Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.27
Polymer concretes, such as methyl methacrylate and unsaturated polyester, in
which a polymer replaces portland cement may have more than double the strength
and modulus of elasticity of portland-cement concrete. Creep is less and resistance
to freezing and thawing cycles is higher with the polymer concretes. After curing
for a very short time, for example, overnight at room temperature, polymer concretes
are ready for use, whereas ordinary concrete may have to cure for about a
week before exposure to service loads.
Monomers and polymers may be used as admixtures for restoring and resurfacing
deteriorated concrete surfaces. Latexes of methyl methacrylate, polyester, styrene,
epoxy-styrene, furans, styrene-butadiene, and vinylidene chloride have been
employed for these purposes. The resulting concrete hardens more rapidly than
normal concrete. A polymer admixture may also be used to improve the bonding
properties of portland cement. Inserted in a mix as an emulsion for this purpose,
the admixture supplies a significant amount of water to the mix, which becomes
available for hydration of the cement. The release of the water also sets the emulsion.
Hence, moist curing is not desirable, inasmuch as the emulsion needs to dry
to develop the desired strength. A grout or mortar containing the bonding admixture
develops a higher bond strength when applied as a thin layer than as a thick one
and the bond may be stronger than materials being joined.
Impregnation of concrete with polymers is sometimes used to harden surfaces
exposed to heavy traffic. Strength and other properties of the impregnated concrete
are similar to those of concrete in which polymers replace portland cement. Impregnation
is achieved by first drying the concrete surface with heat and then soaking
the surface with a monomer, such as methyl methacrylate, styrene, acrylonitrile,
or tert-butyl styrene. It is subsequently cured with heat.
Slab Toppings. At least partly because of excellent adhesion, epoxies are formulated
with sand and other fillers to provide surfacing materials for concrete.
Unlike standard concrete topping, epoxy-based surfacing materials can be thin.
They are especially useful for smoothing uneven, irregular surfaces. The epoxy
cures quickly, allowing use of the surface in a short time.
Grout. Cracked concrete can be repaired with an epoxy grout. The grout is forced
into cracks under pressure for deep penetration. Because of its good bonding
strength, the epoxy grout can largely restore strength, while, at the same time,
sealing the crack against penetration by liquids.
(‘‘Polymers in Concrete,’’ ACI 548; ‘‘Guide for the Use of Polymers in Concrete,’’
ACI 548.1; and ‘‘Polymer Modified Concrete,’’ SP-99, American Concrete
Institute.)
4.19 CONCRETE MASONRY UNITS
A wide variety of manufactured products are produced from concrete and used in
building construction. These include such items as concrete brick, concrete block
or tile, concrete floor and roof slabs, precast wall panels, precast beams, and cast
stone. These items are made both from normal dense concrete mixes and from
mixes with lightweight aggregates. Concrete blocks are made with holes through
them to reduce their weight and to enable masons to grip them.
Nominal size (actual dimensions plus width of mortar joint) of hollow concrete
block usually is 8 � 8 � 16 in. Solid blocks often are available with nominal size