A question for you concrete experts....

[Angus Cowman]

I agree with Dun and AG. AG, I wonder if the mesh wire works as good as rebar in climates with not as much frost as we have. If it isn't heaving on the concrete all winter as you feed, it may be just as good. Just wondering, had any experence with it? gs

Too many so-called concrete folks walk the wire down and dump the concrete on top. Doesn't take a lot of effort to put small concrete blocks under it and keep it inside the concrete. But rebar never hurts it for sure.

the weight of the concrete being poured will mash the wire to the bottom and when it is pulled back up half of the time it wants to come to the top and then it leaves a weak spot and will cause the concrete to chip and leave the wire exposed
if using wire the best is to use wire mats which are heavier gauge wire (almost twice as heavy) than the rolled mesh and install it on chairs of concrete bricks
I still prefer rebar over the wire as it is easier to work with and adds more structural support than the wire

 

[Angus Cowman]

Recommendation... get yourself a good solid section composed of 8 to 10 inches of gravel, 6 to 8 inches of concrete.

Set yourself up a way to keep it at the same elevation... do you have a level?

Your comment about the 8 to 10 inches of gravel is interesting....is that an idea for giving a more solid foundation for the slab or some method for water filtration beneath? As for the comment about keeping it at the same elevation, I´m not sure I understand.

I was under the impression that it would be best to have some slope in order to move water away from the site as quickly as possible. I´m even putting a slight hump down the middle so the water should run towards either edge and then out the far end. Again, this is a big trench-type silo and plenty of rainfall will be pelting it from time to time.

Finally, I´m thinking that a good footing will be important as I plan to use my front end loader right up against the walls as I remove material and clean the thing in anticipation of the next filling.

actually 3-4" of sand will do as much or better than the gravel
the gravel is placed so the ground can move under the concrete and act as a cushion so it doesn't Heave the concrete up
in most areas gravel is used because it is cheaper than sand
in some applications we HAD to use sand especially if we were close to solid rock or if the concrete was going to have uneven weight loads placed on it
such as being empty part of the time with no load and then being filled with a heavy load
if the sand is available I would use it you just need to water and pack the sand before placing the concrete on it
and also you will need to wet the sand pretty well right before you pour concrete on it so it doesn't suck the moisture away from the cocrete and make it harden to fast
some people will but plastic on top of the sand and under the concrete but I don't like doing it that away and we were never allowed to do it that way on the wtaer and sewer treatment facilities that I built

 

[dun]

I agree with Dun and AG. AG, I wonder if the mesh wire works as good as rebar in climates with not as much frost as we have. If it isn't heaving on the concrete all winter as you feed, it may be just as good. Just wondering, had any experence with it? gs

Too many so-called concrete folks walk the wire down and dump the concrete on top. Doesn't take a lot of effort to put small concrete blocks under it and keep it inside the concrete. But rebar never hurts it for sure.

the weight of the concrete being poured will mash the wire to the bottom and when it is pulled back up half of the time it wants to come to the top and then it leaves a weak spot and will cause the concrete to chip and leave the wire exposed
if using wire the best is to use wire mats which are heavier gauge wire (almost twice as heavy) than the rolled mesh and install it on chairs of concrete bricks
I still prefer rebar over the wire as it is easier to work with and adds more structural support than the wire

I guess that's where I do it wrong. I pour to half the depth then lay the wire or rebar on it then pour the upper half

 

[Angus Cowman]

I guess that's where I do it wrong. I pour to half the depth then lay the wire or rebar on it then pour the upper half

that will work if you are pouring small sections for yourself but try doing that on a large pour and you will have a cold joint in your concrete right in the middle
you also can't do that on an inspected project also because the steel has to be inspected prior to the concrete being poured and it also has to be tied together and continous

 

[dun]

I guess that's where I do it wrong. I pour to half the depth then lay the wire or rebar on it then pour the upper half

that will work if you are pouring small sections for yourself but try doing that on a large pour and you will have a cold joint in your concrete right in the middle
you also can't do that on an inspected project also because the steel has to be inspected prior to the concrete being poured and it also has to be tied together and continous

Good point but I don;t pour huge areas. If it's that big I hire it done. We're lucky to have 2-3 really good concrete guys as neighbors, sadly one of their kids is the crappiest concrete guy around.

