A simple question but something I am unfamiliar with: What is meant by slump when referring to concrete? I've seen references to a 5" slump or a 6" slump. Thanks.

-- John Fritz (JohnFritz24@hotmail.com), March 05, 2002

Answers

It is a density test, concrete is placed in a hollow tube about the size of a large coffee can, the can is lifted up and away, the remaining height measurement of the pile of concrete determines the number which is in inches. If its too wet the slump goes too low, meaning low strength.

-- mitch hearn (moopups@citlink.net), March 05, 2002.

Mitch is right on the money! I bet he works at a ready-mix co. Around here they use the same plastic container as for crush strength cylinders.

-- Okie-Dokie (tjcamp6338@aol.com), March 05, 2002.

The slump test, is the most generally accepted method used to measure the CONSISTENCY or workability of concrete. The equipment consists of a slump cone( a metal conical mold 12" high, with an 8" diameter base and a 4" diameter top) and a steel rod (5/8" in diameter, 24 " long) with a hemispherically shaped tip. The dampened slump cone, placed upright on a flat, solid surface, should be filled in 3 layers of approximately equal volume. Each layer is rodded 25 times. Following rodding, the last layer is struck off and the cone is slowly removed vertically as the concrete subsides or settles to a new height. The empty slump cone is then placed next to the settled concrete. The SLUMP is the vertical distance (in inches) that the concrete settles, measured to the nearest 1/4" from the top of the slump cone mold to the displaced original center of the subsided concrete.

What all that means is: A HIGH number means the concrete is soupy or wet 6 to 8 inch slump, a low number 1 to 3 means the crete is very dry (this concrete is usually seen in the curb machines that have no forms, kida like the Play Dough kid toy machines).

For the average job you want a 4" SLUMP. This is easy to work with and cures nicely in good time for the finishers to work with.

As an end note: never call concrete---cement. Cement is an ingredient of concrete along with the aggregate and the sand.

Concrete takes 28 days to cure. Wait 5-7 days befor driving on fresh concrete.

And YES I paid big bucks to learn the above and to be certified in testing concrete. Concrete is a big part of my job.

-- Bonnie Norris (queqid@att.net), March 05, 2002.

Slump is a measure of concrete plasticity. A slump test is taken with a special cone 4" diam at the top, 8" diam at the bottom, 12" tall. The cone is held firmly to a steel plate while being filled. Must be filled in 3 layers, each layer tamped 25 times in a spiral motion. Strike off the top and lift cone slowly (15 seconds). Difference in height between cone and pile of concrete is the 'slump'. For plain old ordinary concrete a slump of 3" to 6" should be fine. Certain applications such as curb and gutter extrusion or pumping operations require tighter (lower) or looser (higher) slumps.

For plain old ordinary concrete, a higher slump does indeed result in lower compressive strength and greater likelihood of spalling, however, there are special additives to increase the slump without reducing strength - called water reducers or plasticizers.

Compression test cylinders are an entirely different beast. They are cylindrical - same diameter top to bottom - and always have a length- to-diameter ratio of 2:1. Mostly, they are 6" diam and 12" tall, but 4" X 8" cyls were approved a few years ago and we are seeing more and more of the smaller size.

The cyls are cured for 28 days and then crushed on a hydraulic press. Total load at failure is converted and reported as 'psi' (pounds per square inch). Concrete used by the average consumer for sidewalks, driveways, and garage slabs is usually 3000 to 4000 psi.

If the concrete will be exposed to freeze-thaw cycles, it should contain an air entraining admixture. This will provide millions of microscopic bubbles in the hardened concrete. Water expands 9% when it freezes and the bubbles give it somewhere to expand into. Concrete plants in cold climates will add it automatically to mix used for weather-exposed structures.

If you have any other concrete questions,I'll be happy to help if I can. Please Email me direct.

Sandy Stone Concrete Inspector for 23 years

-- Sandy in MN (onestonefarm@hotmail.com), March 05, 2002.

WOW

Not 1, but 2 concrete inspectors replied. I am impressed. Guess this is how the forum is supposed to work.

An old concrete finisher says have a good day.

-- Bob in WI (bjwick@hotmail.com), March 05, 2002.

Bob when I worked for a concrete company I used to joke with the finishers and tell them the saying on every finishers tombstone should read "give me 5 more gallons"!

-- Gary (burnett_gary@msn.com), March 05, 2002.