U.S. patent number 3,676,641 [Application Number 05/106,798] was granted by the patent office on 1972-07-11 for apparatus for assisting in the curing of concrete and for heating.
Invention is credited to Wallace A. Olson.
United States Patent |
3,676,641 |
Olson |
July 11, 1972 |
APPARATUS FOR ASSISTING IN THE CURING OF CONCRETE AND FOR
HEATING
Inventors: |
Olson; Wallace A. (Sioux Falls,
SD) |
Family
ID: |
22313315 |
Appl.
No.: |
05/106,798 |
Filed: |
January 15, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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770163 |
Oct 24, 1968 |
|
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Current U.S.
Class: |
219/200;
219/213 |
Current CPC
Class: |
H05B
3/342 (20130101); H05B 3/00 (20130101); H05B
2203/026 (20130101); H05B 2203/003 (20130101); H05B
2203/033 (20130101); H05B 2203/014 (20130101) |
Current International
Class: |
H05B
3/34 (20060101); H05B 3/00 (20060101); H05b
001/00 () |
Field of
Search: |
;219/213,345 ;249/78
;25/10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Albritton; C. L.
Parent Case Text
This application is a division of copending parent application Ser.
No. 770,163, filed Oct. 24, 1968.
Claims
I claim:
1. A flexible rollable heat control blanket comprising:
a first layer composed of a thin, flexible, moisture-impervious,
heat-conductive and electrically-insulative plastic material;
a second layer contiguous with said first layer and composed of a
thin, flexible and electrically conductive material productive of
heat in response to the application of an electric potential across
opposing edge portions thereof;
a third layer contiguous with said second layer and composed of a
thin, flexible moisture-impervious and electrically insulative
plastic material, at least one of said first and third layers being
of the color black so as to inhibit penetration of electromagnetic
waves in the ultraviolet and near-ultraviolet region of the
spectrum;
a fourth layer contiguous with said third layer and composed of a
comparatively substantially thicker, flexible heat-insulative and
resilient material of foam vinyl substance, the exposed surface of
said fourth layer opposite said third layer being of a very light
color including one of the colors yellow and white so as to be
substantially reflective to electromagnetic waves in the visible
region of the spectrum;
and a flexible reinforcing web, composed of interlaced strings of
nylon exhibiting substantial tensile strength, buried within said
foam vinyl substance and spaced from and generally parallel to said
exposed surface and said third layer.
Description
The present invention pertains to apparatus for assisting in the
curing of concrete. It also relates to electrically-energized
heating apparatus.
Numerous techniques have heretofore been employed in an effort to
accelerate the curing or hardening of concrete. For example,
electric current has been actually conducted through the mix
itself, in some cases utilizing included reenforcing bars as at
least one of the electrodes. As another approach, hot water or
steam has been conducted through tubing included in form structures
for the purpose of hastening the curing process.
Accelerated curing of the concrete is particularly important from
an economic standpoint in the fabrication of such large structural
members as pre-stressed pilings, beams and other sections used in
the construction of concrete bridges and the like. These elements
typically are cast in giant forms that occupy substantial space and
in themselves are very expensive. Absent accelerated curing, the
concrete often must remain in the form for a period of perhaps
seven days. Successful acceleration of the curing can permit
removal of the concrete casting within 12 to 48 hours, permitting a
much greater volume of production from each form. Also in field
construction, accelerated curing is more economical in that it
permits form removal after a shorter time interval.
In addition to reducing the time required for curing, it is also
desired, and indeed required, that the member exhibit a certain
minimum compressive strength. That strength typically is specified
on the basis of measurements of core samples taken at the end of
the initial curing period, 3, 7, or 28 days following the original
pouring of the concrete mix. Some of the prior approaches to the
acceleration of concrete have been deficient in that the ultimate
compressive strength is too low. Other approaches have been
excessively costly in terms of the apparatus necessary to implement
them or the cost of the energy employed for the purpose of heating
the mix. Difficulties have also been encountered by such effects
upon the concrete as creep, shrinkage, camber, warpage, cracks,
checks, alligatoring, surface dusting, discoloration, release from
the forms and the like.
