Introduction to concrete and concrete materials

Bogue's compounds are the four main compounds formed in the kiln when limestone and clay are burnt to clinker (about 1450 °C). They are calculated from the oxide analysis by Bogue's equations (IS 4032). In cement chemistry notation C = CaO, S = SiO$_2$, A = Al$_2$O$_3$, F = Fe$_2$O$_3$.

Role in properties and strength

• C$_3$S: hydrates in hours to days. It gives the early strength (up to 7 days) and much of the 28-day strength, and releases a lot of heat and $Ca(OH)_2$. More C$_3$S gives rapid-hardening cement.
• C$_2$S: hydrates slowly, so it contributes little in the first weeks but gives the strength after 28 days (up to 1 year). It releases little heat and gives better resistance to chemical attack, so it is high in low-heat cement.
• C$_3$A: reacts instantly with water and causes flash set, so gypsum is added to control it. It adds little to strength (only some at 1 day), liberates large heat, and is the compound attacked by sulphates (forms ettringite and expands). Sulphate-resisting cement keeps it below 5%.
• C$_4$AF: contributes very little to strength; it acts as flux in the kiln and gives the grey colour. It is fairly resistant to sulphates.

Strength development

Early strength is governed by C$_3$S, later strength by C$_2$S. Approximate share of strength at 28 days: C$_3$S and C$_2$S together form about 70-80 per cent of the cement and produce the C-S-H gel, which is the main strength-giving product.

Admixtures are materials, other than cement, water, aggregate and fibre, added to the concrete in small quantities just before or during mixing to modify its fresh or hardened properties (IS 9103).

Purposes of using admixtures

• Improve workability without extra water, or reduce water at the same workability.
• Accelerate or retard setting and early strength gain.
• Increase strength and durability, reduce permeability.
• Reduce heat of hydration and bleeding/segregation.
• Save cement and give economy.
• Provide air entrainment for frost resistance.

Chemical admixtures (IS 9103; ASTM C494 Types A-G)

Mineral admixtures

Finely divided siliceous materials, used in large quantities (10-40% of cement), which react with $Ca(OH)_2$ (pozzolanic reaction) to form extra C-S-H.

• Fly ash (IS 3812): improves workability, lowers heat, better long-term strength.
• Silica fume: very fine (about 0.1 µm); gives very high strength and low permeability.
• Ground granulated blast furnace slag (IS 12089): sulphate and chloride resistance.
• Rice husk ash, metakaolin: highly reactive pozzolans.

Mineral admixtures improve durability and reduce the cost and CO$_2$ footprint of concrete, while chemical admixtures mainly control the workability and setting.

Bulking is the increase in the volume of sand (fine aggregate) when it contains a certain amount of surface moisture, compared with the same mass of dry sand.

Cause

Moisture forms a thin film around each sand grain. Surface tension of this film pushes the particles apart and keeps them from packing closely, so voids increase and the volume rises. Bulking is about 20-40 per cent at a moisture content of 4-6 per cent for fine sand. When the sand is fully flooded (saturated), the film is broken and the volume falls back to the dry volume. Finer sand bulks more than coarse sand.

Bulking %
  |        _____
  |      /       \
  |    /          \
  |  /              \___ (flooded:
  |/                      bulking = 0)
  +----------------------------> moisture %
     4-6% peak

Effect

• If the mix is batched by volume, the actual quantity of sand is less than intended, so the mix becomes lean in fine aggregate: the concrete is harsh, honeycombed, less workable, lower in strength, and the yield per bag is lower.
• Wrong sand-cement proportion affects economy and quality.

Remedial measures

• Batch sand by weight, not volume.
• Make a bulking allowance: increase the sand volume by the bulking per cent found by test.
• Dry the sand or flood it fully before batching.

Determination (field test)

Fill a measuring cylinder to a height $h_1$ with moist sand, then add water and stir so that the sand is flooded; the height becomes $h_2$.

Role in concrete manufacture

Knowing bulking, the sand quantity measured in volume batching is corrected as: required volume of damp sand = volume of dry sand $\times\ (1 + \text{bulking}/100)$. The free moisture carried by the sand is also deducted from mixing water, so the water-cement ratio stays correct.

Concrete is a composite of cement, fine and coarse aggregates and water that hardens to a stone-like mass. It is the most used structural material in the world, and with steel reinforcement (RCC) or prestressing it carries tension as well as compression.

Advantages as a structural material

• High compressive strength (20-60 MPa and more) at low cost per unit strength.
• Can be moulded to any shape (beams, shells, arches, slabs) and cast in place.
• Local materials are used; cement, sand, aggregate are widely available.
• Durable and needs little maintenance; improves strength with age.
• Fire resistance is good because concrete is a poor conductor and does not burn.
• Gives mass for stability (dams, foundations) and good sound and thermal insulation.
• Steel is protected from corrosion by the alkaline cover (pH about 12.5).
• Monolithic action in RCC gives rigid frames.
• Can use waste products (fly ash, slag).

Disadvantages

Low tensile strength (about 10 per cent of compressive), large self-weight, cracks, shrinkage and creep, long curing time, and difficulty in repair/modification.

Concrete compared with steel

Hence concrete is preferred for foundations, slabs, dams and low to medium rise buildings, and steel for long spans and tall towers.

Hydration of cement is the chemical reaction between cement compounds and water that produces new hydrated compounds which set and harden into a rigid, strong mass. It is exothermic (releases heat).

Hydration reactions (simplified)

The calcium silicate hydrate ($C_3S_2H_3$, called C-S-H gel) is the main binding product, about 50-60 per cent of the paste volume. $Ca(OH)_2$ (calcium hydroxide, about 20-25%) is crystalline, adds little strength but keeps the concrete alkaline. About 23 per cent of the cement mass of water is chemically combined plus about 15 per cent held as gel water, so roughly 38 per cent (w/c about 0.38) is needed for complete hydration.

Role of the compounds in strength gain

Strength
  |            ___C3S+C2S (total)
  |        ___/ ..... C2S
  |     __/  .../
  |   _/ ..../  
  |  /..... C3S
  | / ...
  +------------------------> age (days)
   1  7   28   90   365

Thus C$_3$S is responsible for the early strength and C$_2$S for the long-term strength. By choosing the proportion of these compounds, rapid-hardening, low-heat or sulphate-resisting cements are produced.

Role of water in concrete

• Hydration: it reacts with cement to form the binding C-S-H gel (needs about 0.23 w/c chemically, 0.38 in total).
• Workability: it lubricates the mix so that it can be mixed, placed and compacted.
• Curing: it keeps the concrete moist so that hydration continues and cracking is avoided.
• Washing aggregates and cleaning equipment.

Excess water leaves capillary pores on drying, which lowers strength and durability.

Quality of water (IS 456:2000, cl. 5.4)

Water should be fit for drinking (potable) and free from harmful amounts of oils, acids, alkalis, salts, sugar, organic matter and silt. Potable water is generally satisfactory.