 

[upfrombottom]

As has been stated, concrete can't withstand the stress of tension, that is the reason for steel in the concrete. But contrary to what most believe, the greatest stress on concrete is at the surface whether bottom or top. The problem with steel is its deterioration from oxidation and it must have a good covering of concrete to protect it. The horizontal center of a slab is the most neutral part of that slab and ideally the steel should be placed toward the edge where the greatest tension occurs i.e. the top and bottom. The best solution for surface tension is fibermesh. Its noncorrosive nature allows its placement where it is needed most.

 

[novatech]

As has been stated, concrete can't withstand the stress of tension, that is the reason for steel in the concrete. But contrary to what most believe, the greatest stress on concrete is at the surface whether bottom or top. The problem with steel is its deterioration from oxidation and it must have a good covering of concrete to protect it. The horizontal center of a slab is the most neutral part of that slab and ideally the steel should be placed toward the edge where the greatest tension occurs i.e. the top and bottom. The best solution for surface tension is fibermesh. Its noncorrosive nature allows its placement where it is needed most.

I agree with everything but the fibermesh. On the right soil it works fine. ( the standard for steel coverage is 1 ") On expansive clays I have seen to many structural cracks to have confidence in it. I also disagree to your recommendation of using sand. On a flat surface with good drainage it is great but in this case he said that he was in a sandy soil on a slope. Sand will flow in water. Sand will blow in wind. When compacted with the right moisture content it will be like rock. I will also disagree with the suggestion of not using plastic. In utility work or most anything else outside moisture is not a problem. In a silo it is a different situation. When concrete is placed it contains water. Therefore when the water evaporates there is shrinkage. Depending on how it is cured noticeable cracks may or may not appear. Even if you cannot see them they are present. This makes concrete somewhat porous. The exact mix will also make a difference. Plastic is just cheap insurance.
As far as thickness is concerned, If you have ever parked in a modern parking garage it was probable built wit precast panels that are hollow and only about 4" thick. The exterior and interior beams are what give the main structural strength. A footing is used on soils with low compressive strength to spread the load.
Soil compaction will be highly dependent on the make up of the soil and the amount of water in it. Sand gravel and red clay sound like very stable, none or low expansive soil.
A gasoline driven plate compactor should do the job. But again if is to dry or to wet it will not compact properly. The soil fill should be compacted in 6' layers. When you pick up a hand full and squeeze it, it should be just moist enough to hold together as a ball.
I am not saying that my suggestions will work for you with your particular structure. I do not know your soil or the load that will be place on it. No matter how good you make the foundation you are still dependent on the soil beneath it. I have seen many highly engineered buildings fail. We live on the Good Lords dirt and He's in charge.
When in doubt the wisest investment may be to pay the expert. A registered professional engineer familiar with the soils in your area.

 

[Angus Cowman]

As has been stated, concrete can't withstand the stress of tension, that is the reason for steel in the concrete. But contrary to what most believe, the greatest stress on concrete is at the surface whether bottom or top. The problem with steel is its deterioration from oxidation and it must have a good covering of concrete to protect it. The horizontal center of a slab is the most neutral part of that slab and ideally the steel should be placed toward the edge where the greatest tension occurs i.e. the top and bottom. The best solution for surface tension is fibermesh. Its noncorrosive nature allows its placement where it is needed most.

I agree with everything but the fibermesh. On the right soil it works fine. ( the standard for steel coverage is 1 ") On expansive clays I have seen to many structural cracks to have confidence in it. I also disagree to your recommendation of using sand. On a flat surface with good drainage it is great but in this case he said that he was in a sandy soil on a slope. Sand will flow in water. Sand will blow in wind. When compacted with the right moisture content it will be like rock. I will also disagree with the suggestion of not using plastic. In utility work or most anything else outside moisture is not a problem. In a silo it is a different situation. When concrete is placed it contains water. Therefore when the water evaporates there is shrinkage. Depending on how it is cured noticeable cracks may or may not appear. Even if you cannot see them they are present. This makes concrete somewhat porous. The exact mix will also make a difference. Plastic is just cheap insurance.
As far as thickness is concerned, If you have ever parked in a modern parking garage it was probable built wit precast panels that are hollow and only about 4" thick. The exterior and interior beams are what give the main structural strength. A footing is used on soils with low compressive strength to spread the load.
Soil compaction will be highly dependent on the make up of the soil and the amount of water in it. Sand gravel and red clay sound like very stable, none or low expansive soil.
A gasoline driven plate compactor should do the job. But again if is to dry or to wet it will not compact properly. The soil fill should be compacted in 6' layers. When you pick up a hand full and squeeze it, it should be just moist enough to hold together as a ball.
I am not saying that my suggestions will work for you with your particular structure. I do not know your soil or the load that will be place on it. No matter how good you make the foundation you are still dependent on the soil beneath it. I have seen many highly engineered buildings fail. We live on the Good Lords dirt and He's in charge.
When in doubt the wisest investment may be to pay the expert. A registered professional engineer familiar with the soils in your area.

