IS 13757 : 1993Burnt clay fly ash building bricks-Specification

IS 13757:1993 is the Indian Standard for Burnt Clay Fly Ash Building Bricks — Specification. It covers a hybrid masonry product that combines burnt clay with fly ash — addressing the need to dispose of thermal power-plant fly ash productively while making serviceable building bricks.

Use it when: - Specifying alternative bricks for sustainability projects — fly-ash bricks have lower embodied CO₂ than pure-clay bricks (less clay mining + less kiln-firing energy per unit) - Procuring bricks in fly-ash-rich regions — bricks made within 50-100 km of major thermal power plants (Singrauli, Korba, Vindhyachal, Tuticorin coastal stations, etc.) - Replacing standard IS 1077 bricks where the cost differential is favourable (typically 10-20% cheaper than pure-clay bricks in fly-ash-abundant regions) - Specifying for government sustainability mandates — many state housing schemes mandate fly-ash brick usage above certain quantities

Use alongside: - IS 1077:1992 — Common burnt clay building bricks (the conventional standard) - IS 2117:1991 — Guide for manufacture of hand-made burnt clay building bricks - IS 12894:2002 — Fly-ash lime brick (a different product — uses calcium silicate hydrate chemistry, no kiln firing) - IS 2185 Part 1:2005 — Concrete masonry blocks (the cement-based alternative) - IS 6041:1985 — Code for AAC block masonry

Composition (Clause 4): typical ranges for fly-ash brick under IS 13757: - Clay: 50-70% - Fly ash: 20-40% - Sand: 5-15% - Some manufacturers also add a small amount of rice husk for porosity

The fly ash partially substitutes the clay — reducing clay-mining requirement and incorporating an industrial byproduct that otherwise sits in ash ponds.

Manufacture: 1. Mixing: clay + fly ash + sand + water mixed to a plastic consistency in a pug mill 2. Extruding / moulding: by either: - Hand-moulding (traditional, lower production rate, more variability) - Machine-extrusion through dies (uniform shape, higher production rate) 3. Drying: in shaded yards for 3-7 days; reaches handling moisture 4. Firing: in clamp / bull's trench kiln (traditional) or Hoffmann kiln (modern) at 950-1100°C for 24-48 hours 5. Cooling: 24-48 hours; sorting; despatch

Classification by strength (Clause 5) — similar structure to IS 1077:

| Grade designation | Min compressive strength (MPa) | Use | |---|---|---| | FA-3.5 | 3.5 | Non-load-bearing partitions; low-grade fill | | FA-5.0 | 5.0 | Load-bearing internal walls (G+1, G+2 light) | | FA-7.5 | 7.5 | Load-bearing external walls (low-rise) | | FA-10 | 10.0 | Load-bearing structural walls; G+2 to G+3 | | FA-12.5 | 12.5 | Heavy load-bearing, retaining walls | | FA-15 | 15.0 | Very heavy load-bearing applications |

Acceptance criteria (Clause 6) — tested in accordance with IS 3495:

| Property | Limit | |---|---| | Compressive strength (gross area) | ≥ grade min (above) | | Water absorption (24-h soak) | ≤ 20% by mass | | Efflorescence | ≤ moderate (per IS 3495 Part 3 visual classification) | | Dimensional tolerance | Length, width ± 4 mm; height ± 3 mm | | Soundness (drop test) | No major chip / fracture on 1.5 m drop |

Standard dimensions (Clause 7): - Conventional (large): 230 × 110 × 70 mm (formed for 4 bricks/sq ft of wall area when laid with mortar joints) - Modular: 190 × 90 × 90 mm (for mortarless / modular construction) - Other sizes: brick types per state PWD specifications

Strength: comparable. FA-3.5 to FA-15 covers the same load-bearing range as conventional clay-brick grades.

Water absorption: IS 13757 permits up to 20% (vs 25% for IS 1077). Lower absorption = better resistance to moisture / efflorescence / freeze-thaw — slight advantage to fly-ash bricks.

Density: typically 1700-1900 kg/m³ for fly-ash bricks vs 1800-2000 kg/m³ for pure-clay bricks. Slightly lighter; small advantage for transportation cost.

Thermal conductivity: comparable; fly-ash bricks slightly lower (better insulation) due to air voids and fly-ash microstructure.

Efflorescence: fly-ash bricks slightly lower tendency due to lower soluble-salt content from fly ash chemistry. Significant for visible facing walls.

Embodied CO₂: - IS 1077 clay brick: 200-300 kg CO₂ per tonne of brick (~ 0.4-0.6 kg CO₂ per brick) - IS 13757 fly-ash brick: 150-220 kg CO₂ per tonne (15-25% reduction) - IS 12894 fly-ash lime brick (no kiln firing): 100-150 kg CO₂ per tonne (40-50% reduction)

Cost: - IS 1077 clay brick: ₹5-10 per brick depending on region - IS 13757 fly-ash brick: ₹4-9 per brick (5-15% cheaper in fly-ash-rich regions) - IS 12894 fly-ash lime: ₹4-8 per brick

Visual appearance: IS 13757 bricks tend to be slightly grayer than conventional red clay bricks. For visible / facing walls, this matters; for plastered walls, irrelevant.

