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Concrete cube test on slab day: how many cubes, and what to write down

How many cube samples IS 456 asks for by the volume you pour, how the cubes are made and cured on site under IS 516, and a record sheet that lets you read the lab report when it comes back 28 days later.

By Bulan Sarkar · Updated

Quick Answer

IS 456:2000 clause 15.2.2 sets the minimum number of samples for each grade of concrete: 1 sample for 1 to 5 m³, 2 for 6 to 15 m³, 3 for 16 to 30 m³, 4 for 31 to 50 m³, and one more for each further 50 m³ or part of it, with at least one sample every shift. Each sample is 3 cubes of 150 mm, and its result is the average of the three. If any cube is more than 15% away from that average, the sample's result is invalid. A typical house slab of 14 m³ needs 2 samples, which is 6 cubes for the 28-day test.

Source: IS 456:2000 Plain and Reinforced Concrete, Code of Practice (with Amendments 1 and 2), clauses 15 and 16, checked Oct 2026.

3 cubesmake one sample (IS 456 cl. 15.3)
150 mmcube mould size (IS 516)
28 daysthe test that decides acceptance
15%most a cube may differ from its sample average

How many cubes for the volume you pour

IS 456 counts samples per grade of concrete. If the slab is M20 and the columns under it are M25, each grade gets its own count. Clause 15.2.2 gives the minimum:

Minimum samples by quantity of concrete of one grade (IS 456:2000 cl. 15.2.2)
Concrete in the workSamplesCubes for the 28-day test
1 to 5 m³13
6 to 15 m³26
16 to 30 m³39
31 to 50 m³412
51 m³ and above4, plus 1 for each further 50 m³ or part3 per sample

Two more rules sit under the table. At least one sample is taken from every shift. And sampling is random and spread across the whole pour, so take the first sample from an early batch and the next from a later one, not all six cubes from one mixer load. For ready-mixed concrete, the code lets supplier and buyer agree a frequency; write what you agreed on the order.

The 7-day test is optional (clause 15.1.1). It gives an early idea of how the concrete is going, but only the 28-day result decides acceptance. If you want both, make a separate set of three cubes per sample for 7 days. The 28-day cubes still need their own three.

Samples needed by volume of one grade5 cubic metres: 1 sample. 15: 2 samples. 30: 3 samples. 50: 4 samples. 100: 5 samples. 150: 6 samples.Up to 5 m³1 sample (3 cubes)6 to 15 m³ (a typical house slab)2 samples (6 cubes)16 to 30 m³3 samples (9 cubes)31 to 50 m³4 samples (12 cubes)51 to 100 m³5 samples (15 cubes)101 to 150 m³6 samples (18 cubes)
Samples needed by volume of one grade. Minimum from IS 456:2000 cl. 15.2.2, at the top of each band. Each sample is three 150 mm cubes.

Making and curing the cubes on site

IS 516:1959 is the method most site labs and contracts still quote; BIS has since revised it as IS 516 (Part 1/Sec 1):2021, so ask your lab which edition it follows. The 1959 text says:

  1. Moulds: 150 mm steel cube moulds, clean, lightly oiled, bolted tight.
  2. Fill in layers about 5 cm deep. Move the scoop round the top edge so the concrete spreads evenly.
  3. Compact by hand with a 16 mm steel bar, 0.6 m long, with a bullet point. At least 35 strokes per layer for a 150 mm cube, spread over the whole face, reaching into the layer below. Tap the sides to close any holes the bar leaves.
  4. Trowel the top level with the mould and cover it with a plate so it doesn't dry out.
  5. For the first 24 hours, keep field cubes where nothing shakes, under damp sacks or matting, at 22 to 32 °C.
  6. After 24 hours, mark each cube, take it out of the mould and put it in clean water at 24 to 30 °C until it goes to the lab. Send it packed in damp sand or damp sacks so it arrives wet, at least a day before the test.

The same clause asks for the day's highest and lowest temperature to be written down while the cubes are on site. A cheap min/max thermometer near the curing tank does it.

