Aggregates as Building Materials : Types and Properties.

Aggregates Properties and Tests

This section gives introduction about Aggregates that forms bulk of concrete. Here, We will study about various properties and tests conducted on Aggregates to determine their strength and soundness.

Sources of Aggregates :

Igneous Rocks

They are hard, tough and dense so they yield highly satisfactory concrete aggregates.

These rocks are crystalline in nature or glassy. Aggregates obtained from Igneous formation have tendency to react with alkalis in cement.

Sedimentary Rocks

They are result of weathering forces of nature on Igneous and Metamorphic rocks. Their cementing material can be carbonaceous, siliceous, or argillaceous in nature.

They vary from soft to hard, porous to dense and light to heavy aggregates.

Limestones are examples of sedimentary rocks that yield good concrete aggregate.

Metamorphic Rocks

They are foliated structures formed as a result of high temperature and pressure. Too much layers cannot produce aggregates of good quality.

Some rocks like Quartzite and Gneiss produce very fine quality aggregates.

Properties of Aggregates :

Aggregates give body to concrete.

It reduces shrinkage.

They occupy 70-80 % of volume of concrete.

The maximum size of aggregates is 80 mm.

Types of Aggregates :

Fine Aggregate : Size ≤ 4.75 mm
Coarse Aggregate : Size > 4.75 mm
Rounded : More workability, smooth, good bond
Angular : rough textured, better interlocking, more water required
Flaky : very poor strength
Cubical : Maximum strength

Angularity Number : It is based on percentage voids in aggregate after compaction in specified manner.

Void (44 %) : A.N = 11

Void (33 %) : A.N = 0

If water cement ratio ≥ 0.65 : No difference in strength between round and angular aggregates.

Tests on Aggregates

Strength Tests

Sieve Analysis

In sieve analysis, we divide a sample of aggregates into various fractions each consisting of particles of same size.

We conduct it to determine the particle size distribution in a sample of aggregates, called gradation.

The sieves of different pore sizes are mounted one over another with finer sieves at bottom.

The aggregates are sieved from top container by shaking.

The materials retained on each sieve after shaking, represents the fraction of aggregates coarser than sieve in question and finer than sieves above.

The size distribution is expressed as what we call fineness modulus.

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregates retained on each standard sieves.

The sieves range between 80 mm to 150 microns. Finally we divide this sum by an arbitrary number 100.

The larger the figure, the coarser the aggregate.

Specific Surface and Surface Index

The Surface area per unit weight of the material is termed as specific surface.

This is the indirect measurement of aggregate grading.

Specific surface increases with the reduction in the size of aggregate particle.

Hence, the fine aggregates contribute very much more to the surface area than does the coarse aggregate.

Greater surface area requires more water for lubricating the mix to give workability.

The workability of the mix, therefore, is influenced more by fine fraction than coarser particles in a sample of aggregates.

Surface Index is an empirical number related to the specific surface of particle with more weightage given to the finer fractions.

The total surface index is calculated by multiplying the percentage of material retained on its sieve by the corresponding surface index.

Then to their sum we add a constant of 330 and divide the result by 1000.

Aggregates Crushing value

Cylinder specimen ( d = 25 mm, 25 mm high)
Sieve : passing 12.5 mm and retained on 10 mm
Load of 40 tonnes by plunger
Material crushed finer than 2.36 mm
Percentage of crushed aggregate with respect to original weight
Crushing value : 30 (roads) ; 45 (other)

Aggregates Impact value

Resistance to impact
15 blows (14 Kg hammer) from height of 38 cm
Quantity of finer material (< 2.36 mm) as percentage
It is less than 45 for wearing surface and less than 30 for runway and roads.

Deval Attrition test

Used to test abrasion
Iron cylinder rotated 10000 times
Material crushed finer than 1.7 mm is expressed as percentage

Dorry Abrasion test

Cylinder specimen (H= 25 cm, D = 25 cm) is abraded by rotating metal disk sprinkled with quartz sand.
After 1000 revolution, We take loss in weight.

\large{\color {blue}{Hardness = 20 - \frac {loss\; (in \;grams)}{3}}}

If Hardness > 17 then sample is Ok.

Los Angels Test (Abrasion)

Material with abrasive charge rotated in cylinder
Particle smaller than 1.7 mm separated and expressed as loss in weight in percentage.
Abrasion value < 30 (wearing surface) ; < 50 (other)

Bulk density of Aggregates (Kg/litre)

Angular aggregate show lower bulk density.
It shows packing and depends on particle size distribution and shape
Aggregates are compacted and weight is taken.

\large{\color {blue}{Percentage \:voids = \frac{G_{S}-\gamma }{G_{S}}\times 100 }}

G_S : Specific gravity of aggregate (2.6 – 2.8)

γ : Bulk density

Bulking of Aggregates :

Free moisture forms film around each particles, which exerts surface tension keeping neighboring particles away from each other.
Due to no point of contact, bulking occurs.
Fine sand bulks more than coarse sand. Bulking in coarse sand is what we neglect. Very fine sand bulk as much as 40 %.
Percentage increase in volume against moisture content added.
Maximum bulking (sand) : 11 % (moisture content)

Test for Organic Impurities

It is an approximate method for estimating whether organic compounds are present in the natural sand in an objectionable quantity or within permissible limit.