Nova
as for the sand washing away if you knew how he was building this there is No where for the sand to run as it is a bunker type silo with the concrete extending to the earthen walls and the ends will have access ramps built to the top of the concrete on the ends so there will be no erosion issues
you may have a degree in engineering but from real world experience the plastic is a joke and causes more problems than it eliminates
as for the sand ANYTIME we were anywhere near solid rock we had to add sand as a cushion for the rock to move without moving the concrete so your comment about being able to pour on granite kinda surprises me as the only time we were never allowed to pour directly on solid rock and always had to install a cushion
from talking to WW a footing is not required or needed except on the ends where he is entering and exiting the edges of the slab the edges will have minimal load on it since there is not going to be any walls poured on top of the slab as he is using the existing earth for the side walls

and yes I have installed the prefabbed or as they are called Prestressed concrete slabs for alot of buildings and parking garages they are engineered entirely different than poured in place concrete and the also have tension plates and cables running thru them to help carry the load and are nowhere comparable to PIP concrete buildings in the way they are engineered they are also poured on beds that are heated and cooled so the concrete will set and cure at the exact times and temperatures

as for the moisture content on fill material we were always controlled by moisture spec for certain types of material most clay based materials optimium was around 18% if memory serves me correctly
I am not trying to disagree with you but just because a set of engineering plans says it has to be doesn't neccesarily hold true on every situation without a little more knowledge of exactly what is being done and what the purpose is

 

[shadyhollownj]

Ok I'm a concrete contractor by trade so heres my opinion. I'm on these boards and finally a question I can answer. As far as your sub base sand is no good. If you have a sandy gravel that compacts good that will work just as good as crushed concrete or stone. the key to good compaction doesnt revolve around a wheel loader. It relies on compacting in lifts anywhere from 4" top 8" depending on compaction equipment. A plate tamper will work for 4" lifts or a heavy vibrating roller for 6 or 8" lifts. Even though your base feels tight with a wheel loader dig down a foot and see what you got. As far as your concrete go with nothing less than 4000 psi. The thickness will vary depending upon size of project but 8" seems ok for a silo. The rebar is one thing that you do not want to skimp on. I would use number 4 bar 1/2 inch on 12" centers. The rebar, like any steel, is to give the concrete tensile strength. Not to hold it together after it cracks even though it does keep it together. As far as the fiber if you are going to use a commercial grade with a big strand and its cheap go for it wont hurt. You really dont need it with rebar on 12" centers. My biggest concern would be the sub-base. Wire instead of rebar will do nothing for you besides add headaches later on with the amount of weight you are talking. As far as the footing you will need some type of a haunch footing on the perimeter. I would recommend using number 5 or 6 bar in the footer and a depth of 18 to 24". Just my opinion but its what I do everyday.

 

[novatech]

Ok I'm a concrete contractor by trade so heres my opinion. I'm on these boards and finally a question I can answer. As far as your sub base sand is no good. If you have a sandy gravel that compacts good that will work just as good as crushed concrete or stone. the key to good compaction doesnt revolve around a wheel loader. It relies on compacting in lifts anywhere from 4" top 8" depending on compaction equipment. A plate tamper will work for 4" lifts or a heavy vibrating roller for 6 or 8" lifts. Even though your base feels tight with a wheel loader dig down a foot and see what you got. As far as your concrete go with nothing less than 4000 psi. The thickness will vary depending upon size of project but 8" seems ok for a silo. The rebar is one thing that you do not want to skimp on. I would use number 4 bar 1/2 inch on 12" centers. The rebar, like any steel, is to give the concrete tensile strength. Not to hold it together after it cracks even though it does keep it together. As far as the fiber if you are going to use a commercial grade with a big strand and its cheap go for it wont hurt. You really dont need it with rebar on 12" centers. My biggest concern would be the sub-base. Wire instead of rebar will do nothing for you besides add headaches later on with the amount of weight you are talking. As far as the footing you will need some type of a haunch footing on the perimeter. I would recommend using number 5 or 6 bar in the footer and a depth of 18 to 24". Just my opinion but its what I do everyday.

