The slump test of concrete gives a site its answer in under three minutes: fill a 300 mm steel cone with fresh concrete, lift the cone away, and measure how far the heap sinks. For an ordinary lightly reinforced slab, beam or column, IS 456 suggests a slump of 25 to 75 mm, and for pumped concrete 75 to 100 mm.

That one number tells you whether the concrete in front of you can be placed and compacted properly. It does not tell you the strength; the cube test does that, 7 and 28 days later. Below is the cone, the method as the current Indian standard lays it out, how to read the three shapes the heap can take, what slump to ask for, how far a ready-mix load may drift from it, and when the slump test is the wrong test to use.

What is the slump test of concrete?

It measures consistency, meaning how stiff or runny fresh concrete is. Engineers also call this workability: how easily the concrete can be placed in the form and compacted without separating. A stiff mix stands almost as tall as the cone. A wet mix spreads out and drops a long way.

The Indian method is IS 1199 (Part 2):2018, the Bureau of Indian Standards code for testing the consistency of fresh concrete. It says the slump test works for concrete that slumps between 10 mm and 210 mm. Outside that range the reading stops meaning much, and the same standard gives other tests for very stiff and very fluid mixes.

IS 456:2000, the main code for plain and reinforced concrete, points to this test in two places. Clause 7.1 sets suggested slump ranges for different kinds of work, and clause 10.3.2 says workability should be checked at frequent intervals during concreting. On most Indian sites the slump cone is the first thing the engineer or the ready-mix supplier’s technician reaches for when a transit mixer arrives.

Slump cone size and the tools you need

The cone is a hollow frustum: a cone with its tip cut off, open at both ends. It is made of metal at least 1.6 mm thick, smooth inside, with two handles two-thirds of the way up and foot pieces at the bottom so the tester can stand on it while filling.

Table 1: Slump test apparatus as per IS 1199 (Part 2):2018

Item What the standard asks for
Mould (slump cone) Base diameter 200 ± 2 mm, top diameter 100 ± 2 mm, height 300 ± 2 mm (about 12 inches); metal at least 1.6 mm thick, no dents, countersunk rivets inside
Tamping rod Steel, round, 16 ± 1 mm diameter, 600 ± 5 mm long, rounded ends
Base plate Rigid, flat, smooth and non-absorbent, placed where there is no vibration or shock
Rule Graded 0 to 300 mm, marks no more than 5 mm apart, zero at the very end
Other items Square-bladed shovel, non-absorbent re-mixing tray, scoop, timer reading to 1 second; a filling funnel is optional
Checks Cone, rod and funnel checked once a year for size and condition; cone inspected for set concrete before every test

A cone that has been dropped off a truck a few times is worth checking against these sizes. A dented mould or a lump of old concrete stuck inside changes the volume, and then the reading means nothing.

How to do a slump test, step by step

Take the sample the way IS 1199 (Part 1) describes and turn it over on the tray with the shovel just before testing. Then:

  1. Wipe the inside of the cone and the base plate with a damp cloth, then mop off any standing water. Set the cone on the plate, on level ground away from running machines.
  2. Stand on the two foot pieces so the cone cannot lift. Fill it in three layers, each about one-third of the height once tamped, spreading the concrete evenly round the cone.
  3. Tamp each layer with 25 strokes of the rod, spread over the whole area. For the bottom layer, tilt the rod slightly and work about half the strokes in a spiral towards the centre.
  4. Tamp the second and top layers through their full depth, so the rod just enters the layer below.
  5. Heap the concrete above the rim before tamping the top layer, and keep a little extra above the rim the whole time. Do not keep adding concrete in a way that packs it harder.
  6. Strike the top level with a sawing and rolling motion of the rod, and clean any spilled concrete off the plate around the cone.
  7. Lift the cone straight up in 5 seconds, give or take 2, with no twist and no sideways movement.
  8. Straight away, lay the rod across the top of the empty cone placed beside the heap and measure down to the highest point of the concrete. That gap is the slump. Round it to the nearest 5 mm.

The whole job, from the first scoop to lifting the cone, has to be finished without a break and within 180 seconds. A test that drags on while the concrete starts to stiffen reads low.

One change trips up older site manuals. The 1959 edition of IS 1199 filled the cone in four layers of a quarter height each. The 2018 revision moved to three layers to match international practice, and its own foreword warns that this may give slightly higher slump values. If your quality plan still says four layers, it is quoting the withdrawn method.

True, shear and collapse slump: what each shape tells you

When the cone comes off, the heap settles into one of three shapes. Only one of them gives a valid reading.

