Concrete question

Sam Salvati

Sam Salvati

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Aug 6, 2007
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So, I cast the concrete for my PH base, and everyone keeps telling me "let it cure let it cure! Let it cure for atleast 20-30 days!". I have to wonder, in this situation of the concrete being cast inside a steel box, exactly what problems might I encounter if I set the hammer on and ran it in only say, 5 days? Unless I misunderstand, which is very likely as this is only the first time I have ever worked with concrete, there shouldn't be a problem as it is cast into the box? I would understand if it was a slab or sidewalk not enclosed on 5 sides, it might be likely to crack like a cracker if pressure is applied on top but this is held tight in the box like a muffin in a pan. What problems could I encounter? The block itself is already solid, and lack of heat on the outside means the chemical reaction has stopped (?). I cast it all at once on Saturday, so now it has been 4 days, AND NO, I am not just being impatient :D I just would like to know more about concrete and why everyone says something when logic seems to allude to something else, which in knifemaking and I think everything else doesn't make it true.
 
got any re-enforcing rod ( rebar ) in it ??? or at least concrete wire ?? with low humidity and summer temps i would think 48 hrs would suffice. paul
 
Pure concrete will cure 50 % in 7 days, 75 % in 14 days and 100 % in 28 days. However most bag concrete and some truck concrete have accelerants which cure much faster.
For hammering I'd let it go full cure. Ask if it has an accelerant and if so when is full cure.
 
Listen to Mete. If you set the hammer on it and trap in moisture, it will only take it longer to cure. The longer you can let it cure, the stronger it will be, and the longer it will take to crack. I spent 8 years in the military making and pouring concrete for ever conceivable purpose.....the quickest way to destroy concrete is to not let it cure before use.
 
Sam, Spray some water on it and cover it with some plastic...let it cure some more, especialy if your gonna drill into it and set anchors.
They say that some of the concrete in the Hoover dam is still curing!
Mace
 
Thanks guys, I am just trying to get more info and insight into WHY people say wait 20-30 days.

Sidecutter, there is little bonding fibers in the concrete, but no rebar or wire or anything.

Robert, not sure if there is an accelerant I will have to look on the bag.

Ed, what do you mean by destroy? Will it crack, crumble, what will happen to it?

Mace, I won't be setting in anchors (they were welded to the box and run up through the concrete from the bottom plate). I'll do the water spray thing too, not the first time I have heard that, I asked the concrete guys I sell to at work and they said the same thing. That's awesome about the hoover dam!

And so you guys know, I am going to let it cure for a whole month.
 
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mete is correct about the 100% strength in 28 days figure, however that is for testing and building purposes only. Even with accelerants added to the mix those only affect the set time and only affect the strength in a negative way, the 28 day figure still applies simply because its an industry standard test. The biggest factors for strength and longevity are your water to cement ratio (the water you add upon initial mixing doesn't evaporate it creates a chemical reaction with the cement and bonds together permanently, the better you cure the concrete the better the bond between the water,cement,and aggregate), your placement/finishing (improper placement and finishing alone can cause major damage to concrete), and your curing. If you poured a wet high slump mix without any special admixtures you will end up with a weak slab that will probably crack within for the first year or so, depending on your climate. If you poured anything from a 5" slump, think thick chili, down to free standing curb mix concrete (unless you poured it dry because it started to set to quick), you will have the greatest possibility to have very strong and durable concrete. The placement and finishing is your next biggest concern, if its inside use, non air entrained concrete with a good amount of reinforcement, it should have been poured and vibrated to remove as much air as possible, then after you've put your desired finish, whether brushed or smooth, and the concrete is no longer plastic, it is best to keep the set concrete wet. The concrete will continue to gain strength as long as it is kept wet, even well after 28 days if you'd like to wait that long. The problems you could possibly encounter by even setting any equipment on the concrete before its cured, are not only that it could crack, but completely crumble. It especially depends on what kind of mix you poured and whether you got it from a redi mix company or mixed it yourself with bag mix.
 
Just fyi, there are an infinite combination of materials in concrete which can result in an infinite range of mechanical properties of concrete at both early and late stages. For example, a type III concrete might be fine after only a day or so, but if you're using an ultra-high strength concrete you will need up to a month till it's recommended for use, and years before it really gains full strength.

Edit: Type III cement in the concrete, not type III concrete.
 
Mace said:
Sam, Spray some water on it and cover it with some plastic...let it cure some more, especialy if your gonna drill into it and set anchors.
They say that some of the concrete in the Hoover dam is still curing!
Mace
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Yep, do that! Don't let it dry quickly. If it has fiberglass fibers in it, it should cure up nice and strong.
 
I will have to look again what premix I used, It said it was suitable for slabs and foundations over 2", I will look tomorrow.

Thanks for the info guys!
 
Sam Salvati said:
I will have to look again what premix I used, It said it was suitable for slabs and foundations over 2", I will look tomorrow.

