Concrete Slump Question?

Question

I had a Civil Engineering interview and one of the technical questions I was given about slump. I know what it is, but there are certain aspects I was unsure of. Is the length of the slump measured from the top (how far it slumped) or from the bottom (how high it is after it slumped)? All I could find in my books is that slump is a measure of workability, is that all? In a bridge design, what would be a case where it was ok to use a high slump (8" or more)?

Answer 1

After removing the slump test cone, it is inverted and laid on the base plate adjacent to the slumped concrete mix. A rigid straight edge is placed on top of the inverted cone and used as a reference for measuring how far the concrete mix has dropped down from a standard height of 12 inches ( The slump cone is 12 inches high, 8 inches at the base and 4 inches at the top). The distance it has dropped down is called the slump.

There is a risk in using concrete mix with a slump of more than 6 inches as the aggregates would tend to fall out, so unless for special purposes where such slump is specified, it is not normally accepted.

For class A (3000psi) concrete construction, the slump should be about 3 inches.

8 inches slump is definitely too runny (or soapy) and the water cement ratio, which is a measure of the strength of the concrete, is suspect.

I have no knowledge of any bridge construction where they used a concrete mix with a slump of 8 inches or more. That would be something close to the consistency of quicksand.

Answer 2

Slump is measured by how much it falls.  So you measure from the top of the cone down to the concrete.  Slump is a measure of workability and can be an indicator of Water/Cement ratio.  Adding additives to the concrete such as Super-P or water-reducers can change the slump by increase workability.  8" or more slump on a bridge would probably only be used for grouting situations such as grouting a bonded tendon.  8" could also be used for a very congested area of steel.

Answer 3

Most of your questions are answered at the reference. To answer the last - in general, the lower the slump, the harder it is to work the concrete but the harder it is once set.

Answer 4

Slump is measured from the top (how far it slumped).