How Concrete Goes Bad
Somewhat surprisingly, concrete is porous. While it is very hard, it also has millions of
tiny pores and microscopic cracks. Looking at concrete under magnification reveals the
pore and capillary structure, and it is remarkable how many tiny voids there are.
There are a number of ways that concrete gets damaged and destroyed, but by far the
most common and widespread are salts and freeze/thaw cycles.
Salt and Water Get in the Pores and the Destructive Process Begins
Salts, chemicals like fertilizers, and freeze/thaw cycles compromise concrete.
Salt gets carried into the pores of the concrete by water and creates an acid-like reaction
inside the concrete, eating away the bonds in the cement portion of the concrete, and
creating voids that attract more water and salt. Salt is hydroscopic, which means that it
attracts water, up to 10% more water in the same space. Water is the real catalyst of
damage. Salt just makes it easier for water to get into the pores of concrete.
When water gets absorbed into the pores, it can expand up to 9% when it freezes. This
pressure creates cracks and larger voids in the interior structure of the concrete, creating
pathways for water and chemical intrusion. Sooner or later, the pressure from freezing
water will pop the surface of the concrete, exposing the aggregate, and creating an
unattractive look and a potential safety issue.
Poor Installation Makes Concrete Vulnerable to Damage
Engineering concrete is a lot more complicated than one might think. A lot of attention is
paid to curing conditions, but most of the damage to concrete is started when the mixture
is made. Concrete contains 3 basic ingredients: Portland cement, rocks (aggregates), and
water. The proportions of these ingredients are critical to the strength and performance of
concrete. If these proportions are off, even a little bit, concrete is weakened and durability
The most common mistake made with concrete is adding too much water to the mix to
make it easier to work. Too much water weakens the concrete and creates a pore structure
that is large, easily accessed, and lacking durability. The use of water-reducing
admixtures (plasticizers) in the mix allows one to have a workable mix while using the
appropriate amount of water to get strong and durable concrete.