Ok I'm not a concrete contractor. I was. What I do now is work for engineering companies doing forensic examinations on why concrete structures failed. I formulate a plan to repair them. Then my company does the repair if we are the low bidder or they just want us to do it which is usually the case. At present we are working on 2 churches.
I have no doubt that what has been suggested will work and is what is usually done in the area you live in. But what is done on one piece of ground will not necessarily work on another piece of ground. The man has a gravel sand and clay mix. The ratio is not known. The depth is not known. The type of clay is not known. The depth of the water table is not known. With all these unknowns how can one possibly know what the foundation design should be?
As far as erosion is concerned, What you see on top of the ground is one thing and usually the only thing people address. Where sand is concerned it is more as to what is happening underground where you cannot see.
What is the gain in going to 8" of concrete over 4". It is a common misconception that thickness adds strength. The more concrete, the more weight, the more steel necessary to overcome the additional load. In this area for 8" we use a double mat. For your area and your soil a, single mat may be all that is required.
Anther difference is the use of a footer vs. a beam. Here a beam is where the vertical gives it strength and the width gives it the load bearing on the soil. Usually 1' wide and height to be determined as per load. The steel sized accordingly. In other areas footers are used where the base is usually wide and not so much depth. The soil is not as load bearing. And again the steel is calculated accordingly.
Here often times we must use deep bell bottom piers to keep the foundations in a level plain. In other areas it is not necessary.
I have never seen any harm caused by using a moisture barrier. Somebody needs to explain that a little better. Every foundation in this area gets it. It is in every building code I have ever seen. But I don't get out much.

 

[shadyhollownj]

We are getting a little technical now. Anything he does with 6" or greater concrete and steel should work. I didnt realize it was a bunk silo. In my book thickness does add strength. Try getting 8" with bar out vs 4" and let me know how your back feels after that.
We are not talking about a high rise its a ground level pad. A tight base with concrete and steel reinforcement its a done deal.

 

[novatech]

We are getting a little technical now. Anything he does with 6" or greater concrete and steel should work. I didnt realize it was a bunk silo. In my book thickness does add strength. Try getting 8" with bar out vs 4" and let me know how your back feels after that.
We are not talking about a high rise its a ground level pad. A tight base with concrete and steel reinforcement its a done deal.

Your are right. He needs nothing more than what ever will carry the load of the equipment. And we will have to agree to disagree about the thickness.

 

[upfrombottom]

The technicality of this thread is almost hilarious. I have a BS in Structural Engineering and can get really technical and boring on this subject. I also have alot of common sense and rely on it when I'm working on my own projects. All things considered, this project is a silage bunker, being built by a farmer, whose main concern will be cost and functionality. I seriously doubt that the engineering safety design of a parking garage will even enter in the mind of the guy furnishing the money, much less the guy finishing the concrete. Even less so if they are one in the same.

 

[TexasBred]

The technicality of this thread is almost hilarious. I have a BS in Structural Engineering and can get really technical and boring on this subject. I also have alot of common sense and rely on it when I'm working on my own projects. All things considered, this project is a silage bunker, being built by a farmer, whose main concern will be cost and functionality. I seriously doubt that the engineering safety design of a parking garage will even enter in the mind of the guy furnishing the money, much less the guy finishing the concrete. Even less so if they are one in the same.

:clap: :clap: :clap: I've been rereading the thread to make sure we were still putting a floor in a pit dug into the side of a hill or if we were building a 4 story parking garage. All I can say is that we put silage in a bunker silo for years and it had nothing but a concrete floor 4" (more or less) of concrete and rolled concrete reinforcing wire in it and never had a break....that's been about 15 years total...oh and unskilled mexican boys did the pouring and finishing. .....all you have is trucks loaded with grass driving into it to dump their load and then later a tractor going up and removing the material a bucket at a time.

