Page 117 - Construction Waterproofing Handbook

3.6 CHAPTER THREE
● Differential movement
● Mildew and algae attack
● Chemical and pollution attack from chloride ions, sulfates, nitrates, and carbon dioxide
Chemical and pollution attack is becoming ever more frequent and difficult to contend
with. Chloride ions (salts) are extremely corrosive to the reinforcing steel present in all
structures, whether it is structural steel, reinforcing steel, or building components such as
shelf angles.
Even if steel is protected by encasement in concrete or is covered with a brick facade,
water that penetrates these substrates carries chloride ions that attack the steel. Once steel
begins to corrode it increases greatly in size, causing spalling of adjacent materials and
structural cracking of substrates.
All geographic areas are subject to chloride ion exposure. In coastal areas, salt spray is
concentrated and spread by wind conditions; in northern climates, road salts are used dur-
ing winter months. Both increase chloride quantities available for corrosive effects on
envelope components.
Acid rain now affects all regions of the world. When sulfates and nitrates present in the
atmosphere are mixed with water, they create sulfuric and nitric acids (acid rain), which
affect all building envelope components. Acids attack the calcium compounds of concrete
and masonry surfaces, causing substrate deterioration. They also affect exposed metals on
a structure such as flashing, shelf angles, and lintel beams.
Within masonry or concrete substrates, a process of destructive weathering called car-
bonation occurs to unprotected, unwaterproofed surfaces. Carbonation is the deterioration
of cementitious compounds found in masonry substrates when exposed to the atmospheric
pollutant carbon dioxide (automobile exhaust).
Carbon dioxide mixes with water to form carbonic acid, which then penetrates a
masonry or concrete substrate. This acid begins deteriorating cementitious compounds
that form part of a substrate.
Carbonic acid also causes corrosion of embedded reinforcing steel such as shelf angles
by changing the substrate alkalinity that surrounds this steel. Reinforcing steel, which is
normally protected by the high alkalinity of concrete, begins to corrode when carbonic
acid change lowers alkalinity while also deteriorating the cementitious materials.
Roofing systems will deteriorate because of algae attack. Waterproof coatings become
brittle and fail due to ultraviolet weathering. Thermal movement will split or cause cracks
in a building envelope. This requires that any waterproof material or component of the
building envelope be resistant to all these elements, thus ensuring their effectiveness and,
in turn, protecting a building during its life-cycling.
Finally, an envelope is also subject to building movement, both during and after con-
struction. Building envelope components must withstand this movement; otherwise, designs
must include allowances for movement or cracking within the waterproofing material.
Cracking of waterproofing systems occur because of structural settlement, structural
loading, vibration, shrinkage of materials, thermal movement, and differential movement.
To ensure successful life-cycling of a building envelope, allowances for movement must
be made, including expansion and control joints, or materials must be chosen that can
withstand expected movement.