• pH not less than 6.
• Limits for mixing water (IS 456 cl. 5.4): organic 200 mg/L, inorganic 3000 mg/L, sulphates (as SO$_3$) 400 mg/L, chlorides 2000 mg/L for plain concrete and 500 mg/L for RCC, suspended matter 2000 mg/L.
• Sea water: not for RCC or prestressed concrete (chloride corrodes steel); may be used for plain concrete only.
• Test: initial setting time of cement paste with the water should not differ by more than 30 minutes from that with distilled water, and the initial setting not less than 30 min; the average strength of cubes (28 days, IS 456 cl. 5.4.1.2) should not be less than 90% of that made with distilled water (IS 456 cl. 5.4.1).

Impurities and effects

Field tests give a quick check of cement quality before use; laboratory tests (IS 4031) are needed for final acceptance.

1. Colour: uniform grey with a light greenish tinge, free from lumps.
2. Physical feel: rubbing between the fingers gives a smooth feel; a rough or gritty feel means adulteration with sand.
3. Hand thrust: pushing a hand deep into the bag should feel cool; warmth means it has already started hydrating.
4. Lumps: good cement has no lumps. Lumps that crush easily between fingers may be only partly air-set (the cement is old); hard lumps mean the cement is bad.
5. Float test: a pinch dropped into a bucket of water should sink slowly and not float (floating indicates impurities/adulteration or air-set).
6. Setting test (paste/pat test): make a thick paste (w/c about 0.35) of cement on a glass plate; a pat 75 mm diameter and 12 mm thick should set hard within 24 hours underwater without cracks or distortion (soundness); initial set in about 30 minutes and final set in 10 hours.
7. Strength test: a 25 mm x 25 mm x 25 mm block of neat cement (cube of paste) kept 7 days in water should not be easily broken by hand and should resist a heavy blow.
8. Fineness: a handful of cement should not leave much residue when passed through a fine (90 micron) sieve.
9. Storage and bag checks: bag weight 50 kg, date of manufacture (use within 3 months), manufacturer's brand and grade printed on the bag (IS 269, IS 1489).

Shape

• Rounded aggregates (river gravel) have the least surface area and least interlocking, so they need less paste to coat them and give the best workability.
• Angular, flaky and elongated particles have more surface area and voids, so they need more water and cement paste for the same slump; the mix is harsh and unworkable.
• Flaky particles also lie flat and trap water below them.

Size

• Larger maximum size means smaller total surface area per unit mass, so less water and paste is needed. For the same cement content, workability improves (or the water demand falls) as the size increases.
• Size is limited by section thickness and bar spacing: IS 456 cl. 5.3.1 limits max size to 1/4 of the minimum member dimension and 5 mm less than the minimum clear spacing between bars (and 20 mm is common for RCC).
• Too fine (very small) aggregate has a large surface area and high water demand, so a stiff mix.

A well-graded mix of rounded aggregate of larger size gives the best workability for a given water-cement ratio.

Shape: Particle shapes are rounded, irregular, angular, flaky and elongated (IS 383).

Texture: surface may be glassy, smooth, granular, rough, crystalline or honeycombed.

Flaky and elongated particles reduce workability, strength and durability, since they lie in planes and hold bleed water under them. IS 383 allows a limit on flakiness and elongation index (commonly about 30-40 per cent combined, tested as per IS 2386 Part 1).

Economy: rounded aggregates reduce the paste requirement, but angular crushed aggregates give higher strength with good bond; the designer balances the extra cement against the benefit.

Sphericity is the degree to which a particle approaches the shape of a sphere. It is the ratio of the surface area of a sphere having the same volume as the particle to the actual surface area of the particle:

Its value lies between 0 and 1; a perfect sphere has sphericity = 1. Practically it is also described as the ratio of the particle's smallest to largest dimension (or the ratio of its diameter to the circumscribing sphere).

Example

• A rounded river gravel or a cube-shaped crushed stone has sphericity close to 0.8-0.9.
• A flaky stone (slab-like) or a needle-like elongated particle has sphericity of about 0.3-0.5, i.e. low.
• A glass bead or a marble (sphere) = 1.

Effect of particles with higher sphericity

• Lower surface area per unit volume, so less water and paste is needed.
• Better workability and easier compaction; lower tendency to segregation.
• Better packing, so a denser concrete with a higher compressive strength at the same cement content.
• Less chance of cracking at the interface and in bearing under stress; no splitting of flaky particles.
• Smoother, rounded spheres may reduce bond (interlock), so angular but equidimensional particles (high sphericity with rough texture) are best for high strength.

Soundness is the ability of an aggregate to resist excessive changes in volume due to changes in physical conditions such as freezing and thawing, alternate wetting and drying, or temperature changes. Unsound aggregates swell, crack and disintegrate, causing pop-outs and loss of durability of concrete.

Laboratory measurement (IS 2386 Part 5)

Soundness is tested by accelerated sodium sulphate or magnesium sulphate attack.

1. Take washed and oven-dried, sieved fractions of the aggregate (e.g. 10-20 mm for coarse aggregate; fine aggregate 300 micron to 10 mm), weighed to 100 g (fine) or specified quantity (coarse).
2. Immerse the sample in a saturated solution of sodium sulphate (or magnesium sulphate) for 16-18 hours at 27 ± 1 °C. Salt crystals form in the pores.
3. Remove, drain 15 min, then dry in the oven at 105-110 °C. This is one cycle.
4. Repeat for 5 cycles. Crystallisation pressure in the pores is similar to ice formation.
5. Wash out the salt (barium chloride test shows no sulphate), dry, and sieve over the original sieve.
6. Calculate the percentage loss in weight.

Limits (IS 383)

After 5 cycles, the weighted average loss should not exceed 12 per cent with sodium sulphate and 18 per cent with magnesium sulphate for coarse aggregates; for fine aggregates also 10% (sodium) and 15% (magnesium).

Mechanical properties show how the aggregate resists loads and wear. Four main properties are:

1. Crushing value (IS 2386 Part 4)

Resistance to crushing under gradually applied compressive load. A 12.5-10 mm sample in a cylinder is loaded to 400 kN in 10 minutes; the fines passing 2.36 mm are weighed.

Limit: not more than 45% for concrete in general and 30% for wearing surfaces such as runways, roads and pavements (IS 383).

2. Impact value (IS 2386 Part 4)

Toughness under sudden shock. A sample of 10-12.5 mm is given 15 blows from a 13.5-14 kg hammer falling 380 mm; the fines passing 2.36 mm as a percentage of the total is the AIV. Limit (IS 383): 45% for concrete other than wearing surfaces and 30% for wearing surfaces.

3. Abrasion value (Los Angeles, IS 2386 Part 4)

Resistance to wear. A sample with steel balls is rotated in a drum for 500 revolutions at 30-33 rpm; percentage passing the 1.70 mm sieve is the abrasion value. Limit 30% for wearing surfaces, 50% for others.

