Fresh concrete slump testing

How is a slump test carried out on site?

The damp cone is placed on a level rigid plate, filled in three layers with 25 strokes of the tamping rod to each layer, struck off level, then lifted vertically and steadily clear of the concrete. The drop from the height of the mould to the highest point of the slumped concrete is measured and recorded to the nearest 10 mm. The whole test is done without interruption, because the concrete stiffens while you work and a pause changes the answer.

Which standard is slump testing carried out to?

Usually BS EN 12350-2, with the sample taken to BS EN 12350-1 and the consistence class or target value coming from BS EN 206 and BS 8500 or from the project specification. Highways work may instead point to the relevant series of the Specification for Highway Works. Where your documents name a different method, or a different point of testing, we work to those and the report states the method used.

What do the slump classes S1 to S5 mean?

They are bands of slump in millimetres used to specify consistence: S1 is 10 to 40 mm, S2 is 50 to 90 mm, S3 is 100 to 150 mm, S4 is 160 to 210 mm and S5 is 220 mm or more. A specification either names a class or gives a target value with a tolerance. S2 and S3 cover most structural concrete placed with poker vibration, while S4 is typical of pumped and free-flowing mixes.

What does it mean if the slump is outside the specified class?

It means the concrete as delivered is not at the consistence the specification asked for, which is a supply matter rather than a strength result. We report the figure, the time it was taken and the delivery ticket details, and we tell the site straight away. The decision to accept, reject or hold the load is yours and your engineer's, not ours. Consistence is one of the things a producer's conformity is judged on, so the record matters to both sides.

Can a load be retested if the slump comes out wrong?

Only where the first result was not valid. If the concrete shears off to one side or collapses, that is not a true slump, so the test is repeated on a fresh portion of the same sample. A valid slump that simply falls outside the specified class is not retested: the figure stands, and the next load is a separate sample with its own result. Where two attempts both give a shear or collapse slump, the mix is outside what the test can sensibly describe.

Why is the slump on site lower than the slump recorded at the batching plant?

Concrete loses workability from the moment it is batched, so any gap between the plant and the chute shows up as a lower slump. Time in transit, concrete and ambient temperature, dry or absorbent aggregates and the type and dose of admixture all affect how quickly it stiffens, and a long wait on site between loads makes it worse. This is why consistence is specified and tested at the point of delivery rather than at the plant.

Is a slump test right for every concrete mix?

No, the test is only sensitive between about 10 and 210 mm, so a very stiff mix gives almost no slump and a very fluid one collapses, and in neither case does the number tell you much. Stiff concrete is better described by the degree of compactability or Vebe methods, and high-workability concrete by the flow table test. Self-compacting concrete is assessed by slump-flow instead. Send the consistence clause from your specification and we'll confirm which method fits.

Do you test every load, or only some of them?

That is set by your specification, and testing every load is unusual outside high-risk or low-volume pours. A common arrangement is the first load of a pour, then a proportion of the rest, plus a test alongside every sample taken for cubes and any load that looks different from the ones before it. More frequent testing early in a job, or after a mix change, is often worth it. We'll work to whatever frequency your testing schedule sets out.

How long will you wait on site if the loads are running late?

We plan the visit around the pour, and short waits between loads are expected. Longer waits are chargeable as standby, so it helps to tell us the pour start time, the expected number of loads and the interval between them when the visit is booked. If the pour moves or is cancelled, tell us as early as you can. Same-day and next-day testing dates are available in many areas, though that depends on location and workload.

How is a slump result reported?

As a single figure in millimetres to the nearest 10 mm, alongside the information needed to tie it to a load: the date and time, the delivery ticket number, the mix reference, the sampling point and the pour location. The form of slump is noted, and any departure from the standard method is stated. Most sites want the number at the time, so it is given to your site contact on the day and confirmed in the written record.

Is slump testing on our site carried out under your UKAS accreditation?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, so it applies to the methods on that schedule rather than to every service. Ask us to confirm whether the test you need is covered, ideally before the work is booked, and the report will make the status of each result clear. If your specification requires accredited consistence testing, say so at enquiry stage.

What drives the cost of slump testing, and what do you need to quote?

Cost follows the visit rather than the number of tests, so the main drivers are travel to your site, time on site, the number of visits across the programme and any out-of-hours working. Slump is rarely booked on its own, and it usually works out cheaper per visit when combined with cube making or other fresh concrete tests on the same trip. Send your site postcode, the concrete specification, the pour dates, the expected number of loads and the testing frequency.