Compressive Strength of Concrete – Cube Test, Grades and Factors

Let’s be honest — if you only learn one property of concrete in your entire civil engineering course, make it compressive strength. It’s the number that determines whether a building stands or falls. Every beam, column, slab, and foundation is designed around it. And every time a batch of concrete is made on a real construction site, workers cast little cubes and crush them weeks later to check this single number. This guide explains everything you need to know — from what the number means to how to test it — in a way that actually sticks.

1. What is Compressive Strength of Concrete?

Simply put, compressive strength is the maximum crushing load per unit area that a hardened concrete specimen can take before it breaks. We express it in N/mm² (MPa).

Quick example: You crush a 150 mm cube and the machine reads 900 kN at failure.
Area = 150 × 150 = 22,500 mm². So f_c = 900,000 ÷ 22,500 = 40 MPa → this is M40 concrete.

Here’s the most important thing to remember: concrete is excellent in compression (resists crushing loads of 15–80+ MPa) but terrible in tension (only about 1/10th of its compressive strength). That is the entire reason we embed steel bars in RCC — the steel handles tension while concrete handles compression. They make a perfect team.

2. Cube Test – IS 516:2018 Step by Step

The cube test is simple in concept: cast a concrete cube, cure it, crush it. But the details matter enormously. A poorly cast cube gives a falsely low reading and can get a perfectly good concrete mix rejected on site. Here’s the correct procedure per IS 516:2018.

Apparatus needed

• Steel cube moulds: 150 mm × 150 mm × 150 mm (standard, for aggregate up to 40 mm) or 100 mm × 100 mm × 100 mm (for aggregate up to 20 mm)
• Compression Testing Machine (CTM): Minimum 2000 kN capacity, properly calibrated
• Tamping rod: 16 mm diameter, 600 mm long, bullet-nosed at one end
• Curing tank: Maintained at 27 ± 2°C throughout the curing period

Procedure (step by step)

1. Mould preparation: Clean the mould thoroughly. Apply a thin film of mould releasing oil on all inner surfaces to prevent the concrete from sticking.
2. Filling: Fill the mould in exactly 3 equal layers (about 50 mm each). After filling each layer, tamp it uniformly 35 times with the tamping rod, distributing the strokes evenly across the entire cross-section. Alternatively, use a vibrating table for better, more consistent compaction.
3. Levelling: After the third layer, strike off the excess concrete and smooth the top surface with a trowel. Mark each cube with the batch number, date, and intended test age.
4. Initial curing: Leave the filled moulds in a damp environment (not in direct sun or wind) for 24 ± 0.5 hours. Then demould carefully without damaging the cube edges.
5. Water curing: Immediately immerse the demoulded cubes in clean water maintained at 27 ± 2°C. Keep them submerged until the day of testing (either 7 days or 28 days).
6. Pre-test preparation: Remove cubes from water 30 minutes before testing. Wipe the surface dry. Check dimensions. Weigh the cube if density is needed.
7. Testing: Place the cube in the CTM with the cast face (smooth side) positioned on the sides — not on top or bottom. The load should be applied to the unmoulded surfaces. Align carefully to the centre of the platens. Apply load continuously and without shock at a rate of 140 kg/cm²/min (14 N/mm²/min) until the cube fails. Record the maximum load P.
8. Calculation: f_c = P (N) ÷ 22,500 mm²

Number of specimens – IS 456:2000 Clause 15.2

Always cast a minimum of 3 cubes per sample and test all three. The average of 3 results is reported as the compressive strength. Rejection rule: if any individual cube result differs from the average by more than 15%, discard that result and average the remaining two.

3. Concrete Grades – M10 to M80

The M in M20, M25, etc. stands for Mix. The number is the characteristic compressive strength (f_ck) in MPa at 28 days, measured on 150 mm cubes under standard curing.

Key rule from IS 456:2000 Table 5: Minimum grade for RCC = M20 (mild), M25 (moderate), M30 (severe), M35 (very severe), M40 (extreme exposure).

4. Characteristic Strength vs Target Mean Strength

This is one of the most misunderstood concepts, but it is actually simple once you see the logic.

The characteristic compressive strength (f_ck) is defined as the cube strength value below which not more than 5% of all test results are expected to fall. So if you have M25 concrete, 95 out of every 100 cube tests must give 25 MPa or more.

Here’s the catch: because of natural variability in any concrete production process, you cannot simply target 25 MPa and hope 95% of cubes pass. You need to target a higher mean strength. This is called the Target Mean Strength (f_m):

Worked example: Design M25 concrete with good quality control (S = 4 MPa).
f_m = 25 + 1.65 × 4 = 25 + 6.6 = 31.6 MPa
So your mix is designed to achieve an average of 31.6 MPa, ensuring 95% of cubes still pass the 25 MPa (M25) criterion.

5. Strength Gain with Age

Concrete is not like baking a cake where it’s done after a fixed time. It keeps getting stronger for months and years as hydration continues. The 28-day mark is our standard reference, but here is how strength builds up:

• 1 day: ~16% of 28-day strength — too early for any loading
• 3 days: ~40% — very light loading only
• 7 days: ~65–70% — used for early quality check; formwork removal for non-critical members
• 14 days: ~85–90% — side shuttering can usually be removed
• 28 days: 100% — the design/characteristic strength
• 90 days: ~115–120% — IS 456 allows 90-day strength for certain structures
• 1 year: ~125–135% — long-term strength (especially PPC and GGBS blended cements)

6. Cylinder Strength vs Cube Strength

If you ever read international literature or ASTM standards, you will encounter cylinder strength (f’_c) rather than cube strength (f_ck). The standard cylinder is 150 mm diameter × 300 mm height (L/D ratio = 2).

