Aggregates, Mortars & Concrete

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

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

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.

($h_1$ = height of moist sand in a container; $h_2$ = 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

Typical proportions (cement : sand, by volume)

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

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.

Concrete

Concrete is a composite material of cement (binder), fine aggregate, coarse aggregate and water (with admixtures as needed) that hardens by hydration of cement. Plain cement concrete (PCC) is strong in compression but weak in tension; reinforced cement concrete (RCC) combines it with steel; prestressed concrete applies pre-compression.

Ingredients

• Cement — OPC, PPC, PSC or other types as required.
• Aggregates — fine and coarse, clean, strong and well graded (see Aggregates & Mortars).
• Water — for hydration and workability.
• Admixtures — chemical and mineral (see Concrete Mix Design & Admixtures).

Quality of mixing water

Water fit for drinking is generally suitable. IS 456 requires water to be free from injurious amounts of oils, acids, alkalis, salts, sugar and organic materials, with a pH of not less than 6, and limits on solids such as organic matter (about 200 mg/L), inorganic solids (about 3000 mg/L), sulphates (about 400 mg/L as SO₃) and chlorides (about 2000 mg/L for plain concrete and 500 mg/L for reinforced concrete). Sea water should not be used for reinforced or prestressed concrete (chlorides cause corrosion).

Grades of concrete (IS 456)

Concrete is designated by M followed by its characteristic compressive strength (N/mm²) of 150 mm cubes at 28 days.

• Minimum grade for reinforced concrete is M20 (IS 456).

Nominal mixes

Nominal mixes may be used for concrete up to M20; design mixes are preferred and required for higher grades.

Exposure conditions (IS 456 — reinforced concrete)

The maximum cement content (OPC, excluding fly ash and slag) is generally limited to 450 kg/m³ to reduce shrinkage and thermal cracking.

Production of concrete

1. Batching — measuring ingredients: weigh batching (accurate, preferred) or volume batching (for small works; allowance for bulking of sand; cement always in whole bags).
2. Mixing — machine mixing (tilting or non-tilting drum mixers, pan mixers, batching plants) until uniform; hand mixing on a watertight platform is allowed only for small works with an extra quantity of cement (commonly 10%).
3. Transporting — pans, wheelbarrows, buckets and cranes, chutes, belt conveyors, pumps, transit mixers (ready-mixed concrete) — avoiding segregation and loss of workability.
4. Placing — in clean, oiled, rigid formwork; in layers; avoiding large free fall (which causes segregation); using tremie for underwater placement.
5. Compaction — removal of entrapped air (about 5% air voids can reduce strength by about 30%): needle (immersion) vibrators, surface vibrators (slabs), formwork (external) vibrators, vibrating tables (precast); hand rodding for small works. Over-vibration causes segregation and bleeding.
6. Finishing — screeding, floating, trowelling, texturing.
7. Curing — maintaining moisture and temperature for hydration.

Curing

Minimum curing period (IS 456): at least 7 days for concrete with OPC; at least 10 days where mineral admixtures or blended cements are used; longer periods (e.g. 10 and 14 days respectively) in hot, dry weather.

Methods: ponding (slabs), wet coverings (hessian, sand), sprinkling, membrane curing (curing compounds, polythene sheets), steam curing (precast — accelerated strength), autoclave curing, infra-red curing.

Fresh concrete

Workability

Workability is the ease with which concrete can be mixed, transported, placed, compacted and finished without segregation.

Factors: water content (most important), aggregate–cement ratio, aggregate size, shape, texture and grading, use of admixtures (plasticisers, air entrainment, fly ash), temperature and time.

Segregation and bleeding

• Segregation — separation of coarse aggregate from mortar (or grout from aggregates) — due to excess water, poor grading, large free fall, over-vibration, long transport.
• Bleeding — water rising to the surface after placing (a form of segregation) — due to high w/c ratio, lean mixes, poorly graded or coarse sand; causes weak surface layer (laitance), porous channels and reduced bond with reinforcement.
• Harshness — difficulty in finishing due to lack of fines/paste.

Workability tests

Degree of workability (typical correlations):

IS 456 recommends slump ranges by placing condition — e.g. low slumps for mass concrete and pavements, about 50–100 mm for heavily reinforced sections, higher for pumped concrete or congested reinforcement (commonly using superplasticisers).

