How to Check Water Absorption of Bricks: IS 3495 Part 2 Test & Limits

A brick is basically a sponge made of fired clay — and how much of a sponge it is decides much of its quality. Bricks that absorb too much water are weaker, suffer frost and salt damage, grow efflorescence, and can even pull water out of the mortar before it sets properly. The water absorption test is the simplest, cheapest quality check you can run on a brick consignment, and every site engineer should know how to do it. Learning how to check water absorption of bricks properly — the right procedure, the right formula, and the right limits — is one of the highest-value skills a young site engineer can pick up.

This guide covers the standard procedure (IS 3495 Part 2), the formula with a worked example, the acceptance limits in IS 1077, and what a high result is really telling you.

Why Water Absorption Matters

Water absorption is the mass of water a dry brick takes in when immersed, expressed as a percentage of its dry mass. It is a direct proxy for the brick’s porosity and degree of burning:

• Under-burnt bricks are porous and absorb a lot of water — they are also low in strength.
• Well-burnt bricks are denser, absorb less, and last longer.

High absorption causes real problems in service:

1. Lower strength and durability — more pores mean less solid material carrying load, and the brick degrades faster under wetting–drying cycles.
2. Efflorescence — absorbed water dissolves soluble salts in the brick and mortar; when it evaporates at the surface, white salt deposits appear. Repeated cycles can spall the brick face.
3. Mortar bond problems — a very thirsty brick sucks water out of fresh mortar, leaving a weak, crumbly joint. (This is also why bricklayers wet bricks before laying in hot weather.)
4. Dampness and frost — saturated bricks transmit damp into the wall; in cold climates, freezing water expands in the pores and cracks the brick.

Practical note: Absorption is a “lower is better” property — but only down to a point. Extremely low-absorption bricks (like some over-burnt or engineering bricks) can be so dense and glassy that mortar struggles to bond to them. For ordinary walling bricks, though, the code limits are upper limits, and comfortably passing them is what you want.

Governing Standards

• IS 3495 (Part 2) — Methods of tests of burnt clay building bricks: determination of water absorption (the test procedure).
• IS 1077 — Common burnt clay building bricks: specification (the acceptance limits).
• ASTM C67 — Standard test methods for sampling and testing brick and structural clay tile (the US equivalent procedure, covering both cold-water and boiling-water absorption).

Apparatus

The 24-hour cold-water immersion described above is the IS 3495 Part 2 routine. But ASTM C67 — the US equivalent — also defines a 5-hour boiling test that gives a fuller picture of the brick’s pore structure, and it is worth knowing because consultants on international projects sometimes ask for it.

How it works: after the standard oven-drying and dry weighing, the bricks are placed in cold water, the water is brought to a boil within one hour, boiled for 5 hours, then allowed to cool to room temperature by natural loss of heat (16–19 hours). The bricks are removed, wiped, and weighed — the same arithmetic applies: (Ws − Wd) / Wd × 100.

Why boil? Boiling drives air out of the fine pores that cold water cannot reach in 24 hours, so the boiling absorption is always higher than the cold-water value. The ratio between the two — called the saturation coefficient (cold-water absorption ÷ boiling-water absorption) — is itself a durability indicator in ASTM C62: a saturation coefficient below about 0.78–0.80 signals a brick that resists freeze–thaw damage well, because its pores are not easily saturated.

Practical note: On Pakistani and Indian sites the cold-water test per IS 3495 is the acceptance test — nobody will ask you to boil bricks for a housing project. But if you ever work on a project with ASTM-based specifications, or a client questions a cold-water result, knowing the boiling method (and the saturation coefficient argument) puts you ahead of the conversation.

1. Ventilated oven — capable of holding 105–115 °C.
2. Balance — readable to 1 g (a brick weighs ~3 kg, so 1 g resolution is plenty).
3. Water tank or tray — large enough to immerse bricks fully in clean water at about 27 ± 2 °C.
4. Damp cloth — for wiping surface water before the final weighing.
5. Sample — 5 bricks selected per the sampling procedure in IS 5454, representing the consignment.

Test Procedure (Step by Step)

1. Dry the bricks. Place the sample bricks in the ventilated oven at 105–115 °C and dry to essentially constant mass (normally about 24 hours).
2. Cool and weigh dry. Let the bricks cool to room temperature (cooling in a desiccator or in dry air avoids re-absorbing moisture), then weigh each one and record the dry mass Wd (also written M1).
3. Immerse for 24 hours. Place the bricks fully immersed in clean water maintained at about 27 ± 2 °C, for 24 hours. Keep them from touching each other and the tank walls so water reaches all faces.
4. Remove and wipe. Take each brick out, wipe all surface water off with a damp cloth — surface film only, don’t squeeze water out of the pores.
5. Weigh wet — quickly. Weigh each brick within 3 minutes of removal and record the saturated mass Ws (also written M2). The 3-minute rule matters: water starts evaporating (and draining) the moment the brick leaves the tank.

