Study on Utilization of Fly Ash Aggregates in Concrete

In this study, the fine and coarse aggregates were completely replaced by fly ash aggregates in fly ash concrete. A mix design was done for M20 grade of concrete by IS method. Ordinary Portland cement of 43 grade was selected and fly ash aggregates were prepared by mixing fly ash with cement and water. The properties of fly ash fine aggregates and fly ash coarse aggregates were studied. The aggregate crushing value and aggregate impact value of fly ash coarse aggregates were also studied. The cement and fly ash proportions of 10:90, 12.5:87.5, 15:85, 17.5:82.5, 20:80 and 22.5:77.5 were tried with a suitable water cement ratio 0.3 to get the fly ash aggregates. The concrete cubes, cylinders and beams were cast with the fly ash aggregates obtained from the above six cement fly ash proportions. Then the compressive strength, split tensile strength and flexural strength were tested and compared with control concrete. This paper briefly presents the compressive strength development of fly ash aggregate concrete at different ages. The split tensile strength and flexural strength of all the concrete mixes were also investigated at different days of curing.


Introduction
In conventional concrete, weight of concrete is one of the parameters to compare with weight of fly ash aggregate concrete.Normally density of concrete is in the order of 2200 to 2600 kg/m 3 .This heavy self weight makes an uneconomical structural material compared to low self weight of fly ash aggregate concrete.In order to produce concrete of desired density to suit the required application, the self weight of structural and non structural members are to be reduced.Hence economy is achieved in the design of supporting structural elements which lead to the development of light weight concrete.Lightweight concrete is defined as a concrete that has been made lighter than the conventional concrete by changing material composition or production method.Lightweight aggregate concrete is the concrete made by replacing the usual material aggregate by lightweight aggregates.Though lightweight concrete can't always substitute normal concrete for its strength potential, it has its own advantages like reduced dead load, and thus economic structures and enhanced seismic resistance, high sound absorption and good fire resistance.Because of the above reasons the study on fly ash aggregate concrete is taken in this research work.

Formation of Fly Ash Aggregates
The constituents like cement, fly ash and water produce the fly ash aggregates.Water is the binding material that paves the way for the function of the aggregate with good bond property.

Proportions for Fly Ash Aggregates
Cement and fly ash are constituents for preparation of the aggregates.Also water is the binder when it is added to increase the workability.Six different proportions of cement and fly ash such as 10:90, 12.5:87.5,15:85, 17.5:82.5,20:80 and 22.5:77.5 were tried.

Preparation of Fly Ash Aggregates
Cement and fly ash were mixed in above six proportions in a concrete mixer.Water was added to the mix by adopting the water cement ratio of 0.3.The contents were thoroughly mixed in the drum until the complete formation of fly ash aggregates.This method of formation of fly ash aggregates is called pelletisation.The fly ash aggregates are shown in figure 1.

Drying and Curing of Fly Ash Aggregates
The fly ash aggregates were taken out from the mixer and allowed to dry for a day.Then the aggregates were cured in a water tank for 7 days.

Segregation of Fly Ash Aggregates
After curing, they were segregated into fine and coarse aggregates based on size of pellets as shown in figure 2. The aggregates having size less than 4.75 mm were sieved as fine aggregates and size more than 4.75 mm were sieved as coarse aggregate.From them 20 mm size coarse aggregates were sieved separately to use them as coarse aggregates.

Mix Proportions
Mix design for M20 grade concrete was done by using IS method.As per the design, the mix proportion is 1:1.53:2.68:0.5.

Casting of Test Specimens
The Concrete Cubes (control specimens) of size 15cm x 15cm x 15cm were cast by using conventional fine aggregate (CFA) and conventional coarse aggregate (CCA) and using fly ash fine and fly ash coarse aggregates obtained from the above six cement fly ash proportions.The specimens were demoulded after 1 day and immersed in water for 1, 3, 7, 14, 28, 56 and 90days for curing.
The concrete cylinders (Control Specimens) of diameter 15cm and height 30cm were cast by using conventional fine aggregate (CFA) and conventional coarse Aggregate (CCA).For FAAC, the specimens were cast with fly ash fine aggregates (FAFA) and fly ash coarse aggregates (FACA) obtained from the above six cement fly ash proportions.The specimens were demoulded after 1 day and immersed in water for 7 days and 28 days for curing.
The concrete beams of (control specimens) of size 100mmX100mmX500mm were cast using conventional fine aggregate (CFA) and conventional coarse aggregate (CCA).For FAAC, the specimens were cast with fly ash fine aggregates (FAFA) and fly ash coarse aggregates (FACA) obtained from the above six cement fly ash proportions.The specimens were demoulded after 1 day and immersed in water for 7 days and 28 days for curing.

