Abrasion Resistance 5

[dik]

I'm looking for information on the relationship between concrete strength and abrasin resistance. Does anyone have a link that has published information. I realise that concrete strength is only one of several factors affecting concrete abrasion resistance. The concrete I'm dealing with has about 25% flyash substitution.

A second question related to the first and more of a puzzle:

A large mass of concrete was placed and the specs required that a layer of sand/cement grout be placed between this and the next layer. The layer of grout was out of spec for strength by about 60% but no cylinders were taken. A second layer of mass concrete was placed over the cement/sand grout.

The grout layer was hydroblasted to remove it and in the process, the upper layer of new mass concrete was eroded by the water jet. Water pressure was 12,500 psi (common). The mass concrete on the lower layer was, with the exception of minor surface abrasion, unaffected. It is puzzling why the mass concrete over the grout joint was severely affected; under the same hydroblasting effort as the mass concrete below, the top mass concrete layer was gouged approximately 3/4" deep. There was a distinct change in colouration between the concrete matrix of the outside surface and the inner surface. The inner surface was a dark green-grey colour and the exterior surface was noticeably lighter in colour. Concrete specified was 35MPa with 25% flyash. Both layers of mass concrete were in excess of two months old. The problem appears to be extensive; an area 50' away had a similar erosion with a similar hydroblasting effort and both mass concrete layers had been cast approx 4 months ago.

Can anyone offer a suggestion about why there would be an apparent separation within the concrete mix?

Dik

 

[kslee1000]

Have you talked to people at PCA (Portland Cement Assoc.). You may find more helps there.

 

[dik]

I have a call in to Cement Canada (old Portland Cement)... and am awaiting a reply. I'm attaching a photograph to show the marked difference in surfaces. Both areas were subject to a similar hydroblasting effort.

Dik

 

[kslee1000]

I am not a material engineer, however, from the observation, it looks like the upper concrete layer has one, or both, of the problems below:

1. inadequate mixing that causes layering in the concrete mix (more fly ash in the bottom?)
2. over vibration during pouring that causing excessive sand layer gathered by the forms.

If not the case, take a few samples and analyze for mix compositions. The root of the problem could be in the mixing water, sand, or else.

 

[cvg]

I don't see abrasion of the aggregate, just the cement and fine aggregate mixture. So I would start by analyzing the mix design and actual mix provided for amount of cement and amount of water. It looks like maybe additional water was added at the site to increase workability. Maybe excess fly ash was added. It is also possible that the quality of the fly ash changed so that it threw off the W/C ratio and cure rate.

 

[dik]

The concrete was pumped in and there have been numerous problems with quality control on this project. When I first saw the effects of the hydroblasting my immediate thought was that there had been a separation of material. Also, the concrete matrix appeared to have too many fines and not enough coarse aggregate.

The problem appears to be parallel to the face of the wall, ie., the outside of the form. Where the hydroblasting was carried up the wall, the same condition occurred for a height in excess of 5', starting from 3' above the joint, so the condition exists for a height of at least 8'.

I'll be out at the site tomorrow AM when the hydroblaster will be present and I want to confirm the same effort as well as do a couple of vertical strips. I also want cores taken to do a micro study of the surface and of the interior of the concrete mass. I also want compression testing of other cores as well as have the surface tested for abrasion resistance.

Dik

 

[hokie66]

I would look at: 1) curing--if the lower form was left in place a lot longer than the upper, that would be a big issue. 2) compaction, too little rather than too much as someone else suggested. 3) w/c ratio. 4) The mix in general, maybe it was supposed to be the same as the bottom, but wasn't in fact.

 

[dik]

Improper curing was the first thing that came to mind, in particular with the 25% flyash and the 'soft' concrete being parallel to the formed surface. The area is vertical and exposed to daylight and wind and may have dessicated. I'll be getting samples this week for testing. The mix also appeared to be wrong... not enough aggregate.

Dik

 

[cvg]

with pumped concrete, a superplasticizer is nearly always required. if they couldn't get the dosage right, then they may have added water to the mix at the site so they could get it through the pumper - not a good idea.

 

[hokie66]

Just a comment, dik. You certainly come up with some interesting problems.

 

[Ron]

dik...I agree with hokie66!

The color difference you mentioned might be a key. If the concrete has 25% flyash, then there's a good chance that portions of the concrete are gaining strength at significantly slower rates. This happens with flyash and Ground Granulated Blast Furnace Slag Cement...strength gain can be as much as 50 percent slower, though the ultimate strength is not affected. As oxidation occurs, the colors moderate to a lighter gray color (from dark gray or blue-green).

 

[dik]

Ron:
Glad to see you back... I was thinking that the colour difference was due to either improper curing or some form of segregation with the curing being more likely. To confuse matters, the upper layer of concrete was cast on a sand-cement grout layer and I was thinking that this could have been displaced by the concrete mass above, forcing it to the side of the form... but not 8' high. The 'softness' extends up about 8' and the ocncrete was pumped. A superplasticiser was used, but, no record of water being added. Quality control of both the concrete supply and the contractor has been poor to terrible. Although they have a mix design, I'm not confident that this has been used in more than one place and just by accident (Bob Newhardt's skit about the monkeys and the typewriters comes to mind). Total concrete is about 21,000 cu.m.

