Concrete Glossary.

Accelerator

An admixture that speeds up setting and early strength gain. Used in cold weather, repair work, or shotcrete.

Acceptance Criteria (IS 456)

The statistical criteria in IS 456 Clause 16 for accepting concrete based on cube test results — both individual and group means.

Aggregate Impact Value (AIV)

A test measuring the resistance of aggregate to sudden shock or impact. Lower values indicate stronger aggregate.

Air Entraining Agent

An admixture that creates microscopic air bubbles in concrete, improving freeze-thaw resistance and workability.

Bleeding

The migration of water to the surface of freshly placed concrete. Excessive bleeding weakens the surface and creates a porous top layer.

C-S-H Gel

Calcium silicate hydrate — the primary product of cement hydration and the main contributor to concrete strength. C-S-H is a nano-structured colloidal gel that binds aggregates together.

C2S (Dicalcium Silicate)

Belite — the slow-reacting silicate phase in OPC. C2S contributes mainly to long-term strength beyond 28 days, with low heat generation.

C3A (Tricalcium Aluminate)

The fastest-reacting aluminate phase in cement. C3A controls initial setting and is responsible for flash setting unless tempered by gypsum.

C3S (Tricalcium Silicate)

Alite — the most abundant phase in OPC and the principal contributor to early strength gain. C3S hydrates rapidly to form C-S-H gel and calcium hydroxide.

C4AF (Tetracalcium Aluminoferrite)

Ferrite phase in OPC. Contributes little to strength but gives cement its grey colour and improves resistance to chemical attack.

Cement Hydration

The chemical reaction between cement and water that produces hardened cementitious paste. Hydration generates heat and the binding C-S-H gel that gives concrete its strength.

Chemical Admixture

Liquid or powder additives used in small dosages (<5% of cement) to modify properties of concrete — workability, setting time, strength gain, or air content.

Coarse Aggregate

Aggregate retained on the 4.75 mm IS sieve. Forms the bulk of concrete volume (60-75%) and provides dimensional stability.

Compressive Strength

The maximum compressive load a hardened concrete cube or cylinder can withstand before failure. The defining property of structural concrete.

Cube Test (IS 516)

The standard compressive strength test for concrete. 150 mm cubes are cured in water at 27±2°C and crushed at 7 and 28 days.

Density (Concrete)

The mass per unit volume of concrete. Normal-weight RCC concrete has a density of approximately 2400 kg/m³.

Design Mix

A concrete mix proportioned through laboratory testing to achieve a specified characteristic strength with statistically defined safety margin.

Final Set

The point at which cement paste has fully hardened and can resist a defined indentation. After this, no further finishing or remixing is possible.

Fine Aggregate

Aggregate passing the 4.75 mm IS sieve — sand. Fills voids between coarse aggregates and contributes to workability.

Fineness Modulus

A single number describing the average particle size of an aggregate, calculated from cumulative percentages retained on standard sieves.

Fly Ash

A by-product of coal-fired power plants, used as a pozzolanic addition to concrete. Reacts with calcium hydroxide to form additional C-S-H.

GGBS (Ground Granulated Blast Furnace Slag)

A by-product of iron manufacturing, ground to cement fineness. Used as a supplementary cementitious material at 25-65% replacement of cement.

Gradation

The distribution of particle sizes in an aggregate sample, determined by sieve analysis. Well-graded aggregates pack densely with minimum voids.

Hardened Concrete

Concrete after final set, when it has gained measurable strength and dimensional stability. Properties are tested on cured specimens.

Heat of Hydration

The heat released as cement reacts with water. Important for thermal control in mass concrete to avoid cracking from temperature gradients.

Initial Set

The point at which cement paste begins to stiffen and lose plasticity. Marks the end of the workable period.

M-Sand (Manufactured Sand)

Crushed stone fines manufactured to act as fine aggregate. Replaces or supplements river sand and offers consistent gradation and shape.

Mineral Admixture (SCM)

Supplementary cementitious materials (fly ash, GGBS, silica fume, metakaolin) added to concrete to partially replace cement. Improve durability and reduce environmental impact.

The process of selecting cement, water, aggregates, and admixtures in correct proportions to produce concrete that meets strength, workability, and durability requirements.

NABL Accreditation

Accreditation by the National Accreditation Board for Testing and Calibration Laboratories. NABL-accredited cube testing is mandatory for many large projects in India.

Nominal Mix

A pre-defined volumetric proportioning of cement, sand, and aggregate (e.g., 1:2:4 for M15) used for small-scale work without formal mix design.

OPC (Ordinary Portland Cement)

The most common type of cement, manufactured by grinding clinker with gypsum. Available in three grades in India: 33, 43, and 53 (compressive strength in MPa at 28 days).

PCE Superplasticizer

Polycarboxylate ether-based superplasticizer — the latest generation, offering up to 40% water reduction and extended slump retention.

Permeability

The ability of concrete to allow fluids or gases to pass through it. Lower permeability = better durability against chloride, sulfate, and freeze-thaw attack.

Plastic Concrete

Concrete in its fresh, mouldable state — from mixing until initial set. The window during which placement, compaction, and finishing must occur.

Plasticizer

A water-reducing admixture that improves workability without adding water, or reduces water demand by 5-12% for the same workability.

PPC (Portland Pozzolana Cement)

OPC blended with fly ash (15-35%). PPC offers better workability, lower heat of hydration, and improved long-term durability than OPC.

PSC (Portland Slag Cement)

OPC blended with GGBS (25-65%). Excellent for sulfate and chloride environments — common in coastal, marine, and aggressive soil conditions.

RCPT (Rapid Chloride Permeability Test)

A test measuring the resistance of concrete to chloride ion penetration, indicating durability in marine or de-icing environments. Result reported in coulombs.

Retarder

An admixture that delays the initial setting of concrete. Used in hot weather, long hauls, or large pours to extend the workability window.

River Sand

Naturally occurring fine aggregate sourced from riverbeds. Increasingly restricted due to environmental regulations.

Segregation

The separation of coarse aggregate from cement paste in fresh concrete. Causes honeycombing and weak spots.

Setting Time

The time it takes cement paste to lose plasticity and harden. Measured as Initial Set (start of stiffening) and Final Set (no longer deformable).

Slump

A measure of the consistency and workability of fresh concrete, determined by the slump cone test. Reported in millimetres.

Slump Flow (SCC)

For self-compacting concrete (SCC), slump flow measures the spread diameter of fresh concrete after lifting the cone — typically 550-850 mm.

Slump Test (IS 1199)

A standard field test measuring the consistency of fresh concrete using a 300 mm slump cone. The collapse height after lifting the cone is the slump value.

Specific Gravity

The ratio of the density of an aggregate to the density of water. Used in mix design to convert weights to volumes.

Superplasticizer

A high-range water reducer that can reduce water demand by 15-30%, allowing very low w/c ratios with high workability.

W/C Ratio (Water-Cement Ratio)

The ratio of the weight of water to the weight of cement in a concrete mix. The single most important factor controlling strength and durability.