Information About Concrete Cover (Antirust Margin)

Following two big earthquakes we suffered in Marmara Region citizens living in big cities such as Istanbul have started to emphasize inspection of their buildings against earthquake. During inspections done both by them and by an expert, they encountered a formation called corrosion on concrete carrying elements to be much bigger quantities in basements. Besides there are many reasons of corrosion forming at concrete carrying components the most important reasons is accepted by experts to have the insufficient antirust margin, also called as concrete cover.

“The most important reasons of the construction fall down at earthquake is that using unsufficient and unsuitable antirust margin. Using sufficient and suitable antirust magrin which is good for long live construction and building.”

What Is Antirust Margin?

The antirust magrin mentioned as net concrete cover in specifications related to reinforces concrete is defined as: “The thickness measured from the exterior of the last outfit of existing within a concrete component until the ending place of the concrete.” In the Standard TS 500 used since 1985 there was not much detailed information related of this thickness. Only in the article 12.2.1, an explanation in a way as “The concrete cower at beams must not be less than 1,5 cm for internal components and not less than 2 cm for external components” had been stated. With the new standard of TS 500 that came into force in February 2000 detailed information has been brought related to this matter and, by other words, lack of control that had gone on for very years and years was prevented. Whilst experienced project engineers putting their signatures under serious projects were meeting this lack of TS 500 by considering articles determined in German norms named DIN 1045, it is understood that antirust margin’s having taken a serious place in Turkish standards was sourced from Marmara earthquake. 4 units per m2 will be sufficient in normal outfits.

Importance Of Antirust Margin From The Viewpoint Of Corrosion

Many reasons may be listed fort he corrosion that causes here and there rusting of steel outfits existing in reinforced concrete components and losing its quaility. If we are askes to set down:
Having insufficient net concrete cover (antirust magrin) within comcrete components. Especially at the buildings close to seashore, along with chlorine ions in the air high degree of humidity rate increases the importance of antirust margin from the aspect of corrosion. Having insufficient quality of concrete. For long years while definition of good concrete is being done, concrete with high power of resistance was looked for. Recently, for definition of good concrete is not solely looked for power of resistance but also its durability is taken into consideration. Durability is a feature along with power opf resistance protecting ability of steel outfits against external influences that determines the lifetime of the concrete. In order to prevent corrosion it was aimed to make rust margin at sufficient thickness as well to make impermeable concrete, to ensure using vibrator when pouring concrete, to produce with as less as possible water, to mix chemical additives increasing fluidity of concrete that is decreased because of lack of water are some of the measures to be taken. Undoubtedly, to take care after pouring the concrete not to get it dirty, to water it or cover top of concrete with protective chemicals also may be included in the measures given above. Another important point to take also care is that there is no possibility to meet insufficiently of antirust margin by liquid mortar only without taking necessary measures (such as special fluids made of chemicals). For reinforced concrete components exposing to intensive external effects (such as wind, rain etc.) the thickness of antirust margin acquires much more importance. Because of insufficiency of antirust margin, it was observed in these types of components that stirrups were completely vanished, main carrying outfits were dissolve to half in space, their diameters could increase up to seven times and cracking of the concrete due to this expansion. To keep antirust margin thickness some lee in concrete pre-cast components manufactured in prefabricated manner and prefabricated construction elements is possible. DIN 1045 norm allows to reduce this thickness up to 1 cm.

 

Yalçın KAYA
Graduate Civil Engineer

 

Referanced Bıbliography: TSE 500 Calculation And Making Rules for Reinforced Concrete Buildings, Standards dated 1985-2000. DIN 1045 German Reinforced Concrete Specifications. Article by Prof. Dr. M. Ali Tafldemir, Prof. Dr. Hulusi Özkul “Marmara Earthquake Concrete Research”. Ready-Mix Concrete Review, September-October 1999 Nr. 35 pp 16-17. Article by Prof. Dr. Erbil Öztekin “Concrete A.D.”, the same review pp 36-37.