Civil Engineering Reference
In-Depth Information
•
Spalling
occurs as a definite depression caused by the
separation of surface concrete.
•
Scaling
occurs to a depth of 25 mm from the surface,
resulting in local peeling or flaking.
•
Cracking
occurs as d- or map-cracking and is sometimes
related to the aggregate performance.
Good resistance to frost expansion can be obtained by proper mix propor-
tioning and choice of materials. In addition to w/c ratio, quality of aggregate
and proper air entrainment, the frost resistance depends on the exposure
conditions. Dry concrete will withstand freezing-thawing, whereas highly
saturated concrete may be severely damaged by a few cycles of freezing and
thawing.
According to many workers, frost damage is not, necessarily,
connected with the expansion during freezing of water, although it can
contribute to damage. When a water-saturated porous material freezes,
macroscopic ice crystals form in the coarser pores and water, which is
unfrozen in the finer pores, and migrates to the coarser pores or the
surfaces.
[30]
The large ice crystals can feed on the small ice crystals, even
when the larger ones are under constraint and cause expansion.
Deterioration of plain concrete due to de-icing salts may generally
be termed
salt scaling;
it is similar in appearance to frost action but more
severe. Any theory on salt scaling should account for this increased damage.
The most widely used test for assessing the resistance of concrete to
freezing and thawing is the ASTM test on
Resistance of Concrete to Rapid
Freezing and Thawing
(ASTM C666). The general approach to preventing
frost attack in concrete is to use an air-entraining admixture. Tiny bubbles
of air are entrapped in concrete due to the foaming action developed by the
admixture during mixing.
15.0 SEA WATER ATTACK
The deterioration of concrete due to sea water attack is the result of
several simultaneous reactions. However, sea water is less severe on
concrete than can be predicted from the possible reactions associated with
the salts contained in it. Sea water contains 3.5% salts by weight. They
include NaCl, MgCl
2
, MgSO
4
, CaSO
4
, and possibly KHCO
3
.
