Lelandwelds said:

Funny, I started out wanting a version of this idea. I wanted an epoxy regular garage floor. I also wanted a rock dust or crushed rock and fines floor for welding and working on equipment. It would be impossible to tear up and easy to freshen up messes.

Goto well to know more.

Click to expand...

You can have both if you section the floor out. You don't have to rely on internet myths or "knowledge" here; the ACI document details this out.

Lelandwelds said:

What? Of course thicker slabs are stronger just like taller steel trusses are stronger.

Click to expand...

Not for a "thin" (8-ish inches, or less) slab. It's important not to confuse a thin slab, which is continuously supported by the ground with a simple beam, which is only supported on two ends.

With a simple concrete beam, you rely on the depth and the steel reinforcement to counteract the sheer forces and the large bending forces. (Neither of which should be seen in a slab on ground.)

With a continuously supported slab, you have the ground underneath resisting vertical forces, so there is typically no sheer. The ground underneath also helps the slab resist bending. (The slab will only bend as far as the ground beneath gives way.)

So, the pad depth and the reinforcing steel (on a thin slab) are not structurally significant with regard to vehicle traffic forces, other than spreading out the bearing forces on the ground (like the footing does on a foundation.)


Here's another way you can think about it: If you put a 1" thick sample of psi concrete on the ground and a 6" thick sample of psi concrete on the ground, both are going to break right at the point where you apply psi of pressure. (The 6" thick sample isn't any stronger than the 1" sample in that scenario.)

(Depending on how you use a slab and attach certain machinery/equipment to it, there could be situations where the depth becomes significant to resisting some bending of the slab, but I can't think of any off hand.)

Lelandwelds said:

Every slab I have ever been under or broken has a huge void area underneath.

Click to expand...

You just proved the point. The minute that you develop a void under the slab, things get really weak - you go from a continuously supported slab to the simple beam condition. (Shear happens at the edge of the void and there is room for the slab to bend into the void. Most thin slabs won't have enough reinforcing to deal with this and will crack.) < This situation isn't a slab issue, it's a ground or a sub base issue.


If you are concerned about the base and voids, you can structurally reinforce your slab. The ACI guide specifies how to do this also. My garage is built on top of some really poor quality fill and I went this way. I recall that I had to use #4 bar spaced somewhere around 6" OC for a 5" psi slab.

You can have both if you section the floor out. You don't have to rely on internet myths or "knowledge" here; the ACI document details this out.Not for a "thin" (8-ish inches, or less) slab. It's important not to confuse a thin slab, which isby the ground with a simple beam, which is only supported on two ends.With a simple concrete beam, you rely on the depth and the steel reinforcement to counteract the sheer forces and the large bending forces. (Neither of which should be seen in a slab on ground.)With a continuously supported slab, you have the ground underneath resisting vertical forces, so there is typically no sheer. The ground underneath also helps the slab resist bending. (The slab will only bend as far as the ground beneath gives way.)So, the pad depth and the reinforcing steel (on a thin slab) are not structurally significant with regard to vehicle traffic forces, other than spreading out the bearing forces on the ground (like the footing does on a foundation.)Here's another way you can think about it: If you put a 1" thick sample of psi concrete on the ground and a 6" thick sample of psi concrete on the ground, both are going to break right at the point where you apply psi of pressure. (The 6" thick sample isn't any stronger than the 1" sample in that scenario.)(Depending on how you use a slab and attach certain machinery/equipment to it, there could be situations where the depth becomes significant to resisting some bending of the slab, but I can't think of any off hand.)You just proved the point. The minute that you develop a void under the slab, things get really weak - you go from a continuously supported slab to the simple beam condition. (Shear happens at the edge of the void and there is room for the slab to bend into the void. Most thin slabs won't have enough reinforcing to deal with this and will crack.) < This situation isn't a slab issue, it's a ground or a sub base issue.If you are concerned about the base and voids, you can structurally reinforce your slab. The ACI guide specifies how to do this also. My garage is built on top of some really poor quality fill and I went this way. I recall that I had to use #4 bar spaced somewhere around 6" OC for a 5" psi slab.