Thanks for the info guys!
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Call the manufacturer directly... ask for tech. and state your use. They should have an answer for you in about 2 seconds.

Mike
 
Concrete reaches it's full strength in 28 days. The damper you can keep it the better. This could mean just hosing it down and covering with plastic ... obviously, this can't be done with everything.
 
Sam ,I'm only 5 miles from you , you should have asked me before the fact !! I can smell the typical accelerant. Fibers do nothing for strength ,only prevent shrinkage cracks .I would have used rebar for that application. Considering the size covering with plastic would probably be enough. Concrete cures by chemical reaction NOT by evaporation and if the water evaporates you get poor strength.
 
Sam, it is a moot point now but just for my curiosity what size is your hammer and how big is your foundation? Re-rod is a VERY good idea for PH foundations. Curing time will be very important as well, think about how a brief time of patience can save you from working forever on a lousy foundation (it is a huge block of concrete, it is not like you can easily scoop the bad one out and start over :(). I have worked on hammers all around the country and you can immediately tell the ones that have a good foundation from the ones that don't. When you walk into a shop where one is being used you can literally feel the difference in your feet. I have seen some with so poor a foundation that the floor around it suffered and was cracking up. Take your anvil off of its stand and place it on a 3/4" board supported only on either end and see how much metal you can move, then put it on a 2 foot solid block of seasoned oak and see how much metal you can move. The same principle applies to power hammers, particularly the big ones that have a separate sow block.

You can't go too big on the foundation and I have seen some serious work done with 50lb to 100lb hammers that had a yard or more of concrete directly under the dies. But I have worked on hammers that I knew were only hitting at a fraction of their potential due to an insufficient foundation, particularly when I could see the sow block jumping with every blow, when several inches of soft pine were used as a substitute for concrete. Which brings me to my final advice, which you probably already know, a thin layer of wood will prevent the pulverization of even well cured concrete under the hammer, just don't get carried away like many do, all you need is a couple inches of a good tough (not soft) material to act as a buffer. Concrete beats just about everything in your shop for compressive strength, but it is pathetically weak in impact toughness, give it a thin protection from impact and it will handle all the compression you can give it.
 
Robert, it was my mistake again thinking "it's concrete, how hard could it be?". The directions said simply to mix and pour and avoid a soupy mixture.

Kevin, it is an Anyang 33 pound. The hammer weight is around 650 pounds, and the base it sets on is a 1/4" steel box and it's 14" wide, 20" long, 21.5" tall filled with unreinforced concrete separate from the floor itself. I will have to use a thin layer of sand because the concrete shrunk about 1/4" from the top of the box. There will be a 3/4 plywood buffer between the top of the concrete and the hammer itself like you said.
 
Just add water and mix. Sam, The dryer you mix the concrete the stronger it will be. Is the 650 pounds total weight for the hammer? I thought all of New York State was solid rock anyway. I guess it all depends on how long you can go without scratching your itch.
 
Sam Salvati said:
Robert, it was my mistake again thinking "it's concrete, how hard could it be?".
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I can't help but laugh, because I hear this at least a few times a week. I have delivered concrete to just about every major interstate, highway, or bridge in southwestern Ohio that has been built in the past 10 years, and continue to on a daily basis. I am ACI and ORMCA certified. I am also based out of the only plant allowed to deliver inside AK Steel, yet at least once a week I have to deliver to a house with a man and his son, armed only with a wheelbarrow, a shovel, and a garden rake to pour on average a 6 yard patio (1 yard of concrete=2000lbs). I can immediately tell they have no idea what they are in for, before I unload anything from my truck I give them concrete safety 101, which is very simple, protect yourself, and don't get it on you! it can burn you. They usually then ask how far back in their yard can I go with my truck, and I explain that my truck weighs in well over 40,000lbs even though its only carrying a half load, and would undoubtedly destroy their driveway and yard, its right about that point when they realize they bit off more than they can chew. :D

I have seen hundreds of chemical and material concoctions added to concrete for any number of purposes, from severe heat resistance, to incredible compressive strength, and even nearly indestructible impact strength. For instance did you know adding sugar to plastic concrete can keep it plastic indefinitely. Calcium Chloride is the common accelerant in this area, and really shouldn't be used over 60°F and is difficult to use over 40°F. Rebar is a must in a block like that, fiberglass fibers work fine in sidewalks but they aren't designed for your application. If you used a standard slab/foundation bag mix it is most likely in the 1500-2500psi compressive strength range, so as thick as your slab is even without reinforcement it probably won't crumble, but it will most likely crack, if you put a good solid grade down before pouring that will help tremendously too.
 
Sam, my amateur radio antenna mast is about the same weight as your hammer. It's supported by 2 yds of reinforced concrete !! That's a static load for the most part but yours is a dynamic load and must support a huge number of poundings.
 
Concrete hardens by hydration (the absorption of water into the chemical structure) not by drying out. To make tough concrete you have to keep it moist, but you don't want excess water thinning it out. The full process takes weeks, not hours or a few days.
 
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