It is a general object of the present invention to provide new and
improved apparatus for curing concrete that overcome or at least
minimize the aforenoted deficiencies, inefficiencies and
difficulties.
Another and particular object of the present invention is to
provide apparatus for accelerating the curing of concrete in
minimum time while at the same time maximizing ultimate compressive
strength.
A further object of the present invention is to provide new and
improved electrically-energized heating apparatus.
A feature of the invention is a blanket, which may be laid upon the
exposed upper surface of the concrete mix, that includes a first
layer of flexible moisture-impervious material substantially
reflective to electromagnetic waves in the ultraviolet and near
ultraviolet regions of the spectrum and is shaped to overlie the
upper surface. Also included is a second layer of likewise-flexible
material that is substantially reflective to the electromagnetic
waves in the visible regions of the spectrum and is affixed to and
across the upper surface of the aforementioned first layer. A
related feature is a heating element that may be included in the
blanket or which may be incorporated into apparatus including a
hollow frame and associated layers respectively of heat-conductive
and heat-insulative materials.
The features of the present invention which are believed to be
novel are set forth with particularity in the appended claims. The
organization and manner of operation of the invention, together
with further objects and advantages thereof, may best be understood
by reference to the following description taken in connection with
the accompanying drawings, in the several figures of which like
reference numerals identify like elements, and in which:
FIG. 1 is a perspective view, partially broken away, of a first
embodiment of apparatus for accelerating the curing of
concrete;
FIG. 2 is a fragmentary cross-sectional view taken along the line
2--2 in FIG. 1;
FIG. 3 is a plan view of an electrically energized heating element
utilized in the apparatus of FIG. 1;
FIG. 4 is a perspective view, partially broken away, of an
embodiment alternative to that shown in FIG. 1;
FIG. 5 is a fragmentary cross-sectional view taken along the line
5--5 in FIG. 4;
FIG. 6 is a fragmentary perspective view showing a heating element
of a kind utilized in the embodiment of FIG. 4;
FIG. 7 is a fragmentary cross-sectional view of still another
embodiment of accelerated-curing apparatus;
FIG. 8 is a perspective view, partially broken away, of an
electrically energized heating apparatus;
FIG. 9 is a fragmentary cross-sectional view taken along the lines
9--9 in FIG. 8;
FIG. 10 is a cross-sectional view of a still different embodiment
of electrically-energized heating apparatus; and
FIG. 11 is a plot of curves illustrating one manner of operating
the embodiments depicted by the preceding figures.
FIG. 1 illustrates a giant casting bed or form 20 utilized to mold
a T-shaped casting 21 of pre-stressed concrete. The form includes
spaced upright panels 22 and 23 that define the vertical member of
the T and laterally projecting sections 24 and 25 that define the
underside of the horizontal portions of the T. Moreover, sections
24 and 25 are turned upwardly at their outer ends so as to
constitute side walls of the form for the horizontal T portion.
Sections 24 and 25 are joined respectively to sections 22 and 23 by
still additional slanted sections 26 and 27. A further section 28
is joined between the bottom edges of sections 22 and 23 to
complete the form itself. Section 28 is disposed across a plurality
of spaced horizontal beams 29 joined at their respective ends by
additional beams 30 from which upwardly project studs 31 on the
upper ends of which rest sections 24 or 25, as the case may be, on
opposite sides of the overall apparatus, As such, casting forms
having a cross-sectional shape as shown, as well as others having a
wide variety of different shapes, are well-known in the
pre-stressed concrete industry. Such forms have at times included
fluid-carrying conduits formed into the structure of sections 22-28
in order to convey heat by means of steam or like in an effort to
accelerate curing of the concrete mix placed into the form.
In FIG. 1, however, heating for the purpose of accelerating curing
of the concrete is achieved by means of an electrically energized
element 32 disposed in substantially convective contact with the
external surfaces of the form. Heating element 32 is itself
fabricated so as to produce heat substantially uniformly over the
form surface. To this end, heating element 32 includes a
heat-conductive wire mesh 33 cemented or otherwise affixed directly
to the exposed exterior surface of sections 22-28. A heat-producing
electrically conductive insulatingly covered length of resistance
wire 34 is folded back and forth across the surface of and affixed
to mesh 33.
