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Old 08-18-2008, 08:07 PM   #1
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Best Grade of Concrete Mix


Hello,
Two years ago, I had my pavement replaced. The contractor gave me crushed stone mix. Since then, I have had spalling and its still continuing. I filled all the spalled areas with fast acting cement and plan to put a sealer down which I should have done after it was replaced.
When the pavements on my block were first put in about 55 years ago, the contractors used a gravel mix. In all other old neighborhoods, gravel mix was also used. I checked some of the neighbors pavements and none show spalling after all this time; but most of them are showing the gravel at the surface due to wear and tear which is expected. The gravel mix on my neighbors properties appears to be about 1/4 to 3/4 diameter.
My questions are:
Do you think putting down a sealer will help and keep the concrete from spalling easily.
Which is the superior mix, crushed stone or gravel for pavements, parking areas and driveways?


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Old 08-18-2008, 09:15 PM   #2
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Best Grade of Concrete Mix


The important thing is the proper mix design (strength), air entrained concrete where you have freezing and a minimum of water in the mix.

Where I am from, the best mix is 4000-4500 psi, 5% air and 3-4" slump. Some suppliers with refuse to sell anything less if the know it is for a driveway. If the contractor adds water, it is noted on the delivery slip and guarantees are voided.

If a contractor cannot handle the 3-4" slump without adding water, he has no business doing driveways.

Depending on the aggregates available locally it may be immaterial if the rock is crushed or not. Any crushed fines will increase the amount of water needed for workability, which increases the shrinkage(cracks) and lowers the strength.

If you have spalling, you either had poor finishing, not enough air, too much water or all of the above.

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Old 08-18-2008, 10:19 PM   #3
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Best Grade of Concrete Mix


Very true. Your concrete woes are not the fault of the concrete. It is the way the concrete was installed. There are so many variables that can screw up concrete. The mix design (from a batch plant) typically is not the issue.
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Old 08-18-2008, 11:17 PM   #4
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Best Grade of Concrete Mix


The mix was poured without adding water. The mixing distributor may have added too much water at the plant. What is a 3 to 4 inch slump? This is the first time I heard that word used in relation to concrete work.
During the coldest part of winter, overnight temperatures average 15 to 20 degrees in Philadelphia, Pa. It was about 90 degrees, when I made repairs today and before I applied the cement, I wet down the damaged spots and I noticed that the concrete seem to absorb the water. What does that indicate about the mix?
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Old 08-19-2008, 07:09 AM   #5
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Best Grade of Concrete Mix


As a rule, crushed stone is superior to gravel, since it is sharp as opposed to smooth. However, it also depends upon the composition of the aggregate itself.
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Old 08-19-2008, 07:25 AM   #6
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Best Grade of Concrete Mix


Slump - http://en.wikipedia.org/wiki/Concrete_Slump_Test
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Old 08-19-2008, 07:28 AM   #7
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The fact that your concrete absorbed water indicates nothing really as all non-sealed concrete does that. Concrete is porous and absorption is independent of type of mix. It will absorb water because of physical forces and will turn a darker colour. But then just as easily, it releases it if it's warm or windy enough outside to do so, or it'll freeze - maybe expand - if it's winter...then you have tiny cracks and the start of larger problems.

IMO concrete driveways should be sealed with a penetrating sealer to minimize the absorption of water...but hey that's just me!
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Old 08-19-2008, 07:45 AM   #8
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Best Grade of Concrete Mix


When concrete absorbs water and that water subsequently freezes, the water expands. That can cause spalling, cracking, and damage in general. Expanding water (ice) is an incredibly powerful force of nature.

It indicates nothing about the mix.

Slump is the stiffness of the mix...Essentially a factor of water content.

The flatworker could have added too much water, failed to cover it on a windy day (causing evaporation of the water in the top of the slab), overworked the surface (bringing too much water to the top which makes the top weak), poured when too hot, poured when too cold, failed to use a curing agent if he needed to, or a number of other things.

99.999% of the time when this happens, the aggregate and mix design was not the problem.
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Old 08-19-2008, 08:03 AM   #9
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Thanks All,
This Slump Test must be done at the mixing plant either manually or with machinary because I never saw contractors who do residential concrete work perform this test.
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Old 08-19-2008, 08:08 AM   #10
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And I will make sure I get a contractor with good credentials the next time I need work done. The person that did my job did not mention anything about sealing the concrete.

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