Formula and Worked Example

Water absorption is calculated for each brick as:

Water absorption (%) = (Ws − Wd) / Wd × 100

The average of the 5 bricks is reported as the consignment’s water absorption.

Worked example: One brick from a consignment weighs:

• Dry mass, Wd = 3.120 kg
• Saturated mass after 24 h immersion, Ws = 3.620 kg

Water absorption = (3.620 − 3.120) / 3.120 × 100 = 0.500 / 3.120 × 100 = 16.0%

Suppose the five bricks give 15.2%, 16.0%, 14.8%, 16.5%, and 15.5%. The average is 15.6% — reported as the water absorption of the consignment per IS 3495 (Part 2).

Acceptance Limits (IS 1077)

After 24-hour immersion in cold water, tested per IS 3495 (Part 2), the water absorption shall not exceed:

Brick class (IS 1077)	Maximum water absorption
Up to class 12.5	20% by weight
Class 15 and above (15, 17.5, 20, 25, 30, 35)	15% by weight

So the 15.6% result in our example passes for bricks up to class 12.5, but would fail for class 15 and higher bricks, which are held to the stricter 15% limit.

Practical note: In everyday site language, “first-class” bricks are generally expected to absorb no more than about 20% — and experienced engineers get suspicious of any consignment testing above 18–19%, because kiln variation means the next truckload could easily cross the line. Aim for comfortably below the limit, not just under it.

What High Absorption Indicates

The IS 1077 limits (20% / 15%) apply to common burnt clay bricks. But sites today use several masonry units, each with its own typical absorption behavior. This table is a field reference — acceptance always follows the relevant product standard:

Masonry unit	Typical water absorption	Governing standard
First-class burnt clay brick (well-burnt)	12–18%	IS 1077 (limit 20%)
Second-class burnt clay brick	18–22%	IS 1077
Third-class / under-burnt brick (jhama)	22–25%+	IS 1077 (usually fails)
Engineering brick (Class A / B)	4.5–7%	IS 1077 / BS 3921
Fly ash brick	12–20%	IS 12894 (limit 20%)
Concrete block (hollow, dense)	8–12%	IS 2185
Calcium silicate brick	12–16%	IS 4139

Reading the table: Notice the pattern — the denser and better-manufactured the unit, the lower the absorption. A fly ash brick absorbing 19% is normal; a burnt clay brick absorbing 19% is suspicious. Judge every result against the right standard and the right material, not against a single magic number.

If a consignment fails or tests suspiciously high, it usually points to one (or more) of these:

• Under-burning in the kiln — the commonest cause; the clay never vitrified properly, leaving an open pore structure. These bricks are also typically low in compressive strength — test that too (IS 3495 Part 1).
• Poor raw material — clay with excessive silt, organic matter, or soluble salts.
• Over-sized or irregular pores from bad moulding or too-rapid drying before firing.

A failed absorption test should trigger the full quality workup: compressive strength, efflorescence (IS 3495 Part 3), dimensional tolerance/warpage (IS 3495 Part 4), and a hard look at the supplier.

Quick Site Check (Not a Substitute)

Real consignments rarely fail cleanly — they hover near the limit, and that is where judgment matters. Consider five bricks from a new supplier, tested per IS 3495 Part 2 for a Class 12.5 (20% limit) order:

Brick	Dry mass Wd (kg)	Saturated mass Ws (kg)	Absorption
1	3.050	3.620	18.7%
2	3.110	3.690	18.6%
3	2.980	3.580	20.1%
4	3.080	3.640	18.2%
5	3.020	3.610	19.5%

Average = (18.7 + 18.6 + 20.1 + 18.2 + 19.5) / 5 = 19.0% — technically a pass against the 20% limit.

But look closer: Brick 3 individually exceeds 20%, and the spread (18.2–20.1%) shows inconsistent burning — this kiln is not under control. On site I would treat this consignment as suspicious: accept it only for non-critical work (boundary walls, internal partitions), demand the compressive strength test (IS 3495 Part 1) before using it in load-bearing walls, and put the supplier on notice that the next consignment gets tested on arrival. A pass that close to the line is a warning, not a clean chit.

On site without an oven, you can do a rough field check: weigh a dry brick on a site scale, soak it in a bucket of clean water for 24 hours, wipe, and reweigh. The arithmetic is identical. It won’t replace the IS 3495 procedure for formal acceptance — oven-drying to constant mass and temperature control matter — but it will catch a bad consignment before a thousand bricks go into the wall.