1. Confusing IS 13757 with IS 12894 — they are different products: - IS 13757 (this code): clay + fly ash, KILN-FIRED - IS 12894: fly ash + lime + gypsum, AUTOCLAVED (no kiln) - They look similar but have very different production processes and properties. Specify clearly.

2. No quality testing on receipt — many sites receive 'fly-ash bricks' that don't meet IS 13757 limits. Brick quality varies wildly between manufacturers. Mandate per-batch testing (compressive strength + water absorption) for orders > 10,000 bricks.

3. Soaking time wrong — IS 13757 bricks should be soaked for 4-6 hours before laying (less than IS 1077 conventional bricks which need 24 hours). Over-soaking causes excessive moisture in the mortar joint.

4. Using IS 13757 in seismic Zone V — fly-ash bricks (like IS 1077 bricks) have low tensile strength. In high seismic regions, supplement with reinforced concrete bands per IS 4326:2013. Don't rely on plain masonry alone.

5. Buying bricks from unregistered kilns — Indian brick industry has a tradition of informal manufacturers. Without BIS registration, quality variability is enormous. Mandate procurement from BIS-licensed manufacturers OR conduct extensive pre-procurement testing.

6. No efflorescence assessment — bricks with high salt content (chlorides, sulphates from poor-quality fly ash) develop visible white deposits on exterior walls within months. Visual screening of sample bricks pre-procurement; mandate per-batch efflorescence test (IS 3495 Part 3).

7. Mixed-grade procurement — some kilns produce variable bricks within a single batch — some FA-7.5, some FA-3.5. Mixing in a structural wall creates weak spots. Mandate grade sorting at the kiln level.

8. Skipping BIS hologram check — major BIS-licensed fly-ash brick manufacturers stamp / mark their product with manufacturer name + BIS reference. Unmarked bricks of dubious origin should be rejected.

• IS 1077:1992 — Common burnt clay building bricks (the conventional alternative)
• IS 12894:2002 — Fly ash-lime bricks (no-kiln autoclaved alternative)
• IS 2117:1991 — Guide for manufacture of hand-moulded burnt clay bricks
• IS 2180:1985 — Heavy duty burnt clay building bricks
• IS 2222:1991 — Burnt clay perforated building bricks
• IS 3812 Part 2:2013 — Pulverised fuel ash for use as admixture in cement-mortar (companion code for using fly ash in cement matrix)
• IS 1727:2004 — Methods of test for pozzolanic materials (used to characterise fly ash before brick manufacture)
• IS 3495 Parts 1-4 — Methods of tests for burnt clay building bricks (compressive strength, water absorption, efflorescence, warpage)
• IS 1905:1987 — Code of Practice for Structural Use of Unreinforced Masonry (design for IS 13757 walls)
• IS 2250:1981 — Code of Practice for Preparation and Use of Masonry Mortars
• IS 4326:2013 — Earthquake-resistant design of masonry buildings
• IS 2185 Part 1:2005 — Concrete masonry blocks (the cement-based alternative)
• NBC 2016 Part 6 Section 4 — Masonry chapter (covers fly-ash bricks alongside clay bricks)

IS 13757:1993 is 32 years old but remains relevant. The hybrid brick technology has stabilised and there's no major revision in sight. Minor amendments since 1993 have refined acceptance test methods.

Market reality (2026): - Indo-Gangetic plain and Maharashtra: extensive availability of fly-ash bricks from kilns near thermal power plants. Cost competitive with pure-clay bricks. - South India: less common; fly-ash sources are coastal thermal plants (Tuticorin, Vijayawada, Mangalore) — distribution radius limited. - Hilly regions / North-East: minimal fly-ash brick production; pure-clay bricks dominate. - Specialty applications: some manufacturers produce 'fly-ash bricks plus' grades with > 40% fly ash content — claimed advantages (lower CO₂, higher strength); not formally covered by IS 13757 but accepted by progressive specifiers.

Government push: - Ministry of Environment, Forest and Climate Change (MoEFCC): regulations mandate that any construction within 300 km of a thermal power plant must use at least 25% fly-ash-derived bricks/blocks by weight of total walling material. This is the strongest market driver for IS 13757 (and IS 12894). - Many state housing schemes (PMAY, etc.) specifically mandate fly-ash brick usage for embodied-carbon reduction. - GRIHA / IGBC / LEED rating systems award credits for fly-ash brick use — meaningful for high-rise commercial / institutional projects.

Specifier recommendation: - For routine residential / commercial construction: IS 13757 bricks are perfectly serviceable. Specify grade, compressive strength, water absorption, efflorescence per IS 3495 testing. - For load-bearing masonry: stick with FA-10 or higher grade; conduct pre-construction testing on a sample to verify actual delivered strength. - For public-facing visible walls: verify colour acceptability — fly-ash bricks tend to be grayer. Sample wall first. - For chimneys / fireplaces / high-temperature applications: fly-ash bricks may have different thermal-shock behaviour than conventional. Use specialty refractory products instead.

Future trends: - Sintering technology improvements are reducing fly-ash brick CO₂ further - High-volume fly-ash bricks (50%+ ash content) gaining specification approval - Hybrid bricks with rice-husk-ash, biomass-ash, copper-slag emerging — not yet codified but growing

For a sustainability-conscious project: specifying IS 13757 bricks (over IS 1077) is a low-cost, low-risk way to demonstrate embodied-carbon reduction. Cost differential: small; environmental impact: meaningful.