As Civil Engineer, this is one of the most common tests that one will have to go through. Nevertheless, it is dumbfounded that some engineers could not understand what the reading from this test renders.

The Concrete Slump Test is one of the in-situ concrete tests which will be done before concrete is discharged in-situ. This test is governed by BS EN 12350 Part 2 and the older reference would be BS 1881 Part 102. There are less than 10 pages and it would make many wonders why some cannot comprehend such a simple test.

Question: What is the difference between S5 slump and collapse? (This question was posed by a consulting engineer from either Malaysia or Myanmar working in Singapore; a developed nation.)

Before we go into details about this test, it is good to read the normative references and the literature behind this test. The standard specification mentioned clearly that this test is to be conducted at the site to indicate the workability of the concrete based on trial mix.

If the concrete is designed to have a high slump and designed as a tremie concrete, then it supposes to achieve the desired slump assisted by admixture(s). In this case, it supposes to be under the category of S4 or S5 which is categorized as concrete and indicates collapsed shape when the test was conducted. The design mix was done in such a manner so that high workability will allow concrete to flow easily when using a pump.

Slump range are as follows;

S1 (10-40mm)

S2 (50-90mm)

S3 (100-150mm)

S4 (160mm-210mm)

S5 (>210mm)

The importance of design mix and trial mix procedure will assist the engineer at the site to check compliance of delivered concrete. With a high slump range, it highlights the potential that the concrete can be tempered with water (usually) to ensure it is not hardened or stiff before being discharged.

None of the literature rejects the condition of total collapse or S5 as a fail or rejects or non-compliance. It is up to the discretion of the engineer to interpret the condition of the slump. It is mind-blowing to hear a consultant insisting on a true slump shape when it is designed as a tremie concrete. There is no way that a slump intended to be at 210mm to have true slump shape with a balance of 90mm (when the cone bucket is 300mm).

Nevertheless, the trick here is for engineers to exploit when it comes to shearing of the slump. If the slump collapses in a uniform shape, there will be no problem unless there is a sign of potential shearing based on displacement and spread of the concrete.

It is like trying to find a true slump for grout when testing for its workability. It is futile if one obstinate engineer tries to conduct such tests with high workability. For that reason, Flow Cone and Flow Trough tests are introduced for highly fluid kinds of cementitious products. In this case, it is similar to the concrete slump test. It does not mean the product failed to comply with standard specification which stipulates the required characteristic strength. Workability only indicates the suitability for concrete application and compliance to the design mix and trial mix.

I hope this will help young and fresh engineers from making unnecessary blunders at construction site.