4. Hardness / toughness (and strength)

Hardness is the resistance to scratching (Mohs scale); strength of the parent rock is tested on cores or cubes. Values: usually granite 100-250 MPa; the rock strength should be not less than the required concrete strength.

A lower value of crushing, impact and abrasion values means a stronger, tougher and more durable aggregate. Weak aggregates cannot make strong concrete since concrete fails through the aggregate in high strength mixes.

Fineness modulus (FM) is an empirical number obtained by adding the cumulative percentages retained on the standard sieves (80, 40, 20, 10, 4.75 mm, 2.36, 1.18 mm, 600, 300 and 150 micron) and dividing by 100. It indicates the mean size of the particles in the aggregate.

The 150 micron sieve is the last one included. Larger FM means coarser aggregate. Typical values: fine aggregate 2.0-3.5 (IS 383 Zone I-IV: fine sand 2.2-2.6, medium 2.6-2.9, coarse 2.9-3.2), coarse aggregate 5.5-8.0, all-in aggregate 3.5-6.5.

Impact on concrete

• A higher FM (coarser) means less surface area, so less water and cement paste for given workability; but too coarse sand gives harsh and segregation-prone mixes.
• A low FM (fine sand) increases water demand, needs more cement and reduces strength, but gives a cohesive mix.
• In mix design, the FM of the fine aggregate is used to select the coarse aggregate volume (ACI) — higher FM sand needs a smaller amount of coarse aggregate.
• FM is used to check uniformity of supply: a change in FM by more than 0.2 indicates a change in grading and needs a change in the mix.

Example

For a sand: retained % on 4.75, 2.36, 1.18, 600, 300, 150 micron are 5, 10, 15, 25, 30, 12 (cumulative 5, 15, 30, 55, 85, 97). Sum of cumulative = 287, so FM = 2.87, which is medium sand (Zone II). In the ACI table, with 20 mm aggregate this sand (FM 2.87) takes a dry-rodded coarse aggregate volume of about 0.61 per m$^3$. A finer sand (FM 2.4) allows more coarse aggregate (0.66), and a coarser sand (FM 3.0) allows less (0.60).

Structural concrete is concrete designed and made to carry loads and form part of the load-bearing structure of buildings, bridges, dams, etc. It has a specified minimum grade (e.g. M20 or above in RCC as per IS 456 cl. 6.1.2), controlled mix, and is usually reinforced.

Types of concrete used in civil construction

Based on binder / composition:

• Plain cement concrete (PCC): without reinforcement, for foundation beds, levelling, mass work.
• Reinforced cement concrete (RCC): with steel bars for beams, slabs, columns.
• Prestressed concrete: high-strength concrete with tensioned tendons, for long-span bridges.
• Precast concrete: cast in factory (blocks, pipes, girders).

Based on density:

• Lightweight concrete (below 1800 kg/m$^3$): aerated, no-fines or lightweight aggregate concrete.
• Normal weight concrete (2200-2600 kg/m$^3$).
• Heavy weight concrete (above 3000 kg/m$^3$): barytes or iron ore, for radiation shielding.

Based on strength and special use:

• High-strength / high-performance concrete (above M60).
• Self-compacting concrete (SCC).
• Fibre reinforced concrete (steel, glass, polypropylene fibres).
• Ready-mix concrete (RMC).
• Polymer concrete, shotcrete (sprayed), roller compacted concrete (dams, pavements), mass concrete, and under-water concrete.
• Ferro-cement for thin shells.

Concrete ingredients

• Cement: the binder (OPC, PPC) which reacts with water.
• Fine aggregate: sand (below 4.75 mm); fills the voids between coarse particles.
• Coarse aggregate: crushed stone or gravel (above 4.75 mm, usually 10-40 mm); forms the bulk and strength skeleton (70-75% of the volume).
• Water: for hydration and workability (clean, potable; IS 456 cl. 5.4).
• Admixtures (optional): plasticisers, retarders, air-entraining agents, fly ash, silica fume.

Typical mix: M20 is 1:1.5:3 by volume, with w/c about 0.5.

Concrete as a structural material over steel

• Cheaper and made from local materials; steel is costlier and often imported.
• Fire-resistant, whereas steel loses strength rapidly at 400-500 °C.
• Little maintenance, no painting or corrosion issues (steel needs repeated protection).
• Can be cast to any shape and gives monolithic, rigid structures.
• Good compressive strength and mass for stability, good sound and thermal insulation.
• Works well with steel: concrete resists compression, steel resists tension, and they have nearly equal thermal expansion ($\approx 10-12 \times 10^{-6}$/°C).

Limitations: heavy self-weight, low tensile strength, slow construction, and cracking; steel is better for very long spans and for faster erection.

Ingredients of olden age concrete

Ancient builders (Egyptians, Greeks, Romans) used:

• Lime (slaked lime) mixed with volcanic ash (pozzolana), or crushed burnt clay brick (surkhi), as binder; gypsum was used in Egypt.
• Broken stone, brick bats and sand as aggregates.
• Water, and sometimes organic additives such as animal fat, milk, blood or egg whites to improve workability and durability. Roman concrete (e.g. the Pantheon) with lime-pozzolana is still standing.

Ingredients of modern concrete

• Portland cement (OPC/PPC) as binder (invented by Joseph Aspdin, 1824).
• Graded fine and coarse aggregates.
• Controlled water (w/c ratio).
• Admixtures: chemical (plasticisers, superplasticisers, retarders, air-entraining) and mineral (fly ash, silica fume, slag).
• Steel reinforcement or fibres for tension.

Concrete as a structural material

Concrete is strong in compression, durable, fire-resistant, economical, can be moulded into any shape using local materials, and when reinforced with steel carries both compression and tension. It is therefore the main material for foundations, frames, slabs, bridges, dams and pavements.

Entrapped air

Air voids that are unintentionally trapped in the concrete during mixing, handling and placing. The voids are large (above 1 mm), irregular in shape, not uniformly spread, and amount to 1-3 per cent of the volume (more in poorly compacted concrete). They reduce strength and should be removed by proper compaction.

Entrained air

Microscopic air bubbles (10-1000 µm, usually below 0.3 mm) that are intentionally introduced by an air-entraining agent. They are spherical, well-spaced and not connected to each other. The usual entrained air content is 3-6 per cent (about 4.5% for 20 mm aggregate) of the volume of the concrete.

Why an air-entraining agent (AEA) is used

• Improves resistance to freezing and thawing (frost damage) and de-icing salt scaling.
• Improves workability (acts like ball bearings), reduces bleeding and segregation.
• Reduces permeability somewhat and allows water reduction.
• Examples: Vinsol resin, wood rosin, fatty acid salts, synthetic detergents (IS 9103).

