Note: Descriptions are shown in the official language in which they were submitted.
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UNREINFORCED SHRINKAGE-COMPENSATING
CONCRETE FLOOR SLAB
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application relates to U.S. Provisional
Application Serial No. 60/663,201, filed March 21, 2005, the priority of
which is hereby claimed.
BACKGROUND OF THE INVENTION
[0002] The advent of shrinkage compensating concrete (circa
1959) brought the concept of overcoming the disadvantages of concrete
drying shrinkage to reality. By chemically increasing the mortar volume
between the aggregate in concrete and combining it with an elastic
restraint, the concrete structure self-compressed, compression being the
ideal condition for concrete because it is weak in tension. The mortar
enlarged either because the cement chemistry was modified (such as by
using Type K cement) or supplemental chemicals were added to the
concrete (such as Type K, S, and M cements where an additive is
introduced to the concrete or pre-blended with ordinary Portland cement).
This process has often been referred to as 'chemically prestressed
concrete' or more commonly known as shrinkage-compensating concrete.
[0003] Ordinarily concrete undergoes a very small expansion once
hardened after placement, followed by a significant evaporation process,
90% of which usually occurs in the first year, with perhaps as much as
50% in the first few weeks. Evaporation results in a volume change in
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the concrete (drying shrinkage) and the concrete shrinks towards its
centroid. Floor slabs on ground, in particular, but slabs on any substrate,
are restrained from shrinking because of friction between them and the
substrate. Thus, all non-shrinkage-compensating (ordinary) concrete
floor slabs are in tension (trying to shrink but prevented from doing so)
shortly after hardening, and would crack randomly save for joints sawn or
tooled into the floor slab to create weakened planes that organize the
cracks into neat and manageable joint patterns. A notable exception is
when slabs are post-tensioned, put into compression after placement and
some curing/hardening by the use of post-tensioning cables. Weakened
planes (typically sawcuts) with insufficient frequency and weakness result
in floor slabs that crack randomly between the weakened planes.
[0004] Shrinkage-compensating concrete was conceived to
enlarge an elastic restraint (such as a deformed reinforcing bar mat) for a
time period of about a week at a rate greater than the rate of volume
decrease due to drying shrinkage. After this time the rate of drying
shrinkage (volume change due to water evaporation) exceeds the rate of
expansion and the concrete begins to lose volume. The stretched
reinforcing bars in the slab compress the concrete until the concrete
shrinkage exceeds the initial elongation. In practice, when the residual
shrinkage is small enough and the concrete's tensile strength is not
exceeded, it doesn't crack. Hence, the term shrinkage-compensating as
opposed to 'shrinkage eliminating' since shrinkage is not eliminated, nor
is it often fully compensated. Shrinkage-compensating concrete permits
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the elimination of joints (weakened planes made by tooling or sawcuts) in
the slabs.
[0005] Customary practice is to construct shrinkage-compensating
concrete floor slabs in nearly square placements without any interior
joints within a placement. Exceptions usually relate to restraints such as
from plumbing or barricades penetrating the floor. In such cases interior
joints are added to eliminate re-entrant corners. The length to width,
rectangular, aspect ratio of the placements is ideally close to 1:1 because
concrete shrinks toward its undivided (un-jointed) centroid. If circular,
rather than rectangular, concrete slabs were useful, then circles, as
opposed to ellipses, would be ideal because uniformly distributed stresses
around the centroid of the slab would affect the slab uniformly (i.e., no
corner conditions). The American Concrete Institute compiles much of
the general industry knowledge in its publication ACI Manual of Concrete
Practice document ACI 223 Standard Practice For Shrinkage-
Compensating Concrete, and for the purposes of this document, the ACI
1998 edition has been referenced here as ACI-223.
[0006] Kalman Floor Company, founded in 1916, first
commercially constructed shrinkage-compensating concrete floor slabs in
1964. Over the past 40 years it has used most, if not all, of the
commercially produced expansive additives and expansive cements. Of
course, other construction firms constructed shrinkage-compensating
concrete floors and used many of the same cements and cement additives
to do so.