 

[shadyhollownj]

The one thing is that just like everything else they dont make concrete like the used too. Now its chemicals and fly ash with a dash of cement.

 

[Lon]

" I would recommend rebar and fiber mesh ( little fiberglass pieces mixed in with the concrete)"

thats how i have always seen done when around here. not saying its only way just what i have only seen done.

 

[Lon]

The technicality of this thread is almost hilarious. I have a BS in Structural Engineering and can get really technical and boring on this subject. I also have alot of common sense and rely on it when I'm working on my own projects. All things considered, this project is a silage bunker, being built by a farmer, whose main concern will be cost and functionality. I seriously doubt that the engineering safety design of a parking garage will even enter in the mind of the guy furnishing the money, much less the guy finishing the concrete. Even less so if they are one in the same.

:clap: :clap: :clap: I've been rereading the thread to make sure we were still putting a floor in a pit dug into the side of a hill or if we were building a 4 story parking garage. :lol2: All I can say is that we put silage in a bunker silo for years and it had nothing but a concrete floor 4" (more or less) of concrete and rolled concrete reinforcing wire in it and never had a break....that's been about 15 years total...oh and unskilled mexican boys did the pouring and finishing. .....all you have is trucks loaded with grass driving into it to dump their load and then later a tractor going up and removing the material a bucket at a time.

i would agree here with TB. i would worry more about making sure packed real good than anything.

 

[novatech]

The technicality of this thread is almost hilarious. I have a BS in Structural Engineering and can get really technical and boring on this subject. I also have alot of common sense and rely on it when I'm working on my own projects. All things considered, this project is a silage bunker, being built by a farmer, whose main concern will be cost and functionality. I seriously doubt that the engineering safety design of a parking garage will even enter in the mind of the guy furnishing the money, much less the guy finishing the concrete. Even less so if they are one in the same.

So what exactly did you find technical? Everything seemed pretty basic to me. I cannot count the times I have seen people tearing out driveways and parking lots because not near enough thought was given to it. You are exactly right he is a farmer. He knows nothing about concrete. He asked a question. My thoughts were that he did not want to have to do it again because of taking advice from some bone head that knew nothing.
What is hilarious to me is that you of all people find nothing else to say but a cut on what was said. With a BS in structural Engineering I would think you should have a lot more than something like this to contribute. Like, hay you guys should have mentioned expansion or control joints.(Probably to technical) Instead you found an opportunity to cut people down that were only giving an opinion. And I might add had just previously admitted that it was getting technical. Yes it is only a bunker silo. I actually could not find where he stated that. He probably did but I did not see it. So given that most of the info. suggested is overkill. Often subjects on the boards get off track. That does not mean we cannot learn a little something with the information given. Personally I found it very interesting in how people in different parts of the country do concrete.

 

[Angus Cowman]

[ Personally I found it very interesting in how people in different parts of the country do concrete.

heck around here the specs are different from one part of the state to another

the last treatment facility I did was in the KC area and we had to contend with gumbo and gray clay
here in the southern part of the state we have to contend with red clay,flint rock and then solid limestone rock

the soutwest part of the state has a shale type of rock that is a pita to work with because it crumbles so badly

I also know Texas is the same way drive a 100 miles and everything changes

 

[upfrombottom]

So what exactly did you find technical?

Steel for beams should be no less than 5-#5 with stirrups at 2 ft. The slab should have a minimum of #3 at 18" oc each way. Concrete should be pored 4' thick. These sizes are minimum.

If you have ever parked in a modern parking garage it was probable built wit precast panels that are hollow and only about 4" thick. The exterior and interior beams are what give the main structural strength

With a BS in structural Engineering I would think you should have a lot more than something like this to contribute.

With all these unknowns how can one possibly know what the foundation design should be?

The first thing I was taught in college by a very respected engineer was:" State only what you believe to be fact and be prepared to defend it, because as a professional you will be challenged."

I actually typed a long and drawn out response to the original question soon after it was posted based on my engineering education . Then I sat and read my response with my common sense education. The latter made me delete it.