Hardened concrete

Compressive strength

• Measured on 150 mm cubes (IS 516) cured in water and tested at 7 and 28 days (cylinders 150 × 300 mm are used in some codes; cylinder strength is roughly 0.8 of cube strength).
• Characteristic strength ($f_{ck}$) — the strength below which not more than 5% of test results are expected to fall.
• Target mean strength for mix design: $f_{ck}^{\prime }=f_{ck}+1.65s$ ($s$ = standard deviation).
• Typical strength gain with OPC: about 65–70% of 28-day strength at 7 days.

Water–cement ratio law and gel–space ratio

Other factors: degree of compaction, curing and age, cement type, aggregate properties and bond, temperature, admixtures, specimen size and shape, loading rate.

Tensile and flexural strength

• Concrete's tensile strength is only about 8–12% of its compressive strength.
• Flexural strength (modulus of rupture) — tested on beams (e.g. 150 × 150 × 700 mm) under two-point loading; IS 456 gives

• Split tensile strength — cylinder loaded along its length (Brazilian test): $f_t=\frac{2P}{\pi DL}$.

Modulus of elasticity

Poisson's ratio of concrete ≈ 0.15–0.20.

Shrinkage

• Plastic shrinkage — rapid evaporation from fresh concrete (hot, windy weather) → surface cracks; prevented by early curing, windbreaks, fog spraying.
• Drying shrinkage — loss of water from hardened concrete; IS 456 suggests a total shrinkage strain of about 0.0003 in the absence of data.
• Autogenous shrinkage (self-desiccation in low w/c concretes), carbonation shrinkage.

Creep

Creep is the gradual increase in strain under sustained load. It increases with higher stress, earlier age at loading, higher w/c, lower humidity and more paste. IS 456 gives creep coefficients of about 2.2 (loading at 7 days), 1.6 (28 days) and 1.1 (1 year). Creep causes long-term deflections and loss of prestress but relieves stress concentrations.

Durability

Durability — ability to resist weathering, chemical attack and abrasion while retaining its properties. Key threats:

Sampling and acceptance

IS 456 specifies sampling frequency based on quantity of concrete (e.g. 1 sample for 1–5 m³, 2 for 6–15 m³, 3 for 16–30 m³, 4 for 31–50 m³, and 4 plus one for each additional 50 m³ beyond that); each sample consists of 3 cubes whose average is the test result. Acceptance criteria compare the mean of consecutive results and individual results with the characteristic strength as specified in the code.

Non-destructive tests

Maturity of concrete

Strength development depends on time and temperature:

(Nurse–Saul function with a datum temperature of about −10 °C.) Concretes of the same mix with equal maturity have approximately equal strength — useful for estimating strength under different curing temperatures and for steam curing.

Worked examples

Frequently tested points

• Potable water generally suitable; pH ≥ 6; chlorides ≤ 500 mg/L for RCC; no sea water for RCC.
• Ordinary M10–M20; standard M25–M55; high strength ≥ M60; RCC minimum M20.
• Nominal mixes: M10 1:3:6, M15 1:2:4, M20 1:1.5:3.
• Exposure: mild 300 kg/m³, 0.55, M20 … extreme 360 kg/m³, 0.40, M40; max OPC content 450 kg/m³.
• Weigh batching preferred; hand mixing needs extra cement; compaction removes air (5% air ≈ 30% strength loss).
• Curing: 7 days (OPC), 10 days (blended/mineral admixtures).
• Slump mould 300 mm high, 100/200 mm diameters; true, shear, collapse slumps; compaction factor for low workability; Vee-Bee for very stiff mixes.
• Bleeding → laitance; segregation → honeycombing.
• 150 mm cubes; $f_{ck}$ = 5% fractile; $f_{ck}^{\prime }=f_{ck}+1.65s$.
• Abrams' w/c law; gel–space ratio $240x^3$.
• $f_{cr}=0.7\sqrt{f_{ck}}$; $E_c=5000\sqrt{f_{ck}}$; shrinkage strain ≈ 0.0003; creep coefficients 2.2/1.6/1.1.
• Durability: carbonation, chlorides, sulphates, ASR, freeze–thaw.
• NDT: rebound hammer (surface hardness), UPV (quality, cracks), cores.
• Maturity $\sum (T+10)\Delta t$.