How it works

1. AEA is a surface-active chemical with a hydrophilic and a hydrophobic end. It lowers the surface tension of water.
2. During mixing, air is drawn in and forms stable, tiny bubbles, whose surface is coated by the agent so they do not coalesce.
3. These bubbles remain in the hardened paste as tiny voids. When water in the capillary pores freezes it expands by 9 per cent; the water is pushed into the nearby air bubbles, which act as pressure-relief chambers, so the paste does not crack.

Side effect: every 1% of air reduces compressive strength by about 4-6%, so the water-cement ratio is lowered to compensate.

The basic ingredients of concrete are:

1. Cement: the binding material (OPC 33/43/53 grade, PPC). It reacts with water (hydration) to produce the paste that glues the aggregates; typically 10-15 per cent of the concrete volume.
2. Fine aggregate: natural sand or crushed stone sand passing the 4.75 mm sieve (IS 383 Zones I-IV). It fills the voids in coarse aggregate and improves cohesion.
3. Coarse aggregate: crushed stone or gravel retained on 4.75 mm, with nominal size 10, 20 or 40 mm. It gives bulk, strength and volume stability (60-75 per cent of volume).
4. Water: clean potable water (IS 456 cl. 5.4) for hydration and workability; water-cement ratio normally 0.4-0.6.
5. Admixtures (optional): chemical or mineral admixtures to modify the properties of fresh or hardened concrete.

Cement + water forms the paste; paste + fine aggregate is mortar; mortar + coarse aggregate is concrete. Steel bars are added in reinforced concrete.

Concrete is strong in compression (20-60 MPa) but weak in tension (only about 8-12 per cent of its compressive strength, $f_{cr} = 0.7\sqrt{f_{ck}}$ as per IS 456 cl. 6.2.2). When a beam or slab bends, the tensile zone cracks at a low load. Steel bars are placed to carry the tension. The reasons are:

1. High tensile strength of steel: Fe 415/500 has a yield strength of 415/500 MPa, so it takes all the tension after cracking while concrete takes the compression.
2. Good bond: concrete grips the steel (deformed bars give greater bond), so the two act together without slip; bond is developed over the development length ($L_d$, IS 456 cl. 26.2).
3. Nearly equal coefficients of thermal expansion (concrete 10-14, steel 12 $\times 10^{-6}$/°C), so no large internal stress from temperature change.
4. Corrosion protection: the alkaline nature of concrete (pH about 12.5) forms a passive film on steel; the cover (IS 456 cl. 26.4) protects the bars from corrosion and fire.
5. Ductility: reinforcement gives warning before failure; under-reinforced sections fail gradually.
6. Steel also resists shear (stirrups), controls shrinkage and temperature cracking, and confines columns (ties).

Hence reinforced concrete combines the economy and durability of concrete with the tensile strength of steel.

Grade of cement is the minimum compressive strength, in MPa (N/mm$^2$), of 70.6 mm mortar cubes (1:3 cement : standard sand, w/c = (P/4 + 3.0)%) at 28 days, tested as per IS 4031 (Part 6). For example, in IS 269:2015, OPC 33 grade means a minimum 28-day strength of 33 MPa, OPC 43 gives 43 MPa and OPC 53 gives 53 MPa. Higher grades are finer and have more C$_3$S, giving higher early strength.

Role of Bogue's compounds

C$_3$S and C$_2$S together make about 70-75 per cent of cement and provide the strength through C-S-H gel; C$_3$A and C$_4$AF mainly influence setting, heat and durability. More C$_3$S gives rapid-hardening cement, more C$_2$S gives low-heat cement, and low C$_3$A gives sulphate-resisting cement.

Physical properties of OPC (tested as per IS 4031; limits as per IS 269:2015):

Also, the loss on ignition (maximum 5%) and insoluble residue (maximum 4%) show quality.

OPC is made by grinding clinker with gypsum (IS 269). PPC is made by inter-grinding (or blending) OPC clinker with 15-35% fly ash or other pozzolana and gypsum (IS 1489 Part 1).

PPC is preferred for plastering, mass concrete and marine works; OPC where fast strength gain is required (precast, formwork removal).

Compounds are given by Bogue's calculation, so the comparison is made on C$_3$S, C$_2$S, C$_3$A, C$_4$AF and free lime (IS 269, IS 4031).

Cement-A

• Very high C$_3$S: rapid-hardening, high early strength; quick strength at 3 and 7 days, useful for early formwork removal and cold weather.
• High C$_3$A (11.7%) and C$_3$S: very high heat of hydration, quick setting (needs gypsum, SO$_3$ is 2.4%), more shrinkage and thermal cracking; poor sulphate resistance.
• Free lime of 3.3% is high (IS 269 limits are based on lime saturation factor; excess free lime causes unsoundness, expansion and cracking later). The high CaO suggests over-limed, risky for soundness.
• Low C$_4$AF, so lighter in colour.
• Use: precast work, repair, emergency works; not for mass concrete or sulphate soils.

Cement-B

• High C$_2$S (56.6%) and low C$_3$S: slow strength gain, but high long-term strength (after 28 days); this is a low-heat cement (like IS 12600 low-heat cement, where C$_2$S is at least 40% and C$_3$S at most 35%).
• Low C$_3$A (5.7%): good sulphate resistance, lower heat, less shrinkage; moderate C$_4$AF 9.1%.
• Low free lime (1.0%): sound cement, lower risk of expansion.
• Use: mass concrete (dams, raft), hot weather, marine and sulphate-bearing environments, where thermal cracking must be limited.

Conclusion: Cement-A is a high-early-strength, high-heat cement; Cement-B is a low-heat, durable, sulphate-resistant but slow-gaining cement.

Setting is the stiffening of the cement paste from a fluid, plastic state to a rigid state, losing its plasticity, but with no significant strength (measured by Vicat needle, in minutes/hours). Hardening is the subsequent gain of strength of the set paste over days and months as hydration continues, measured by compressive strength tests. Setting is thus a change of state, and hardening is the development of strength.

i) Setting and hardening

• Setting: change of the fresh paste from plastic to a rigid (but weak) state; takes hours; tested by Vicat apparatus.
• Hardening: gain of strength after setting, taking days to years; tested by compressive strength.

ii) Initial and final setting time

• Initial setting time: time from adding water to the point where the paste begins to lose plasticity (Vicat needle 1 mm square section penetrates to 5 ± 0.5 mm from the bottom of the mould). IS 269: not less than 30 minutes (OPC). Concrete must be placed and compacted before this time.
• Final setting time: time at which the paste has become hard enough that the needle with the annular attachment makes an impression but the cutting edge does not leave a mark. Not more than 600 minutes (10 h).

iii) Admixture and additive

• Admixture: added to the concrete (or mortar) at the mixer, in small amounts (usually below 5% of cement mass), to change its fresh or hardened properties; e.g. superplasticiser, retarder.
• Additive: added to the cement at the factory (during grinding) or in large quantity by weight of cement, e.g. gypsum, fly ash, slag; it becomes part of the cementitious material.

