Engineering Materials & Building Components

Classification of rocks

Geological classification

Physical classification

• Stratified rocks — in layers (sedimentary rocks such as sandstone, limestone); can be split along bedding planes.
• Unstratified rocks — no layers; compact crystalline structure (igneous rocks such as granite, trap).
• Foliated rocks — tend to split along a definite direction (metamorphic rocks such as slate, gneiss, schist).

Chemical classification

Common building stones and uses

Qualities of a good building stone

1. Crushing strength — adequate for loads; good structural stones commonly have high strength (e.g. granite and trap rank highest; many sandstones and limestones are lower).
2. Appearance — uniform colour and texture for face work; free from clay holes, spots and cracks.
3. Durability — resistance to weathering (rain, frost, wind, temperature changes, chemicals).
4. Hardness — resistance to abrasion (important for floors, pavements, steps); measured by Mohs' scale.
5. Toughness — resistance to impact (road metal).
6. Low porosity and water absorption — porous stones absorb water and deteriorate by frost and chemical action (water absorption for good stones is low, commonly below about 5%).
7. Specific gravity — heavier stones (specific gravity above about 2.7) are generally more compact and durable, suited to dams and retaining walls.
8. Fire resistance — stones with minerals of different expansion (e.g. granite with quartz) crack in fire; argillaceous stones resist fire better.
9. Workability and dressing — ease of cutting and dressing (economy).
10. Texture — fine, compact, crystalline structure is desirable.
11. Seasoning — freshly quarried stones contain quarry sap and should be seasoned (allowed to dry) before use, commonly for several months.
12. Availability and cost.

Tests on stones

Mohs' scale of hardness

Brick earth

Composition of good brick earth

Harmful ingredients

• Lime lumps — expand on slaking after burning, causing bricks to split (lime bursting).
• Iron pyrites — cause crystallisation, discolouration and splitting.
• Alkalis (salts of sodium, potassium) — cause efflorescence (white patches) and melting.
• Pebbles and gravel — prevent uniform mixing and weaken bricks.
• Organic matter and vegetation — leave pores after burning, making bricks porous.

Manufacture of bricks

1. Preparation of clay

Unsoiling (removal of top soil), digging, cleaning (removing stones, roots), weathering (exposure to atmosphere to soften and mature the clay), blending (mixing ingredients) and tempering (mixing with water and kneading — in a pug mill for large-scale production) to obtain a homogeneous plastic mass.

2. Moulding

A frog — a shallow depression on one face of a hand-moulded brick — provides a key for mortar, reduces weight and may carry the manufacturer's mark. Bricks with frogs are laid frog upward.

3. Drying

Green bricks are dried (naturally in the shade/open air, or artificially in drying chambers) to reduce moisture before burning, preventing cracks and distortion. Natural drying commonly takes one to two weeks depending on weather.

4. Burning

Burning gives strength, hardness and durability through dehydration and partial vitrification of clay; bricks are commonly burnt at around 900–1100 °C.

Sizes of bricks

For modular bricks, about 500 bricks make one cubic metre of masonry (including mortar joints).

Classification of bricks

Traditional (field) classification

IS classification (IS 1077)

Common burnt clay bricks are classified by average compressive strength into classes (designations in N/mm²) such as 35, 30, 25, 20, 17.5, 15, 12.5, 10, 7.5, 5 and 3.5 — the class designation is the minimum average compressive strength.

Tests on bricks

Laboratory tests (IS 3495)

Field tests

• Sound test — two bricks struck together should give a clear metallic ringing sound.
• Hardness test — no impression when scratched with a finger nail.
• Soundness/strength test — a good brick should not break when dropped flat on hard ground from a height of about 1 m.
• Shape, size and colour — uniform, with sharp straight edges and uniform colour.
• Structure — a broken brick shows a homogeneous, compact structure without holes or lumps.