Also disposed at least substantially in convective contact with
individually different ones of sections 22-28, as by being affixed
directly to mesh 33 in the manner illustrated, are a plurality of
temperature-sensing elements 35 distributed over the outer surface
of the form in correspondence with separately controlled units of
resistance wire 34. Capillary tubes 36 extend individually from the
respective different sensors 35 to thermostatic control units 37.
The length of each separate unit of resistance wire 34 is selected
in view of the supply source potential and available current
capacity, its own heat dissipation per unit length and the desired
heat output per unit of surface area. Depending upon the routine
selection from among these different variables, physically separate
sections of the folded heating wire may be joined together in
appropriate series or parallel combinations. However, each such
unit is separately energized from a conventional power source,
typically a 110 or 220-volt supply of 60 cps alternating current,
by the actions of the individually different thermostatic
controllers 37 in accordance with their different thermal settings
and the respective different temperatures determined by sensors 35.
To conserve energy, the side of resistance wire 34 opposite the
form preferable is covered by a layer 38 of heat-insulating
material such as asbestos.
As shown in FIG. 1 for clarity of illustration, wire 34 is folded
back and forth with a comparatively wide-spacing between the folds.
In actual practice, the spacing between adjacent folds is
preferably more narrow in order to obtain evener heating. In a
typical example, the fold spacing is only about 11/2 inches. The
wire, covered with a vinyl or other insulating sleeve, is in either
solid or stranded form of a conductor such as copper typically of
about 16 guage. The total length of each section is selected to
achieve a dissipatation of between 10 and 35, and preferably about
the latter, watts per square foot of form area heated. Mesh 33 in
this example is a section of ordinary window screen of about 26
guage galvanized steel wire. The mesh is pressed into channels 39
as shown in FIG. 2. At spaced portions along the length of the
wire, a metal staple 40 projects through the mesh and clips around
insulation 41 covering the conductive wire, as depicted in FIGS. 2
and 3. The resulting construction serves admirably to ensure
dissipation of the heat into the concrete mix substantially
uniformly over the entire heated area of the form.
Preferably covering the upper surface of mix 21 is a sheet 42 of
moisture-impervious material in order to prevent evaporation to the
atmosphere of water from the mix. Disposed on top of sheet 42 is
another electrically energized heating element 43 that in this case
again is in the form of a resistance-wire conductor 44 folded back
and forth across and affixed to a heat-conductive wire mesh 45.
Temperature sensors 46 are distributed over mesh 45 in order to
sense the temperature in the region of each different heater unit
and supply a corresponding signal to a thermostatic controller 47
that controls the supply of energizing power to the heating element
as indicated schematically in FIG. 1. Finally, a layer 48 of
heat-insulating material is disposed to overlie conductor 44 and
mesh 45. As shown, but a single energizing connection is provided
between controller 47 and conductor 44; in practice, conductor 44
is segmented into a plurality of smaller units each individually
controlled and energized in response to the respective different
temperature sensors.
Because details in the manner of operating the FIG. 1 apparatus
apply also to others of the structural embodiments to be discussed
herein, those other structures will next be described after which
attention will be directed to the operation. Looking then at the
apparatus of FIG. 4, a form 50 is shaped to define a pair of
adjacent U-shaped channels 51 and 52 in which concrete mix is
placed to mold a corresponding pair of square beams 53 and 54. The
exposed ends of beams 53 and 54 have been offset in FIG. 4 to more
clearly depict the shape of the channels; in use, of course, an end
plate encloses the ends of each of the channels and the mix
completely fills the space bounded by the forms.
Secured against the exterior heat-conductive channel walls are a
plurality of electrically energized heating panels 55 each of which
have power-connecting leads 56. Disposed in thermally conductive
contact with the channel wall adjacent to each panel 55 is a
temperature sensing element 57 from which extends a capillary tube
58 that, in the manner of FIG. 1, leads to a thermostatic
controller which controls the supply of power by way of leads 56 to
panels 55.