"Water should be fit for drinking"

IS 456:2000 cl. 5.4 says that water used for mixing and curing should be clean and free from injurious amounts of oils, acids, alkalis, sugar, salts, organic materials and other substances harmful to concrete or steel. Potable (drinking) water is regarded as suitable because it has been shown by long experience to contain acceptable amounts of these impurities, so it needs no special testing. But the reverse is not always true: some water not fit to drink (e.g. slightly salty or with a bad smell) may still be fine for concrete, and a sweet-tasting water with sugar, or the water with high sulphates, may not be suitable. The clause is therefore a simple, practical guide. Sea water is not allowed in RCC, and water with pH below 6 is unsuitable.

Standard tests on water (IS 456 cl. 5.4, IS 3025)

• Chemical tests: pH (not less than 6), organic (max 200 mg/L), inorganic solids (3000 mg/L), sulphates as SO$_3$ (400 mg/L), chlorides (2000 mg/L plain; 500 mg/L RCC), suspended matter (2000 mg/L).
• Physical comparison tests: the average 28-day compressive strength of three 150 mm cubes with the test water should be not less than 90% of those with distilled water, and the initial setting time should not differ by more than ± 30 min and should not be less than 30 min.
• Acid test: 200 mL of the sample should need not more than 2 mL of 0.1 N NaOH to neutralise using phenolphthalein; alkalinity test: not more than 10 mL of 0.1 N HCl using methyl orange.

Functions of water in a concrete mix

1. Reacts with cement (hydration) to form binding gel.
2. Acts as a lubricant, giving workability for mixing, placing and compaction.
3. Used in curing to keep hydration continuing and control shrinkage and heat.
4. Washes aggregates, and helps in bonding of paste and aggregate.
5. Excess water, however, evaporates leaving pores that decrease strength, so w/c ratio must be controlled.

Role of water

Water hydrates the cement, gives workability by lubricating the particles and is required for curing. About 0.23 of the cement weight is chemically combined and 0.15 is physically held in the gel pores (total w/c of about 0.38 for full hydration); anything more only supplies workability.

Advantages of high water content

• Higher workability, easy mixing, placing and compaction, which helps in congested reinforcement and long pumping distances.
• Easier finishing; lower labour effort.
• Provides enough water for complete hydration and reduces plastic cracking in very dry mixes.

Drawbacks

• Lower strength: strength falls rapidly with w/c (Abrams' law, $f = A/B^{w/c}$).
• Excess water leaves capillary pores on evaporation, so concrete is more porous and permeable, less durable (more chances of corrosion, frost and chemical attack).
• Increases bleeding and segregation, laitance and a weak top surface; water pockets under aggregate and bars reduce bond.
• Larger drying shrinkage and creep, causing cracks.
• Longer setting time and higher formwork pressure.
• Wastes cement when the cement is added to keep w/c constant.

Workability should be increased with plasticisers, not extra water.

Definitions (IS 383, IS 2386)

• Nominal size: the size by which the aggregate is designated, i.e. the sieve size through which most of it passes. A "20 mm nominal size graded aggregate" (IS 383 Table 7) has 100% passing 37.5 mm, 85-100% passing 20 mm, 0-20% passing 4.75 mm.
• Single size aggregate: most particles pass one sieve and are retained on the next smaller one. E.g. 20 mm single size: 85-100% passes 20 mm, 0-20% passes 10 mm, 0-5% passes 4.75 mm. It is used for making up a grading by blending.
• Maximum size: the smallest sieve on which practically no material is retained, i.e. the sieve through which 100% of the aggregate passes. E.g. an aggregate with 100% passing 40 mm and 90% passing 20 mm has maximum size 40 mm and nominal size 20 mm.

Importance of grading

Grading is the distribution of particle sizes (sieve analysis, IS 2386 Part 1).

• A well-graded aggregate has least voids, so less cement paste fills the voids: economy.
• Better workability with less segregation and bleeding.
• Higher density and strength, lower permeability, so more durable.
• Less shrinkage because less paste and water is required.
• Poor grading (too much of one size or gap-graded) gives harsh mixes, high water demand, honeycombing, and more cement.
• Grading uniformity is checked by the fineness modulus and grading curve limits of IS 383 Table 7/9.

Requirements of coarse and fine aggregates (IS 383:2016)

Common to both

• Clean, hard, strong, durable and chemically inert; free from clay, silt, organic matter, coal, mica, salts and shale (deleterious material kept within the limits of IS 383 Table 2).
• Free from alkali-reactive and unsound material (soundness loss within 12% for sodium sulphate test).
• Proper grading and shape.

Coarse aggregate

• Crushed stone or gravel; nominal size 10, 20 or 40 mm chosen for the section (IS 456 cl. 5.3.1: not more than 1/4 of the minimum thickness and 5 mm less than clear bar spacing).
• Crushing value at most 45%, impact value at most 45% (30% for wearing surface), abrasion at most 50% (30% for wearing surface).
• Flakiness and elongation within limits; angular, rough texture preferred; water absorption below 2% generally.
• Specific gravity 2.5-3.0.

Fine aggregate

• Natural sand, crushed stone sand or crushed gravel sand, passing 4.75 mm sieve, in grading zones I-IV (IS 383 Table 4).
• Fineness modulus 2.0-3.5; organic impurities test by colour (should not be darker than standard); bulking considered.

Need for grading

Grading is the particle size distribution. It is needed to make smaller particles fill the voids between larger ones, giving the least voids and a dense, strong, workable and economical concrete that needs the minimum cement paste and water. It prevents segregation, harshness and honeycombing, and ensures uniform quality.

Size of aggregate

Aggregates are classified by size (IS 383):

• Fine aggregate: passes 4.75 mm, retained on 150 micron.
• Coarse aggregate: retained on 4.75 mm sieve.
• Standard coarse sizes: 40 mm, 20 mm, 12.5 mm, 10 mm, either graded or single size.
• Maximum size is selected by the thickness of the member, spacing of bars and the method of placing: 20 mm for ordinary RCC, 40 mm and above for mass concrete and foundations, 10-12.5 mm for thin sections or pump mixes.
• Larger size gives less surface area, so less water and cement, but too large a size reduces strength due to weak bond and also is limited by cover and bar spacing. For high strength concrete, smaller maximum size (10-20 mm) is better.
• Size is determined by sieve analysis using IS sieves 80, 40, 20, 10, 4.75 mm, 2.36 mm ... 150 micron.

Judging well-graded from the grading curve

Plot the sieve size on a log x-axis against percentage passing on the y-axis.

 % pass
 100|          ____.-----  upper limit
    |      _.-'  ..--- curve of sample
    |   .-' ..--'
  50| .' .-'      lower limit
    |.'-'
   0+-----------------------> sieve size (log)

• The aggregate is well graded if the curve is smooth, continuous (S-shaped) and lies within the upper and lower envelope limits given in IS 383 (Tables 7, 9 for graded aggregate and Zone I-IV for sand).
• A curve with a flat portion (steps) shows gap grading; a steep curve shows a single-size (uniformly graded) material.
• Curves lying above the upper limit mean too fine, and below the lower limit mean too coarse.

