Concrete Mix+ Reinfocing for Exterior Composit Deck 1

[ELIofVA]

I am an architect in Virginia. I am interested in recommendations for concrete mix for exterior composite deck.

Recently I had a concrete finisher advise a client of mine that 4000psi concrete was more likely to shrink than 3000psi. In that case, it was being placed in an interior non composite steel deck. It seems to me that the higher strength mix would have less water to evaporate and therefore would shrink less.

I am building an exterior deck to replace funky 30 year old wood deck on my house using a galvanized composite steel deck. Though not required for loads, I am considering a higher Fc (Concrete Compressive Strength) value because it would have less water and be less porous, making it more waterproof to create a roof for lower porch. I am considering using Fc of 5000psi and a superplastisizer to get the workability and slump needed for smooth slab finishing. I am also considering the fiberglass reinforcing to create a continuous tension capacity. Does the fiberglass in anyway reduce the woven wire fabric requirement. The catalog calls for WWF of 6x6 - w1.4 x W1.4 with a steel area
.028 square inches. Would epoxy coated #3's (.11sq inch) at 2'-0"oc in the direction of the span be a reasonable alternative to WWF. I wonder about the durability of exterior concrete decks using uncoated WWF. The load capacity listed in the "Wheeling Deck Products" Catalog specifies only Fc of only 3000psi for loads listed. Are there any issues with greater strength concrete?

Also, I have a 24 inch cantilever. The catalog shows my gage and depth slab can take a point load a given distance from support. Can I assume that the negative moment capacity at the support is equal to that load # x distance shown on table. My application features a composite slab of 1 1/2" depth with concrete slab to be 4" at thickest. Point loads for guard rail post will create a negative moment. Should I design reinforcing for 2 1/2" thick slab (net thickness between ribs). My post do align with grooves, making it a full 4" for width of groove.

All helpful hints will be greatly appreciated.

 

[dbuzz]

Drying shrinkage in concrete is influenced by many factors associated with its constituents, its geometry and the environment. Properties of the constituents will affect shrinkage properties of concrete in different ways. Suitably spaced contraction joints permit movement from shrinkage. Air temperature, relative humidity and wind velocity affect the evaporation rate and therefore the loss of moisture from the concrete surface.

The best way to reduce the shrinkage is to: use minimum water content; use highest possible volume fraction of good quality aggregate and maximum possible aggregate size; use shrinkage limiting cement; not use admixtures known to increase shrinkage, such as those containing calcium chloride; ensure concrete is properly placed, compacted and cured.

If you do replace the mesh with longitudinal bars, make sure you also include some transverse reinforcement (i.e. bars across the span rather than along it).

With the cantilever span, remember that the reinforcement is always placed on the tension side and must be adequately anchored in the compression zone. You'd be surprised at how many builders make this mistake.

 

[ELIofVA]

Thanks dbuz for your awnswer. The specs on the deck warn me not to use calcium chloride because of corrosion. From your statement I learn that calcium cloride will increase shrinkage. What about a super plastisizer?. I had hands on experience replacing a wood deck on my bridge using superplastisizer and was very impressed with its ability to improve worakability and slump without adding water. Your statement implies that you agree with me that the higher strength concretes that have a lower portion of water will shrink less. This slab is 10'x18'. deck is spanning in 10' direction (8'span and 2'cantilever). I doubt that a small amount of shrinkage would be a probem. My detail features a 1" gap next to framed wall so no moister will be trapped. 1 1/4" gap would not be a problem.

Still looking for advice on sizing negative moment on composit decking.

 

[Taro]

ELIofVA,
Be careful with how high you let your concrete strength get. Higher strength concrete generally DOES shrink more than lower strength concrete. That's because higher strength is usually achieved by adding more cement to the mix which requires more water for hydration. More water means more shrinkage. Always specify the lowest strength concrete that meets your strength and durability requirements. See ACI 318 Chapter 4 for durability issues including strength, water/cement ratio, air entrainment, etc.

Refer to any reinforced concrete text to determine reinforcing sizing for flexure.

 

[ELIofVA]

Thanks Taro for your response.

What mix would be non absorbing of water. One of my purposes in this deck is to be a roof for a porch below. What are the consequences of some shrinkage on composit deck. My understanding that the lower the water/cement ratio is the stronger the concrete is and the less absorptive of water when cured. Can the superplastisizer reduce the amount of water required for workability and therefore reduce shrinkage and enhance strength.

 

[ERV]

SuperPlastisizers do improve workability of concrete, and can raise the slump of the concrete to a more fluid like consitency without the addition of water. Higher compressive strength concrete does not necessarily mean that the concrete will be "non absorbing of water". A concrete design mix of 5000 psi will mean higher compressive strength, however, will also mean that the concrete is more brittle. High compressive strength is desirable in larger concrete sections (particularly comlumns). For thin sections such as your deck it may be less than desireable, particularly at the cantilever. One needs to fully understand the stress/strain curves of concrete and the advantages and disatvantages of high compressive strength in order to specify concrete over 3000 psi for thin sections.
The finish of the concrete has more to do with the absorbtion rate of moisture than does the compressive strength. A steel trowel finish (as slick as can be) is less susceptible to absorbtion than a broom finish. If a waterproof deck is desired, than one should consider coating the top of the slab with a water impervious coating or membrane.
I am not aware of any concrete mix that is water proof or even close to it.

