 25% RBA
50% NA + 50% RBA
Absorption Capacity (%)
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Fig 3.15: Comparative values of bulk specific gravity in aggregates
3.4 Mix Design
Concrete mix design is the process of finding precise proportions of cement, water, fine and coarse aggregates for concrete to achieve desired target strength in structures.
Various properties of constituent materials are required for concrete mix design and these properties have already been evaluated. In this research work, mix design for natural stone aggregate with target strength of 20.7 MPa (3000 psi) is taken as control and 14 different mixes are prepared varying type and percentages of coarse aggregate.
RAC with various replacement ratios (0%, 25%, 50% and 100%) of natural aggregate by recycled aggregate were prepared. Water cement ratio has been kept constant in all the concrete mixes. Half of the mixes have been admixed with superplasticizer and water content of these concrete mixes has been reduced by 15%. The quantity of superplasticizer used was 0.7% of the weight of cement content in the concrete mixes.
Table: 3.2 and 3.3 show the quantity of different constituent materials of the concrete mix design.
0 0.5 1 1.5 2 2.5 3
FA NA RSA RBA 75%
NA+25%RSA 50%
NA+50%RSA 75%
NA+25%RBA 50%
NA+50%RBA
Bulk Specific Gravity (OD) Bulk Specific Gravity (SSD)
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Table 3.2: Quantity of materials for different concrete mixes
Table 3.3: Quantity of different type of coarse aggregates used for preparation of concrete specimens
Specimen ID
Coarse Aggregate (%) Natural Stone
Aggregate (NA)
Recycled Stone Aggregate (RSA)
Recycled Brick Aggregate (RBA)
NAC 100 - -
RSAC100 - 100 -
RSAC50 50 50 -
RSAC25 75 25 -
RBAC100 - - 100
RBAC50 50 - 50
RBAC25 75 - 25
NACA 100 - -
RSACA100 - 100 -
RSACA50 50 50 -
RSACA25 75 25 -
RBACA100 - - 100
RBACA50 50 - 50
RBACA25 75 - 25
Mix-01 NAC 345 179.4 732 1195 0 0 0
Mix-02 RSAC100 345 179.4 732 0 1195 0 0
Mix-03 RSAC50 345 179.4 732 597.5 597.5 0 0
Mix-04 RSAC25 345 179.4 732 896.25 298.75 0 0
Mix-05 RBAC100 345 179.4 732 0 0 1195 0
Mix-06 RBAC50 345 179.4 732 597.5 0 597.5 0
Mix-07 RBAC25 345 179.4 732 896.25 0 298.75 0
Mix-08 NACA 345 152.5 732 1195 0 0 2.415
Mix-09 RSACA100 345 152.5 732 0 1195 0 2.415
Mix-10 RSACA50 345 152.5 732 597.5 597.5 0 2.415
Mix-11 RSACA25 345 152.5 732 896.25 298.75 0 2.415
Mix-12 RBACA100 345 152.5 732 0 0 1195 2.415
Mix-13 RBACA50 345 152.5 732 597.5 0 597.5 2.415
Mix-14 RBACA25 345 152.5 732 896.25 0 298.75 2.415
Water (kg/m³)
Fine Aggregate
(kg/m³) Mix Specimen ID Cement
(kg/m³) Natural
Stone
Recycled Stone
Recycled Brick
Superplasticizer (kg/m³) Coarse Aggregate (kg/m³)
49 3.5 Concrete Casting, Compacting and Curing
Quality concrete is a prerequisite of a good research work. A small variation in concrete mixing, compacting and curing may lead to contradictory results. The process of batching, weighting and mixing of necessary materials were performed according to ASTM standards. The aggregates (both fine and coarse aggregates) were brought to saturated surface dry (SSD) condition before mixing with other ingredients. As the water absorption capacity of recycled aggregates was much higher than natural stone aggregates, they were immerged in water for 24 hours and then brought to SSD condition. At first, necessary amount of coarse and fine aggregates and cement were placed in a drum-mixer and were mixed for about 2 minutes (Fig 3.16). Then the required amount of potable water was added gradually into the blended materials and the mixing operation was continued for about 2 minutes to produce a uniform mix (Fig 3.17). If the concrete mix needed to be admixed with superplasticizer then 50% of water was added in the mix first and then superplasticizer was mixed in the rest 50% of water. When a test on fresh concrete (i.e. slump test) had to be performed, necessary sample was taken from fresh concrete, test was executed and then the utilized amount of concrete was poured back to the source, blended once again to make homogeneous mix and then concrete was poured into the moulds (ASTM C192/C192M).
The concrete in the moulds were compacted in three layers with a vibrator nozzle (Fig 3.18). The vibrator was allowed to penetrate 25 mm in each layer. After each layer is vibrated, outside of the mold was tapped at least 10 times with the help of a hammer to close the holes and to release the entrapped air voids (ASTM C192/C192M). They were demolded after 24 hours of casting and were cured under water by immersing in a curing tank in the lab until their testing age (Fig 3.19).
