Posted: 6/30/2009 10:07:46 AM EDT
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Recently I've used some of the standard yellow bag Quikrete mix and have a question. I've mixed concrete in a pan before, and thought that it sucked so bad that I'd never do it again. Thus, I got one of those $109 Harbor Freight 1.5 cbft mixers (My back is worth much more than that). Using the correct amount of water, the concrete appears too stiff. So, I added up to the amount the bag describes as 'maximum', and the mix didn't really change much.
It does not flow well at all. I was just setting posts, so I just tamped it with a 2*4 after every few shovels I put it. I expected it to be self leveling to a degree, but it wasn't about to happen. I cannot see it working well in forms as it is without leaving tons of voids. Stabbing it with a piece of re-bar (to settle it) would leave a hole that wouldn't fill back in much on its own. I know about vibrating it, but I though that technique wasn't recommended because it could cause the gravel to settle to the bottom too much. With this mix, I would be afraid to use a form tube because who the hell knows what type of swiss cheese the cured concrete would look like when you cut it off... What gives, is there a technique, or a different mix used in forming things like curbs... |
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the main culprit is probably hot weather.
if you are not concerned about strength, you can try this trick. in our concrete lab we added some dish soap to improve workability, but it was in low water mixes. sometimes the mix would run out of the slump cone it was so liquid. i would experiment in some small batches to verify the consistency. then scale up to a bag, once you are satisfied. |
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The water to cement ration determines the strength and workability of concrete. IIRC it can range from .25 to .7. The higher the ratio, the more workable it is. However, strength decreases, due to excess water creating voids. You will also run into mix segregation problems with too high of w/c ratio. Most concrete is kept between around a .35-.4 ratio. Admixtures known as superplasticizers can be added that increase workability (ie simulate high w/c ratio) without decreasing strength. You could try mixing early in the morning or later at night to help decrease your water loss. Lots of paving jobs in hot regions are done at night to avoid high temps. |
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Quoted: Ok, Thanks... It was less than 5 minutes between the mixing and the shoveling it into the hole. Are you saying I lost water from eveporation in that period of time? Hmm, how carefully did you measure the water? They usually put a starting amount of water and then you can add a little more if you need extra workability. Was it just standard quikrete, or was it fast setting? Either way its best to lay concrete when its cool because the curing process creates a lot of heat, which combined with high ambient air temp can create some problems. |
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Quoted:
Quoted:
Ok, Thanks... It was less than 5 minutes between the mixing and the shoveling it into the hole. Are you saying I lost water from eveporation in that period of time? Hmm, how carefully did you measure the water? They usually put a starting amount of water and then you can add a little more if you need extra workability. Was it just standard quikrete, or was it fast setting? Either way its best to lay concrete when its cool because the curing process creates a lot of heat, which combined with high ambient air temp can create some problems. Very carefully, I was actually really suprised how little water the instructions called for... 3 quarts for an 80 pound bag (4 max). It was warm, but not blazing. Probaly about 85 degrees... |
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disclaimer: I'm not a concrete expert and I do not work in concrete or construction.
