IS 516:1959 Part 2 is the Indian Standard (BIS) for methods of tests for strength of concrete - part 2: determination of flexural strength. This code standardizes the testing methods for determining the strength of concrete, specifically detailing the procedure for finding the flexural strength (modulus of rupture) using beam specimens. It is extensively used by QA/QC engineers to verify concrete tensile strength for rigid pavements and flat slabs.
Covers the procedure for determining the flexural strength of hardened concrete by testing beams.
Core diameter, L/D corrections, capping, soaking, loading rate and IS 456 acceptance link.
| Reference | Value | Clause |
|---|---|---|
| Method | Cores cut from hardened concrete for compression test | Cl. 1 |
| Min core diameter | 100 mm (preferable); 50 mm (where unavoidable) | Cl. 4.1 |
| Core diameter — relation to max aggregate | ≥ 3 × max nominal aggregate size | Cl. 4.1 |
| Length-to-diameter (L/D) — preferred | 1.0 (for direct cube-equivalent strength) | Cl. 4.2 |
| L/D — acceptable range | 1.0 to 2.0 (with correction) | Cl. 4.2 (Table 1) |
| L/D correction — 2.00 | 1.00 (no correction) | Cl. 5.2 (Table 1) |
| L/D correction — 1.50 | 0.96 | Cl. 5.2 (Table 1) |
| L/D correction — 1.00 | 0.87 | Cl. 5.2 (Table 1) |
| Capping — required for end planeness | Sulphur capping or HA mortar | Cl. 4.3 |
| Soaking before test | 40 hours in water before crushing | Cl. 5.1 |
| Loading rate | 140 kg/cm²/min (~14 N/mm²/min) | Cl. 5.3 |
| Equivalent cube strength | fc(cube) = (5/4) · fc(core) · L/D correction | Cl. 5.4 |
| Number of cores per location | 3 (mean reported) | Cl. 4.4 / IS 456 Cl. 17.4 |
| Acceptance — IS 456 Cl. 17.4 reference | Mean equivalent cube ≥ 0.85 fck; individual ≥ 0.75 fck | IS 456 Cl. 17.4.3 |
IS 516 (Part 2):1959 specifies the method for determination of length change of concrete due to moisture variations — i.e., drying shrinkage and moisture-related volume change of hardened concrete. It is a specialist supplement to IS 516 Part 1 (compressive strength).
Use it when: - Specifying low-shrinkage concrete for water-retaining structures, large slabs-on-grade, restraint-prone applications (long walls, podium slabs) - Diagnosing cracking in suspect concrete — shrinkage cracks have a characteristic pattern (random, hairline, in areas of restraint) - Qualifying a mix design with admixtures (SRA — shrinkage-reducing admixtures) where shrinkage performance must be proved - Pavement / airport runway PQC mixes where shrinkage drives joint design and curing duration
Distinct from autogenous shrinkage (chemical, even in sealed concrete) which is more relevant for high-strength / low-W/C mixes — IS 516 Part 2 measures drying shrinkage specifically.
For routine RCC building work, drying shrinkage is rarely directly tested. The mix-design rules in IS 10262:2019 and IS 456 durability clauses indirectly control shrinkage via aggregate and W/C limits.
Specimens (Clause 3): prismatic concrete specimens 100 × 100 × 500 mm, with reference studs cast into each end face for length measurement. Three specimens per mix.
Procedure:
1. Casting: prepare specimens per IS 1199 fresh-concrete sampling. Cast in steel moulds with end-plate inserts that hold reference studs.
2. Initial cure: 24 h in mould at 27 ± 2°C, then demould.
3. Saturated curing: 28 days in saturated lime water at 27 ± 2°C. Establishes a 'saturated reference' length.
4. Initial reading: measure length at end of 28-day cure using a length comparator with 0.002 mm sensitivity. This is the 'saturated length' L₀.
5. Drying environment: transfer specimens to 27 ± 2°C, 50% ± 5% relative humidity environment chamber. This is the standardized drying condition — actual ambient conditions vary across India and don't apply.
6. Subsequent readings: take length measurements at 7, 14, 28, 56, 90, 180, 365 days of drying. Each measurement is taken after specimens equilibrate at the lab temperature briefly (~30 minutes outside the chamber).
7. Compute drying shrinkage at each age: ``` Drying shrinkage (%) = (L_t − L₀) / L_ref × 100 ``` where L_t = length at drying age t, L₀ = saturated length, L_ref = gauge length between studs (typically 460-480 mm depending on stud setup).