 

[ELIofVA]

My use of deck as roof for porch below is not as high a standard as a finished room. The beams supporting the steel pans have a 3/8" per foot slope to drain water. Regardless of compressive strength, a lower water/cement ratio the mix, the higher the density of the resulting cured concrete. Lower density can result in two problems depending on the curing (shrinkage or lower density). Therefore, am I correct to specify low water/cement ratio with superplastisizer, air entrainement, of 3000psi. 3000psi is what the tables for the pans are specifying.

You are saying the lower compressive strength concrete has higher tensil characteristics. Does the fiberglass additive reduce the brittleness of the resulting concrete.

 

[ERV]

Fiber mesh does not reduce brittleness, it does however, provide some degree of keeping cracks from opening. It should be noted that fibermesh is not a substitute for reinforcement and should only be used as a safety factor for cracking due to shrinkage and not loading.

 

[JAE]

I would question the use of composite steel decking on an exterior "roof". Especially without any true roofing on top of the concrete. Over time, the concrete will absorb moisture and soak into the concrete, eventually creating a rusty mess...even with the galvanizing. You are in Virginia - close the coast? If so, I would avoid metal at all costs.

I agree with Taro about the higher strength concrete possibly adding to shrinkage rather than reducing it.

I also agree with dbuzz in the second paragraph above except I would question the use of shrinkage reducing cements (K cement) as this requires very very VERY careful dsign in terms of the correct amount of reinforcing to properly work.

ERV - I have never heard of higher strength concrete being more "brittle". The higher the f'c, the higher is your modulus of rupture (tensile strength). What do you mean by brittle?

If you have to use plain concrete for the roof, I would:

- not use metal deck (form the deck instead)
- use air entrainment to reduce porosity
- use reinforcing bars (especially for your cantilever) and use epoxy coated if you are near the coast.
- include fiber reinforcing as well as it helps to keep cracks tight
- get a mix that represents Taro's and dbuzz's comments above.

 

[ERV]

EliofVA,

An excellent referrence for concrete is Design of Reinforced Concrete, by Jack C. McCormac, available from John Wiley & Sons, ISBN 0-471-39576-5:

http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471395765.html

This book will answer all your questions and will be an invaluable guide to future concrete projects. The book comes with a program (on CD) called ConCad:Concrete Analysis and Design.

 

[ELIofVA]

I live in the moutains of Virginia. Therefore, salty air is not a problem. My primary task is the deck. The cover for lower patio is secondary. Jae, you raise an issue of the vulnerability of using even galvanized composite decks for exterior applications. How do the road engineers resolve this conflict? I am looking for a technology to replace the highly vulnerable exterior wood decks. This project is now underconstruction with a pan on site nearingg installation.

I have appreciated all the comments. Each response brings up new questions.

 

[JAE]

Road Engineers? Most bridges do NOT use galvanized metal deck - but either formed concrete slabs or precast concrete.
Some bridges do use open grating but this is rarely used anymore.

Instead of concrete - there is now on the market a composite-type product that looks like wood, but isn't - I think it is some type of plastic. Will never rot as it is not wood-based.

What do you mean by "a pan on site"???

 

[bikerdon]

The galvanized SIP (stay in place) forms that you see under bridge decks are only forms. Once the concrete is cured their job is done. Usually, the bottom or the corregations are filled with strips of styrofoam to reduce deadload so they're not composite. This also gives a uniform slab depth.

Your composite pans won't help in the cantilever area. You'll need to design and place reinforcement in the top of the slab there. I'd expect the loads are light enough that you may be able to used welded wire fabric and carry it across the whole top of the slab, which would help with any shrinkage/temperature reinforcment needs. The cantilever reinf. will need to run parallel to the direction of the cantilever span, which may or may not be in the same direction of your positive moment reinforcement of your 10 foot span. Hard to tell without a sketch.

Do what you can to reduce the shrinkage cracking but if it does crack their are products you can use to try to seal the cracks. Also, as far as permeability goes, have you considered applying a sealer to the top of the cured concrete?

JAE's suggestion to use composite lumber would make your life a lot easier. It's liable to bring other design questions forth such as deflection and creep issues. In New York State, some park bridges have been built using this material. These are lower load, low vehicle volume structures used in parks for pedestrians and maintenance vehicles.

 

[ELIofVA]

JAE When I said "a pan on site" I meant I have "the pans or are on site.

Following the input from this thread, my main goal is to develop a dense concrete that is a low water/cement ratio that will reduce shrinkage. Fiberglass additive will help prevent cracking. Air entrainment will reduce permeability. As advised I will place a slick finish. When cured, I will put a concrete sealer. I am not going to put a membrane roof because that will wear out and require maintenance. The whole assembly has a 3/8" / ft slope. I am somewhat surprised to hear that galvanized composit pans are vulnerable to rusting. I did not know that bridge pans are forms only. However I am commited to my SB 1.5" 22gage galvanized pan that are cut to lenght and on site. My loads are way under the capacity of the pans. Therefore, I expect and hope it will be a long time before the pans rusting will become a factor.

I like the composit decks because the steel becomes the reinforcing and the concrete does not need to be as thick becuase concrete does not need to go below reinforcing. In my own house I can except some risk. However, for my clients, I must be more sure. To build concrete decks, I could use stay in place forms with all structure in the concrete and reinforcing as bikerdon described for bridges. I would consider leaving the concrete in the grooves becasue that added depth would increase strength. Although calculating it is more difficult.

I will order book referenced above by ERV

The problem I have with the synthetic decks are the wood framing that supports them. The new Pressure Treated woods are more corrosive causing vulnerability with the fasteners.

I appreciate all the comments above, even when it has challenged my assumptions.

Eli