Aggregates
Aggregates are inert granular materials (sand, gravel, crushed stone) that form about 70–80% of the volume of concrete. They provide bulk, dimensional stability (reduce shrinkage), strength and economy.
Classification
| Basis |
Types |
| Size |
Fine aggregate — passes the 4.75 mm IS sieve (sand); coarse aggregate — retained on 4.75 mm (gravel, crushed stone); all-in aggregate — natural mixture of both |
| Source |
Natural (river sand, pit sand, gravel, crushed rock); manufactured sand (M-sand) (crushed rock fines); artificial/industrial (slag, fly ash aggregates, expanded clay); recycled aggregates (from construction and demolition waste) |
| Shape |
Rounded, irregular (partly rounded), angular, flaky (thin), elongated |
| Surface texture |
Glassy, smooth, granular, rough, crystalline, honeycombed |
| Unit weight |
Normal-weight (about 2.5–2.7 specific gravity), lightweight (pumice, expanded clay, cinders), heavyweight (barytes, magnetite, haematite) |
Effect of shape: rounded aggregates give better workability for a given water content; angular aggregates give better interlock and bond (higher strength) but need more cement paste; flaky and elongated particles reduce workability and strength and should be limited.
Properties of aggregates
- Strength — measured by crushing value, impact value and abrasion value (IS 383 limits: e.g. crushing and impact values not more than about 45% for concrete in general and 30% for wearing surfaces such as runways, roads and pavements; Los Angeles abrasion value not more than about 50% and 30% respectively).
- Specific gravity — commonly about 2.6–2.7 for natural aggregates.
- Bulk density and voids — depend on grading and compaction; well-graded aggregates have fewer voids.
- Porosity and water absorption — affect water demand and durability; many good aggregates absorb less than about 2%.
- Soundness — resistance to weathering (sodium or magnesium sulphate test).
- Chemical stability — no harmful reaction with cement.
Moisture conditions
| Condition |
Description |
| Oven dry |
All moisture removed |
| Air dry |
Surface dry, pores partly filled |
| Saturated surface dry (SSD) |
Pores filled, surface dry — reference condition in mix design (neither absorbs nor adds water) |
| Moist (wet) |
Pores filled plus free surface water (adds water to the mix) |
Mix design water is adjusted for absorption (dry aggregates take water) and free moisture (wet aggregates add water).
Bulking of sand
Moist sand increases in volume because thin films of water around particles push them apart. Bulking increases with moisture up to a maximum (commonly around 4–6% moisture, with bulking of about 20–40%, finer sands bulking more) and then decreases; saturated sand does not bulk.
When sand is measured by volume, bulking must be allowed for — otherwise the mix has less sand than intended.
Bulking (%)=h2h1−h2×100
(h1 = height of moist sand in a container; h2 = height after inundation with water.)
Deleterious materials
- Clay, silt and fine dust — coat particles, reduce bond, increase water demand.
- Organic impurities — interfere with hydration (colorimetric test).
- Coal, lignite, mica, shale — weak or unstable particles.
- Salts — chlorides (corrosion of steel), sulphates (sulphate attack).
- Reactive silica — alkali–aggregate (alkali–silica) reaction: reactive silica in aggregates reacts with alkalis from cement to form an expansive gel that absorbs water and cracks concrete (map cracking). Prevention: non-reactive aggregates, low-alkali cement, pozzolanas (fly ash, slag, silica fume), keeping concrete dry.
Grading of aggregates
Grading is the particle size distribution, determined by sieve analysis using IS sieves (80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 µm, 300 µm, 150 µm).
- Well-graded aggregates have particles of all sizes — minimum voids, less paste needed, dense and economical concrete.
- Gap-graded aggregates lack intermediate sizes — used in special concretes (e.g. exposed aggregate finishes, SMA in roads).
- Uniformly graded aggregates — mostly one size (e.g. no-fines concrete).
Grading zones of fine aggregate (IS 383): sand is classified into Zone I (coarsest) to Zone IV (finest) according to the percentage passing specified sieves; Zones I to III are generally suitable for concrete (Zone IV is very fine and needs mix adjustments).
Maximum size of coarse aggregate
The largest practicable size reduces paste requirement, but it is limited by member dimensions and reinforcement spacing — commonly not more than one-fourth of the minimum thickness of the member, and smaller than the clear spacing between bars and the cover by a margin (about 5 mm) so that concrete can pass between bars. 20 mm aggregate is most common for reinforced concrete; 40 mm or larger for mass concrete.
Tests on aggregates
Sieve analysis; specific gravity and water absorption; bulk density and voids; flakiness and elongation indices; crushing value, impact value and Los Angeles abrasion value; soundness; silt/clay content (field settling test and laboratory tests); organic impurities; alkali reactivity; bulking of sand.
Mortars
Mortar is a workable paste of a binding material (cement, lime, gypsum or clay), fine aggregate (sand, surkhi) and water, which hardens after application.
Functions
- Binds bricks or stones into a monolithic mass and distributes loads uniformly.
- Fills joints and makes masonry weather-tight.
- Provides a smooth, protective finish as plaster.
- Holds coarse aggregate together in concrete (as the matrix).
- Improves appearance (pointing).