Compression Test
15cm x 15cm x 15cm concrete cubes were tested as per IS 516-1959.The test was conducted in 120T compression testing machine.The load was applied at the rate approximately 140kg/cm 2 /min until the failure of the specimen.The maximum load applied to the specimen until failure was recorded and shown in Table -5.Fly ash aggregate concrete with fly ash aggregates prepared from cement fly ash proportions 15:85 showed 15%, 13%, 11%, 15%, 14%, 13% and 8% increase in Compressive strength at the ages of 1day, 3days, 7days, 14days, 28days, 56days and 90days respectively over the control concrete.

Split Tension Test
Concrete cylinders of 15cm diameter and 30 cm height were tested for Split tensile strength as per IS 5816-1976.
The specimen was placed horizontally between the loading surfaces of the compression testing machine and the load was applied without shock until the failure of the specimen.The maximum load at failure was tabulated in Table -6.The fly ash aggregate concrete beam with fly ash aggregate made from cement fly ash proportion 15:85 showed an increase in split tensile strength of 12% and 15% respectively at the ages of 7days and 28days compared with control concrete beam.

Flexural Strength Test
The concrete beams of size 10cm x 10cm x 50cm were tested as per IS 516-1959.The load was applied through two similar rollers mounted at one third points of the supporting span.The load was applied without shock until the failure occurs.The maximum load at failure was tabulated in Table -7.Control concrete 5.36 The fly ash aggregate concrete containing fly ash aggregate made from cement fly ash proportion 15:85 showed increase in flexural strength of 11% and 16% respectively at the ages of curing in 7days and 28days than the control concrete specimen.
The cement fly ash proportions 10: 90, 12.5:87.5, 17.5:82.5, 20:80 and 22.5:77.5 showed reduction in flexural strength of 17%, 9%, 11%, 15%, and 20%, respectively at the age of curing in 7days and 18%, 11%, 14%, 18%, and 23%, respectively at the age of curing in 28days than control concrete specimen.ash with aggregate is technically feasible and are found to be light weight in nature.They found fly ash aggregate concrete up to 20Mpa can be used for production of concrete blocks for masonry construction in structures.They also suggested to select mortar which can produce fly ash aggregate concrete of strength up to 40Mpa.However concrete strengths more than 40Mpa can also be produced using less content of fly ash aggregates.Dr.J.B.Behera, Dr.B.D.Nayak, Dr.H.S.Ray and Dr.B.Sarangi examined the use of sintered fly ash aggregate in concrete as a partial replacement of granite aggregate.They concluded that in addition to light weight characteristics, the sintered fly ash concrete possesses strength and deformation characteristics similar to concrete with natural granite aggregate.
Gao Li-Xiong, Yaoyan and Wang Ling, China building materials Academy, Beijing found light weight aggregate concrete made with sintered fly ash aggregate showed good workability, high compressive strength and low absorption of water.
Mehnet Gesoglu, Turan Ozturan and Erhan Gunegisi found the compressive strength of concrete ranging from 20 to 50Mpa was practically produced by using light weight fly ash aggregates.They also found the increase in splitting tensile strength of concrete due to increase in aggregate crushing strength of fly ash aggregates.
Haydar Arsian and Gokhan Baykal investigated the fly ash aggregates produced from fly ash and cement mixing by pelletization method and evaluated Engineering properties such as crushing strength, specific gravity, water absorption, particle size distribution, surface characteristics and shear strength properties of the manufactured aggregates experimentally.The experimental investigation showed that these aggregates are a good alternative for wide range of civil Engineering applications.
Rudolph Klotten, Metallurgical Engineer, Bakerstown road, Tarentum expressed that the largest potential outlet for light weight aggregates(LWA) manufactured from fly ash is in concrete, concrete products, block and masonry units.LWA offer better thermal and acoustical insulation, high fire resistance, easy cutting and drilling.
Gokhan Baykal and ATA Gurhan Doven studied the properties of fly ash aggregates made by pelletization process such as unit weight, specific gravity, water absorption, crushing value test, California bearing ratio (CBR), Direct shear tests, soundness tests, compressive and flexural strength and obtained good results when compared to conventional aggregates in concrete.

Figure 2 .
Figure 2. Segregation of fly ash aggregates

Table 1 .
Physical properties of Conventional Fine Aggregate (CFA) and Fly ash fine aggregate (FAFA)

Table 2 .
Physical properties of Conventional Coarse Aggregate (CCA) and Fly ash Coarse Aggregate (FACA)

Table 3 .
Fly Ash (Class F) at Mettur Thermal Power Station (MTPS) Fly ash (Class F) obtained from Mettur Thermal Power Station (MTPS) was used in the experimental work.The comparison of chemical composition of fly ash produced at MTPS to that of world's standard is as below.

Table 4 .
Comparison of Workability of FAAC with Conventional Concrete

Table 5 .
Compressive strength of fly ash aggregate concrete and control concrete with different ages of testing

Table 6 .
Split tensile strength of fly ash aggregate concrete and control concrete with different ages of testing

Table 7 .
Flexural strength of fly ash aggregate concrete and control concrete with different ages of testing