Fibre Mesh v Steel Reinforcement in Slabs - Eng-Tips

The primary methods to prevent early age cracking are concrete quality, finishing, and curing. The primary method to control longer term cracking is subgrade preparation. For heavily loaded slabs, increased thickness and discrete reinforcement are the best controls.

The use of fibers is limited to non-structural purposes, with few exceptions. This is primarily because the distribution and orientation of fibers is not assured. Distributed fibers help reduce crack width by encouraging development of multiple, small cracks, or in some cases, helping to improve the properties of concrete which reduce cracking. Once cracks start to grow, they are controlled by the modulus and area of fibers and reinforcement crossing the cracks. The modulus of plastic fibers is far below that of steel (- ksi for plastic v 29,000 ksi for steel.) The use of discrete (conventional) reinforcement assures that crack width is controlled once they start to grow. In slabs, this is usually relatively less important, but we have been seeing quite a few large companies move back to conventionally reinforced slabs after finding that fiber and unreinforced slabs did not perform well enough. Some slabs where poor performance was seen were well prepared and cracking was the result of loading and/or restraint, and the lack of reinforcement allowed wider crack propagation and/or vertical faulting.

In reinforcing an indoor slab, small, closely-spaced bar or wire will generally control cracks better than larger, widely-spaced reinforcement. Having reinforcement closer to a surface will restrain cracks on that surface better.

State of the practice for design of slabs is provided in ACI 360:

Construction practices are covered by ACI Committee 302:
This is due for an update, and the committee is now doing that. hokie66....
TXstructural....

EXACTLY!!

Fiber is an enhancement to the concrete mix....it is not reinforcement
It increases both the flexural (good) and compressive strength (not so much) (steel fiber only...polypropylene is worthless...as noted by hokie66)

I spent about 10 years on the ACI Committee for fiber concrete. It was a constant effort trying to keep the BS out....fiber provides some help, but it does not replace reinforcement and its crack limiting capabilities are limited as well.

The most significant effect that I've seen with fibers (steel fibers, again), is a reasonable increase in the modulus of rupture. Thanks Folks,

Seems to be a hotly debated topic.

The application I am considering is for a sand & aggregate storage shed (quarry). The floor (on grade) will be subject to loads from plant and materials. My approach is to design a floor slab which is suitable for use and to simplify its construction as much as possible.

Therefore, I am considering a deep slab (200mm / 8") with 1 no. layer A252 mesh (bottom) with fibre mesh reinforcement within the concrete mix. This will allow me to increase control joint spacings and allow for larger pours. The slab will be laid on graded hardcore well compacted in 9" layers. The slab does not need to be a super flat floor nor is the application the interiro of a 5* hotel, with a tiled finish, therefore, it does not need to be over-designed in terms of crack control. Primary objective is functionality & durability.

The viewpoints above are all valid and informative, many thanks.



murdydrum
chartered engineer
Depending on the soil type, an 8" slab for moderate loading should be OK... you can check the design. A good high strength concrete, low slump, sawcut into proper sizes at the right time and curing should all be used. I'd probably use regular reinforcing steel properly chaired and not WWM/WWF. I'd skip the fibres completely, steel and/or polypropylene. The former should be used if there is heavy equipment moving on the slab. In addition to the rust noted by MM, they are really difficult to work with.

Dik I stick by my recommendation to use steel fibers, based on the desire to minimize the need for crack control joints in an industrial slab where they're storing sand and aggregate. The slab will be subject to heavy abuse, both from the product itself, and it's likely that front end loaders will be used... they'll tear up the joints! The use of steel fibers allows larger slab areas to be poured without joints.

For normal industrial floors, I'm all for reinforcing bars (mesh is a joke), but you generally cannot stretch the joint spacing with reinforcing, and it does nothing for durability and abrasion resistance. Steel reinforcing is normally placed in the upper 1/3 of the slab to control cracking, rather than being used for bending strength, so it generally doesn't help you there. Murdy stated his "primary objective is functionality and durability". Steel fibers give you that... improved bending strength, crack control, fewer joints, abrasion resistance, lower maintenance costs, etc., etc..