The individual heating panels are composed of a layer 59 of
electrically conductive material such as carbon sandwiched between
a pair of layers 60 and 61 of electrically insulating material,
such as asbestos, with at least layer 61, which is in convective
contact with the wall of the thermally conductive form material,
being also substantially heat-conductive. Also included between
layers 60 and 61 in electrical contact with spaced portions of
layer 59 are electrically conductive strips 62 to which individual
different ones of each pair of leads 56 are respectively
electrically coupled by connectors 63. The application of electric
power by way of leads 56 effects the flow of current in conductive
layer 59 and the resultant dissipation of heat therefrom by virtue
of resistance heating. To conserve heat, external layer 60
preferably is covered with a sheet 64 of thermally insulating
material.
Covering the upper surface of the mix from which beams 53 and 54
are formed preferably is a moisture-impervious sheet 65 such as a
plastic, metal foil or, conveniently for handling, a thin sheet of
aluminum. A plurality of additional heating panels 66, formed in
the same manner as panel 55, are distributed across sheet 65 with
each panel 66 having its respective power-connecting leads 67.
Similarly in this case, one or more temperature sensors 68 are in
thermal contact with sheet 65 and serve through thermostatic
controllers to control or regulate the supply of energizing power
to panels 66.
A modified electrically energized heating panel is shown in FIG. 6
wherein a layer 69 of electrically conductive material is
sandwiched between a pair of layers 70 and 71 that are preferably
of plastic in order to be both highly flexible and
mositure-impervious. By reason of the latter character of layers 70
or 71, the resulting heating panel may, as for example in the case
of covering and heating the exposed upper surface of the mix in the
forms of either FIGS. 1 or 4, be placed directly against the mix.
Included between layers 71 and 70, running along opposed edges of
the panel, are strips 72 and 73 of electrically conductive material
that are in contact with conductive layer 69 so as to serve as
spaced electrodes for conducting current through the conductive
layer. To that end, connector terminals 74 are secured by rivets 75
to strips 72 and 73 respectively. Instead of an actual sandwich
formation, the electrically conductive material in the devices of
either FIGS. 6 and 7 may be directly impregnated in and distributed
throughout a carrier material such as a plastic. Another suitable
insulating material is fiberglass or so-called glass cloth.
To facilitate quick and simple attachment of the heating panels to
the walls of the form, the external surface of layer 70 in this
instance is coated with a contact-type adhesive 76. For ease of
handling the panel prior to installation, adhesive layer 76 is
covered with a thin sheet 77 of a protective material such as
paper. To apply the panel to a surface of the form, the installer
merely peels off layer 77 and presses the panel directly against
the wall of the form whereupon adhesive 76 holds it in place. In
this way, defective panels may be quickly substituted and, after
use with temporary forms such as those often set up in field
construction, the panels may be removed for subsequent use
elsewhere. The flexibility of layers 69-71 together with the use of
adhesive 76 permits attachment of the panels to form surfaces of
irregular shapes.
For particular use directly upon exposed concrete mix surfaces,
such as over the top of the mix placed into the forms of FIGS. 1
and 4 or on the upper surface of concrete slabs in the case of
pavement or sidewalk construction, the heating unit of FIG. 7 is in
the form of a lightweight flexible blanket 78. Like in the case of
FIG. 6, the underside of blanket 78 includes an electrically
conductive layer 79 of a material such as graphite across spaced
portions of which electrically conductive power-connecting strips
(not shown) are connected. Conductive layer 79 is sandwiched
between a pair of layers 80 and 81 with at least layer 81 being
both heat-conductive and moisture-impervious. For simplicity of
fabrication, both layers 80 and 81 are in this case formed of
plastic. Secured to and disposed over the surface of upper layer 80
is a sheet 82 of heat-insulative material preferably composed of
foam vinyl or other resilient substance. Imparting strength to
blanket 78 against tearing of sheet 82 is a preferably included web
83 or reenforcing material formed by interlaced strings of a tough
material such as nylon. For purposes to be further discussed
hereinafter, at least the upper, exposed surface 84 of sheet 82 is
a bright color, such as white or yellow, in order to be highly
reflective to light or electromagnetic waves in the visible portion
of the spectrum. At the same time, preferably both layers 80 and
81, as well as conductive layer 79 when composed of graphite, are
of a black color in order to be highly reflective of
electromagnetic radiation in the ultraviolet and near-ultraviolet
regions of the spectrum.