Table 2: The three types of slump and what to do

Type What you see What it usually means What to do
True slump The heap sinks evenly and stays in one piece, roughly the shape of the cone The mix holds together; the reading is a fair measure of its consistency Record the slump to the nearest 5 mm
Shear slump One side slides off at an angle, leaving a slanting face The mix lacks cohesion, so the paste does not hold the stones together Take a fresh sample and repeat the whole test
Collapse slump The heap falls flat and spreads out The mix is too wet for this test, or it is a very fluid mix such as self-compacting concrete Take a fresh sample and repeat; for a very fluid mix, use a flow test

The 2018 standard is strict here. If two tests in a row shear or collapse, the result is reported as invalid, because the concrete lacks the plasticity and cohesion the test needs. The 1959 edition allowed a sheared slump to be measured and noted. That is no longer the rule.

A shear on a site mix is a warning worth acting on. A mix that cannot hold its shape on a steel plate is likely to separate in a deep column too, with the stones settling to the bottom and the paste running out through gaps in the shuttering. That is how honeycombing starts; the causes and repairs are in the guide to honeycombing in concrete.

The old 1959 standard also carried a quick check that still works. After measuring, tap the side of the heap gently with the rod. A well-proportioned mix with a fair slump sinks a little further and stays together. A badly proportioned one tends to fall apart.

What slump should M20 concrete have?

IS 456 does not tie slump to the grade. M20 in a lightly reinforced house slab and M20 in a crowded column need different slumps, because what decides the slump is where the concrete goes and how it gets there. Strength comes from the mix design and the water-cement ratio, not from how runny the concrete is.

Table 3: Suggested slump by placing condition (IS 456:2000, clause 7.1)

Placing condition Degree of workability Slump
Blinding concrete, shallow sections, pavements laid by pavers Very low Not measured by slump; compacting factor 0.75 to 0.80 (clause 7.1.1)
Mass concrete, lightly reinforced slabs, beams, walls and columns, floors, hand-placed pavements, canal lining, strip footings Low 25 to 75 mm
Heavily reinforced slabs, beams, walls and columns Medium 50 to 100 mm
Slipform work, pumped concrete Medium 75 to 100 mm
Trench fill, in-situ piling High 100 to 150 mm
Tremie concrete Very high Not measured by slump; flow test (clause 7.1.2)

Figure 1: IS 456 slump ranges on one scale, 0 to 150 mm

Low workability: lightly reinforced work, 25 to 75 mm
  
Medium workability: heavily reinforced sections, 50 to 100 mm
  
Medium workability: pumped concrete and slipform, 75 to 100 mm
  
High workability: trench fill and in-situ piles, 100 to 150 mm
  
Each bar starts and ends at the slump figures in Table 3. The full width stands for 150 mm.

So the honest answer for M20 is: 25 to 75 mm for an ordinary lightly reinforced house slab, beam or column placed by hand, 50 to 100 mm where the steel is packed tight, and 75 to 100 mm if it is going through a pump. The ranges overlap on purpose. The engineer picks a target inside the range and writes it into the order.

IS 456 adds a note under the same table: for most of these placing conditions, internal needle vibrators are the right tool, sized to the steel spacing and the thickness of the section. Tremie concrete, placed under water through a pipe, needs no vibrator.

How much can ready-mix slump vary from the order?

IS 4926:2003, the Indian standard for ready-mixed concrete, allows the measured slump to differ from the specified slump by 25 mm or by one-third of the specified value, whichever is less. The test for acceptance is done on the concrete as it comes out of the supplier’s truck at site.

Table 4: Allowed slump band on a ready-mix load (IS 4926, clause 6.2)

Slump in the order One-third of it Tolerance that applies Acceptable reading
50 mm about 17 mm ± 17 mm about 33 to 67 mm
75 mm 25 mm ± 25 mm 50 to 100 mm
100 mm about 33 mm ± 25 mm 75 to 125 mm
150 mm 50 mm ± 25 mm 125 to 175 mm

Time matters too. Under IS 4926, if discharge from the supplier’s truck is held up at site for more than 30 minutes because the site was not ready, responsibility for the concrete passes to the buyer. The standard asks the buyer to plan so that a full load can be emptied within 30 minutes of the truck arriving.

The delivery ticket is your first check. IS 4926 lists what it must carry, including the grade, the specified target workability, the nominal maximum aggregate size, any admixtures and the time of loading. Compare the slump you measure against the target on that ticket, and the time on the ticket against your watch.

Ready-mixed and site-mixed concrete each have their place; the trade-offs are set out in RMC vs site mix concrete.