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[0007] Between December 1996 and May of 1997, Kalman Floor
Company (Kalman) fully realized that the industry's spotty performance
record relating to cracking of shrinkage-compensating concrete floor slabs
was due to the standard practices in composition and integrity of the
substrate supporting the slabs, especially the relationship of subgrade
friction to slab performance. Consequently, Kalman began using
polyethylene slip sheets to minimize or completely prevent bonding of the
slab to the substrate and permit the slabs to move more freely. The
result was a substantial reduction in shrinkage-compensating concrete
(SCC) slab cracking. Theretofore, slip sheets were common only in the
post-tensioned slabs on grade installed in the USA and were otherwise
recommended against because it was believed (and is true for ordinary
concrete) they prevented water from escaping into the ground increasing
the moisture gradient across the slab thickness which is the source of
upward lift at slab joints known as curling. ACI has subsequently adopted
slip sheets as standard practice for SCC slabs 'in its upcoming publication.
[0008] Apparently, until then, no one had substantiated subgrade
friction as a primary factor in cracking of SCC slabs (other than obvious
fixed restraints like posts in the slab) that Kalman became convinced of
and has demonstrated is so. Before May 1997, it was uncommon to
witness, with the naked eye, the compressible linings between SCC slabs
and other relatively immovable objects, like walls and columns actually
being compressed by slab enlargement. Thereafter, Kalman regularly
witnessed the actual expansion of the slabs indirectly by the compression
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of that surrounding foam. Thus, the original invention circa 1959,
patented in 1964, by Professor Alexander Klein of chemically prestressed
concrete, was finally installed with repeated success.
[0009] Reference to ACI 223 reveals that the suggested internal
restraint mechanism (a rectangular, deformed reinforcing bar mat) is not
only intended to restrain the growth of the concrete, but is also to offset
subgrade friction. In other words, knowing the bottom of the slab was
restrained by the ground, it is recommended by ACI 223 to restrain the
upper half of the slab with rebar to provide a more uniform condition of
expansion in the slab (so the top Wouldn't expand very much more than
the bottom of the slab). On that basis, it appears that the monograph in
ACI 223 which relates the amount of reinforcing required to the slab
thickness and anticipated expansion was developed. In theory, when the
amount of expansion is zero (but not less than zero), the amount of
reinforcing required to restrain it is zero. Since drying shrinkage always
occurs and subgrade friction is never zero, one would assume from ACI
223 that reinforcing must always be used.
[0010] Given that ACI 223 required reinforcing to offset subgrade
friction and to elastically restrain expansion, Kalman's use of polyethylene
sheeting led to an associated reduction in the required amount of
reinforcing. Subgrade friction was greatly reduced by slip sheets. This
conclusion was reached after field observations of the shrinkage-
compensating concrete slabs Kalman installed with one joint that had the
bottoms of adjacent slabs closer together than their tops. By introducing
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a lower subgrade friction using polyethylene slip sheets, the slab bottom
was able to expand more than the top. Obviously less rebar in the top
was needed and construction was begun by Kalman in that manner by
widening the spacing between the perpendicular reinforcing bars.
[001I.] Guidelines for determining the bar size and for determining
the bar spacing of the rebar mat (on center, each way) are usually.
expressed by ACI and others in percent, by the area of a cross-section of
a reinforcing bar relative to the cross-sectional area of the floor slab.
Thus, for a six inch thick floor slab and an 18" spacing of ASTM A615 #4
rebar, the area of steel is .20 in2 and the respective area of concrete is
18" x 6" or 108 inZ. Dividing .2 by 108 and multiplying the result by 100
equals 0.185% steel by cross-sectional area of concrete. For a time,
Kalman increased the spacing of the rebar to decrease the cross-sectional
area of steel relative to the cross-sectional area of concrete, providing
less reinforcing bar restraint, allowing the upper 1/2 of the floor slabs to
expand more easily and increase the uniformity of expansion of the top of
the slab relative to the bottom of the slab.