Really depends on what you are using it for. Setting posts, I just dry-pack. If you are pouring something that needs to be strong, like a foundation or a garage floor, (well, my first choice would be to buy it from a conc co) it is completely imperative to determine the "slump" as mentioned. That is the best way to make sure it is properly mixed. From what I've seen, concrete is supposed to be stiff, as you describe. If it "self levels" it is too wet. The best way to get it to run out (again, from what I've seen) is to use a concrete vibrator. If you need high strength concrete, follow the mixing directions. Plus... be particularly anal about following the curing procedures. Curing is where it gets its strength. (Concrete ppl... please correct me where I may be mistaken. :-) ) -3D |
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Quoted:
disclaimer: I'm not a concrete expert and I do not work in concrete or construction. Really depends on what you are using it for. Setting posts, I just dry-pack. If you are pouring something that needs to be strong, like a foundation or a garage floor, (well, my first choice would be to buy it from a conc co) it is completely imperative to determine the "slump" as mentioned. That is the best way to make sure it is properly mixed. From what I've seen, concrete is supposed to be stiff, as you describe. If it "self levels" it is too wet. The best way to get it to run out (again, from what I've seen) is to use a concrete vibrator. If you need high strength concrete, follow the mixing directions. Plus... be particularly anal about following the curing procedures. Curing is where it gets its strength. (Concrete ppl... please correct me where I may be mistaken. :-) ) -3D This is pretty good general advice. If you just putting in a footer for a deck its not really a big deal. Use more water if you need to. If the post is supporting a roof its a bigger deal. In which case you should probably just get concrete premixed from a concrete co. One thing I think is a mistake many people do when installing a post on a footer is to sink the wood post into the concrete. This will rot out the bottom of the post a lot faster than if you use a $10-20 metal bracket. Also it will allow you to replace the post a lot easier if it gets damaged. I would bolt the bracket to the concrete footer and then put a couple of screws in the post to attach it to the bracket. |
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For setting posts I throw some gravel in the bottom of the hole, set the post, throw in the concrete mix dry and add water via a water hose, then mix with shovel. It's just fence posts. If you are using for more sturdier projects you may have to mix for 10-15 minutes or more to get the water thouroughly mixed into the concrete, while slowly adding more water into the mix. It shouldn't flow out of the mixer but instead move out of the mixer with help from a shovel. |
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Quoted: For setting posts I throw some gravel in the bottom of the hole, set the post, throw in the concrete mix dry and add water via a water hose, then mix with shovel. It's just fence posts. If you are using for more sturdier projects you may have to mix for 10-15 minutes or more to get the water thouroughly mixed into the concrete, while slowly adding more water into the mix. It shouldn't flow out of the mixer but instead move out of the mixer with help from a shovel. Yep |
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When you do the dry mix. i.e. dump in the dry mix and pour water in on it. Does the concrete get wet all the way to the bottom? I know it's a dumb question but I've always had my doubts. Who cares, I set posts for 10x10 or smaller decks the same way. Dry mix is fine for no to light load bearing post. If you just have to wet set, then use more water, it isnt that structural, it is just holding the post in the ground. Hell if you really wanna be anal, drive piece of rebar thru the side of post, about 6-8 inches from bottom, both directions. Fill bottom of hole with a couple of inches of gravel for drainage also. But that is only if you wanna be ANAL! Travis |
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Quoted:
When you do the dry mix. i.e. dump in the dry mix and pour water in on it. Does the concrete get wet all the way to the bottom? I know it's a dumb question but I've always had my doubts. I am under the impression that the moisture and ground water in the soil is sufficient to cure the dry mix at the bottom of the hole. By wetting the top, you are basically just holding the post in place as the stuff below slowly sucks water out of the soil. I deffer to the experts for a better answer though. -3D |
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Quoted:
Quoted:
When you do the dry mix. i.e. dump in the dry mix and pour water in on it. Does the concrete get wet all the way to the bottom? I know it's a dumb question but I've always had my doubts. I am under the impression that the moisture and ground water in the soil is sufficient to cure the dry mix at the bottom of the hole. By wetting the top, you are basically just holding the post in place as the stuff below slowly sucks water out of the soil. I deffer to the experts for a better answer though. -3D You should mix the water a little better. Although it probably wouldn't matter for the type of construction or purpose we are talking about........ If the top portion of sakcrete sets up and the bottom portion doesn't set up for a couple of days or less you will have a cold joint between the two. So it will be a little weaker. probably not enough to worry about. Just use a shovel and mix it a little more. |
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Quoted:
Quoted:
Quoted:
When you do the dry mix. i.e. dump in the dry mix and pour water in on it. Does the concrete get wet all the way to the bottom? I know it's a dumb question but I've always had my doubts. I am under the impression that the moisture and ground water in the soil is sufficient to cure the dry mix at the bottom of the hole. By wetting the top, you are basically just holding the post in place as the stuff below slowly sucks water out of the soil. I deffer to the experts for a better answer though. -3D You should mix the water a little better. Although it probably wouldn't matter for the type of construction or purpose we are talking about........ If the top portion of sakcrete sets up and the bottom portion doesn't set up for a couple of days or less you will have a cold joint between the two. So it will be a little weaker. probably not enough to worry about. Just use a shovel and mix it a little more. Hardly matters on a post with minimal loading. All you are trying to do is increase the area at the base of the post for better side loading resistance, and the increase in area for some vertical loading (for decks). |