A cube's 28 days, from the chute to the lab1 sample from a batch during the pour; 2 fill three 150 mm moulds in 5 cm layers, 35 strokes a layer; 3 24 hours under damp sacks at 22 to 32 degrees; 4 mark, demould, cure in water at 24 to 30 degrees; 5 send to the lab packed damp; 6 crushed at 28 days, average of three.1Take the sample from a batch during the pourRandom batches, spread over the whole pour2Fill three 150 mm moulds5 cm layers, at least 35 strokes a layer324 hours under damp sacksSomewhere still, 22 to 32 °C4Mark, demould, into waterClean water at 24 to 30 °C5Send to the lab packed dampArrive at least a day before the test6Crushed at 28 daysSample result = average of the three cubes
A cube's 28 days, from the chute to the lab. Field cubes under IS 516:1959 clause 3. Temperatures are the site ranges in that clause; at the lab the cubes sit in water at 27 ± 2 °C.

The record sheet: one line per cube

A lab report comes back weeks after the pour, listing cube IDs and loads. It only means something if you can match each ID to where the concrete went. Keep one line per cube with these columns:

Cube record columns (fill the first eight on slab day)
ColumnExample
Cube IDS1-A, S1-B, S1-C
Pour date and time the sample was taken10 Oct 2026, 11:40
Grade orderedM20
Where it wentFirst-floor slab, grid lines B to C and 2 to 3
Truck or batch numberRMC challan 4471, or mixer batch 6
Made byName of the person who filled the moulds
Site min/max temperature, first 24 h24 °C / 31 °C
Demoulded and into water11 Oct, 12:00
Sent to lab, lab name6 Nov
Test date and age7 Nov, 28 days
Load (kN) and strength (N/mm²)607 kN, 26.98 N/mm²
Sample average and the ±15% check26.99, all three inside 22.94 to 31.04
Lab report numberKeep the report with the sheet

Strength is the load divided by the face area. A 150 mm cube has a face of 22,500 mm², so every 22.5 kN on the lab sheet is 1 N/mm². IS 516:1959 also asks the lab to report each cube's ID, test date, age, curing, size, maximum load and anything unusual about how it broke.

Reading the results when they come back

First, check each sample on its own. Average the three cubes, then check that none of them is more than 15% above or below that average (IS 456 cl. 15.4). If one is, the sample's result is invalid. That usually points to a cube made, cured or handled badly, not to the slab, so make all three cubes the same way.

Then the grade check. Clause 16.1 says concrete complies when two things hold: the mean of any group of four consecutive sample results meets column 2 of Table 11, and every individual result meets column 3. Table 11's limits have been amended since 2000, and a small job may have fewer than four samples, so the pass line is your engineer's call against the current amended table. You bring the record sheet and the numbers.

If the concrete doesn't comply, clause 16.4 says the structural adequacy of the affected parts is investigated. Clause 17 describes the next steps: cores (at least three), then a load test if needed. Your engineer decides these. The record sheet tells everyone which slab bay a failed sample came from.

Six cube results against the ±15% bandSample 1: 26.98, 26.00 and 28.00 newtons per square millimetre, all inside the 15 percent band. Sample 2: 26.98, 26.00 and 20.00; the 20.00 cube is outside its band, so the sample is invalid.S1-A, 607 kN26.98 N/mm²S1-B, 585 kN26.00 N/mm²S1-C, 630 kN28.00 N/mm²S2-A, 607 kN26.98 N/mm²S2-B, 585 kN26.00 N/mm²S2-C, 450 kN20.00 N/mm², outside the band
Six cube results against the ±15% band. The third worked example. Sample 1 averages 26.99 N/mm², so its band runs from 22.94 to 31.04 and all three cubes sit inside it. Sample 2 averages 24.33. Its band starts at 20.68, and the 20.00 cube falls below it.