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Fig 3.16: Mixing of concrete ingredients in the drum-mixer
Fig 3.17: Freshly prepared concrete out of drum-mixer
51
Fig 3.18: Compaction of concrete with vibrator nozzle
Fig 3.19: Curing condition of the prepared concrete specimens
52 3.6 Tests on Fresh Concrete
3.6.1 Slump Test
The only test done on fresh concrete mixes was slump test. Slump test measures the consistency of fresh concrete before it sets. It is performed to check the workability of freshly made concrete and therefore the ease with which concrete flows. It can also be used as an indicator of an improperly mixed batch. The slump test is carried out as per procedures mentioned in ASTM C143/C143M-05. The metal mold used for this test is shaped as frustum of a cone. The frustum has a base diameter of 203 mm and top diameter of 102 mm. The height is of 305 mm. It has open base and top perpendicular to the axis of the cone. It is provided with foot pieces and handles. The frustum should have smooth interior surface free from any projections. The associated tamping rod has 16 mm diameter round cross-section and approximately 600 mm length. A measuring device also needed to record the value of slump. The interior of the frustum was cleaned and oiled. The mould was filled with prepared concrete mix in 4 approximately equal layers. Each layer was tamped with 25 evenly distributed strokes with the tamping rod. The mold was lifted upwards without disturbing the mix in vertical direction. The value of slump was measured as the difference between the height of the mould and that of height point of the slumped mix (Fig 3.20).
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Fig 3.20: Slump test of freshly prepared concrete
3.7 Tests on Hardened Concrete
To evaluate the mechanical and durability properties of recycled aggregate concrete and to observe the effect of superplasticizer on it, a total of six tests have been conducted on hardened concrete. They are compressive strength test, split tensile strength test, ultrasonic pulse velocity (UPV) test, density, absorption and voids in hardened concrete test, rapid chloride permeability (RCPT) test and static modulus of elasticity test. Details of each test and testing procedures have been explained in following subsections.
3.7.1 Compressive Strength Test
The most common test for hardened concrete is compressive strength test. It is widely used to evaluate the strength of concrete. The tests of concrete cylinders were conducted using specifications provided in ASTM C 39 and test of concrete cubes were conducted using specifications provided in BS EN 12390-3:2009. The test is
54
done by applying a compressive axial load to cylinders or cubes until the specimen is crushed. Compressive strength is calculated by dividing the failure load with the area of application of the load. The prepared concrete cylinders and cubes were tested at 7, 28 and 90 days. The specimens were tested in a compression machine as shown in Fig 3.21 with a loading rate of 3.0 KN/sec. The load was applied continuously until the specimen fails and the maximum load carried by the specimen during the test was recorded.
Fig 3.21: Compressive strength test setup in the laboratory
3.7.2 Splitting Tensile Strength Test
Splitting tensile strength test is an indirect method of measuring tensile strength of concrete using a cylinder specimen which spits across the vertical diameter. The tensile strength of concrete is one of the basic and important properties which greatly affect the extent and size of cracking in a structure. The following equation was used to calculate the splitting tensile strength of concrete specimen:
55 T = 2P/πDL
Where,
T = Splitting tensile strength (psi)
P = Maximum applied load indicated by the testing machine (lb) L = Length (in)
D = Diameter (in)
Splitting tensile strength test is conducted by following the procedures specified in ASTM C 496. The test was carried out for concrete cylinder specimens at the age of 28 days. Fig 3.22 shows the set up of splitting tensile strength test.
Fig 3.22: Splitting tensile strength test setup in the laboratory
3.7.3 Density, Absorption and Voids in Hardened Concrete Test
The mechanical and durability properties of concrete are highly influenced by its density. A denser concrete generally provides higher strength and fewer amount of voids and porosity. Smaller the voids in concrete, it becomes less permeable to water and soluble elements. So water absorption will also be less and better durability is
56
expected from this type of concrete. Density, absorption and voids in hardened test is done according to procedures prescribed in ASTM C 642. At first, two inch thick slices of concrete cylinders were made from sample specimens. Each slice has a volume not less than 350 cm3 and free from observable cracks, fissures, or shattered edges. Oven dry mass, A of the concrete slice was taken after keeping it in an oven at temperature of 100 to 110°C for not less than 24 hours. Then the specimen was immersed in water at approximately 21°C for not less than 48 hours. The concrete specimen was then surface dried with the help of a towel and saturated mass after immersion, B was then determined. The specimen was then placed in a suitable receptacle, covered with tap water and boiled for 5 hours. It was allowed to cool by natural loss of heat for not less than 14 hours to a final temperature of 20 to 25°C.
Surface moisture was removed with a towel and saturated mass after boiling, C was determined. Finally, the specimen was suspend and immersed in water and the immersed apparent mass, D was determined. Fig 3.23 shows various steps of the test conducted in the laboratory. The following equations were used to calculate the density, absorption and voids in hardened concrete.
Absorption after immersion, % = [(B – A)/A] × 100
Absorption after immersion and boiling, % = [(C – A)/A] × 100 Bulk density, dry = [A/(C-D)] × ρ
Bulk density after immersion = [B/(C – D)] × ρ
Bulk density after immersion and boiling = [C/(C – D)] × ρ Apparent density = [A/ (A – D)] × ρ
Volume of permeable pore space (voids), % = (C – A)/(C – D) ×100
Where,
A = mass of oven-dried sample in air, g
B = mass of surface-dry sample in air after immersion, g
C = mass of surface-dry sample in air after immersion and boiling, g D = apparent mass of sample in water after immersion and boiling, g ρ = density of water = 1 Mg/m3 = 1 g/cm3