why does everyone hate post concrete (qwikcret and sakret)?

[FJ and G]

I frequently hear people saying to stay away from it b/c of the gypsum content and it leaching out over time, when using for bridges, viaducts, building and so on. But I've never seen a picture of this stuff falling apart (except on RR roadbeds where it's not reinforced with rebar).

Didn't the Romans use concrete and their stuff is still standing?

[tangerine-jack]

I use quick crete and similar brands all the time.  I normally keep two bags on hand.  The problem is not the concrete, but the consumer that does not use it properly, mixing it wrong or using it too thin for example.

The Roman engineer had an advantage with his concrete; he could have put to death a foreman that allowed a weak mix to be used on the job site.   Ancient quality control.

[FJ and G]

that's pretty funny about Roman quality control; but not humorous to the poor mason who is having a bad day!

I read somewhere that concret never really sets fully, but gets stronger and stronger with age.

 

Anyways, even if it did deteriorate some, it would add to the weathering effect 

[altterrain]

Dave,

 I use lots of the bagged stuff (70 - 60# bags on a job last week) and have never had a problem. On that same job we excavated the old sidewalk and discovered not one but three layers of concrete! The oldest was probably 100 years old and still pretty intact but odd looking. I also make alot of my own especially when it comes to mortar mixes because its a lot cheaper to do so. I do not think I would use the bagged concrete for casting garden railway bridges and such because the aggregate can be fairly large. Here's an interesting page on making your own -  http://www.pavingexpert.com/. Its british so the techniques are a bit different from what you normally see here in the US.

-Brian 

[cabbage]

David,

As a person who has to specify types of concrete for my designs -let me say something! The conventional "quick setting post concrete" -isn't concrete!!! I has plastcisores in it and more accurately reambles an epoxy glue than a concrete. Roman style Hydraulic Concrete does keep getting harder while Portland style reaches a peak over 8 days.

My computer designs sit on a floating bed of 20cm thick concrete -but this is a mixture of chopped fibreglass strands and spray mixed with the cement powder into a 10 "slump" concrete which is pumped into the mould (about 10m square). When set, the whole is far stronger than steel re-bar concrete and more importantly non magnetic!!! (It is also 25 times more expensive)...

regards

ralph

[tangerine-jack]

Cabbage brought up a good and valid point, that not all concrete is created equal.  There are as many mixes of concrete as there are recipeis for stew.

Even Roman concrete went through several mixes, a cheap kind for plebian public projects like apartments and daily use baths and other community buildings, then another for the "grand-I-love-me-because-I-can-afford-to" mix that was used for the super projects like aqueducts and ampitheaters that carried a name on it.  Both mixes in my opinion are very good as 2,000 years later most of it is still standing.  Even in those mixes there was a lot of variation as most ingredients for concrete were gathered localy, so some had more sand, others more rock, depending on the geography.  As far as the poor mason's foreman was concerned, he got paid very well for his efforts (provided the building stayed up) and I can assure you that he double and triple checked EVERYTHING before certifying it as good.

[FJ and G]

ok, so not all concrete is concrete; but I've never before seen a picture of a model building or bridge falling apart because they used post concrete. Some of this stuff has big aggregates in it but if you properly mix it in a mold and pour it in the right consistency, I notice that the sides can be really smooth.

[tangerine-jack]

You can also buy a finer grade of quickrete than the post hole mix, look around your local big box and you will see they sell 8 or more types of mix.  For a model I would go with the mix that has the smallest aggregate (normaly the most expensive).

Whichever you decide, make it correctly, I will be coming up your way fairly soon and If I see an inferior job....well, I am Italian by heritage and I do have sharp objects........

[ttrigg]

David:
Who ever told you to stay away from quickcrete/sackrete had a big burr under his saddle blanket.  I have used the stuff for years, the sidewalk all the way around my house and the lower patio are made from it.  I would almost say it is the best thing since someone invented the peanut butter and jelly sandwich!  I've also used it for the foundation of my mountain waterfall.

[altterrain]

Dave,

 I think you are referring to quick set type concrete? I have never used that stuff so I can't really comment on it specifically except at twice the price of normal high strength concrete, I never saw the need. I have set hundreds of posts with the normal stuff usually by setting them in with dry concrete and them watering it in. Never had one go askew.

 -Brian

[FJ and G]

thanks, guys

T-J,

 

You're invited any time

 

I made my bridge supports with quikqrete post concrete, the $3 bag of 80 pounds that sets up quickly and has huge aggregate. I used a drain pipe as a form. As you can see, it's smooth as a baby's as*.

 

 

Some deterioration over time would be welcome; rebar is in center and sticking out the bottom about 2 feet to anchor in ground 

[Ray Dunakin]

First off, I'm not an expert. But I have learned quite a bit since I started my In-ko-pah Railroad project.

"Quikrete" and "Sakrete" are not a type of concrete, they are brand names. Each of these brands sells several different types of premixed concrete, for different purposes and needs. I have not heard the complaints you mentioned; the only things I've heard are that some types of concrete shrink or expand when they cure. Also, supposedly some concrete can leach minerals into concrete-lined ponds and streams which may be harmful to fish. (I don't know if that's true or not.)

For a while I was using Quikrete brand vinyl concrete patcher for casting abutments and bridge piers. I've also been using it mized with mortar to cement rocks together, though lately I'm relying more on the mortar alone for that. I've used Quikrete 5000 high strength concrete for pouring slabs to cover my tunnels.

Currently I'm building a wooden form to cast a solid concrete, curved, multiple-arch bridge. I will probably use a 50/50 mix of morart and vinyl patcher. I don't want regular concrete which has a lot of chunky aggregate (rocks) because I'm afraid it might not fill the details and leave voids. I've heard of someone else using this 50/50 mix with good results on bridge piers.

If anyone out there IS a concrete expert, please let me know if you foresee any problems with using the mix I've described.

[cutlass12001]

Hello, Ray

I am currently also building a concrete form for a curved concrete bridge for my garden railroad.  I have 5 foot diameter curves, so this should be a challenge.  I will welcome any advice you could give