Fresh concrete flow testing

What does the flow table test measure?

It measures the consistence of fresh concrete as a spread diameter: the concrete is jolted on a hinged table and the width it spreads to is the flow value in millimetres. It does the same job as a slump test, but over a more workable range where slump has stopped discriminating usefully. Flow describes how the concrete handles, not how strong it will be, and the result is compared with the flow class or target in your specification.

When should we use flow testing instead of a slump test?

When the concrete is too workable for slump to say anything useful, or when your specification sets a flow class rather than a slump class. Slump loses sensitivity above about 210 mm, so free-flowing and heavily plasticised mixes all come back looking much the same. The flow table covers that range. If the concrete is genuinely self-compacting, neither test applies and the slump-flow method is used instead.

How is the flow table test carried out?

A cone-shaped mould 200 mm high, 200 mm across the base and 130 mm across the top is filled on a 700 mm square hinged table in two layers, each tamped ten times, then struck off and, after a pause of about 30 seconds, lifted clear. The upper leaf of the table is raised to its stop and dropped 15 times, which spreads the concrete out. Two diameters at right angles are then measured and averaged to give the flow value, recorded to the nearest 10 mm.

Which standard is flow table testing carried out to?

Usually BS EN 12350-5, with the sample taken to BS EN 12350-1 and the flow classes and tolerances coming from BS EN 206, BS 8500 or the project specification. The method is not intended for concrete with aggregate above 63 mm, and it is only sensitive across flow values of roughly 340 to 620 mm. Where your specification names a different consistence method we work to that, and the report states the method used.

What are the flow classes F1 to F6?

They are the bands used to specify consistence by flow: F1 is 340 mm or less, F2 is 350 to 410 mm, F3 is 420 to 480 mm, F4 is 490 to 550 mm, F5 is 560 to 620 mm and F6 is 630 mm or more. Most flow-specified concrete sits in F4 or F5. F1 and F6 fall outside the range the test discriminates well in, so a requirement at those extremes is usually better served by another method.

Is the flow table test the same as the slump-flow test for self-compacting concrete?

No, they are different methods: both report a spread in millimetres, but the flow table test compacts the concrete into its own cone and then jolts it on a table, while the slump-flow test uses the ordinary slump cone on a flat plate with no compaction and no jolting. Slump-flow is normally carried out to BS EN 12350-8, often with a t500 flow time recorded alongside it. Booking the wrong one gives a result that cannot be compared with your acceptance limits.

Does a higher flow class mean weaker concrete?

Not in itself, because high workability achieved with a plasticiser or superplasticiser does not reduce strength, as the water content has not changed. High workability achieved by adding water is a different matter: it lowers strength and affects durability. That is why a flow result is read alongside the delivery ticket and any record of water or admixture added after batching. A high flow with visible bleeding or segregation is the one to be suspicious of.

What makes the flow result change between the batching plant and the point of placing?

Mostly time and temperature: superplasticised mixes hold their workability for a defined window and then lose it quickly, so a load that sits in the queue can arrive well below the flow it left the plant with. Concrete temperature, ambient conditions, dry aggregates and the admixture type all shorten or extend that window. Consistence is specified and tested at the point of delivery for exactly this reason, so the test is timed around discharge rather than around arrival on site.

What happens if the concrete segregates or the spread is not round?

We record it, because the shape of the spread and what happens at its edge say as much as the number does. A ring of paste running ahead of the aggregate, coarse aggregate heaped in the centre or free water at the rim all point to a mix that is flowing because it is wet rather than because it is well designed. The flow value is still reported, with the observation alongside it, so the number is not read on its own.

Can the supplier re-dose the load with plasticiser on site and have it retested?

That is between you and the concrete producer, and where it is done it should be the producer's decision and dose, recorded on the delivery ticket. The load then has to be remixed properly, usually several minutes at mixing speed, before a new sample is taken. We will test the concrete again and report both results with the times, so the record shows what was tested and when. Whether the re-dosed load is acceptable stays with you and your engineer.

Is flow testing covered by 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 covers the methods named in that schedule rather than everything we do. Ask us to confirm whether the consistence method your specification calls for is covered, and the report will state the status of each result. Worth checking early where accredited results are needed for consistence as well as strength.

What do you need to know to quote for flow testing on our pours?

Your site postcode, the concrete specification or the consistence clause from it, the pour dates and volumes, the expected number of loads and how often testing is required. Tell us whether cubes or other fresh concrete tests are needed on the same visit, because most of the cost is the visit rather than the individual test. Out-of-hours pours, long waits between loads and work spread over several addresses all affect the price.