We fill 350 ml graduated clear glass bottle up to 75 ml mark and add 3 % solution of sodium hydroxide in water.

Then we gradually add sand until the volume measured by sand layer is 125 ml. The total volume then becomes 200 ml by adding more solution.

Then we shake the bottle vigorously and leave it for 24 hours. Now the colour of the liquid is compared with standard solution freshly prepared by mixing tannic acid, alcohol and sodium hydroxide in proper compositions.

Coloured acetate sheets can also serve as comparison medium.

Moisture Content Test

Drying Method

Oven dry the sample
Loss in weight after drying measured
Accelerated by methylated spirit or acetone

Displacement Method

Principle : specific gravity of normal aggregate is higher than that of water. Given weight of wet aggregate will occupy a greater volume than same weight of dry aggregate.

Calcium Carbide Method

CaCN reacts with surface moisture in aggregate to produce acetylene gas.
We measure the pressure which is proportional to moisture content.

Soundness of Aggregate :

Alternate immersion of sample in solution of sodium or magnesium sulphate and over drying it.
Salt crystals grows in pores. We measure the loss in weight.
Loss in weight : not greater than 12 % ( when Na₂SO₄) ; 8 % (when MgSO₄)
Alkali content in cement < 0.4 %

Flakiness Index

Percentage by weight of particles whose least dimention is less than 3/4^th of their mean dimention.
Not applicable to size < 6.3 mm
British standard limit : 50 (gravel) ; 40 (Coarse aggregate)

Elongation Index

Percentage by weight of particles whose greatest dimention is greater than 1.8 times their mean dimention.
Not applicable to size < 6.3 mm
Presence of elongated particles in excess (10-15) % is undesirable.

Specific Gravity :

Sample ( < 2 Kg) is placed in wire basket and immersed in distilled water ( 22 – 32 )°C for period ( 24 +/- 1/2 hours)
We take their weight after jolting (A₁)
Then We remove the basket from water, dry the aggregate. Empty basket is immersed, weight is taken (A₂)
Dried aggregate weight is taken in air (B)
Oven dried sample (24 +/- 1/2 hours) is weighted ( T = 100-120 °C)(C)

\large{\color {blue}{Specific \:Gravity = \frac{C }{B-A}}}

\large{\color {blue}{Apparent\:Specific \:Gravity = \frac{C }{C-A}}}

\large{\color {blue}{Water \:Absorbtion = 100 \times\frac{B-C }{C}}}

Mortar

This section gives introduction about Mortar. It provides detailed account of types and properties of Mortar along with various proportions that we use in construction.

Mortar :

Workable paste prepared by adding water to mixture of binding material and fine aggregate. It should be plastic enough to hold bricks together.

Addition of lime improves workability of Mortar.

Types :

Bricklaying Mortar
Cement Mortar : High strength and resistance to water. Its composition varies between 1:2 to 1:6
Finishing Mortar : For pointing, plastering and ornamental finishing
Lime Mortar :Fat lime or hydraulic lime is the binding material; It has high plasticity due to which it shows no cracks and does not sets quickly.
Gypsum Mortar : Plaster and Soft Sand; Low durability in damp climate.
Gauged Mortar : Combination of lime, cement and sand is used; high plasticity
Surkhi Mortar : Surkhi (crushed bricks) is used as aggregate with lime as binder.
Aerated Cement Mortar : Air entrailing agent is in use.
Mud Mortar : Mud binder, saw dust, dung and husk as fine aggregate
Heavy Mortar : Bulk density (> 15 KN/m³); heavy Quartz are is in use as fine aggregate
Lightweight Mortar : Bulk density (< 15 KN/m³); lime and cement as binder; Sound and heat proof
Type M Mortar : Highest strength mortar (17.2 MPa); exterior masonry work; takes lateral load
Type S Mortar : Medium strength (12.4 MPa); high bonding and used for ground work
Fire resisting Mortar : aluminous cement to fine; powder of fire bricks.
Type N Mortar : reinforced interior
Type O Mortar : Non lateral bearing
Packing Mortar : Cement sand, Cement-loam; Pack oil well
Sound Absorbing Mortar : cement, lime, gypsum and slag as binder; Pumice rock and cinder as fine aggregate
X Ray Shielding Mortar : rocky fine aggregate; bulk density > 22 KN/m³
Chemical resistant Mortar : Silicate type, Sulphur (poor alkali resistance)
Composite Mortar : Mechanical grinding; not for tall building

Mortar Proportions : (Cement : Fine aggregate)

For Normal Brick Work : 1:6

For Plastering : 1:4

Grouting rocks : 1:1.5

Guniting : 1:3

Mica in sand reduce strength of mortar.

Cement mortar richer than 1:3 is not used as there is no gain in strength and there is high shrinkage.

Saw dust in mortar acts as pozzolana.