IS 3085 : 1965Method of Test for Permeability of Cement Mortar and Concrete

IS 3085:1965 is the method of test for permeability of cement mortar and concrete — measuring how readily water passes through hardened concrete under pressure, expressed as a coefficient of permeability. It matters because durability, not strength, is what usually limits concrete life: chloride and sulphate attack, carbonation and reinforcement corrosion are all transport problems, and permeability is the property that governs transport.

It sits in the durability stack:

• IS 456 — durability provisions (cover, W/C, min cement, exposure) that this test verifies the *result* of
• IS 2645 / IS 3370 — integral waterproofing / liquid-retaining structures · IS 516 / DIN 1048-style — water-penetration (depth) tests

IS 3085 forces water through a sealed concrete/mortar specimen under a known pressure head and measures the steady-state flow to compute a Darcy coefficient of permeability (m/s). The governing facts:

• Permeability is dominated by water-cement ratio, compaction and curing — a low-W/C, fully compacted, well-cured concrete can be orders of magnitude less permeable than a high-W/C or poorly cured one of the *same strength*
• Cracks and honeycombing short-circuit the matrix — a sound mix, badly placed, fails on permeability while passing cubes
• Low permeability is the *common root* of durability: it slows chloride ingress (corrosion), sulphate attack, carbonation and leakage simultaneously

The engineering point: the IS 456 durability rules (max W/C, min cement, cover, exposure class) are essentially a *recipe for low permeability*. This test is how you confirm the recipe actually produced impermeable concrete — strength alone does not.

Scenario: mix qualification for a water tank / basement raft where leakage and durability govern.

Step 1 — design for low permeability: low W/C, adequate cement content, good grading; full compaction and extended moist curing are non-negotiable inputs.

Step 2 — cast & cure specimens representative of the works (same mix, compaction, curing).

Step 3 — IS 3085 test: seal the specimen in the permeability cell, apply the specified water pressure, run to steady flow, compute the coefficient of permeability.

Step 4 — judge: compare against the project's permeability acceptance for the exposure (water-retaining / aggressive ground). A pass confirms the IS 456 durability intent was *built*, not just specified; a fail sends you back to W/C, compaction or curing — usually curing.

A mix can hit 35 MPa and still be too permeable for a tank — which is exactly why this test exists alongside the cube.

1. Assuming strength implies impermeability. Two concretes at the same MPa can differ hugely in permeability — curing and W/C decide it, not strength.

2. Under-curing then testing. Curing has a first-order effect on permeability; a mix sabotaged by poor curing fails this test even with good materials.

3. Testing flawed specimens. Micro-cracked, segregated or honeycombed specimens short-circuit and read meaninglessly high — and reveal a *placing* defect, not a mix defect.

4. Leaky cell seal. Bypass flow around the specimen reads as high permeability — a test-setup error.

5. Ignoring permeability in durability specs. Specifying only strength for aggressive/water-retaining work omits the property that actually controls service life.

• IS 456 — durability requirements (W/C, cover, min cement, exposure) this test verifies
• IS 3370 (Parts 1–4) — liquid-retaining structures (low permeability is central)
• IS 2645 — integral cement waterproofing compounds · IS 9103 — admixtures (water-reducers cut W/C → permeability)
• IS 516 Part 1 — strength (the *other* half; not a durability proxy)
• IS 10262 — mix design (low-W/C durable mixes)

IS 3085 is old (1965) and rarely run on routine jobs, but it measures the property that actually decides how long concrete lasts. The single most useful idea it encodes: strength and permeability are not the same thing, and durability tracks permeability. Most real-world concrete failures — corroding rebar, leaking basements, sulphate-damaged foundations — are low-permeability failures dressed up as something else, and the usual culprit is curing, the cheapest input and the first one site cuts. For water-retaining, marine or aggressive-ground work, specify and verify permeability, not just MPa; the IS 456 durability clauses are a low-permeability recipe and this is the test that confirms you cooked it right.