Crack Control For Fiber Reinforced Concrete
Crack Control For Fiber Reinforced Concrete
(OP)
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.
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.






RE: Crack Control For Fiber Reinforced Concrete
RE: Crack Control For Fiber Reinforced Concrete
RE: Crack Control For Fiber Reinforced Concrete
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
www.americanconcrete.com
RE: Crack Control For Fiber Reinforced Concrete
RE: Crack Control For Fiber Reinforced Concrete
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
www.americanconcrete.com
RE: Crack Control For Fiber Reinforced Concrete
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Crack Control For Fiber Reinforced Concrete
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.
RE: Crack Control For Fiber Reinforced Concrete
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Crack Control For Fiber Reinforced Concrete
A few paper links in case you are interested are below:
http://homepage.tudelft.nl/p3r3s/MSc_projects/repo...
http://tnodiana.com/node/245
RE: Crack Control For Fiber Reinforced Concrete
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.
RE: Crack Control For Fiber Reinforced Concrete
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.