Crack Control For Fiber Reinforced Concrete

[AFCbuilder]

I am wondering how to handle the issue of watertightness for a manhole design using fiber reinforced concrete (looking into both steel or synthetic fibers). I have seen that watertightness implies that rebar must be used to control cracking (based on ACI code) but does anyone have experience with CSA regarding this?

Since manhole design is a concrete ring under pure compression, do I need to really worry about crack control? In my analysis, I had considered compressive loads for earth pressure and hydrostatic pressure. This led to very reasonable stresses and loads based on my preliminary design. Are there specific pressure testing which is standard for manholes? The spec had no mention of this but higher pressures may cause leakage.

 

[cvg]

most circular manholes use steel reinforced precast sections. if you are designing a rectangular one, it will be heavily reinforced with rebar. I have not seen fibers used for manholes. gaskets and waterstops at joints is the most important thing for water tightness.

 

[AFCbuilder]

I was considering the case for a cast-in-place circular manhole. I thought I had seen some projects where conventional reinforcement was replaced entirely by fiber reinforcement. Would wire mesh be suitable for crack control?

 

[TehMightyEngineer]

I looked into replacing reinforcement in a manhole through use of steel or synthetic fibers and came to the conclusion that fibers will increase the shear and tensile strengths of concrete, provide addition ductility, and provide increased resistance to cracking; but it will not be sufficient to replace all the reinforcement and thus the manhole would have to be designed as plain concrete. Given the forces involved in a typical manhole I concluded that, without more research into using fibers to replace reinforcement, there was no feasible way to use fibers in place of rebar.

We typically use WWR for manholes barrels as the higher yield strength and closer wire spacing helps flexure and crack control. The concrete will be relatively watertight as long as cracking is controlled. Joints are sealed to prevent leakage. If a truly watertight structure is needed (such as for sewage) then liners and additional joint sealant is used.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[AFCbuilder]

Thank you both for the input!

 

[TehMightyEngineer]

One last thing, I emailed a synthetic fiber reinforcement company for a different reason but they insinuated that they had research that would allow for the replacement of reinforcement with fibers. It would be worth it to inquire with some fiber manufacturers and see what they might have for research.

That said, when I looked through the ASTM codes for manholes they clearly didn't allow for such a substitution so that may control unless you get a EOR that will play nice with your experiment of replacing reinforcement with fiber.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[dicksewerrat]

Fiber does NOT reinforce concrete or all the structural engineers would be using it in high rise bldgs.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[KootK]

Reinforce, no; mitigate cracking, yes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dicksewerrat]

How big is this manhole? And where is it?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Brad805]

Some in Europe are using steel fibers as the reinforcement in tunnel segments. I was told about a case at a conference in the Netherlands where it has been done in practice in England. With a three point bending test and NLFEA software you can back calculate the stress strain curve for that particular steel fiber dosing. With that you can predict the strength reasonably for other cases. Bekaert has done a lot of research with Dramix (

http://www.bekaert.com/en/products/...fibre-reinforcement-for-tunnel-applications).

There are other applications as well. Whether your jurisdiction recognizes this is another matter.

A few paper links in case you are interested are below:

http://homepage.tudelft.nl/p3r3s/MSc_projects/reportBurger.pdf

http://tnodiana.com/node/245

 

[Ron]

TME is correct.

Fiber in concrete is not a reinforcement....it is only a mix enhancement. Depending on the fiber type, the aspect ratio of the fibers, the dose rate of fiber used and its compatibility within the mix design matrix, fiber will generally increase strength properties of concrete, including compressive, tensile and shear strengths. This is the primary enhancement provided by fiber inclusion in the mix that affects cracking.

Do not use fiber enhancement to delete control joints. Do not use fiber, EVER, to replace reinforcement.

Control joint spacing can sometimes be increased, but in my opinion, do not increase spacing by more than 25%. Timing of control jointing DOES NOT change. Still requires early sawing in same way as conventional concrete.

 

[Gumpmaster]

There is actually some research being done to replace rebar with steel fibers in very congested areas such as at coupling beams between openings in special reinforced shear walls. The dosages of the fibers are very large (on the order of 250 lb/cy). The report I saw showed similar performance between the conventionally reinforced coupling beam and the fiber reinforced coupling beam.

Also, as Brad805 has noted, underground construction has been using fibers to completely replace conventional reinforcing. It is typically used in conjunction with rock bolts. This is much more advanced in the European and Australian markets than in the USA. The goal in these designs is the absorption of energy rather than a flexural capacity, so it's an apples/oranges thing compared to conventionally reinforced sections. The loading and nature of the load is also significantly different than above ground structures.

So, steel fibers can replace conventional reinforcing, but it's not yet accepted practice except in the mining/tunneling industry. Quite a bit more research needs to be done to apply it to other types of structures such as manholes.