Sorry bro. A little correction here. Compressive strength and tensile strength is two totally different thing. Tensile test is carried out for steel materials such as rebar, I-beam, Steel pipe casing etc. It is a "pulling" test so that we can know how much is the yield strength and tensile strength before failure.

Lets say we are going to construct a single storey building. Inside the columns, the designers will on use the minimum amount of steel as per the BS's requirement. This is because the column does not have to resist much moment. Heck, you don't really need rebar for single storey columns. Some people even used bricks for their columns and there are no issues as long as the foundation is stable. My senior engineer once said the only time that the rebar will come to work is when there is a crack in your column which is wide enough for you to see your rebar. Otherwise, your structure will rely on your concrete's compressive strength only. That is why for bigger and heavier structures, we specify higher concrete grades. At the same time, it will also reduce the amount of required steel area. Hence please dont just add or use bigger rebars when the specified sizes are unavailable. There is a limit too.

You can also think of it this way. Why rebars are allowed to use lappings for extensions. If it is used to handle compression such as normal steel structure (H-Sections), lapping is definitely not acceptable. Imagine it sliding down because it is tied together with a wire only. The reason lapping is adequate because we only need them for their tensile strength to resist moment. You will also never see a steel structure being built by using rebars. All of them are built using proper structural steels which have enough strength to resist buckling during compression. Rebars are too slender, hence it will buckle easily. It can be used as bracing to prevent the steel structures from buckling though.

The compressive strength for steel is too high. That is why we never even measured their strength. I bet the compressive machine will fail first if you try to compress a steel plate or cube. The reason we add rebar into concrete beams is to reinforce them. Hence the reason why we called them reinforced concrete. The concrete here is the main character and the rebar is just the support. We also never measure the yield strength of the concrete because it will fail almost immediately when pulled (Very low tensile strength) and that is the reason why we add rebars into concrete. For beams, when subjected to dead loads and live loads, it will bend downwards. However, the concrete's strength is from compression. If bending, then it will crack and thus the reason why we add rebars. To enhance the tensile strength of the concrete.

Furthermore, you need to check on which load combinations that your beam failed. During uplift or normal combinations? This is because adding rebars for top part of beam does not help much if the beam is facing downwards force. Furthermore it will only congest the top part of the beam and makes it harder for the concretor and probably end up with a beam full of honeycombs.

However, back to rebars, you can eliminate the need for rebars in your concrete products IF your concrete has enough ductility. One of the latest products are Ultra high performance ductile concrete (UHPDC). Recently went to their seminar and was very impressed by this technology. They manage to make their concrete ductile and hence eliminating the needs for any rebar. Furthermore, the concrete strength is also increased by a lot and hence can be stressed further. Very ideal for long span prestressed bridge. You can read about it here. http://www.dura.com.my/uhpc/dura-uhpdc

Btw, you are a QS right?

This post has been edited by vxv: Oct 23 2014, 01:36 PM