Types of mortar
| Mortar |
Features / use |
| Cement mortar |
Cement and sand (e.g. 1 : 3 to 1 : 6 by volume); strong and durable; masonry, plastering, pointing, damp situations |
| Lime mortar |
Fat or hydraulic lime with sand or surkhi; good workability; fat lime mortar for dry situations; hydraulic lime mortar for damp situations |
| Composite (gauged / cement–lime) mortar |
Cement, lime and sand (e.g. 1 : 1 : 6); combines strength of cement with workability and water retention of lime; less cracking |
| Surkhi mortar |
Lime with surkhi (burnt clay powder) — hydraulic properties |
| Mud mortar |
Clay with water (and chopped straw/cow dung) — cheap, for low-cost and temporary buildings |
| Gypsum mortar |
Plaster of Paris with sand — interior plasters, quick setting |
| Special mortars |
Fire-resistant (with aluminous cement and fire-brick powder), lightweight (with sawdust, pumice), sound-absorbing, X-ray shielding (with barytes), packing mortar (oil wells), waterproof mortar |
Typical proportions (cement : sand, by volume)
| Use |
Proportion (typical) |
| Pointing |
1 : 2 to 1 : 3 |
| Ceiling plaster, external plaster in severe exposure |
1 : 3 to 1 : 4 |
| Internal and external wall plaster |
1 : 4 to 1 : 6 |
| Brick masonry — load-bearing, exposed |
1 : 4 to 1 : 5 |
| Brick masonry — general |
1 : 6 |
| Stone masonry (rubble) |
1 : 6 |
Properties of good mortar
- Workability — easy to spread and to fill joints.
- Water retentivity — retains water against suction by bricks (important for bond and hydration).
- Strength — adequate compressive strength (but mortar should not be much stronger than the units — cracks should occur in joints, which are easier to repair).
- Adhesion (bond) with masonry units.
- Durability — resistance to weathering, frost, salts.
- Low shrinkage and cracking.
- Setting time suitable for construction.
Tests on mortar
Compressive strength (e.g. 70.6 mm cubes), adhesion/bond test (bricks joined by mortar pulled apart), crushing and tensile strength, water retention (flow before and after suction), consistency (flow table), setting time and soundness of binders.
Preparation and use
- Mixing — dry mixing of binder and sand to uniform colour, then adding water gradually (hand mixing on a clean platform, or machine mixing).
- Cement mortar should be used within a short time after adding water (before its initial set, commonly within about 30 minutes); retempering after setting starts is not allowed.
- Lime mortar can be used over a longer period (hydraulic lime mortar should be used the same day).
- Masonry units are wetted before laying to prevent suction of water from mortar; mortar joints are cured.
Quantity of materials
The dry volume of materials required for a unit volume of wet mortar is greater because voids in sand are filled by paste and water is lost; a factor of about 1.27–1.33 (commonly 1.30) is used.
Worked examples
✎Worked ExampleExample 1 — fineness modulus
Cumulative percentages retained on the 4.75 mm, 2.36 mm, 1.18 mm, 600 µm, 300 µm and 150 µm sieves are 2, 10, 30, 55, 80 and 96. Find the FM and classify the sand.
Solution. FM=1002+10+30+55+80+96=2.73 → medium sand
✎Worked ExampleExample 2 — bulking of sand
Sand measured by volume in the field shows 25% bulking. How much loose moist sand should be taken to supply 1 m³ of dry sand?
Solution. 1×1.25=1.25 m3 of moist sand (otherwise the mix would contain only 1/1.25=0.8 m³ of sand).
✎Worked ExampleExample 3 — cement mortar quantities
Find the cement (bags) and sand for 1 m³ of 1 : 6 cement mortar.
Solution. Dry volume =1.30 m³; parts = 7
Cement =1.30/7=0.186 m³ → 0.186/0.0347=5.35 bags
Sand =0.186×6=1.11 m3
✎Worked ExampleExample 4 — water absorption of aggregate
An aggregate sample weighs 1025 g in SSD condition and 1000 g when oven dry. Find its water absorption.
Solution. 10001025−1000×100=2.5%
Frequently tested points
- Fine aggregate passes 4.75 mm; coarse retained on 4.75 mm; aggregates occupy 70–80% of concrete volume.
- Rounded aggregates — better workability; angular — better bond and strength.
- SSD is the reference moisture condition.
- Bulking of sand: maximum around 4–6% moisture (20–40%); saturated sand does not bulk; finer sand bulks more.
- Alkali–silica reaction: reactive silica + alkalis → expansive gel; controlled by low-alkali cement and pozzolanas.
- FM: fine sand 2.2–2.6, medium 2.6–2.9, coarse 2.9–3.2; IS 383 Zones I (coarse) to IV (fine).
- Max aggregate size ≤ 1/4 of member thickness and less than bar spacing and cover.
- Crushing/impact ≤ 45% (general), ≤ 30% (wearing surfaces); LA abrasion ≤ 50% / 30%.
- Composite mortar (cement–lime) combines strength and workability; surkhi gives hydraulicity.
- Pointing 1 : 2–1 : 3; brickwork 1 : 6; mortar should not be stronger than bricks.
- Cement mortar used within about 30 minutes; dry volume factor ≈ 1.30.
⚠Common MistakeCommon mistakes
- Ignoring bulking when batching sand by volume.
- Using mortar that has begun to set after retempering with water.
- Treating flaky and elongated aggregates as acceptable in any quantity.
✔Revision SummaryChapter summary
- Aggregates are classified by size, source, shape and density and strongly influence concrete properties.
- Strength, specific gravity, absorption, moisture condition, bulking and deleterious materials must be evaluated.
- Sieve analysis, fineness modulus, grading zones and maximum size govern aggregate selection.
- Mortars bind masonry and form plasters; cement, lime, composite, mud, gypsum, surkhi and special mortars have distinct uses and proportions.
- Good mortar is workable, water-retentive, adhesive and durable; quantities are computed using a dry volume factor.