[0012] Due to continued successful installations of shrinkage-
compensating concrete floor slabs on slip sheets using less rebar,
resulting in minimal cracking, it became possible to reevaluate the
manner in winch a concrete slab placement enlarges. It is my
understanding that the enlargement of a given concrete floor slab
placement is the cumulative effect of the incremental expansion of each
portion of concrete between the slab edge and the slab centroid (devoid
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of considerations for restraint, creep, temperature, relative humidity,
drying shrinkage, and etc.). Thus, it is easy to predict the amount of
actual displacement of any point on the floor slab from its centroid
relative to its distance from the centroid.
[0013] Kalman's U.S. Patent No. 6,470,640 is based on the
conclusion that the cumulative volume change of the floor slabs really
resulted in the perimeter of the slabs moving a significant amount, while
the interior - held back by the effects of restraint, creep, relative
humidity, and drying shrinkage on itself and on the surrounding concrete
between itself and the slab edge - didn't move significantly and the
reinforcing in the center of the slabs was wasted. The waste occurs
because deformed rebar is "connected" to the concrete periodically along
its entire length. After a certain development length, the rebar will act as
a restraint mechanism whether it is continuous across the slab or just in
portions of the slab. The idea of eliminating the central reinforcement of
the floor slab has been demonstrated to work as predicted. The now un-
reinforced centroidal area of the slab, if it moves at all, experiences
insufficient tension to result in cracking. The slab perimeter moves and is
restrained by the internally developed tension ring,
SUMMARY OF THE INVENTION
[0014] According to the present invention a shrinkage-
compensating slab of a concrete building structure has no reinforcing bars
embedded therein, but instead is elastically restrained around its
perimeter by connected steel or plastic plates (or a channel) which are
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thus tangentially aligned with the expansive and shrinkage forces of the
concrete and act as an externally developed tension ring. The plates can
include attachment studs on their inwardly facing side or be connected to
attachment cables. The concrete itself can include a gradation of
aggregate to restrain the expansion of the mortar.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Fig. 1 shows a schematic plan view of a generally square
concrete slab with perimeter steel plates having inwardly-extending
connecting studs and diagonal gussets (optional) according to a preferred
embodiment of the present invention.
[0016] Fig. 2a shows an inner face of one of the steel plates used
in Fig. 1,
[0017] Fig. 2b is an end view of the steel plate of Fig. 2a,
[0018] Fig. 3 is a plan view of a larger generally rectangular
concrete slab with a perimeter of steel plates according to the invention,
and
[0019] Figs. 4a, 4b and 4c are respectively inside faces, an end
view and exterior faces of multiple steel plates welded together to provide
a perimeter boundary for the larger concrete slab of Fig. 3.
Example
[0020] Building requirements 100,000 sf of 4,000 psi, 8" thick,
concrete floor slab on ground with #4 reinforcing bar spaced at 16" on-
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center, each way. Sub-base is compacted granular road-base. Coarse
and fine aggregates are to be proportioned so that 8 to 18% is retained
on standard ASTM sieves; and a top-size coarse aggregate of 3/4" (#57).
Sawcut the floor slabs to a minimum depth of the thickness divided by 4,
for crack control at a joint spacing of 15' on-center, each way. An
alternate for the use of shrinkage compensating concrete is permitted.
An example of this approach:
[0021] 1. Develop a 4,000 psi concrete mix design containing
6.25 sacks of cement. Fifty-eight percent of the volume of aggregate to
be coarse, the remainder to be concrete sand. Top-sized coarse
aggregate of 3/4" (ASTM C33 #57 stone) composed of locally available
stone such as river rock.
[0022] a. Incorporate shrinkage compensating cement or
component adjusting dosage for a desired volumetric expansion of
0.03%.
[0023] b. Reproportion the mix to develop internal restraint
[0024] i. Change size and shape of coarse aggregate to
ASTM C33 #467, crushed quarried stone. Adjust the dosage of shrinkage
compensating component to again achieve 0.03% expansion.
[0025] ii. Adjust gradation of coarse and fine aggregate by
increasing the coarse aggregate to a typically practical limit of 63% by
volume of aggregate. Adjust the dosage of shrinkage compensating
component to again achieve 0.03% expansion.
