A NEW APPROACH TO CRACK CONTROL FOR REINFORCED CONCRETE - An investigation of crack widhts close to the reinforcement and the correlation to service life

Abstract

One of the most important factors governing durability of concrete structures is

reinforcement corrosion, and to prevent degradation associated with it, it is desirable

to use large concrete covers. However, if the concrete cover is increased, the majority

of design methods will predict that the crack widths at the concrete surface increase

significantly. This can be contradictory, since the same design methods both specify

smallest concrete cover and require crack width limitations to prevent reinforcement

corrosion.

To fulfil crack control demands current European structural codes imply that the

engineer tend to use as small covers as possible, use small diameter reinforcement

bars with narrow spacing and place excessive amounts of crack reinforcement in

sections with high bending moments. All these measures are disadvantageous with

respect to durability, which is best achieved by increasing the concrete cover and

providing space for effective casting of the concrete. The structural codes focus on

limitation of crack widths at the concrete surface, which is well defined and

measurable. However, in view of durability the crack width close to the bars should

be more decisive for the durability and service life of the structure. The width of the

cracks close to the bar can be expected to control the total amount of free oxygen,

moisture and other corrosive agents transported via the crack to the reinforcement.

This thesis examines the crack width close to the bar surface and the correlation to

parameters such as steel stress, concrete cover, steel diameter and concrete quality.

The test results show that the crack widths on the concrete surface are of the order

twice the crack width close to the bar surface and that the crack width near the

reinforcement bar is much less affected by a change in concrete cover than the crack

width at the concrete surface.

The test results also showed that the crack width close to the bar is affected only to a

limited extent by bar diameter and concrete quality and that steel stress is the

dominating parameter. During the tests it was noticed that small inclined cracks

developed at the bar interface and in the vicinity of the crack so that the concrete near

the bar was separated from the rest of the concrete and more or less followed the bar.

This mechanism has similarities with pull-out failures, but is limited by the

continuous reinforcement. The formation of the small inclined cracks induced a cone

shaped concrete piece, which is the main reason for the different development of

crack widths close to the reinforcement and at the concrete surface.

Non-linear FE-calculations have also been performed, showing similar relations

between the crack width close to the reinforcement bar and the crack width at the

concrete surface, as the experimental tests described above. The FE-analysis

confirmed that the crack width close to the reinforcement bar was almost unaffected

by the thickness of the concrete cover. In the results from the FE-calculations the

formation of the cone shaped concrete piece during the tensile loading was described

in more detail.

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To evaluate the total effect of crack widths and concrete cover on the corrosion risk,

cracked beams exposed to chlorides have also been tested. The main conclusion from

this test series was that large concrete covers protect the reinforcement better than

small concrete covers from chloride induced corrosion for exposure from below, even

if the crack widths are larger at the concrete surface. It was also shown that the

corrosion risk increases for chloride exposure from above and for high steel stresses.

• author: Tammo, Kristian
• supervisor: Sven Thelandersson
• opponent: Professor Emborg, Mats, Institutionen för Väg- och Vattenbyggnad, Luleå Tekniska Universitet
• organization: Division of Structural Engineering
• publishing date: 2009
• type: Thesis
• publication status: published
• subject: Building Technologies
• keywords: concrete cover, corrosion, Crack width, steel stress and service life
• pages: 161 pages
• defense location: Lecture hall C, V-building, John Ericsson väg 1, Lund University Faculty of Engineering
• defense date: 2009-03-27 13:15:00
• language: English
• LU publication?: yes
• id: bdcebdb7-ea16-4fd9-ba57-ca1468564d18 (old id 1301937)
• date added to LUP: 2016-04-01 14:49:16
• date last changed: 2018-11-21 20:30:29