To enable placement over a generally flat and horizontal surface to
be heated, the apparatus of FIG. 8 includes a hollow, rectangular
frame 85 composed of channels 86 secured together and preferably
formed of a lightweight material such as aluminum. Diagonally
opposite corners of frame 85 and oppositely spaced other portions
of the frame are interconnected by struts 87 in order to give
rigidity to the unit. The resulting frame thus has its major
dimensions horizontal and its sidewalls define both an open top and
an open bottom.
Disposed between those sidewalls across the open top is a layer 88
of heat-conductive material. In this case, layer 88 is a wire mesh
of the kind shown and described with respect to FIG. 3 secured
around its periphery to the upper surface of channels 86.
Similarly, an electrically conductive resistance wire 89 having an
electrically insulative covering is distributed across the surface
of mesh 88 by being folded back and forth and pressed into channels
formed in the mesh. Located so as to overlie mesh 88 and conductor
89 is a sheet 90 of heat-insulating material, in this case a sheet
of conventional building insulation having an insulating filler 91
and an aluminum-foil backing 92 which is placed directly over the
heating wire 89. Disposed across the upper or outer surface of
filler 91 is a thin sheet 93 preferably again of a lightweight
material such as aluminum. Sheet 93 thus also is
moisture-impervious so as to shield against the effects of
precipitation in an outdoor environment.
In use, the FIG. 8 unit may be placed directly over a horizontally
disposed quantity of concrete mix such as exists in the case of the
formation of a slab. In this case, the actual heating unit,
composed of mesh 88 and resistance wire 89, is spaced by the width
of channels 86 from the mix. This is particularly advantageous in
certain field applications wherein the concrete mix includes an
extremely course aggregate such as stones of considerable size. In
that situation, a stone near the upper surface of the mix can act
to concentrate and reflect back into the heating element an intense
quantity of heat, in some cases sufficient to scorch or even
destroy a section of the heating element when the latter is placed
directly upon the mix surface. That result is avoided with the FIG.
8 apparatus by virtue of the fact that the heating element is
spaced a short distance away from the upper mix surface and thus is
not in direct thermal contact therewith.
An auxillary use for the FIG. 8 apparatus is particularly valuable
during conditions of freezing weather. With the ground frozen, it
is, of course, difficult to dig into the earth in order, for
example, to excavate the earth in preparation for pouring a
concrete slab. However, simply by placing the FIG. 8 apparatus over
the region to be excavated and energizing resistance wire 89,
sufficient heat is dissipated into the ground comparatively quickly
to thaw the earth even to a depth of several feet. In a typical
practical embodiment for that purpose, the sizing and layout of
resistance wire 89 is chosen so as to dissipate approximately 70 to
100 watts per square foot. As compared with laying a blanket such
as that shown in FIG. 7 directly upon the ground, the apparatus of
FIG. 8 also is advantageous when used from ground thawing, again
because channels 86 space the heating element a short distance
above the ground so that large rocks or other objects on or near
the earth's surface cannot concentrate and reflect back sufficient
heat to damage the heating unit. In use of this apparatus for
thawing the ground, it is also advantageous in that the thawing
occurs with a dispersal of the moisture that is unfrozen. This
contrasts with the use of steam for ground thawing which results in
the earth becoming muddy.
FIG. 10 illustrates a heating unit generally similar in shape to
that of FIG. 8 and particularly adapted for the curing of concrete
slabs molded in a horizontally disposed form 94 having its major
dimensions horizontal and its sidewalls 95 defining an open top. In
at least most cases involving the formation of a slab whose
thickness is small compared with its length and width, sufficiently
uniform accelerated curing may be effected by heating only the
exposed upper surface of the mix, as in the case of mix 96 in FIG.
10. To this end, the heating unit includes a rectangular frame 97
in this case formed of angle iron shaped to mount upon the upper
periphery of sidewalls 95. Stretched horizontally across the frame
so as to overlie the upper surface of mix 96, across the open top
of form 94, is a layer 98 of a heat-conductive material such as a
sheet of aluminum. An electrically energized heating element is
disposed substantially in convective contact with layer 98, and in
order to produce heat substantially uniformally over this surface
the element is again composed of a length or lengths of resistance
wire 99 folded back and forth so as to be distributed over layer
98. Overlying heating element 99 is a layer 100 of heat-insulative
material. To strengthen the entire assembly as well as to permit
rougher handling, layer 100 preferably is covered by a sheet 101 of
a material such as aluminum the outer edge portions of which are
bent downwardly and secured to frame 97.
