Hussam Ali Mohammed

If we use, high strength concrete, the column size reduces. Hence, it becomes very difficult to provide the development length to the reinforcing bars of beams. With increase of axial load carrying capacity of concrete core, the force in the stirrup increases. Also, reduced cross section of column concrete results in congestion of rebar (even if the rebar requirement less). Ultimate failure strain of high strength concrete is less as compared to normal concrete, which affects the ductility.

Thanks Dipti Ranjan Biswal ... for answering..

The term "very high strength exceeding 60MPa" now is old. RPC reaches 800 MPa.

Esraa Mubder Edaan ... you are totally right..

But I mean traditional common building construction... For sure no one will need 800MPa, ... Also, you know.. in general sites, engineer works with concrete strength ranged between 21-45MPa.. not more..

Please tel us, .. where the 800MPa used... ?

I think the main problems are:

1. Cost: because of high cement content or type of cement.

2. Shrinkage: for the same previous reason and the need for more expensive curing methods.

Does this make sense?

Dear Ahmed S. H. Suwaed ... Thanks for your answer...

Cost is a reasonable reason.. but its not the main ... many researches carry on to reduce the cost of concrete in the same time improve its strength... as a results (High strength concrete with acceptable cost)... So the reason of cost will be neglected there...

Thanks again for answering..

From structural aspects there are some problems with High Strength Concrete (HSC) as follows:

1. Structural elements become more brittle. It is not good because we normally want ductile concrete members, which give a warning before they can be failured. The redistribution of load is also limited. The plastic capacity is limited. This is critical expecially by earthquake design. Structures receive more load due to earthquake.....

2. For schlanke members, such as column made of HSC, there is a problem with stabilitity.

3. Depending on strength of aggregate, the fracture energy of HSC can be smaller than that of NSC. This may lead to a reduction of shear capacity of concrete members (espeically without shear reinforcement).

Regards,

A problem with mixture, high strength concrete needs large amount of cement, therefore Ca(OH)2 will product from hydration reaction ,so that Pozolana like (Silica Fume with High SiO2 %) is necessary to react with excess Ca(OH)2 and then products CSH strong Gel.

Dear Ngoc Linh Tran ... Thanks for answering..

1. Concrete already considered as a brittle materials, its behavior in tension almost consider zero, ... right its not zero, .. but we neglected for safety requirements..

2. How columns reduced its stability when used HSC...? Why..?

3. The fracture energy of HSC could be less than NSC, ... but in final....in total... HSC is more higher strength in shear

I'm glad for your answer.. wait for your reply...

Dear Saddam Kh Faleh ... Thanks for answering...You have point of view.. but HSC is efficient technique used for a lot of structures.. many and many kind of structures... So... the use of higher amount of cement was not the problem, ... can you explain more..

Dear Hussam,

1. As I mentioned above, HSC shows more brittel behaviour than NSC. It is in compression. The ultimate compressive strain of NSC is about 3.5%o, while it shows smaller by HSC (about 3.0%o, depends also on fc). The plastic strain of HSC is also smaller than NSC. By HSC the linear behaviour is dominated.

2. By the same design load, HSC column requires smaller compression area (cross section) than NSC. Assuming that a NSC has a fc = 40 MPa, and HSC has fc = 80 MPa (two times larger), the required area of cross section of HSC is equal to 1/2^0.5 = 0.7 times of that of NSC. The moment of inertia of HSC cross secion is thus equal to 1/4 = 0.25 times of that of NSC cross section. The flexural stiffness EI_HSC = EI_NSC / 2. Therefore, slender HSC columns show a potential problem of stability than that made of NSC.

3. Let's talk a bit more about shear strength of RC members without shear reinforcement as a typical case. Two main shear strength parts are the shear strength in the compression zone and in the tension zone. These two shear parts are not the same in values or have a constant ratio between them. They are influenced by many parameters including reinforcement ratio, concrete compressive strength fc, tensile strength fct, elastic modulus Ec, fracture energy GF (and also maximum aggregate size ag). It is noted that by RC members with small depth the shear strength strongly depending on fracture energy GF or maximum aggregate size ag. By HSC members with aggregate strength smaller than fc, the fracture energy is very small compared to that of NSC. Therefore, the shear strength of HSC can be smaller than that of NSC members. For more details you may have a look at the following paper of mine regarding calculation of shear capacity of reinforced concrete members, in which fracture energy is of particular interest.

Chapter A New Shear Model for Fibre-Reinforced Concrete Members With...

Best regards,

40MPa is the most economical concrete for building structure. otherwise steel is the best selection.

The question

What are the potential problems associated with the use concrete of very high strength exceeding 60 MPa?

Based on my research experience, the potential problems of the use of a very high concrete exceeding 60 MPa is the high temperature rise at early age concrete as this kind of concrete used more cement than that of normal strength concrete. The high temperature rise at earlier age produced crack in concrete, lots of unhydrated cement caused the target strength of concrete could not achieve. Do those make sense?

Dear Ngoc Linh Tran ... Thanks so much for your Excellent answer..

Dear Chengzhi Wang ... Thanks for answering...

So.. the reason is the cost ... and how to get an economical concrete ...? does you think that the only reason...?

Dear Gidion Turuallo ... Thanks for answering ...

So, the main reason is the predicted cracking..... what if these cracks could be handled .. You know, the HSC have a widely used over many structures.. and they overtake the problems of early cracking..

You give a reasonable reason her... Thanks so much