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WHY DID THE REINFORCED CONCRETE ADJACENT TO THE REPAIRED AREA DETERIORATE SO SOON AFTER A MAJOR RESTORATION PROJECT?

12/15/2022

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I believe the logical answer to the posed question is that the selected restoration strategy did not achieve electrochemical compatibility of the steel reinforcement network within the repaired deck. This was likely because the concrete repair material accelerated the rate of corrosion of the adjacent rebar, which was corroding, but had not advanced to the stage where it had caused delamination of the concrete.
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The full technical reason for this phenomenon - often referred to as the ring or halo effect - is too involved to explain briefly within this blog. However, I’ve always found the Figure I have included below as very helpful in understanding what can happen.
Often the approach to restoration is to simply remove visibly deteriorated concrete while ignoring the fact that corrosion will continue within the surrounding concrete. However, the repaired areas will change from anodic to cathodic sites, thereby increasing the remaining cathode to anode ratio - this will increase the rate of corrosion of the remaining anodic sites. (as illustrated by the Figure) For this reason, I believe that the use of corrosion evaluation testing methods in conjunction with a delamination survey can be essential.

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WHAT IS THE SOLUTION?
Experiences both in the laboratory and on restoration projects have convinced me that a very effective and economical way to achieve electrochemical compatibility is to apply a zinc rich epoxy primer to the exposed reinforcing steel within the repair area; the zinc acts as a sacrificial anode, thereby mitigating corrosion activity within the steel.

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This and other restoration issue are included in
INVESTIGATING CONCRETE PROBLEMS
Learning from Those Who Learned the Hard Way!

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AN EXAMPLE OF WHY CAUSE ANALYSIS CAN BE CRUCIAL FOR THE SUCCESSFUL CONSERVATION OF HERITAGE MASONRY

12/15/2022

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PictureClick on photo above for a larger view.
Why did the natural stone units and joints continue to crack within this beautiful heritage building’s masonry - even though several attempts were made to repair them over decades using conventional crack repair and repointing materials and practices?
·  Well, the first clue is that the cracks are predominantly located close to window openings.
·  The second clue is that the window lintels and sills do not project beyond the jambs - thereby concentrating gravity loads from natural movement, wind, snow loadings, etc., at the jambs, rather than distribute them into the adjacent mass of masonry.

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Click on photo above for a larger view.
WHAT IS THE SOLUTION?

Well, for new buildings, designers can ensure the lintels extend at least beyond the first vertical (head) joint adjacent to the jambs - or they can install hidden steel lintels to achieve a similar effect.
For existing buildings, lengths of stainless steel helical rods can be installed into two horizontal  joints above the windows. The rods should be continuous across elevations, and located roughly at the centre of the exterior wythe, and embedded within a high strength grout. (The grouted rods perform the function of ‘transfer beams’ to distribute loads more uniformly within the masonry.) The front of the joint should then be face-pointed using a hydraulic lime-based mortar and the cracks repaired. The mortars should of course be selected to be compatible with the natural stone masonry.
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Click on photo above for a larger view.

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This and other restoration techniques are included in
CONSERVATION OF HERITAGE MASONRY
Canadian Project Case Studies

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    Paul Jeffs

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    Based in Guelph, Ontario, Paul Jeffs founded PJ Materials Consultants in 1989 to provide specialist consulting services for North American construction-related industries. Prior to this, he lived and worked in several countries, including the U.K., Bahrain, Iran and Japan, and provided technical advice to many construction projects within the regions surrounding those countries. Paul specializes in providing materials-related technology advice for the construction, protection and restoration of concrete and masonry - and especially for the conservation of heritage buildings and structures. 

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PJ Materials Consultants was formed by Paul Jeffs in 1989 to provide a wide range of specialist consulting services for concrete and masonry structures. The main business activity is divided between the provision of materials technology expertise and professional development technical training.