No slump concrete 1

novembertango88 · Feb 18, 2020

Hello all
I am working on a site where they are using a concrete mix of 1 cement 1.5 sand 3 aggregate and water:cement ratio of 0.4.
The mix is workable enough for columns, slabs etc when vibrated with a poker and the visible quality is good.
It is achieving 20N/mm2 and 24N/mm2 for 3- and 7-day stengths with a target strength of 35N/mm2.

However - when I carry out a slump test there is no slump (see attached photo - I didn't measure the slump as it didn't move at all).

Are the main issues (1) not being workable enough for a particular application, and (2) not having enough water to hydrate the cement.
If so is it acceptable?

Your advice would be greatly appreciated.

Kind regards

r13 · Feb 18, 2020

I remember it was taught at our concrete lab, but forgot the applications. Here is a quote.

According to ACI's Cement and Concrete Terminology (ACI 116R-90), no-slump concrete is freshly mixed concrete exhibiting a slump of less than 1/4 inch. Zero-slump concrete is defined in the same document, as concrete of stiff or extremely dry consistency showing no measurable slump after removal of the slump cone. And negative-slump concrete is concrete of a consistency such that it not only has zero slump but still has zero slump after additional water is added.
It was determined that the rollers were very effective in compacting the dry-mix concrete and that the strength properties, compressive strength and flexural strength, were somewhat higher than would have been obtained with a conventional mix of the same cement content having a slump in excess of 1 inch. The surface smoothness, surface texture, and riding quality of the pavement were considered adequate for wearing surfaces of secondary roads and streets, haul roads, service entrances, tank trails, ect., and as a base for any pavement system. Indications are that considerable cost reductions in the construction of portland cement pavements can be realized by use of dry (zero-slump) concrete mixtures placed and spread by base course or asphalt spreaders and compacted with heavy vibratory rollers.

bridgebuster · Feb 18, 2020

~1/2" slump is typical for slip forming highway barriers and curbs. Your dry materials are in line with slip forming although the w/c ratio seems a hair low. Are you using water reducer in the mix?

cvg · Feb 18, 2020

you generally can't pump a stiff mix like this so probably not ideal for columns. no slump is generally used for slip forming and roller compacted or slab on grade paving applications, not so much for formed vertical construction. as far as having enough water to hydrate, you should have a mix design report that will show the required mixture for adequate hydration and strength gain.

novembertango88 · Feb 19, 2020

Thanks for your responses.
It appears to be working fine for columns. When vibrated it's fluid enough. I've read that a w/c ratio of 0.35 is adeqate to hydrate the cement and I'm concerned that a higher ratio is more risky with regards to the strength. The surface is rough as the ACI note suggests but it is to be screeeded/rendered so it's not an issue.

darthsoilsguy2 · Feb 21, 2020

rule of thumb: never worry that a mix doesn't have enough water to hydrate. if the mix looks blended and doesn't have pockets of dry cement, it has enough water to hydrate. Same rule applies to pancake batter and its milk content.

r13 · Feb 21, 2020

Agree with darthsoilsguy2, most of mixing water is required for concern of workability.

STrctPono · Feb 25, 2020

Darthsoilsguy2 is correct. You can have below 0.35 w/c ratio and still hydrate the mix. Won't be workable at all but there is enough water to hydrate the mix.

With that being said, if your aggregates are not temperature/moisture controlled then they could be sucking up a lot of the free water causing your mix to be overly stiff. Are they shoveling the aggregate from large piles that are just sitting out baking in the sun? If your local aggregates are naturally porous, it will make matters worse.

Sounds like you are at about 740 lbs of cement per cubic yard and about 35 gallons of water. Normally, I like to be below 700 lbs of cement for most mixes, but allow some types of mixes to go over as extra paste is typically needed for things such as added fibers or pumping. Since it sounds like you are proportioning mixes by the old 1:1.5:3 methodology, I'm guessing you don't have access to plasticizers?

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