Turning now to the mode and manner of operation of the various
different apparatus described above, it is helpful first to
consider certain background material. The binding material utilized
in the formation of concrete is commonly called cement. The
concrete itself is the resulting hard mass that is formed from a
mixture of cement, certain additives, an aggregate and water. The
aggregate constitutes a filler and typically consists of various
different proportions of sand, gravel and stone. The additives may
include air entraining compounds and materials that are intended to
produce better or quicker initial setting of the mix. A typical mix
is composed of 94 pounds cement, 169 pounds of moist sand, 336
pounds of moist gravel and 41/2 gallons of water. In general, most
building construction involves the use of five to seven 90 pound
sacks of cement for each cubic yard of aggregate.
The cement itself is manufactured by driving out the moisture from
natural chemical compounds in a high temperature kiln. Such
compounds are composed primarily of lime, silica, certain clays and
alumina. The resulting mass derived from the kiln is a solid
clinker that subsequently is ground and screened to provide a
powdery material. In driving out the moisture from the natural
chemical by the use of heat, the chemical and physical change in
the natural material involves the absorportion of energy by the
material. The molecular structure is significantly changed so as to
no longer have a link of water; it is dehydrated. Thus, the
resulting cement, even though in powder form, retains latent
energy. When this energy subsequently is released, as water is
added to the mix, the reaction is exothermic so that the mix gives
up heat. That is, water is taken back into the cement mixture as a
hydration reaction so as again to become an integral molecular part
of the cement as a result of which a binding material is created.
The energy given off during the hydrating chemical reaction is
sometimes termed the heat of hydration.
To obtain proper compressive strength in freshly mixed and placed
concrete, it is necessary that moisture be retained in the concrete
in a sufficient amount during its curing to allow complete chemical
reaction (hydration) between the water and the cement to take
place. Thus, one of the purposes of placing moisture-impermeable
sheet 42 in FIG. 1 over the upper surface of the mix is to protect
the as-yet-unformed upper surface from loss of moisture. When,
instead, moisture is lost from a mix by reason of improper bedding
under the mix, capillary attraction to the forms or covers or
evaporation to the atmosphere as in the case where heated air is
passed over the surface in attempted accelerated curing, the
desired chemical balance in the ultimate concrete is upset. As a
result, cracks and other undesirable surface blemishes typically
appear.
In the usual concrete mix, more than a sufficient amount of
moisture is included initially to properly complete the hydration
process. However, when the care indicated is not taken to retain
the moisture in the concrete so that the hydrating action is not
permitted to become complete, the internal structure of the
resulting concrete is disrupted as a result of which it exhibits
insufficient compressive strength. Another concern in the placing
of concrete arises during cold weather. When the temperature of
fresh concrete is low, as may occur in cold weather, the initial
setting is delayed as a result of which the mix may freeze so that
proper hydration cannot take place. Even when initially set but not
yet well cured, exposure of the mix to a freezing atmosphere while
the compressive strength is still low may result in rupture of the
internal structure. Consequently, the different heating apparatus
described not only is advantageous from the standpoint of
accelerating curing but also in permitting the pouring and
formation of concrete structures during cold weather. This is
especially significant in the case of huge forms such as that
illustrated in FIG. 1; by permitting year-round operation even
though located outdoors, the return from the investment in the form
is, of course, substantially increased.
A different effect, most likely to occur in the summer, can also
cause disruption of the internal structure of the concrete. The
natural hydration action can be undesirably disturbed by the
addition of excessive heat, particularly when the excess persists
for a period of several hours. This disturbance may be encountered
as a result of direct rays emanated by the sun. The damaging rays
include both those in visible portion of the spectrum and those in
the ultraviolet and near ultraviolet regions. The consequence of
permitting sun rays to strike the concrete in an excessive amount
is to effect a regression of compressive strength gain during the
curing period with a consequent loss in ultimate compressive
strength. This, of course, can be troublesome with regard to forms
such as those in FIGS. 1 and 4 that, because of their large size,
often are located in the open. The trouble is also encountered
necessarily in the pouring-in-place of concrete slabs during the
construction of highways and sidewalks.