A well-graded aggregate has particles of all sizes from large to small, in proportions that fill the voids.

Why it is better

• Minimum voids: small particles fill the gaps between large ones, so the paste required is less and cement is saved.
• Better workability with lower water: reduces segregation and bleeding.
• Higher density and strength for the same cement content and lower permeability, so better durability.
• Less shrinkage and creep (less paste).
• Uniform, cohesive mixes without honeycombing; easier compaction and finishing.
• Compared with single-size (many voids, high paste demand) and gap-graded (risk of segregation) aggregates, well graded gives the most economical mix.

Basis for judging well graded

1. Sieve analysis (IS 2386 Part 1): the percentage passing each IS sieve should lie within the IS 383 limits (Table 7 for coarse aggregate, Table 9 for fine zones I-IV).
2. The grading curve is smooth and continuous with no flat steps.
3. Fineness modulus within the required range (e.g. 2.6-3.0 for medium sand; all-in 3.5-6.5).
4. Coefficients: uniformity coefficient $C_u = D_{60}/D_{10}$ (large, above 4-6 for well-graded) and curvature coefficient $C_c = D_{30}^2/(D_{10} D_{60})$ between 1 and 3.
5. Practical check: lowest void content (loose/rodded bulk density test) among combinations.

Gap-graded aggregate is an aggregate in which one or more intermediate size fractions are missing or present in small amounts; the grading curve shows a flat (horizontal) portion between two sizes. Example: coarse aggregate of 20 mm and 4.75 mm with little material between 4.75 and 10 mm sizes, plus sand.

% passing
100|              _______
   |             |
 50|  ___________|    <- gap (flat)
   | /
   +--------------------> log size

Features

• Can give dense packing with less sand in the aggregate when the sand fraction is correct (e.g. 4.75-10 mm omitted), giving good strength and economy in high-strength and exposed aggregate finishes.
• Requires a lower sand content (about 30 per cent compared to 40 per cent) for the same workability.
• Prone to segregation (large particles separate from the mortar), so needs low slump and careful handling; not suitable for pumped or highly workable mixes.

Role of grading in the strength of concrete

• Grading controls the void content, and thus how much paste is needed and the resulting w/c ratio for a given workability.
• A well-graded mix reaches the required workability with less water, so a lower w/c ratio and higher strength.
• A dense, well-packed aggregate skeleton reduces voids and permeability and gives uniform stress transfer, so there are fewer weak zones and honeycombs.
• Poor grading (too many fines or too coarse) increases water demand or causes bleeding and segregation, and strength falls.

Gradation (grading) is the distribution of aggregate particle sizes in a sample, found by sieve analysis (IS 2386 Part 1) and expressed as the percentage passing (or retained) on a series of standard sieves. Good gradation, within IS 383 limits, means a smooth continuous grading curve with minimum voids, giving workable, economical and strong concrete.

Function of admixture: An admixture (IS 9103) is a material added in small quantities to concrete during or before mixing, to modify its properties. Its functions include:

• Increase workability or reduce water content (plasticisers, superplasticisers).
• Accelerate setting and early strength (accelerators) or delay setting (retarders) for hot weather and long transport.
• Entrain air to improve frost resistance (air-entraining agents).
• Improve durability, reduce permeability and heat (pozzolans, fly ash, silica fume).
• Reduce bleeding, segregation and shrinkage, and save cement.

Effect of aggregate shape on hardened concrete

• Rounded particles pack well, but have weaker bond with paste, so slightly lower flexural strength.
• Angular, rough aggregate has better interlock and bond: higher compressive and flexural strength and a better abrasion resistance, but needs more water and paste.
• Flaky and elongated particles lie flat, trap bleed water beneath them, and cause weak planes, lower strength and lower durability (higher permeability).
• Shape also affects shrinkage and creep indirectly via paste content.

Effect of grading on water requirement

• A well-graded aggregate has fewer voids and a smaller surface area for a given workability, so less water and paste is required.
• Too many fines (fine grading or excess sand) increase the surface area and water demand; too coarse or gap-graded mixes cause harshness and bleeding.
• Hence good grading lowers w/c ratio for the same slump and improves strength.

Alkali-aggregate reaction (AAR)

It is the chemical reaction between alkalis (Na$_2$O, K$_2$O) from cement and reactive silica (opal, chert, strained quartz) or certain carbonates in the aggregate, in the presence of moisture.

• It forms an expansive alkali-silica gel that absorbs water and swells, producing cracks (map cracking), expansion, pop-outs and loss of strength and durability, appearing after several years.
• Control: use non-reactive aggregate, low-alkali cement (below 0.6% Na$_2$O equivalent), pozzolans (fly ash, slag) and keep concrete dry.

Flaky aggregate: a particle whose least dimension (thickness) is less than 0.6 times its mean dimension (the mean of the sieve sizes between which it falls) (IS 2386 Part 1). The flakiness index is found using the thickness gauge.

Elongated aggregate: a particle whose greatest dimension (length) is more than 1.8 times its mean dimension; its elongation index is found with a length gauge.

Flaky + elongated particles are also called poorly shaped particles. IS 383 requires aggregate to be cubical and limits flaky and elongated pieces (a combined index of about 30-35 per cent is a common limit in practice).

Effect on concrete

Therefore, flaky and elongated aggregate is limited and cubical, well-shaped crushed stone is preferred.

Classification of aggregate by shape (IS 383 / BS 812)

Angularity number

The angularity number measures the voids in a compacted aggregate sample and thus its angularity (IS 2386 Part 1). The sample is compacted in a cylinder with a tamping rod (100 strokes in three layers), and the percentage of solid volume in the container is calculated. The value for a perfectly rounded aggregate packed in this way is taken as 67 per cent solids (33% voids).

The percentage of solid volume is $\frac{100\,W}{C\,G_a}$ where $W$ is the mass of aggregate in the cylinder, $C$ the mass of water filling the cylinder and $G_a$ the specific gravity. The range is 0 (rounded) to about 11 (most angular); the greater the number, the more angular the aggregate, the more voids, and the more paste is required.

Given sample (58% solid volume)

The angularity number is 9, which is high (close to the upper limit of 11), indicating a highly angular aggregate (crushed stone) with about 42% voids. It has good interlock and strength, but will have lower workability and need more water and cement paste (about 8-10 kg/m$^3$ more water, and the mix will be harsh).

Retempering of concrete

Retempering is the addition of water and remixing of concrete that has started to stiffen (partially set, with a loss in slump) so as to restore its workability to a level allowing placing.

• Allowed only if the concrete has not reached its initial set; IS 456 cl. 13.2 requires concrete to be placed and compacted before initial setting begins, and good practice is to place it within about 30 minutes of mixing.
• The extra water should be limited, and mixing must be continued; the original w/c ratio must not be exceeded.
• Effects of added water: lower strength (about 5-10 per cent), more shrinkage and permeability. Concrete that has set should never be retempered. Use of a superplasticiser (re-dosing) is the better method of restoring workability.