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Fiber Reinforced Concrete

Fiber Reinforced Concrete

abusementpark

(Structural)

(OP)

25 Aug 09 22:36

What are the primary uses for Fiber Reinforced Concrete?

Does anyone have any good experience with FRC?

Any good references would be greatly appreciated.

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RE: Fiber Reinforced Concrete

msquared48

(Structural)

25 Aug 09 22:43

Small Crack control, and to create fuzzy, hairy concrete. (smile)

Mike McCann
MMC Engineering

RE: Fiber Reinforced Concrete

Ron

(Structural)

25 Aug 09 22:53

abusementpark....fiber is good to enhance the properties of concrete, but not to replace rebar.  Steel fiber does a good job of increasing the modulus of rupture for flexural strength.  It also increases the compressive strength, but not in the same proportion.

Polypropylene fibers, in my opinion, are oversold.  They do not do as much for the concrete as the manufacturers would have you believe.  Yes, they will decrease the early crack potential.  This means that the space between cracks will increase, but also the width of cracks will increase.  They do not stop cracking altogether.

I have had good experience with steel fibers in concrete, not so good with polypropylene.  I know of one pavement section used as a loading dock and dumpster pad that I designed over 20 years ago and it is still performing quite well, with no cracks other than at control joints.  We added steel fiber to the asphalt on the same project.  The asphalt has performed amazingly well for over twenty years.

RE: Fiber Reinforced Concrete

SlideRuleEra

(Structural)

26 Aug 09 08:09

See thread507-: Fiber Reinforced Concrete - Current Status

www.SlideRuleEra.net
www.VacuumTubeEra.net

RE: Fiber Reinforced Concrete

abusementpark

(Structural)

(OP)

26 Aug 09 18:18

Ron, what specific problems have you had with polypropylene fibers?

RE: Fiber Reinforced Concrete

Ron

(Structural)

26 Aug 09 19:10

ABP...just that many ready mix producers will use poly fibers to cure their mix deficiencies (high water-cement ratio particularly).  Shrinkage of the mixes has been high, resulting in wide, but longer spaced shrinkage cracks.

Workability is often an issue with any fiber.

RE: Fiber Reinforced Concrete

thruthefence

(Aerospace)

26 Aug 09 19:16 http://www.fibrebond.com/

precast shelters

precast shelters

RE: Fiber Reinforced Concrete

concretemasonry

(Structural)

26 Aug 09 22:37

One reason for the variable answers is the wide range of ability to handle the fibers and use the correct mix design. Some producers treat the fibers as a generic commodity despite the range of fibers and their configuration.

One major problem is the mixing time. Too short, you get poor distribution. Too much mixing time, they can ball up and cause problems.

A good ready mix producer may have mix designs for use with the specific fibers used. Some even adjust the plant/yard mixing time depending on the distance/time to the job site.

The fuzz can be eliminated by a quick singe with a torch a few days or more after placement.

fibers are not intended to be a substitute for rebar or WWF.

I had a driveway installed with psi air entrained concrete with WWF and joints sawed late the same day. Since I did not have a broom finish, I did not find any fuzz, but may have found some if I really hunted. The concrete was from a small local plant within a mile of the huge Lafarge cement plant, so he had all the cooperation and testing he needed, since he frequently allowed Lafarge into the plant for promotional purposes.

For the sidewalk (easement) and the apron to the street,I could not use rebar or WWF due to the city code written to make access to utilities easier, used the smae mix design. Water and sewer were too shallow, so repairs were frequent if you did not keep the water trickling in the winter.

There is definitely a place for them, but there is a wide range of fibers and configurations that frequently get lost in the promotion.

Dick

RE: Fiber Reinforced Concrete

hokie66

(Structural)

27 Aug 09 07:31

Promotion.  That's the word.  I've always just called it sales spin.

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