Worked examples

1. Cubes for a 14 m³ house slab

  • Slab and beams in M20: 14 m³ (the slab-day example in our material order system)
  • 14 m³ falls in the 6 to 15 m³ band: 2 samples
  • 2 samples × 3 cubes = 6 cubes for the 28-day test
  • Optional 7-day test: 3 more cubes per sample, 6 more
  • Take sample 1 from an early truck or batch and sample 2 from a late one

Result: Keep 12 clean moulds on site for the pour: 6 for 28 days and 6 for 7 days. If you skip the 7-day test, 6 are enough.

2. A 120 m³ building, poured over several days

  • All in one grade, M25: 120 m³ in total
  • First 50 m³: 4 samples
  • Remaining 70 m³: one per 50 m³ or part, so 2 more
  • Total: 6 samples = 18 cubes, and never fewer than one sample per shift
  • Six pour days of one shift each means at least 6 samples anyway

Result: 6 samples, 18 cubes for 28 days, one sample from each pour day. If a column grade is different, count its volume separately.

3. Is this sample's result valid?

  • Lab loads for S1: 607, 585 and 630 kN on 150 mm cubes (22,500 mm²)
  • Strengths: 26.98, 26.00 and 28.00 N/mm²; average 26.99
  • ±15% band: 22.94 to 31.04 N/mm². All three inside: valid, result 26.99
  • Second case: 607, 585 and 450 kN gives 26.98, 26.00 and 20.00; average 24.33
  • Band 20.68 to 27.98: the 20.00 cube is outside

Result: The first sample's result is 26.99 N/mm². The second sample is invalid under IS 456 cl. 15.4, so check the record sheet for that cube: who made it, how it was cured, whether it dried out.

Tips and tricks

  • Fill the cube moulds from the concrete going into the slab, at the point of placing, not from a bucket kept aside at the start of the pour.
  • Never top up a cube from a different batch to finish it. Each sample comes from one batch.
  • Photograph the marked cubes on the day they come out of the moulds. The photo is your proof of the ID if a label wears off in the tank.
  • Ask the lab to report the load in kN as well as the strength. You can then check the arithmetic yourself: load ÷ 22.5 = N/mm².
  • With ready-mixed concrete, write the challan number on the cube line. Ask for the plant's own cube results for that batch too.
  • Keep the curing tank in shade with a lid. Water left in full sun can run past the 30 °C top of the range.

What we would do

We'd count the samples from the slab volume before the pour and keep the moulds, the 16 mm bar and a marker ready on site. We'd take the samples from early and late batches and fill the record sheet's first eight columns before the day ends. When the report comes back, we'd do the ±15% check on each sample ourselves, then hand the sheet and the report to the engineer for the Table 11 check. We'd spend a little extra on the 7-day set for the first slab only, to catch a problem with the mix early. The slab casting day checklist puts this into the order of the whole day, with a sheet to print.

Frequently asked questions

How many cubes are needed for a concrete test?
One sample is three 150 mm cubes. IS 456:2000 cl. 15.2.2 asks for 1 sample for up to 5 m³ of a grade, 2 for 6 to 15 m³, 3 for 16 to 30 m³, 4 for 31 to 50 m³, and one more for each further 50 m³ or part, with at least one sample each shift.
Is the 7-day cube test compulsory?
No. IS 456 cl. 15.1.1 calls the 7-day test optional, for a quicker idea of quality. Only the 28-day compressive strength decides whether the concrete is accepted or rejected.
How do I convert the lab load in kN to N/mm²?
Divide the load by the face area. A 150 mm cube has 22,500 mm² of face, so 450 kN is 20 N/mm² and every 22.5 kN is 1 N/mm².
What if one cube in a set is much lower than the other two?
Work out the average of the three. If any cube is more than 15% above or below that average, IS 456 cl. 15.4 says the test result of that sample is invalid. Look at how that cube was made and cured.
How should cubes be cured on site?
Under IS 516:1959 cl. 3.3: 24 hours in the mould under damp sacks at 22 to 32 °C, then marked, demoulded and kept in clean water at 24 to 30 °C until they go to the lab. They should arrive damp, at least a day before the test.

Sources and date checked

This guide covers counting, making, curing and recording test cubes. The concrete grade, the acceptance check against IS 456 Table 11 and any action on a failed result belong to your structural engineer.

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