huangyhg

abrasion resistance
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
check out our whitepaper library.
have you talked to people at pca (portland cement assoc.). you may find more helps there.
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
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.
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.
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
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.
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
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.
just a comment, dik. you certainly come up with some interesting problems.
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).

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