Why water should not be added at site to raise slump

When a load arrives stiff, the quickest fix is a bucket of water in the drum. It is also the most common way good concrete is turned into weak concrete. Extra water does raise the slump, but it raises the water-cement ratio with it, and that ratio is what decides strength and how well the concrete resists water and salts over the years. IS 456 caps the water-cement ratio by exposure condition for exactly this reason.

IS 4926 closes the door on it for ready-mix. No water beyond the amount needed to reach the specified workability may be added to the truck mixer drum before discharge, unless the buyer specifically asks for it and signs for it. Many transit mixers carry an onboard water tank, mainly for washing out the drum. Under IS 4926 it is not a way to top up a load the buyer has not asked for and signed for.

The route the standard does allow is an extra dose of admixture. IS 4926 permits an additional dose at site to regain workability, in special cases, by agreement between supplier and buyer. A plasticiser (a chemical that makes concrete flow more without extra water) brings the slump back while the water-cement ratio stays where the mix design put it. IS 456 limits retarders, plasticisers and superplasticisers to 0.5, 1.0 and 2.0 percent of the cementitious materials by weight, unless a higher dose is agreed on the basis of trials. The types are explained in the guide to concrete admixtures, and the cement and water side of a mix is worked through in the building material calculation guide.

Why concrete loses slump between the plant and the pour

Concrete starts stiffening the moment water meets cement. The longer the trip, the hotter the day and the drier the stone, the more slump is lost before the chute opens. That is why IS 4926 has the acceptance test taken at discharge, not at the plant. A long wait at the gate on a summer afternoon can take a load that left the plant in range and deliver it stiff.

The machine that carries the concrete shapes how much of this you see. A transit mixer keeps the drum turning on the road so the mix stays even, and the number of trips it can make in a day is covered in transit mixer productivity. Some are built for stiff work: the Conmat MR 60C is listed for handling extra-low-slump concrete of the kind used in slipform paving and dam work.

A self-loading mixer such as the AJAX ARGO 4500 changes the picture. It loads, weighs, mixes and pours on site, so there is almost no travel time to lose slump in. The flip side is that every batch is a new mix made by the operator, so test the first batch of the day and any batch after the water or sand changes.

Comparing transit and self-loading mixers for your next pour? Browse concrete mixer models and prices and connect with a dealer near you.

Pumped concrete is where slump matters most. A mix outside the 75 to 100 mm band for pumping, or one that shears on the cone, is the usual cause of a choked line. A stationary pump such as the Putzmeister BSA 1407 D Classic will push what it is given, and the slump test at the hopper is the last chance to stop a bad load. What happens when it is missed is in the guide to concrete pump line blockage.

When is the slump test the wrong test?

IS 1199 (Part 2) says the slump test does not apply to foam concrete, to no-fines concrete, or to concrete whose nominal maximum aggregate size is over 40 mm. If a mix does contain stone bigger than 40 mm, the sample is wet-sieved through a 40 mm screen before testing.

Very stiff mixes fall below the 10 mm the slump test can read. Pavement quality concrete, dry lean concrete and roller-compacted concrete sit here, and IS 456 clause 7.1.1 suggests the compacting factor test for them, with a value of 0.75 to 0.80. The 2018 standard also includes the Vee-bee test and a newer degree of compactability test for such mixes. Road pours of this kind are covered in the guides to dry lean concrete and slip form paving.

Very fluid mixes run past the top of the range. Tremie concrete and self-compacting concrete are judged by a flow or slump-flow test, where the spread of the concrete is measured instead of its drop; IS 1199 (Part 6) covers self-compacting concrete.

And no slump test, however good, replaces the cube test. A load can show a perfect 75 mm and still be weak if the wrong cement or too little of it went in. Slump tells you the concrete can be placed. Cubes tell you it will carry the load.

What to keep on site for slump testing

Keep a clean, undented cone, a proper 16 mm rod and a flat plate on site, and test the first load, every load that looks different, and any load that has waited. Write down the time and the reading against the delivery ticket. On a site with a quality engineer that record is part of the job; the role is described in the post on the QA QC engineer in construction.

More concrete and material topics are in the building materials guides. Planning a pumped pour for a slab or column? Compare concrete pump models and prices and talk to a dealer near you.

The method and figures on this page are from IS 1199 (Part 2):2018, Fresh Concrete: Determination of Consistency and IS 456:2000, Plain and Reinforced Concrete, Code of Practice, with the ready-mix tolerance, timing, water and delivery-ticket rules from IS 4926:2003 and the older four-layer method from IS 1199:1959, all read in October 2026.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication. Code requirements on this page are as published in IS 1199 (Part 2):2018, IS 456:2000 and IS 4926:2003, read in October 2026; the slump for a structural pour should be set by the project’s engineer.

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