It is, then, to the end of avoiding such difficulty as a result of
the direct rays of the sun that, in FIG. 7, sheet 82 of blanket 78
is colored so as to reflect the visible light waves and layers 80
and 81 are formed of a black-colored plastic so as to inhibit
penetration of the ultraviolet and near ultraviolet rays. The use
of blanket 78 may serve several purposes at the same time,
particularly in some climates such as those at high altitudes where
the day-time hours may be featured by comparatively high
temperatures with bright sunlight while during the immediately
following night the temperature drops to a value below freezing.
That is, blanket 78 forms a moisture-impervious seal over the
concrete surface in order to prevent escape of the necessary water
of hydration, its heating element when energized functions to
prevent freezing of the curing mix and to accelerate curing and, at
the same time, the reflective features of the blanket prevent
overheating by sun radiation.
Generally speaking, the hydration process which is an essential
part of hardening or curing of the concrete continues at a
significant rate for a number of days. As related to unaccelerated
curing at normal ambient temperatures, the compressive strengths of
the concrete typically are measured and rated in terms of the
strength at fixed intervals of time such as at 3 days, 7 days and
28 days following the time of pouring. A reasonably-high ultimate
compressive strength would be a value over 6,000 pounds per square
inch.
As indicated, the overall aim of accelerating the curing of
concrete is to cause the concrete to reach at least near its
ultimate compressive strength in a shorter period of time, and it
is to that end that an external heat source is employed to apply
heat to the mix. The function of the heat is to cause the water to
be hydrated into the cement more quickly as a result of which what
conventionally would be termed 3, 7 or 28 days compressive
strengths are obtained in a fraction of that time. It is believed
that greater hydration of water results during accelerated curing.
The consequent formation of a greater bulk of cement paste, in
turn, forms a less thick coat around each aggregate particle, and
this accounts for the greater compressive strengths exhibited in
the end.
However, an attempt to obtain even greater acceleration of the
curing, by dissipating very large quantities of heat into the mix
in order to greatly elevate its temperature, leads to an actual
regression in compressive strength. In order to avoid the occurence
of such regression while at the same time obtaining
maximum-possible compressive strength in the minimum time interval,
the manner in which the heat is applied is carefully controlled.
This is the primary function of the temperature sensors and
thermostatic controllers described above. Optimum results are
obtained by first applying the heat substantially uniformly over
the outer surface of the mix and then terminating application of
the heat when the measured temperature reaches a preselected level.
That level is such that the immediately subsequent exothermic
heating, which continues within the mix, further increased the mix
temperature only to a predetermined maximum value. That maximum
value corresponds to the attainment of maximum ultimate compressive
strength at the end of the curing period. Thus, the application of
the heat is terminated earlier than might otherwise be the case
when a temperature level is reached that anticipates the subsequent
further exothermic heat rise so that the ultimate temperature the
mix reaches is the most consistent with obtaining the highest
compressive strength in the end.
While the ultimate compressive strength of concrete varies with
respect to cements obtained in different geographical sections of
the country, and even to some extent in the output from a single
cement plant, the maximum value to which the mix temperature is
caused and permitted to rise, including the increase due to
exothermic reaction continuing after terminating the heating step,
is found to be generally between 150.degree. and 160.degree.. In
order to anticipate the subsequent exothermic temperature rise, it
is also found that the application of heat from the external source
must be terminated when the temperature rises to a level of about
140.degree.. In addition, it is found preferable to include what
may be termed a pre-setting period before heat from the
electrically energized heating element is applied. That is, the
application of the heat is delayed until the initially fluid mix
reaches the semi-fluid condition typified as that when the exposed
surface may first be meaningfully smoothed by trowelling.
Additionally, the form may be pre-heated where necessary so that
its temperature initially is approximately the same as that of the
mix.