Common Errors to Avoid

• Not drying to constant mass — residual moisture in the “dry” brick understates absorption and can pass a bad consignment.
• Weighing late — beyond 3 minutes after removal, evaporation skews the wet weight downward.
• Testing a single brick — one brick proves nothing; the standard requires the average of 5, and a wise engineer looks at the spread too.
• Immersion in dirty or warm water — use clean water near 27 °C; hot water accelerates absorption and cold slows it.
• Confusing absorption with porosity — related but different: absorption is mass-based and test-specific; porosity is volume-based. Don’t interchange the terms in a report.

Frequently Asked Questions

The lab test takes 48 hours. Long before the numbers arrive, an experienced eye can already smell trouble:

1. The ring test. Strike two bricks together — a well-burnt brick gives a clear metallic ring; an under-burnt brick gives a dull thud. Dull sound and high absorption almost always travel together.
2. The colour check. Uniform deep red or copper colour suggests even burning. Pale, yellowish, or patchy bricks are usually under-burnt and will absorb more.
3. The scratch test. Try scratching the brick with a fingernail or coin. If it marks easily, the surface is soft — expect high absorption and low strength.
4. The drop test (rough). Drop a brick flat from about 1 m onto firm ground. A good brick survives; a bad one breaks or cracks. Crude, but it correlates surprisingly well with both strength and absorption.
5. Check the frog and edges. Sharp, well-defined edges and a clear frog (the maker’s indent) indicate good moulding and firing. Rounded, crumbling edges mean soft, porous brick.
6. Ask about the kiln. Clamp-burnt (puzawah) bricks vary far more than kiln-burnt bricks from a continuous kiln. If the supplier changed kilns, retest — the last consignment’s result no longer applies.

Practical note: None of these replaces the IS 3495 test for formal acceptance. But they decide which consignments get tested urgently and which get rejected at the gate without wasting two days.

What is the water absorption limit for first-class bricks?
Per IS 1077, bricks up to class 12.5 must not absorb more than 20% by weight after 24-hour cold-water immersion (tested per IS 3495 Part 2). Higher classes (15 and above) are limited to 15%.

How long are bricks soaked in the water absorption test?
24 hours in clean cold water at about 27 ± 2 °C, after oven-drying at 105–115 °C to constant mass.

Is lower water absorption always better?
Almost always for ordinary walling bricks — it signals better burning, higher strength, and better durability. The only caveat is at the extreme: glassy, near-zero-absorption bricks can bond poorly with mortar, which is why engineering bricks have their own specifications.

What is the difference between IS 3495 and IS 1077?
IS 3495 describes how to test (Part 2 covers water absorption); IS 1077 is the product specification that sets the acceptance limits the test results are judged against.

How many bricks are tested for water absorption?
Five bricks per consignment, selected per the sampling procedure in IS 5454. The average of the five is reported — and as the borderline example above shows, the spread between them matters too.

What is the boiling water absorption test for bricks?
A 5-hour boiling immersion per ASTM C67 that fills pores cold water cannot reach. It is not the IS acceptance test, but the ratio of cold-water to boiling-water absorption (the saturation coefficient) is used internationally as a freeze–thaw durability indicator.

What is the water absorption limit for fly ash bricks?
IS 12894 limits fly ash bricks to 20% average water absorption after 24-hour immersion — the same headline number as clay bricks, but tested per IS 3495 Part 2 all the same.

Can bricks be used if water absorption is 25%?
Not for structural or exposed work — 25% exceeds every standard limit and signals severely under-burnt, weak brick. Such bricks (often called jhama) are at best fit for temporary works, brick-bat filling, or soling — never for load-bearing walls.

Does high water absorption mean low compressive strength?
Almost always, yes. Both properties trace back to the same cause: incomplete vitrification in the kiln leaves open pores (high absorption) and weak inter-particle bonding (low strength). A failed absorption test should always trigger a compressive strength test (IS 3495 Part 1).

Should bricks be wetted before laying?
Yes — especially in hot weather. Dry, absorptive bricks suck water out of fresh mortar and weaken the joint. Soak bricks briefly (or sprinkle them) so they are damp but not dripping when laid. This is standard good practice, not a substitute for using good bricks.

Final Thoughts

The water absorption test costs almost nothing — an oven, a scale, a water tank, and 48 hours — yet it exposes the commonest brick defect there is: under-burning. Make it a habit on every new consignment, keep the 20%/15% limits in your site diary, and never let a failed result be overruled by a supplier’s assurances.

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Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.