Shape index of aggregate

Shape index is a quantitative measure of the shape of the aggregate, which indicates how much it deviates from a cube. It may be expressed by the flakiness index (percentage by weight of particles whose thickness is below 0.6 of the mean size), elongation index (length above 1.8 times mean size) or in IS 2386 by the angularity number. Another way is in terms of sphericity or the ratio of length : width : thickness of a particle:

A value close to 1 means a cubical particle (good shape). Higher values show flaky or elongated aggregate that gives poor workability and strength.

Aggregate crushing value (ACV) measures the resistance of an aggregate to crushing under a gradually applied compressive load (IS 2386 Part 4). It indicates the strength of the aggregate.

Apparatus

A steel cylinder of 150 mm diameter with a plunger and base plate, compression testing machine, tamping rod (16 mm dia, 45-60 cm long), balance, IS sieves 12.5 mm, 10 mm and 2.36 mm.

Procedure

1. Take aggregate passing the 12.5 mm and retained on the 10 mm sieve, oven-dried at 100-110 °C for 4 hours and cooled.
2. Fill the cylinder in three layers of equal depth, tamping each layer with 25 strokes; level the surface; weigh the sample ($W_1$, about 3 kg).
3. Place the plunger on the sample and position the assembly in the compression machine.
4. Apply the load at a uniform rate so that 400 kN is reached in 10 minutes; then release.
5. Take the crushed aggregate out and sieve over the 2.36 mm IS sieve; weigh the portion that passes ($W_2$).
6. Repeat with a second sample and take the average.

Limits (IS 383)

ACV not more than 45% for aggregates used in concrete other than wearing surfaces and 30% for wearing surfaces such as roads and pavements. A smaller value means stronger aggregate.

Mechanical properties of aggregate are those that describe how an aggregate behaves under load, shock and wear. They include:

• Crushing value (strength under gradual compression),
• Impact value (toughness under sudden load),
• Abrasion value (resistance to wear, Los Angeles test),
• Hardness and toughness (resistance to scratching, to breaking). These are tested by IS 2386 Part 4 and they affect the strength and durability of concrete, particularly in pavements and wearing surfaces.

Ranking the grading of aggregate in the laboratory

Grading is checked by sieve analysis (IS 2386 Part 1):

1. Oven-dry a representative sample (e.g. 5 kg of coarse, 500 g of sand).
2. Sieve through the stack of IS sieves arranged in decreasing size (80, 40, 20, 10, 4.75 mm, 2.36 mm, 1.18 mm, 600, 300, 150 micron) on a mechanical shaker for about 10 min.
3. Weigh the material retained on each sieve; calculate % retained, cumulative % retained and % passing.
4. Plot % passing against sieve size (log scale) to get the grading curve.
5. Rank/classify: compare with the limits in IS 383 (Tables 7 and 9): well graded if within limits; classify sand into Zone I-IV; compute the fineness modulus (coarser = higher FM) to rank samples from fine to coarse. The sample whose curve lies inside the limits and is smooth is ranked best; gap-graded or single-size samples rank lower.

Properties of aggregates for concrete

Physical: particle size and grading, shape, surface texture, specific gravity, bulk density, porosity, water absorption, moisture content (bulking), voids. Mechanical: crushing, impact, abrasion value, hardness and strength. Chemical/other: soundness, alkali-aggregate reactivity, deleterious matter (clay, silt, coal, salts), thermal properties, fineness modulus.

Influence on concrete properties

Fineness modulus = (sum of cumulative percentage retained on the standard sieves 80, 40, 20, 10, 4.75, 2.36, 1.18 mm, 600, 300 and 150 micron) / 100 (IS 2386 Part 1).

Total weight of sample = 0 + 0 + 6 + 5 + 4 = 15 kg.

Answer: Fineness modulus of the coarse aggregate = 7.13 (a single-size 20-4.75 mm aggregate; the range for coarse aggregate is about 5.5-8.0).

i) Admixtures and additives

• Admixture: a material added to the concrete batch at the time of mixing (small dose, usually below 5% of cement mass) to modify properties of fresh or hardened concrete: e.g. plasticiser, retarder, air-entraining agent.
• Additive: a material added to the cement, usually at the manufacturing (grinding) stage or in larger quantities as a cementitious part: e.g. gypsum, fly ash, slag, silica fume. It forms part of the binder.

ii) Apparent and absolute specific gravity

• Absolute specific gravity: ratio of the mass of the solid material (without the pores) to the mass of an equal volume of water. The volume includes only the solid matter, with no pores (found with the aggregate crushed to powder).
• Apparent specific gravity: ratio of the mass of the oven-dry aggregate to the mass of water of volume equal to the solid volume plus the impermeable (sealed) voids; the water-permeable pores are excluded.

Since the apparent volume includes the closed pores, apparent specific gravity is slightly lower than absolute specific gravity (equal only if there are no impermeable pores). For most aggregates the two are close (2.6-2.7).

Following IS 2386 Part 3 (pycnometer method).

Given: oven-dry mass $A = 480$ g; saturated surface-dry mass $S = 490$ g; mass of pycnometer full of water $B = 1400$ g; mass of pycnometer + sand + water (topped up) $C = 1695$ g.

Taking $B$ as the pycnometer filled with water only (as given) and $C$ with sand and water topped up:

Mass of water displaced by the sand in the SSD condition (equivalent weight) = $S - (C - B)$, with $C - B = 1695 - 1400 = 295$ g (net gain due to sand under water).

Apparent specific gravity

Bulk specific gravity (oven-dry basis)

Bulk specific gravity (SSD basis) = $490/195 = 2.51$. Water absorption = $(490-480)/480 \times 100 = 2.08\%$.

Answer: Apparent specific gravity = 2.59; bulk specific gravity (oven-dry) = 2.46 (SSD basis 2.51).

Chemical admixtures (IS 9103, ASTM C494)

1. Water-reducing admixture (plasticiser), Type A: reduces water by 5-12% for the same slump, or raises slump. Example: lignosulphonates, hydroxycarboxylic acid salts.
2. High-range water reducer (superplasticiser), Type F/G: reduces water 15-30% or gives flowing concrete. Example: sulphonated naphthalene formaldehyde (SNF), sulphonated melamine (SMF), polycarboxylate ether (PCE).
3. Retarding admixture, Type B: delays setting for hot weather, long transport and mass pours. Example: sugar, calcium lignosulphonate, gluconates.
4. Accelerating admixture, Type C: speeds up setting and early strength in cold weather, early formwork removal. Example: calcium chloride (not for RCC), calcium nitrite, triethanolamine.
5. Air-entraining admixture: entrains 3-6% fine bubbles, improving frost resistance and workability. Example: Vinsol resin, fatty-acid salts.
6. Water-proofing / damp-proofing admixtures: reduce permeability (stearates, silicones).
7. Other chemical admixtures: corrosion inhibitors (calcium nitrite), shrinkage reducers, viscosity modifying agents (for SCC), alkali-silica reaction inhibitors, pumping aids, bonding agents.