A typical time-temperature plot of such a heat cycle appears in
FIG. 11 from the accelerated curing of concrete formed in a giant-T
casting bed similar to the apparatus of FIG. 1. The same bed also
included originally installed duct work that had been utilized for
the purpose of conveying steam in an effort to accelerate the
curing. The four curves grouped rather closely together represent
temperatures as measured in individually different sections along
the length of the form. The other curve, that which appears to
depart from the norm and is the lowest at the 4 o'clock position,
indicates a measurement taken on a vertical test cylinder. Its
lesser mass permitted a small loss of heat of hydration as a result
of which its temperature was slightly reduced after deenergization
of the heating element.
A type II cement was utilized in the mix that was placed into the
casting bed at a mix temperature of 70.degree. F. The ambient
temperature of the atmosphere surrounding the bed at the time of
placement was 50.degree. F. For the first two hours after
placement, no external heat was applied from any of the heating
elements. After that 2-hour period, at 5:00 p.m., the heating
elements were fully energized so as to dissipate heat at a rate of
approximately 35 watts per square foot. After approximately 41/2
hours, at 9:30 p.m., the mix temperature reached an average value
of about 140.degree. F. as a result of which the thermostatic
controls automatically deenergized the heating elements. However,
the subsequent exothermic heating as a result of the continuing
hydration process caused the temperature to continue to rise to a
level of approximately 150.degree. at which the temperature
remained on through the night. It appears that the described
electric curing results in a phenomena within the mix that produces
a continuation of exothermic reaction throughout a complete initial
curing cycle that continues long after deenergization of the
heating elements.
A core sample taken at the end of the initial curing period shown,
5:00 a.m. the next morning, exhibited a compressive strength of
4200 pounds per square inch. This compared favorably with a core
sample that previously had been taken after the same time interval
from a mix the curing of which was accelerated by utilizing the
steam conduits affixed to the mold. That sample had yielded a
compressive strength of only 3,500 pounds per square inch. Repeated
tests of core samples, taken at the end of 28 days following the
initial accelerated curing according to the schedule shown in FIG.
11, resulted in the attainment of compressive strengths
consistently about 800 pounds per square inch above that which
previously were regularly obtained by the use of steam curing in
the same forming apparatus.
In other comparative tests, application of the heat in the manner
of and according to the schedule herein described has resulted in
concrete that, after only 72 hours, exhibited a compressive
strength 600 pounds per square inch greater than the strength
exhibited by concrete cured for 7 days in accordance with so-called
standard laboratory curing. That is a form that previously might
have been able to turn out but one casting per week may, by use of
the principles herein discussed, have its production increased to
two or three castings per week. Such increased productivity results
in lower curing cost, and the close control of the curing as a
result of the measured application of the heat results in more
uniform high compressive strengths in successively produced
castings.
The illustrated apparatus may take many different shapes and forms
in correspondence with the production of a wide variety of
differently-shaped concrete structures. While steel forms have been
illustrated in FIGS. 1 and 4 as are typical to hold the heavy
weights of concrete mix involved, other materials may be more
advantageous for use in field construction. Wood, appropriately
treated to reduce capillary attraction to the moisture in the mix,
is a typical form material which is heat conductive. Another
advantageous material is fiber glass in which case the
heating-element resistance wire conveniently may be imbedded
directly therein. In addition to forming structural members as have
herein been illustrated, the techniques are equally applicable to
the formation of complete structures such as concrete
cattle-feeding bunkers.
Although the use of integral sheets of insulation have been
particularly illustrated for the purpose of minimizing heat loss
from the side of the heating element opposite the concrete mix,
such insulation may be formed in any convenient manner. For
example, after affixing the heating panels, such as those in FIG.
6, directly against a form wall, insulation then may be simply
sprayed into place over the heating panels. This is particularly
advantageous, for example, when affixing the panels to various
steel form sections in the fabrication of vertical building walls
and wherein the panels are of a variety of sizes and contours.
Numerous features of the present invention are claimed in the
aforementioned parent application. Other such features are claimed
in copending applications Ser. No. 106,784, filed Jan. 15, 1971 and
Ser. No. 110,512, filed Jan. 28, 1971, which also are divisions of
the parent application.
While particular embodiments of the invention have been shown and
described, it will be obvious to those skilled in the art that
changes and modifications may be made without departing from the
invention in its broader aspects and, therefore, the aim in the
appended claims is to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
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