Civil Engineering Reference
In-Depth Information
(CO
2
) in the exhaust must be vented to the outside and
prevented from reacting with the fresh concrete
(Fig. 14-23). Direct-fired units can be used to heat the
enclosed space beneath concrete placed for a floor or a
roof deck (Fig. 14-24).
Hydronic systems transfer heat by circulating a
glycol/water solution in a closed system of pipes or hoses
(see Fig. 14-25). These systems transfer heat more effi-
ciently than forced air systems without the negative effects
of exhaust gases and drying of the concrete from air move-
ment. The specific heat of water/glycol solutions is more
than six times greater than air. As a result, hydronic heaters
can deliver very large quantities of heat at low temperature
differentials of 5°C (10°F) or less between the heat transfer
hose and the concrete. Cracking and curling induced by
temperature gradients within the concrete are almost elim-
inated along with the danger of accidentally overheating
the concrete and damaging long-term strength gain.
Typical applications for hydronic systems include
thawing and preheating subgrades. They are also used to
cure elevated and on-grade slabs, walls, foundations, and
columns. To heat a concrete element, hydronic heating
hoses are usually laid on or hung adjacent to the structure
and covered with insulated blankets and sometimes plas-
tic sheets. Usually, construction of temporary enclosures
is not necessary. Hydronic systems can be used over areas
much larger than would be practical to enclose. If a
heated enclosure is necessary for other work, hydronic
hoses can be sacrificed (left under a slab on grade) to
make the slab a radiant heater for the structure built
above (
Grochoski 2000
).
Any heater burning a fossil fuel produces carbon
dioxide (CO
2
)
; this gas will combine with calcium hydrox-
ide on the surface of fresh concrete to form a weak layer of
calcium carbonate that interferes with cement hydration
Fig. 14-25. Hydronic system showing hoses (top) laying on
soil to defrost subgrade and (bottom) warming the forms
while fresh concrete is pumped in. (68345, 68344)
(
Kauer and Freeman 1955
). The result is a soft, chalky
surface that will dust under traffic. Depth and degree of
carbonation depend on concentration of CO
2
, curing
temperature, humidity, porosity of the concrete, length of
exposure, and method of curing. Direct-fired heaters,
therefore, should not be permitted to heat the air over
concreting operations—at least until 24 hours have
elapsed. In addition, the use of gasoline-powered con-
struction equipment should be restricted in enclosures
during that time. If unvented heaters are used, immediate
wet curing or the use of a curing compound will minimize
carbonation.
Carbon monoxide (CO), another product of combus-
tion, is not usually a problem unless the heater is using
recirculated air. Four hours of exposure to 200 parts per
million of CO will produce headaches and nausea. Three
hours of exposure to 600 ppm can be fatal. The American
National Standard Safety Requirements for Temporary
and Portable Space Heating Devices and Equipment
Used in the Construction Industry (ANSI A10.10) limits
concentrations of CO to 50 ppm at worker breathing
levels. The standard also establishes safety rules for venti-
lation and the stability, operation, fueling, and mainte-
nance of heaters.
Fig. 14-24. A direct-fired heater installed through the
enclosure, thus using a fresh air supply. (69875)