Common admixtures available in the market

• Chemical: plasticisers (water reducers), superplasticisers (SNF, PCE), accelerators, retarders, air-entraining agents, water-proofing compounds, corrosion inhibitors, shrinkage reducers, viscosity modifiers, bonding agents.
• Mineral: fly ash, silica fume, ground granulated blast furnace slag (GGBS), rice husk ash, metakaolin.
• Others: pigments, fibres, expansive agents, pumping aids.

Accelerating admixture (ASTM C494 Type C, IS 9103)

An accelerator is an admixture that speeds up the setting and early strength gain of concrete by speeding up the hydration of C$_3$S and C$_3$A.

Types

• Soluble inorganic salts: calcium chloride (most common and cheap), calcium nitrate, calcium nitrite, sodium thiocyanate, sodium carbonate.
• Organic: triethanolamine, calcium formate.
• Rapid-setting shotcrete accelerators: aluminates, silicates.

Dosage: calcium chloride up to 2% of cement mass (IS 456: chlorides in the mix limited, so it is not used in RCC and prestressed concrete).

Effects

• Initial and final setting times are shortened; 1-3 day strengths rise, with lower or similar 28-day strength.
• Heat of hydration is liberated earlier.
• Chlorides cause corrosion of steel; also shrinkage, creep and sulphate attack may increase.

Uses: cold weather concreting, early removal of formwork, emergency repair, precast production, shotcrete, concreting under water or against flowing water, plugging leaks.

Precautions: avoid in RCC when chloride-based; trial mixes and accurate dosage; quick placing; sufficient curing.

Superplasticisers (high-range water reducers, ASTM C494 Types F and G, IS 9103) are chemical admixtures that disperse the cement particles so effectively that they permit a large reduction in water (15-30%) at the same workability, or produce very high workability with the same water.

Types: sulphonated naphthalene formaldehyde (SNF), sulphonated melamine formaldehyde (SMF), modified lignosulphonates, and polycarboxylate ethers (PCE, latest and most efficient).

Mechanism: cement particles in water attract each other and form flocs that trap water. The long polymer molecules adsorb on the cement surface and give a negative charge (electrostatic repulsion) or a steric barrier (PCE), so flocs break up, trapped water is released and the particles disperse.

Role in concrete

1. High workability (slump 150-250 mm, flowing concrete) without bleeding and segregation: used for congested reinforcement and pumping.
2. Water reduction of 15-30 per cent, giving lower w/c ratio, so higher strength (for the same cement content).
3. High-strength and high-performance concrete (M60 and above) with w/c as low as 0.25-0.35.
4. Cement saving: reducing both water and cement at the same w/c.
5. Lower permeability, better durability; self-compacting concrete is made with it.
6. Better surface finish, faster placing.

Precautions: the effect lasts 30-60 minutes (slump loss), so it is added at site just before placing; correct dosage (0.5-2% of cement); compatibility with cement; may cause segregation at overdose.

Admixture: a material, other than water, aggregate, cement and fibre, added to the concrete in small quantities immediately before or during mixing to modify the properties of fresh or hardened concrete (IS 9103).

Role of admixtures in concrete

• Improve workability (plasticisers) or reduce water.
• Control setting (accelerators, retarders) and early strength.
• Improve strength and durability (superplasticisers, silica fume, fly ash); reduce permeability.
• Entrain air for frost resistance.
• Reduce heat of hydration, shrinkage, bleeding and segregation.
• Give special properties: waterproofing, corrosion inhibition, colour, pumpability, gas formation in grouts.
• Economy: cement saving.

Use of superplasticiser

Superplasticisers (high-range water reducers, Type F/G) are polymers (naphthalene, melamine, polycarboxylate) that disperse cement flocs and release trapped water.

1. Reduce water by 15-30 per cent at the same workability, so w/c falls and strength and durability rise (high-strength concrete).
2. Produce flowing concrete (slump above 150 mm) with no extra water, for heavily reinforced sections, pumping, tremie placement and self-compacting concrete.
3. Save cement at the same strength.
4. Dosage is 0.5-2.0% of cement mass; added with the last part of mixing water or on site, since the slump loss occurs within 30-60 minutes. Over-dose can cause segregation and retard setting.

Chemical admixtures (IS 9103 / ASTM C494)

1. Water reducers / plasticisers (Type A): lignosulphonates, hydroxylated carboxylic acids.
2. Retarders (Type B): sugar, gluconate, phosphates.
3. Accelerators (Type C): calcium chloride, calcium nitrate, triethanolamine.
4. Water-reducing retarders / accelerators (Type D and E).
5. Superplasticisers (Type F and G): SNF, SMF, PCE.
6. Air-entraining agents: Vinsol resin, fatty-acid soaps.
7. Water-proofing, corrosion inhibitor, shrinkage-reducing, viscosity modifying admixtures.

Mineral admixtures

Finely divided pozzolanic or cementitious materials added in large dosage: fly ash, silica fume, ground granulated blast furnace slag, rice husk ash, metakaolin, natural pozzolanas (volcanic ash), and inert fillers (limestone powder).

How plasticisers reduce the water content

Cement particles in water carry opposite electric charges at their surfaces and attract each other, forming flocs that trap part of the mixing water inside. This trapped water does not help workability.

1. The plasticiser molecule has a hydrophilic and hydrophobic end; it adsorbs on the cement surface.
2. It gives all the particles the same negative charge, so they repel each other (dispersion), and a lubricating water film forms around each grain.
3. The flocs break up and the trapped water is released to lubricate the mix, so the paste becomes more fluid.
4. Therefore, for the same slump, mixing water can be reduced by 5-15 per cent (up to 30 per cent with superplasticisers), giving a lower w/c, higher strength, lower permeability, or for the same w/c a higher slump.

Chemical admixtures per ASTM C494 (and ASTM C260, C1017)

Recommendations

(i) Hot weather - Type B or D (retarder / water-reducing retarder), or Type G. High temperature speeds hydration, reduces slump and gives flash set and cold joints. A retarder delays setting and holds workability for transport and placing, and the water reducer cuts the water needed for the slump, reducing heat and cracks.

(ii) Cold weather - Type C or E (accelerator / water-reducing accelerator), non-chloride for RCC (e.g. calcium nitrate). Low temperature slows hydration, so setting and strength gain are delayed and early concrete may freeze. An accelerator gives faster setting and strength gain that protects against frost damage and allows earlier formwork removal. Air entrainment is also helpful.

(iii) Freezing and thawing exposure - Air-entraining admixture (ASTM C260; 4-7 per cent air), preferably with a water reducer to hold w/c to about 0.45 or lower. The tiny air bubbles act as relief chambers for the 9 per cent expansion of freezing water, preventing cracking and scaling of the paste.