Msquared... you seem real hung up on the fuzzy/rusty floor thing. Have you ever seen a properly constructed steel fiber floor? The fibers to not stick out. Sure you'll get a little surface rust, but who cares? This is an industrial building that somebody is dumping rock and sand on, and running over with heavy equipment.

Another suggestion to increase strength and limit joints, Is the post-tension the slab. Post-tensioning can also be used to reduce the thickness of the slab. While I agree with spats's basic premise, I do not agree that the use of fiber will allow no joints. It will allow different spacing of the joints (distance between joints can be a bit more), but it will not preclude the use of joints.

An hokie66 noted, the toughness of the slab will be increased by the use of fibers.

As dik alluded, don't let the fiber addition substitute for sound concrete mix design and placement practices.

I am beginning a self build journey early next year (Project thread to come, no doubt plenty of mistakes to learn from!)

I have our engineer drawings back now, and the 300mm deep foundations need 2 layers of A252 steel. How is best to connect these, mainly keeping the upper one in place?

I've seen 'top hats', 'deck chairs' and 'hi-chairs'. What's the best way to do it? Engineer hasn't detailed anything, just that 2 layers are required, 50mm from top and 50mm from bottom.

If I use the hi-chairs, do I need to run them continuously or can they be chopped up and tied along the foundation? I understand they are only there to keep the mesh in place while the concrete is poured and set. Foundations are mostly 600mm wide

And photos of steel mesh installation in progress would be much appreciated!

Thanks Is this a strip footing? Sounds to me like it is and the mesh is just to help resist differential movement.

We wouldn’t usually specify on the drawings how to support the reinforcement, but it is mentioned in our construction notes - have you checked the ones I assume your engineers provided?

Anyway, the high chairs come in long lengths as they are usually used for slabs , so cut them to around 500mm wide and put them at regular enough centres to prevent the mesh sagging significantly.

You could support the bottom layer on pieces of concrete brick or block paver.

He should specify how the reinforcement is placed. What if there is a failure and you blame the engineer and he blames your method and advice from DIYnot?

I'd assumed decisions on how to connect them were to be made by the builder, and the engineer just specified where they were to go.

Is this a strip footing? Sounds to me like it is and the mesh is just to help resist differential movement.

Yes it is

We wouldn’t usually specify on the drawings how to support the reinforcement, but it is mentioned in our construction notes - have you checked the ones I assume your engineers provided?

Their 'General design notes' covers a lot of things, but not this!

Anyway, the high chairs come in long lengths as they are usually used for slabs , so cut them to around 500mm wide and put them at regular enough centres to prevent the mesh sagging significantly.

You could support the bottom layer on pieces of concrete brick or block paver.

Fantastic, I've been looking online for a couple of weeks and not found anything near to a robust answer like that!

I'm thinking that every 1m, with the high chairs fixed to the meshes would be fine? I'll decide on the day, depending how much sag there is (Or get the engineer to spell it out).

Dare I ask, where there is a stepping in the foundation, should I be bending some 8mm bars and fixing them between the differing height bottom layers, and top layers?

My only concern going back to the engineer, is they seem to over specify..... neighbours who have managed to not need an engineer have built without any mesh and years on it 'seems' to be fine.

I'm thinking that every 1m, with the high chairs fixed to the meshes would be fine? I'll decide on the day, depending how much sag there is (Or get the engineer to spell it out).

1m sounds fine but yeah, eye it in, and wouldn’t hurt to get your engineer to confirm.

Dare I ask, where there is a stepping in the foundation, should I be bending some 8mm bars and fixing them between the differing height bottom layers, and top layers?

That’s a question for your engineer as they should definitely provide a detail for this, but yes, you need to maintain continuity at the step so tying the bars together is essential.

My only concern going back to the engineer, is they seem to over specify..... neighbours who have managed to not need an engineer have built without any mesh and years on it 'seems' to be fine.

Have you had a ground investigation carried out? There may be something in the report that guides the engineer towards specifying reinforcement in the strips.
Reinforcement in strip foundations is usually found where there is a risk of differential settlement, or where the foundations are likely to bear onto particularly weak or loose ground.
It often seems like engineers over-specify, but usually it’s working to within the constraints of the British Standards or NHBC Standards (which most engineers refer to even when there is no warranty provider).
Plus you only get your fingers burnt once or twice by cutting a design too close to the bone and after that you add a little extra comfort factor