Properties & testing of engineering materials (brick, stone, M-sand, aggregates, cement, timber)

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.

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.

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

Classification of trees

Softwoods and hardwoods

Structure of a tree (cross-section)

Felling of trees

• Trees are felled when mature (maturity varies from a few decades to over a century depending on species); immature timber has more sapwood, and over-mature timber may have decay.
• Felling in a season when sap is at its minimum is preferred (to reduce decay and speed seasoning) — commonly mid-winter in plains and mid-summer in hilly regions.

Qualities of good timber

Sound (clear ringing sound when struck); uniform colour and straight, close grain; free from defects (knots, shakes, decay, insect attack); adequate strength, hardness, toughness and elasticity; durability against fungi and insects; well seasoned (appropriate moisture content, no warping); heavy for its species (denser timber is stronger); fibrous and not brittle; sweet smell (fresh cut); good workability and ability to hold nails and screws; fire resistance for large sections.

Seasoning of timber

Seasoning is the controlled drying of timber to reduce its moisture content to a level in equilibrium with the atmosphere where it will be used.

Objectives

• Reduces shrinkage, warping and cracking in service.
• Increases strength, hardness, stiffness and durability (more resistance to decay).
• Reduces weight (economical transport).
• Makes timber easier to work, paint, glue and treat with preservatives.
• Improves resistance to fungi and insects.

Freshly felled (green) timber may contain moisture exceeding its dry weight; well-seasoned timber commonly has about 10–15% moisture content (lower for furniture in dry interiors, somewhat higher for outdoor/structural use).

Methods

Conversion (sawing) of timber

Conversion is sawing logs into marketable sizes (planks, battens, scantlings).

Defects in timber

Defects due to natural forces

Defects due to fungi

• Dry rot — fungi attack timber in damp, poorly ventilated, warm conditions (e.g. floor boards on damp walls); timber becomes dry, brittle and powdery; can spread through masonry.
• Wet rot — decay under alternate wetting and drying (e.g. posts at ground level); timber becomes soft and spongy.
• Brown rot, white rot, heart rot, sap stain, blue stain.

Defects due to insects

Termites (white ants) — hollow out timber internally; beetles (powder-post beetles) — reduce sapwood to powder; marine borers (shipworms) — attack timber in sea water.

Defects due to seasoning

Defects due to conversion

Chip mark, diagonal grain (improper sawing), torn grain (tool action), wane (original rounded surface/bark on the edge of a sawn piece).

Preservation of timber

Objectives: increase durability and life; protect from fungi, insects and marine borers; reduce fire hazard (with fire retardants).

Preservatives

Methods of application

• Brushing and spraying — surface treatment; limited penetration; maintenance treatments.
• Dipping and steeping — immersion for minutes to days.
• Charring — surface burnt to form a protective charcoal layer (e.g. ends of posts in the ground).
• Hot and cold open tank process — timber heated in preservative, then cooled so that preservative is drawn in.
• Pressure impregnation — in closed cylinders:
  • Full-cell (Bethell) process — vacuum then pressure — maximum retention (marine piles).
  • Empty-cell processes (Rueping and Lowry) — deep penetration with less retention (economical).
• Boucherie process — sap displacement in green poles.

Fire resistance

Timber is combustible, but large sections perform relatively well in fire: the surface chars, and the char layer insulates the inner wood, which retains strength. Fire resistance is improved by fire-retardant chemicals (e.g. ammonium phosphate, ammonium sulphate, borax, boric acid), fire-retardant paints, larger sections, and protection with plaster or boards.

Properties of timber

• Anisotropic — properties differ parallel and perpendicular to the grain: compressive and tensile strengths parallel to grain are much higher than perpendicular to grain.
• Strength decreases with increasing moisture (below fibre saturation point) and with defects (knots, sloping grain).
• Density — denser timber is generally stronger and harder.
• Good strength-to-weight ratio, low thermal conductivity (good insulator), good sound absorption, easy to work.
• Shrinkage: tangential shrinkage > radial shrinkage > longitudinal shrinkage (negligible).

Common Indian timbers

Wood-based products

Measurement of timber

Worked examples

Frequently tested points

• Exogenous (softwoods, hardwoods) vs endogenous (bamboo, palm).
• Softwoods: conifers, distinct rings, resinous, light (deodar, pine); hardwoods: broad-leaved, strong (teak, sal, shisham).
• Heartwood — dead, durable; sapwood — living, less durable; cambium forms new wood; medullary rays radial.
• Seasoning reduces moisture (well-seasoned about 10–15%); fibre saturation point ~25–30%.
• Kiln seasoning — quick and controlled (compartment and progressive kilns).
• Quarter and radial sawing reduce warping; flat sawing is economical.
• Shakes: heart, cup (ring), star, radial; knots reduce strength.
• Dry rot — damp unventilated conditions; wet rot — alternate wetting and drying.
• Seasoning defects: bow, cup, twist, checks, splits, honeycombing, case hardening.
• Preservatives: creosote (oil type), CCA/ASCU (water-borne); pressure impregnation — Bethell (full-cell), Rueping and Lowry (empty-cell).
• Large timber sections char and retain strength in fire.
• Strength much higher parallel to grain; tangential shrinkage > radial > longitudinal.
• Plywood: odd number of veneers, cross-grained; glulam, LVL (parallel grains), CLT (cross layers); compreg = compressed impreg.
• Quarter girth formula $V=(G/4{)}^{2}L$.