Reducing Concrete Strength

[Zambo]

I am in a remote location using 20MPa concrete (about 3,000 PSI, I think!)

This concrete mix design used to show about 70% strength at 7 days and gain strength normally until the 28 day cubes were crushed. Now this has become random with either a few % rise in strength or sometimes a small strength reduction between 7 and 28 days.

Any explanations apart from dodgy crushing machine?

 

[asixth]

Did the concrete achieve the specified design strength at 28 days. The strength achieved at 28 days affects the durability as well. Typically the higher grade the concrete the harder the surface and more abrasive resistant.

 

[Ron]

Has the mix designed changed (fly ash addition, ground blast furnace slag cement, etc)? If not, has the cement supplier changed. Could just be a coarser grind of the cement, assuming the strength ultimately comes up.

It was not clear in your post if the strength gain curve has changed and the ultimate has not, or if the ultimate strength has been affected as well. Please clarify.

 

[Zambo]

Nothing has changed with the mix design (although there could of course be some spec change in the contents such as aggregate source or something in the cement grinding). But the point is that the 7 day and 28 day cubes are from the same source.

Yes the strength gain has changed from normal i.e about 70% at 7 days and 100%+ at 28 days to very flat. This means either very little gain of sometimes a strength loss between 7 and 28 days.

I have never in 25 years seen a strength loss so can only assume the cube crushing machine is at fault. Have you ever seen such a phenomena caused by the contents of the mix?

BTW the concrete almost achieves the 28 day strength at 7 days and then generally shows little increase (or as stated slight loss) at 28 days. So there are some close failures and some very marginal results.

 

[BAretired]

What size cubes are you using for the test? How are they cured?

BA

 

[Ron]

Zambo...the only time I've actually seen a reduction in strength at 28 days compared to the 7 days was a case where a reactive aggregate was involved. It caused microcracking in the concrete and reduced the strength.

Certainly take a look at the testing procedure and equipment. Could be something simple like a bad load cell or gage on the equipment. Get them to recalibrate if possible.

Not sure where you are located, but curing room conditions can also cause the strength gain curve to flatten. Particularly if the curing room temperature drops lower than about 22C.

 

[concretemasonry]

Zambo -

Your curing room temperature could be going up a bit at this time of the year (especially if the curing room was being filled with a lot of fresh, hot sample) and the outside temperatures could be going up.

Aggregates may change seasonally where water for washing is limited but increased availability can was out some of the fines that hold back the strength.

Also look at who the cement supplier actually is. Even with the same cement chemistry, freshly ground cement can be "hot" and more reactive. Cement is not always ground when it is fired into a clinker for inventory purposes.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[apsix]

The mix DESIGN might not have changed, but what control is there on the actual proportions used?
If you are relying on a labourer just shovelling in the correct amount, the resulting concrete is bound to be variable.

 

[Zambo]

Thanks for all the input - some interesting suggestions.

What's impotant is that the 7 and 28 day cubes are from the same barches, so the mix design, source of cement etc seems to be ruled out.

Apart from the crushing machine the other factors which seems relevant are the "reactive aggregate" suggested by Ron and the possibility in a change in curing arrangment at a time between the 7 and 28 day periods.

 

[concretemasonry]

Zambo -

If the concrete is from the same batch (other deliveries on the same day are not always the same), the site samples were probably made by the same technician and handled similarly before getting to the lab curing facility. A technician can easily increase or decrease the strength of a prism even withing the general site casting parameters, which I learned from a professor while in college and as a concrete technician. Hnadling of samples can cause erratic results, especially with a limited number of tests.

Take a good look at the testing machine and the procedures. An improperly capped (different technician) or placement on the platen (especially on an older machine) can give some erratic results - unparallel surfaces or eccentric loads especially.

At least they are apparently not low, but the cause is always troubling. There may be a combination of factors that are acting together.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.