Typical values: - Good-quality OPC concrete: 365-day shrinkage ≈ 0.04% to 0.06% - High-W/C or rich-paste concrete: 0.06% to 0.10% - Concrete with SRA admixture: 0.02% to 0.04% (50% reduction typical) - Conservative design limit (Eurocode 2): 0.06% at 365 days
IS 516 Part 2 is silent on acceptance limits — refer to IS 456 Clause 6.2.4 (drying shrinkage of 0.0003 = 0.03% for design purposes) or specific structural design codes.
Restraint cracking: when concrete is restrained against free shrinkage (by adjacent older concrete, foundations, walls), tensile stress develops:
``` σ_t = E × ε_sh × R ```
where ε_sh = free drying shrinkage strain (e.g., 0.0005 = 500 microstrain), E = concrete modulus (~30 GPa for M30), R = restraint factor (0 free; 1 fully restrained, typical interior slab corners ~ 0.5).
For a typical restraint factor 0.5 and shrinkage strain 0.0005: σ_t = 30,000 × 0.0005 × 0.5 = 7.5 MPa
This exceeds concrete tensile strength (f_cr ≈ 0.7√f_ck = 3.5 MPa for M25). Hence cracking is inevitable in restrained concrete unless you (a) reduce shrinkage, (b) provide expansion joints, or (c) provide enough reinforcement to keep crack widths within IS 456 limits (0.3 mm typical, 0.2 mm for aggressive exposure).
This is why long walls, foundation slabs, and water-retaining structures need either: - Pour joints every 6-8 m - Shrinkage-compensating concrete (Type K cement, or expansive admixtures) - Heavy reinforcement (P_min ≥ 0.35% for cracked-controlled design per IS 3370 Part 2)
1. Inconsistent ambient conditions during measurements — if the lab temperature drifts 5°C between readings, the thermal expansion (α ≈ 10 × 10⁻⁶ /°C × 500 mm × 5°C ≈ 0.025 mm) masks shrinkage signals (typical shrinkage is 0.1-0.5 mm over 500 mm). Strict temperature control during measurement is non-negotiable.
2. Wrong cure period — Part 2 requires 28-day saturated cure before the drying period starts. Specimens dried from early age give artificially high shrinkage values because some of what's measured is 'early-age self-desiccation', not pure drying.
3. Mixing 'free' shrinkage with 'restrained' performance — Part 2 measures FREE shrinkage of unrestrained specimens. Real structures have restraints, so the relevant 'shrinkage cracking' depends on free shrinkage × restraint × elastic modulus. Don't quote free shrinkage as 'crack risk'.
4. Ignoring aggregate — concrete shrinkage is dominated by the cement paste; aggregates restrain it. Sand and coarse aggregate type, quantity, and quality all affect shrinkage by 30-50%. A mix with 35% paste shrinks ~30% less than a mix with 45% paste. Reducing paste (higher coarse aggregate fraction) is the most effective shrinkage control.
5. Using 56-day or 90-day data as 'final' shrinkage — drying shrinkage continues for years (some studies show 70-80% at 1 year, 90% at 5 years). Specifying based on 56-day data alone under-predicts long-term shrinkage by 25-40%.
IS 516 Part 2:1959 is 66 years old and one of the oldest unrevised IS concrete-testing standards. The methodology is sound but the specimen size (100 × 100 × 500 mm) is non-standard internationally — most modern labs use ASTM C157 or BS EN 12390-16 geometries.
For routine RCC building work: shrinkage testing per Part 2 is rarely done. Designers accept the IS 456 Clause 6.2.4 nominal value εcs∞ = 0.0003 and design accordingly.
For water-retaining structures, large slabs, restraint-sensitive applications: shrinkage testing per Part 2 is occasionally specified in the design basis report — usually when SRA admixtures or shrinkage-compensating cement is being qualified. Test cost: ₹15,000-25,000 per mix per lab, with 365-day turnaround (which is the actual practical constraint).
Modern alternative: many designers now estimate concrete shrinkage from mix proportions + admixture data sheets using fib Model Code 2010 / 2020 shrinkage models. These are calibrated against extensive international datasets and give shrinkage strain as a function of paste content, W/C, cement type, relative humidity, and section thickness. The IS 516 Part 2 test is then run only to verify the model prediction on a critical mix — not as routine QA.
BIS revision overdue: there's no public sign of revision as of 2026. When it eventually happens, expect alignment with ASTM C157 or EN 12390-16 specimen geometries and possibly inclusion of autogenous shrinkage measurement methods (currently absent from Indian standards).