Concrete cover is about 6".

I've put together a tech memorandum on the condition and in it I suggest (I have no control over the work) that samples be taken to determine the basic mix, strength and microscopic comparison of the two materials. At this point, I don't know if they are the same... appearance says otherwise. I'm also a little concerned about the apparent lack of coarse aggregate. For the mass concrete involved, I'd have used 1-1/2, 2 or even 3" aggregate. I was not involved in either the design or spec writing.

 

[Ron]

dik...I agree with your testing protocol. That should give some answers. One other thing to look at...is there a chance that the sand-cement grout became contaminated with form oil, thus preventing its proper setting, followed by the co-mingling/displacement you suspect?

 

[dik]

Although I suspect the cause, I don't know why it's happened and the testing, in particular the microscopic testing, may provide info to further the review. Some people want to have the concrete removed to beyond the rfg and a patch installed and I'm not sure this will stay. If the concrete will eventually achieve strength and hardness (in particular hardness) then I'd just as soon leave it and have the Contractor provide curing for the existing exposed surfaces.

In my tech memo, I've raised the question about form release agents, thanks for the supplemental info to the forum though. If it weren't for a client, I'd have posted it for general info.

I use eng-tips for learning about stuff that I wouldn't encounter elsewhere and the info provided by members is invaluable at times. One quick and extensive reference book.

Dik

 

[nuche1973]

dik,
Looking at your photo, two questions come to mind:
1. When was the fly ash added?
2. Was the aggregate wet when added to the mix?

I ask the first question because the photo shows the flyash at the bottom of the mix. This suggests that the flyash was not throughly mixed into the concrete. Secondly, the large aggregate is clean, indicating that there was little to no bonding between the cement and the aggregate.

Most release agents are water based and any issues would be on the surface of the conrete and not to the depths shown in your picutre. It is true that petroleum base agents stain the concrete, but that's also on the surface. The presence of "bug-holes" would be an indication if the contractor used a petroleum based release agent. I agree with cvg, the w/c ratio is wrong.

 

[civeng80]

dik

Regarding abrasion resistance and concrete strength, curing is important and not many contractors do it properly, but in my opinion (and Ive been in charge of many concrete pours in my lifetime as owner/builder/designer) the most important factor is finishing. The concrete must be finished to a smooth and hard surface using a trowling machine. The harder the concrete is when applying the final trowling the better.

 

[dik]

CCB1:
I understand the flyash was added at the time of the initial batching. I don't know the degree of saturation of the aggregate, and from general quality control matters on the project, it's possible that the supplier doesn't know either.

The surface at the joint was to have a grout trowelled in prior to placing the upper level. For this to happen, the workers would have to have the grout placed after the formwork was erected and the workers would have to work within the form. I'm not too sure of the clearances, but with the size of the pour this is possible.

One explanation for the difference in material is the sand-cement grout was placed and then the concrete was dumped on top, forcing the material up the face of the wall.

There is serious reluctance in doing microscopic studies and the people in charge of the project are looking at doing some compressive testing of the interior... which will not tell me much... They are also looking at removing the soft material and using a 'patch'.

civeng80:
As far as curing, I understand that there was none and that the forms were loosened after the first day and that they were stripped off a day or so later.

I may not be able to obtain the info that I've requested and the cause may not be determined.

Dik

 

[nuche1973]

dik,
That's too bad on not getting the information you need. After 7+ years in the concrete industry, I am still amazed how some contractors treat concrete. What is the thickness of this pour? Where are your test cores going to be taken from? It will be interesting to see where your cores will break.

 

[KCRatnayake]

dik,
As far as I know abrasion resistance varies with regions or countries. Grade of concrete has very little effect on abrasion resistance. I.e. same grade of concrete in different countries have different abration resistance. It's abration resistance of agrregate which is decisive.

People involve in diamond cutting industry are experts in this. If you have some contacts with Hilti guys, you should be able to get some good information.

 

[cvg]

clefcon - actually, strength of concrete definitely does have an impact on abrasion resistance of a concrete mixture - up to a point. A weak cement matrix will allow individual rocks, pebbles and sand grains to be "plucked" out of the cement matrix by flowing water. As the cement matrix gets stronger, it provides a stronger bond with the aggregate which reduces the "plucking" effect. At some point the bond becomes strong enough that aggregate cannot be plucked at all from the matrix. Beyond this point, the resistance to abrasion is more directly related to the aggregate qualities such as strength and hardness. For "very high" strength concrete mixes such as silica fume, the cement paste mixture can reach compressive strengths higher than the aggregate and so it again plays a greater role in the abrasion resistance than the aggregate quality.

See attached paper which illustrates some of the research into a variety of concrete mixtures for abrasion.