Portland cement

Cement is a finely ground hydraulic binder that sets and hardens by chemical reaction with water and remains stable under water. Joseph Aspdin patented Portland cement in 1824, naming it after its resemblance to Portland stone.

Raw materials

Manufacture

Processes

Burning in the rotary kiln

The kiln is a long inclined steel cylinder lined with refractory bricks, rotating slowly; material moves down towards the hot end.

The clinker (dark nodules, a few mm to 25 mm) is cooled, stored and ground with about 3–5% gypsum (and other additions for blended cements) to a fine powder, then stored in silos and packed (commonly 50 kg bags) or dispatched in bulk.

Composition of cement

Oxide composition (typical ranges for OPC)

Bogue compounds

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 $\sum (T+10)\Delta t$.

Terms in brickwork

Rules for bonding

Bond is the arrangement of bricks in successive courses so that vertical joints do not coincide, distributing loads and binding the wall together.

1. Bricks should be of uniform size; length about twice the width plus one joint.
2. Lap should be at least one-quarter brick along the length of the wall and half a brick across the thickness.
3. No continuous vertical joints in the face or within the thickness.
4. Stretchers are used mainly on the faces; the hearting should be filled with headers as far as possible.
5. Minimum use of bats; closers are placed next to quoin headers to develop lap.
6. Vertical joints in alternate courses should be in the same vertical line (perpends).
7. Each alternate header should be centrally placed over the stretcher below (in appropriate bonds).

Types of bonds

English bond versus Flemish bond

Construction practice and precautions

1. Use good-quality bricks (well burnt, uniform); soak bricks in water before use (to prevent absorption of water from mortar) but lay them surface-dry.
2. Lay bricks with frogs upward on a full bed of mortar; fill all vertical joints ("buttering").
3. Keep mortar joints uniform — commonly about 10 mm.
4. Build walls plumb and courses level using a mason's line, spirit level and plumb bob; check with a straight edge.
5. Raise the wall uniformly along its length; limit the height built in a day (commonly about 1 m or less) to allow mortar to gain strength; use racking back rather than toothing at junctions where possible.
6. Break joints according to the bond; use closers at quoins; avoid excessive use of bats.
7. Cure brickwork by keeping it moist (commonly at least 7 days for cement mortar).
8. Rake joints (about 10–12 mm deep) while mortar is green if the wall is to be plastered or pointed.
9. Provide holdfasts for door and window frames, and bond walls at junctions.
10. Protect fresh work from rain, hot sun and frost; do not load newly built walls prematurely.
11. Leave scaffolding holes only as permitted and fill them afterwards.

Reinforced brick masonry and brick nogging

• Reinforced brick masonry (RBM) — steel bars or hoop iron embedded in mortar joints or grouted cores to resist tension, shear and seismic forces — lintels, slabs (reinforced brick concrete), walls in earthquake zones.
• Half-brick walls (partitions) are often strengthened with hoop iron or reinforcement in every third or fourth course.
• Brick nogging — brickwork filling a framework of timber or concrete posts and rails (panel walls).
• Earthquake-resistant masonry — horizontal RCC bands (plinth, lintel, roof bands) and vertical reinforcement at corners and junctions as per IS 4326.

Strength of brickwork

The strength of brick masonry depends on:

• Compressive strength of bricks and mortar grade (masonry strength is less than brick strength).
• Workmanship — filling of joints, joint thickness (thicker joints reduce strength), wetting of bricks, bond.
• Slenderness ratio of walls and columns — effective height (or length) ÷ effective thickness; higher slenderness reduces permissible stress (stress reduction factor).
• Eccentricity of loading.
• Shape and size of units.

IS 1905 (Code of practice for structural use of unreinforced masonry) gives basic compressive stresses for masonry based on unit strength and mortar type, stress reduction factors for slenderness and eccentricity, area reduction factors, limits on slenderness ratio (commonly up to about 27 for walls in cement or cement–lime mortar), and minimum wall thicknesses.