Prestressed Concrete VS Reinforced Concrete 8

[ENGR_2321]

I have researched this and I get mixed answers. But I would like to see if there's a simpler way to know which instances are best to use either pre-stressed or reinforced concrete in your practical expereince ?

 

[Ingenuity]

When you state "prestressed" that refers generally to both PRE-tensioned and POST-tensioned - which are different 'beasts' - one being most often a PRECAST product (in the case of pre-tensioning), vs field-placed (post-tensioned concrete) and is therefore sort of similar to cast-in-place reinforced concrete but not really.

Give us a context so we can narrow down the responses.

Typically, 'prestressed' concrete will result in large spans, less depth, weight savings (and therefore foundation cost savings), better serviceability (primarily cracking and deflections), ....

 

[dik]

You may want to talk to a couple of contractors in your area to maybe get a handle on the relative costs. Both work well if done correctly.

Dik

 

[concretemasonry]

The selection between pre-stressed or reinforced concrete ultimately depends on the schedule, construction logistics location and the economics (as usual) and several can be eliminated or minimized.

Pre-stressed can allow using the experience of the possible suppliers to make the decision easier. Normal reinforced concrete can be based on the usual economics and logistics since it can be used on most structures.

Generally, normal generic structures usually fall into the reinforced concrete category.

Being able to use your past experiences is very valuable.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[KootK]

In my opinion, prestressed concrete is better concrete unless:

- Competitive, skilled supply and labor for prestressing is not available locally.
- Something about the construction environment would lead to construction safety concerns with prestress.
- Something about the service environment and/or prestressing technology would lead to durability concerns.
- Local facilities insurers have some nut-bag objection to prestressing.

In the markets where I've worked that, the following are commonly prestressed:

- Precast concrete elements (plank, beams, columns, wall cladding panels).
- Slabs in multi-story buildings (unbonded)
- Aggressively slender transfer beams (bonded).
- Bridges.
- The odd high seismic shear wall.





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.

 

[ENGR_2321]

Thank you everyone for all your helpful and valuable insights.

@Ingenuity and @kootK that's especially what I was looking for like a general comparison of the uses as you guys described. I agree there's many factors to take into account but just looking at it from a general view. Normally I use reinforced concrete but I've been looking into pre-stressed. It makes sense bridges and structures with long spans and less depth is best to use pre-stressed. Situations in which we are trying to keep it at a particular size but with high strength. Basically high rise construction and real tall slender columns I am assuming also fit the category ?

In a case of a let's say concrete garage whose 3 walls(except the entrance) are also retaining walls and has a "deck load" on top of the top slab (roof): Pre stressed is better than reinforced concrete design in this case I am assuming ? Walls being about 8-10 ft tall

 

[rapt]

That would depend on the size of the garage. A small garage (couple of cars) with retaining walls on 3 sides would probably be better as RC.

But if you want sensible answers, you need to provide sufficient information to base the answer on.

 

[ENGR_2321]

Yes sorry a small 2 car garage 20 ft x 20 ft

 

[SlideRuleEra]

Here is change from long ago: Beginning more or less in the the 1950's, (Driven) precast, prestressed concrete piles have essentially replaced (driven) precast, reinforced, concrete piles.

The prestressed piling are smaller and lighter for the same load capacity, they are better at withstanding the stresses of pile driving, can be manufactured quicker, and provide superior resistance to corrosion (e. g., in sea water).

http://www.SlideRuleEra.net

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[dik]

Span is too short and the building is too small...20' is hardly enough length to stress the strand... Consider reinforced concrete only.

Dik

 

[rapt]

I would do it with RC beams in one direction, one over doorway and one half way to back wall and one way slab transverse.

Flat slab would be too thick in RC.

 

[ENGR_2321]

Okay I got it. THANK YOU everyone.

 

[Teguci]

Plant Precast concrete:
- add about 10% to ship to site
- more site logistics to stage, lift and install
- complicated connections
- joints are susceptible to wear and need to be carefully reviewed for waterproofing if needed
- member sizes are limited to plant bed sizes and shipping limitations (about 16ft x 65ft for a modern highway system).
- modular and well suited for repetitious designs
- produced under plant conditions which allows for tighter tolerances (ACI provides for shallower reinforcing depths)
- Shallower structural depth due to prestressed strands
- Fast onsite erection. Concrete is already cured and there is typically no formwork time needed.
- Inspections of elements occur in the plant
- Elements will be cambered and thicknesses will be consistent (4" slab will be 4" at the midspan too)
- Not post installed anchor friendly

For a 20ft x 20ft slab, I'd probably detail it for cast in place and then ask a local precaster to price it up with a solid slab system (I don't recommend hollow core for exterior exposures).

 

[MacGruber22]

Ignoring this small 2-car garage example...Consider a well-constructed post-tensioned garage with a high-quality mix design (integral corrosion inhibitor, high A/C, low W/C, etc.). All else equal, they require less long-term maintenance due to the number of joints and inter-component connections.


-Mac