The relationship is approximately: f’_c (cylinder) ≈ 0.8 × f_ck (cube)

Why the difference? A cube is squat (L/D = 1) and the platens of the testing machine provide lateral confinement at the top and bottom through friction. This confinement artificially boosts the measured strength. A slender cylinder fails in pure uniaxial compression without this effect, giving the true material strength.

7. Key Factors Affecting Compressive Strength

Many students memorise these as a list. Instead, think of them as variables you can control to increase or decrease strength.

• Water-cement ratio (most important): Lower W/C → denser C-S-H gel → fewer capillary pores → higher strength. Abrams’ Law: every 0.05 increase in W/C reduces 28-day strength by roughly 4–6 MPa.
• Degree of compaction: Every 1% entrapped air reduces strength by 5–6%. Honeycombed concrete can lose up to 50% of its potential strength. Never skip vibration.
• Curing: Concrete cured for only 3 days achieves only ~60% of properly 28-day cured strength. Minimum curing per IS 456: 7 days for OPC, 14 days for blended cements.
• Cement type and grade: OPC 53 grade gives higher strength than OPC 43. RHC achieves in 3 days what OPC achieves in 28 days.
• Aggregate quality: Weak, porous, or flaky aggregates set a ceiling on achievable concrete strength. Hard, angular, well-graded aggregates give stronger concrete.
• Age: Strength continues to increase with time, especially for PPC and GGBS blended cements.
• Admixtures: Superplasticizers allow very low W/C (0.25–0.35) at good workability, enabling high-strength concrete. Silica fume dramatically densifies the paste and ITZ.

8. Diagram – Cube Test Process & Strength Gain

9. Exam Tips (RTMNU)

• ✅ IS 516:2018 = cube test standard. IS 456:2000 Clause 15.2 = acceptance criteria. Always cite both.
• ✅ Cube dimensions: 150 mm × 150 mm × 150 mm (standard). Tamping: 35 strokes per layer, 3 layers.
• ✅ CTM loading rate: 14 N/mm²/min (140 kg/cm²/min) — this number comes up every exam.
• ✅ Formula: f_m = f_ck + 1.65S — memorise with units and practice at least 2 numericals.
• ✅ 28-day = 100%, 7-day = 67%, 3-day = 40%, 1-day = 16% — put these in a table in your answer.
• ✅ Cylinder strength ≈ 0.8 × cube strength — explain WHY (friction confinement at platens).
• ✅ Minimum grade for RCC: M20 (mild), M25 (moderate), M30 (severe) — IS 456 Table 5.

10. Key Takeaways

• Compressive strength = maximum load ÷ area. Tested on 150 mm cubes per IS 516:2018.
• Grades range from M10 (lean concrete) to M80+ (ultra-high performance). M20 is minimum for structural RCC.
• f_ck is the 5th percentile strength. f_m = f_ck + 1.65S is what the mix is designed to achieve.
• 28-day strength is the design reference. Strength continues to grow beyond 28 days.
• W/C ratio, compaction, and curing are the three most controllable factors on site.
• Cylinder strength ≈ 0.8 × cube strength due to the L/D ratio effect.

11. Frequently Asked Questions (FAQs)

Q1. What is the loading rate in the cube compressive strength test?

As per IS 516:2018, the compressive load is applied continuously and without shock at a rate of 14 N/mm² per minute (equivalent to 140 kg/cm²/min) until the specimen fails. Faster loading gives a higher apparent strength, which is why the rate is strictly controlled.

Q2. Why is the 28-day strength used as the design strength?

By 28 days, OPC concrete reaches approximately 99–100% of its design-intended strength. The remaining strength gain beyond 28 days is a built-in safety margin. This period is also practically convenient — 28 days is long enough for meaningful quality control without delaying construction schedules excessively.

Q3. What does M25 grade concrete mean?

M25 means the characteristic compressive strength of the concrete is 25 MPa at 28 days, measured on standard 150 mm cubes. “Characteristic” means 95% of all cube test results from a given batch should equal or exceed 25 MPa. For mix design purposes, the target mean strength would be approximately 31.6 MPa (for S = 4 MPa).

Q4. What is the minimum concrete grade for RCC?

As per IS 456:2000 Clause 6.1 and Table 5, the minimum concrete grade for Reinforced Cement Concrete is M20 for mild exposure conditions. For moderate, severe, very severe, and extreme exposures, the minimum grades are M25, M30, M35, and M40 respectively.

Q5. How many cubes should be cast per concrete sample?

As per IS 456:2000 Clause 15.2, a minimum of 3 cubes must be cast from each sample. All three are tested at the specified age (usually 28 days). The average of three results is reported. If any cube result differs from the average by more than 15%, that result is rejected and the average of the remaining two is used.

Q6. Why is cylinder strength lower than cube strength?

A 150 mm × 300 mm cylinder (L/D = 2) fails in near-pure uniaxial compression. A 150 mm cube (L/D = 1) experiences end-friction confinement from the testing machine’s platens, which artificially boosts the measured strength. As a result, cube strength is about 1.25 times the cylinder strength for the same concrete, or: cylinder strength ≈ 0.8 × cube strength.

🔗 Related: Water-Cement Ratio Law – Abrams Law Explained

🔗 Related: Factors Affecting Strength of Concrete

📚 Reference: IS 516:2018 – Method of Tests for Strength of Concrete, Bureau of Indian Standards