lecture 3, constituents of concrete-fine aggregate

Constituents of concrete-Fine Aggregate (Sand)

Presented by: Dr. H.M.A.MahzuzAssistant Professor, Department of Civil and Environmental EngineeringShahjalal University of Science and Technology, Sylhet

Sand or Fine aggregate in Concrete, where???

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Aggregates

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Granular materials, such as sand, gravel, crushed stone used with cementing medium to form concrete or mortar.

Aggregates act as inert filler, providing stability against volume changes and influencing strength.

Fine Aggregate Diameter< 4.75mm

Coarse AggregateDiameter > 4.75mm

Aggregates

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Fine aggregate (Sand)•Particles having diameter in between 4.75 mm to 75 micron is called Sand.•Fills the voids within coarse aggregates•Assists in hardening of cement by allowing water to seep through its voids•Minimizes shrinking and cracking of concrete•Economizes concrete by varying its proportion for strength

Coarse Aggregate (Stone chip/ singles/ brick chip)•Acts as main filler, forms the main bulk mass•Binding material adheres on its surface•Imparts volumetric stability and durability to concrete•Increases crushing strength, resistance to wear and tear, and water tightness of concrete•Economizes concrete since it is cheaper than cement

SandSand is an important building material used in the preparation of mortar, concrete, etc.

Sources of Sand: Sand particles consist of small grains of silica (Si02). It is formed by the decomposition of sand stones due to various effects of weather. The following are the natural sources of sand.

a. Pit Sand: This sand is found as deposits in soil and it is obtained by forming pits to a depth of about 1m to 2m from ground level. Pit sand consists of sharp angular grains, which are free from salts and used for making mortar, clean pit sand free from organic and clay should only be used.

b. River Sand: This sand is obtained from beds of rivers. River sand consists of fine rounded grains. Colour of river sand is almost white. As the river sand is usually available in clean condition, it is widely used for all purposes. c. Sea Sand: This sand is obtained from sea shores. Sea sand consists of rounded grains in light brown colour. Due to all such reasons, sea sand Sea sand consists of salts which attract the moisture from the atmosphere and causes dampness, efflorescence and disintegration of work. It is not recommendable for engineering works. However be used as a local material after being thoroughly washed to remove the salts.

Classification of sand according to size

08/27/17 CE- Properties of Concrete 7

Fine sand- size less than 1/16 inch (passes through No16 ASTM sieve). Used for plastering work.

Medium/ moderate coarse sand- size 1/16 -1/8 inch (passes through No 8 ASTM sieve). Used in masonry work.

Coarse sand: size 1/8-3/16 inch (passes through No 4 ASTM sieve). Used in concrete work.

According to BNBC-2006


Sand availability in Bangladesh

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Sand in available in all over Bangladesh. Mainly rivers are the principal sources of sand in Bangladesh. Good quality river sand in available in Dhaka, Mymensing, Sylhet (Jaflong , Volagonj).

Unit weight of sand: sand = 1450 t0 1500 kg/m3Price of sand = 18-25 Tk/cft

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Characteristics of sand:

1. It should be chemically inert (non-reactive).2. It should be clean and coarse. It should be free from organic matter.3. It should contain sharp, angular and durable grains.4. It should not contain salts, which attract the moisture from atmosphere.5. It should be well graded (i.e.) should contain particles of various sizes in suitable proportions.6. Bulking


Bulking of Sand:

The presence of moisture in sand increases the volume of sand.

Sand Gradation/ Sieve analysis

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A sieve analysis (or gradation test) is a practice or procedure used (commonly used in civil engineering) to assess the particle size distribution (also called gradation) of a granular material.The size distribution is often of critical importance to the way the material performs in use. A sieve analysis can be performed on any type of non-organic or organic granular materials including sands, crushed rock, clays, granite, feldspars, coal, soil, a wide range of manufactured powders, grain and seeds, down to a minimum size depending on the exact method. Being such a simple technique of particle sizing, it is probably the most common.

grading of sand

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Example of Gradation curve and FM (ASTM 136)

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sum(without pan) = 3.1+10.4+29.1+60.9+85.7+99.3 = 289, FM = 289/100=2.89

ASTM Seive No Sieve size (mm) Sieve size (inch) Mat Retain(gm) % mat Ret Cumul % ret % finer FM

4 4.75 0.187 (3/16) 31 3.1 3.1 96.9

2.89

8 2.36 0.094 (1/8) 73 7.3 10.4 89.6

16 1.18 0.047 (1/16) 187 18.7 29.1 70.9

30 0.6 0.023 318 31.8 60.9 39.1

50 0.3 0.012 248 24.8 85.7 14.3

100 0.15 0.006 136 13.6 99.3 0.7

pan --- 7 0.7 100.0

Sum 1000 100

Example

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Question:Three types of sand A, B and C are mixed in the ratio of 1:2:3. The FM of A, B and C sand are 2.62, 2.32 and 2.80. Compute the combined FM.

Answer:

Combined FM= (1x2.62+2x2.32+3x2.8)/(1+2+3)= 2.61

Example

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Question:A 1200 gm sand is the mixture of two different types of sand from Savar (FM- 2.28) and from Durgapur (FM- 2.74). The combined FM is 2.51. Find the amount of each sand.

Answer:

Combined FM= (Durgapur- combined)/(combined-Savar) = (2.74-2.51)/(2.51-2.28)=0.23/0.23=1/1

Durgapur=600gm and Savar= 600 gm


WHAT IS FINENESS MODULUS OF AGGREGATE?

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves and dividing this sum by 100.

Type of Sand Fineness Modulus Range

Fine Sand 2.2 – 2.6

Medium Sand 2.6 – 2.9

Coarse Sand 2.9 – 3.2

Oven dry. Surface and pores are bone-dry and fully absorbent.

Air dry. Surface is dry but contains some interior moisture and is therefore somewhat absorbent.

Saturated, surface-dry. Pores are saturated but surface is dry--neither absorbing water from nor contributing water to the concrete mix.

Damp or wet. Aggregate contains an excess of moisture on the surface.

Some basic terms:

Thanks a lot

Surkhi Surkhi is used as a substitute for sand for concrete and mortar, and has almost the same function as of sand but it also imparts some strength.

It is avoidable in important elements since it absorbs water more than sand.