Building Management Hawaii - April/May 2012 - (Page 10)
Corroding Rebar and Spalling Concrete
Why zinc can be a spalling solution.
By Damien Enright
s we are all too aware, corrosion and spalling are huge maintenance problems for building managers in Hawaii. Our frequent morning rains and salt-laden breezes take their toll on unprotected surfaces. In fact, corrosion is the single most important factor in the deterioration of reinforced concrete structures and buildings. If concrete is not proactively maintained, maintenance fees will creep up and the cost for concrete repairs will increase dramatically. You risk the loss of use of your structures, or, even worse, open yourself up to possible accidents, liability and unwanted risk.
The Rusty Facts
Two key factors—chloride (salt) contamination and dissimilar metals—are at the root of our corrosion issues.
• Chloride in concrete cancels out the alkalinity of Portland cement (used in most concrete construction). Salt creates a low pH level, which makes reinforcing steel susceptible to corrosion. • Dissimilar metal corrosion is when different materials make contact with each other and cause oxidation and corrosion. This is especially an issue on lanai and walkways where a piece of rebar embedded in the concrete is adjacent to, or in contact with, an aluminum railing post. The contact between the dissimilar metals creates a galvanic cell (or a chemical reaction that produces electrical energy) that accelerates corrosion. Corroding rebar can grow to 10 times its original cross section. It expands within the concrete and causes it to crack and spall. Proper repair requires removal of unsound concrete with light chipping hammers, thorough cleaning of rust and corrosion on the
remaining rebar, protecting new rebar with a corrosion inhibitor, and then placing a high quality repair mortar to match the existing construction.
No Quick Fix
Unfortunately, the story does not end there. The placement of new chloride-free patching mortar next to the existing chloride-contaminated concrete creates another galvanic cell. The electrochemical current generated at the bond line speeds up corrosion in the immediate area. This phenomenon is called the ring anode effect or the halo effect. Innumerable association board members complain, “We completed a big spall repair project just a few years ago and already I’m starting to see cracking and I swear we did repairs in that area.” Sadly, they are experiencing the ring anode effect. The repair mortar consists of good new material, but failures
Rebar embedded in the concrete is in contact with an aluminum railing causing the dissimilar metals to oxidize, rust and create a dangerous situation, as shown looking up through the bottom of this lanai.
April - May 2012
Table of Contents for the Digital Edition of Building Management Hawaii - April/May 2012
Contents Building Management Hawaii, April/May 2012
DEPARTMENTS: Concrete Restoration
If You’re Not Testing, You’re Guessing. Diagnose your concrete woes to save time and money.
Corroding Rebar and Spalling Concrete. Why zinc can be a spalling solution.
Top Five Fixes for Spalling. Preventing the corrosion of your building.
Trees – Our Green Assets. How not to plant the wrong tree in the wrong place.
Watts Up? Watts Down!. What you don’t know about parking lots could cost you big bucks.
Covered Parking & Photovoltaics: A Symbiotic Relationship. Covered Parking & Photovoltaics: A Symbiotic Relationship
Fire Prevention & Response
Top 10 Fire Fighters. Preventing fires is a job for everyone, but condo boards and property managers have a particular responsibility.
Keep a Clean Chute. Maintainance of trash chutes prevents serious high-rise fires
Stop, Drop & Go Wireless. New technology aids in fast response and saving lives.
Insurance: Do Condo Owners ‘Get’ Your Master Policy?
EDITORIAL: Editor’s Note
Ask An Expert: Epoxy vs. Regular Rebar
Movers & Shakers
Resource Guide: Concrete Restoration & Asphalt
Building Management Hawaii - April/May 2012