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Concrete damage requires complete restoration.

There is no simple solution or fast fix to solve the problems of water penetration, a primary cause of the deterioration of a building's facade. Because water damage is directly related to the design of the building, it is critical for certain elements of the structure to be correctly positioned and properly constructed to manage the direction of the flow of water.

In the case of reinforced concrete construction, there are several factors that lead to the deterioration of the concrete material. For one, many concrete buildings have exposed spandrel beams or columns as part of their design vocabulary, leaving the materials subject to the wear and tear of the elements. And concrete is a porous material, sometimes highly permeable to water. Precipitation, particularly acidic in the New York City area, slowly alters the chemistry of the concrete and reduces its alkalinity, a process known as carbonation. Carbonated concrete no longer acts as an anti-corrosion buffer for the embedded reinforcing steel, thereby encouraging the formation of rust.

Normally, concrete can adequately weather the storm, provided there is a sufficient thickness of concrete covering the reinforcing steel bars. But when steel reinforcing bars are incorrectly placed during construction, they can be too close to the surface leaving them susceptible to the effects of acid rain. (The accepted standard of design and construction industries is a minimum of one and one-half to two inches of concrete over steel.)

Another problem arises from the presence of chlorides in the concrete that are added during construction to accelerate hardening. The chlorides change the electrolytic balance of the concrete resulting in accelerated corrosion on the steel. Still another problem occurs when repeated freeze/thaw cycles act directly on the concrete disrupting its matrix. The deterioration process begins on a microscopic level but with repeated exposures to the fluctuating temperatures, the concrete eventually crumbles.

As these various damages take place, associated cracking that occurs allows more water to get in, further accelerating the actions of water against the concrete. Steel reinforcing bars continue to corrode and rust builds up exerting pressure against the concrete and pushing it away from the building, resulting in visible cracking and spalling.

WASA is currently involved in a concrete rehabilitation project on an award-winning cooperative apartment project built in the 1970's. This multi-building complex in Manhattan has been chronically suffering from spalling concrete; repeated repair projects using cementitious patches have failed to provide a lasting solution.

A major condition assessment was undertaken by WASA, evaluating the reinforced concrete structures with exposed spandrel beams at every floor, brick panels composed of eight-inch square bricks between spandrels, and aluminum frame windows. All of the buildings have projecting balconies, each with its own brick parapet.

The study revealed a number of problems. There were approximately 11,000 locations of spalling, half of which were patches going bad. The remaining areas of concrete were spalling for the first time. Other conditions were brought to light. The original concrete had been made with lightweight aggregate, rendering it more permeable to water and susceptible to carbonation. And in many areas, the reinforcing steel was placed much too close to the surface of the concrete.

Future spalling was inevitable unless corrective action was undertaken, action that provided a permanent solution. This meant replacing all the surface concrete, involving removal of the face of the concrete between two to three inches in depth on all the spandrel beams. The steel reinforcing bars were cleaned of all corrosion and coated with corrosion-inhibiting paint. The spandrel beams were recast using a cementitious patching material. A slightly modified design to the spandrel beams provides greater coverage over the existing steel reinforcing bars. The new concrete was designed with a steep wash to permit water to run off and prevent snow build-up above the face.

During the study, WASA also determined that the flashings were inadequate, and thus the brickwork above the concrete spandrels was absorbing water and allowing it to penetrate the concrete on which the bricks were sitting. To correct this, one course of brickwork was removed above every spandrel beam and flashing was replaced, with weep holes, to ensure water in the wall system would be led away from the building.

The project is being accomplished in phases and is expected to be completed in 1993, giving the residential complex a new lease on life.

As this example illustrates, repairing concrete damage is often an involved process. A full investigation is needed to determine the underlying causes responsible for what appear to be "obvious" problems that require thoughtful appropriate attention.
COPYRIGHT 1992 Hagedorn Publication
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Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Popkin, Bruce W.
Publication:Real Estate Weekly
Date:Oct 28, 1992
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