How much reinf. in slabs on grade 15

[IJR]

There seems to be differing opinions on the min. amount of reinforcement and on how it should be placed.

It will be nice to learn more from experienced friends:

1. I understand that the reason behind such reinforcement is temperature and shrinkage, but definitely not the same amount as is specified in ACI318 for structural slabs. How much? How placed.

2. A friend argues that since vapor barriers are placed beneath there will be no shrinkage problems. I can not find logic in this argument since as I understand shrinkage and temperature effects are considered for the sake of limiting cracks and vapour barrier may cut down shrinkage but if no further cover is made above slab, wont temperature changes affect this unreinforced construction.

 

[bvanb]

sc,
Keeping in mind that we're talking about slabs on grade here, I think if the subgrade is poorly prepared or the slab is exposed to extreme loads then we should be looking at a structural slab. However, for slabs on grade, the main concern is shrinkage and temperature cracking. I recently constructed a workshop and RV storage area 36'x48' and used fibre (poly not steel). Since this was my personal building I was particular about the outcome. It is located in an area with moderately severe winters and has been in place for two of them. So far not one crack!

I'm ashamed to admit that on clients projects I haven't pushed hard enough for use of fibres. As was earlier mentioned, I've also never seen wwf show up where the drawings indicate. Could be because the worker hooks the fabric, pulls it approximately into place then turns around and steps on it. If the mud is fairly high slump, which for flatwork it usually is, then he probably is pushing the fabric back down.

bvanb

 

[phuduhudu]

I'd like to add my own concerns. I have never heard of fibres being used here in concrete and I was discussing a slab for heavy plant with a client. I mentioned that WWF never ends up in the right place and he jumped on that ans said "well let's leave it out". Obviously it is a cost saving but I am concerned that any problems with the slab would be attributed to the lack of reinforcement because it is religiously put in here on every project. Any thoughts as to the risk of leaving it out and using plain concrete with saw cut joints at 5m centres both ways. Carl Bauer

http://www.bauerconsultbotswana.com

 

[Ron]

Carl...you should not have a problem leaving the WWF out. Concentrate on good subgrade prep (flat as possible to minimize thickness variations in the slab...remember, you can make the top flat, it's the bottom that causes cracking problems!), good concrete mix design with as large an aggregate as practicable, and keep both the cement content and the water as low as you can and still achieve durability. Strength will not likely be an issue if you accommodate durability.

If you want to use fiber, I suggest steel fiber. Polypropylene is OK, but in my opinion, is not as effective as steel fiber.

Ron

 

[Isa001]

Thanks Ron et al for the info, just getting to grips with this site. Alot of info to look at.

Ron can see that you are not a great fan of fibres particularly synthetic.

Reading the threads there seems to be alot of generalisation about fibres - there are of course many different types all held with varying claims by the manufacturers.

1. The monofilament polyprpolene fibres that are commonly used in concrete to reduce plastic shrinkage, increase impact resistance etc etc are "non structural" and am in agreement with the general consnsus that they do nothing that a good quality concrete should do in relation to cracking etc. Although nowadays they are being used in enhancing the fire resistance of concrete.( they melt and let the water in the concrete - now steam- escape through the capillaries so reducing spalling)which is probably worth paying for in u/g structures.

2. Second type of fibre we can term is structural. As far as I am aware this is about the "post-crack performance" of concrete - so how the fibre performs after the concrete cracks. Esentially if you do a flexural test on a beam the first crack will be the same whatever the fibre reinforcement - the flexural strength is dependant on the concrete matrix not the reinforcement.

There are a number of tests that can compare the strength of fibre after cracking - known as toughness.

As for the argument for/against steel or polypropylene fibres with regard to toughness there are poly fibres on the market nowadays that can equal steel. eg

7-8kg polypropylene fibre = 40kg steel hooked end fibre.

There are obvious advantages of using poly in that they don't rust, lighter, cheaper etc etc. Whether this better than steel is upto the designer.

The issue for me is that if I take out the steel mesh what is the replacement for steel fibres.

Interested in MERBAS.What are you using the monofilament fibres in precast for?

Cheers

 

[Ron]

Isa001...I would disagree that the flexural properties of concrete are not enhanced by fiber addition, even before first crack. The properties are significantly enhanced by fiber...that is one of the main reasons it was developed and initially used.

As for the amount of fiber to use to replace wire mesh...in steel fibers, about 1 percent by volume is usually used.

As for the polypropylene fibers, you'll have to check with the supplier for "equivalence".

One clarification....I am not against the use of fibers. I am against the use of fibers (or anything else) as a means to compensate for poor construction practices.