Double rebar arrangement of Concrete wall

Rebars are arranged into concrete wall both vertically and laterally. “Double rebar arrangement” means that the 2 pairs of these vertical and lateral rebars are arranged into concrete wall.

□■□ Question ■□■

In some brochures of condominium, it is insisted that “This building is applying Double rebar arrangement, so that stronger than the building which is applying Single rebar arrangement”. How should we consider that insist?

□■□ Answer ■□■

It is fact that Double is better than Single. But these days, Double rebar arrangement is not special, but usual. It doesn’t mean that the building with Double rebar arrangement is designed with special care regarding earthquake. Single rebar arrangement is limited to a few insignificant cases like a very thin wall. Therefore it won’t be a good indicator to make comparison with other condos.

Know the reason why rebars are necessary 

First of all, let’s think about the reason why rebars are necessary.

In the field of architecture in Japan, RC stands for “Reinforced Concrete”, and “Reinforce” means rebar.

Concrete itself is very old material and has been used for many buildings. Pantheon in Roma is one of the most famous buildings made of concrete. But it doesn’t have rebar. Reinforced concrete has shorter history than you may expect, is invented in 19 century.

Concrete is mainly composed of cement, sand, gravel and water, and is coagulated by hydration reaction of cement and water. As a feature, very strong against compression, weak and brittle against tension. Then, concrete is combined with rebar which is strong and ductile against tension to supplement its weakness.

 Are the rebars adequate?

Generally we can say “a lot of rebars → supplemented weakness adequately → strong”. If using same diameter of rebar, Double rebar arrangement means twice of Single rebar arrangement, so it sounds like true that insisting “Double is strong”.

But it’s a not problem of “quantity” like a Double or Single, but a problem of “ratio” to the wall thickness. Even if same amount of rebars, the performance varies depending on the wall thickness (e.g. 120mm or 300mm). Thick wall has to have more rebars than that of thin wall.

The wall is thin, but Double rebar arrangement?

There is physical limitation, too.

Rebar must be embedded to concrete some degree to avoid getting rust. This embedded depth is called “covering depth” and it varies depending on the finish of wall, location (e.g. indoor or outdoor) and so on.

Supposing that the cover depth is 40mm, Sum of cover depth is 80mm (considering both front and back side). In the case of Single rebar arrangement, because of thickness of rebar both vertical and lateral, minimum thickness of wall is over 100mm even if using thin rebars. A space is required between vertical rebars or between lateral rebars, so that we can understand that Double rebar arrangement is impossible for wall of 120mm thickness.

On the contrary, consider the case of thick wall (300mm) with Single rebar arrangement. Rebars are arranged into center of wall and there are 140mm width spaces without rebar both front and back side. As described above, concrete has a brittle feature, so that there is a fear of chipping and crack if no-rebar zone becomes wide. Therefore wall of 300mm thickness is automatically applied Double rebar arrangement.

Then, how is the case of thickness between 120mm and 300mm? In the case of newly built condo, almost all walls thicker than 180mm have Double rebar arrangement. Around 150mm may have “CHI-DORI” rebar arrangement which is the middle of Single and Double.

"Double rebar arrangement" strong?

 Structural engineers try to design buildings achieving targeted performance with low cost as much as possible. The more rebars, the much construction cost. Generally, rebar arrangement of each wall is proper quantity, not large quantity. It is not proud thing even if all walls have Double rebar arrangement by chance result of cost-efficient design.

It is not a sophisticated conclusion, but it can be said that “Double rebar arrangement” is not important indicator to compare earthquake safety.