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[0026] 2. Establish the size and shape of a floor slab placement
[0027] a. Based on crew availability and finishing requirements,
select a pour size the crew can practically install, such as 15,625 square
feet. To maintain an aspect ratio of the length and width of the
placement as near to 1: 1 as practical, define the boundaries of each slab
placement by bulkhead construction joints spaced at 125' on-center, each
way.
[0028] b. Reconsider the joint layout for the building use. If the
owner prefers small joints, the re-dimension the size of the slab
placement to account for anticipated operating environment. To control
joint width account for:
[0029] i. Thermal conditions include seasonal charges and
controlled temperature environments. The hardened concrete slab will
expand and contract as the temperature rises and falls, for example. This
is simply calculated at 10 millionths per degree F.
[0030] ii. As relative humidity rises and falls the hardened
concrete slab will expand and contract
[0031] iii. To reduce the joint width due to these factors,
change the joint spacing to be 115' on-center, each way.
[0032] c. Dimension the size of the slab placement to account for
friction between the slab and the subgrade supporting it. As friction rises
slab placement size should fall if cracking is to be minimized, which is the
purpose of shrinkage compensating concrete. It is essential for this
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concept that the friction be reduced as much as possible, and preferably
uniform.
[0033] i. Prevent the slab from bonding to the subgrade by
installing a break between the slab and the base such as polyethylene
sheeting.
[0034] ii. Decrease the friction between the slab and the
base by using, for instance, multiple layers of polyethylene sheeting, and,
for example, talc between sheets.
[0035] iii. Whenever practical maintain a uniform slab
thickness. Especially avoid thickened edges.
[0036] iv. Assume for this example items i, ii, iii above still
raise concern the slab may crack. Reduce the joint spacing to 100' on-
center, each way.
[0037] d. Account for undesired slab restraint from walls,
columns, posts, dock levelers, drains, and etc., by isolation with closed-
cell polyethylene foam, wooden forms, sawcut joints, etc.
[0038] 2. Constrain the expansion of the slab without using a
distributed mat of reinforcement or using an internal tension ring of
reinforcement.
[0039] a. Surround the slab with 10 foot long steel plates with 6"
x 1/2" headed. "Nelson" studs at 12" c/c top and bottom, offset 6" top to
bottom.
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[0040] b. Assemble the perimeter of plates by standing each plate
on one long edge with studs toward the slab interior. Continuously weld
every plate to plate junction on the outside and inside faces, forming a
square 100' x 100'.
[0041] c. Choose thickness and height for the plate:
[0042] i. The height of the plate may be as high or less
than the height of the slab off the sub-base. If it is less than the slab
height, it may be purposely located nearer the sub-base or slab top.
Since floor slab tops have no friction and floor slab bottoms do, it is
advantageous to install the plate nearer the top. In this example, the
plate is to be 8", the full slab depth so as to simultaneously act as the
bulkhead.
[0043] ii. The thickness of the plate is sufficient to provide
desired slab restraint. The thickness can be gradually changed to adjust
the restraint. For economy, the thickness in this example will be uniform
for every plate at 3/8".
[0044] iii. The stiffness, durability, abrasion resistance and
impact resistance is sufficient to provide protection of the concrete edge
from anticipated traffic. A 3/8" x 8" x 10' steel plate will have these
properties.
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Internally Restrained Shrinkage-Compensating Concrete
Made Without the Use of the Non-Concrete Restraints
[0045] Circa 2000, Kalman Floor Company conducted its usual
preliminary job expansion testing for one job, monitored by the apparatus
described in U.S. Patent No. 5,487,307, but did so for two significantly
different sizes of coarse aggregate in the concrete. Typically, materials
from the local ready mix concrete plant are shipped to a laboratory
contracted by Kalman to mix the materials in a prescribed mix design,
and dose the mix with varying amounts of expansive component. The
results are used to predict the amount of concrete expansion on the job
and, along other things, to discern which dosage rate provides the desired
volumetric expansion of the concrete. In this particular case, a No. 3
stone combined with #57 coarse aggregate was incorporated in some
mixes while only a #57 sized coarse aggregate was incorporated in other
mixes. These two sizes of aggregates were available for the project.
Since Kalman wanted to use either aggregate, it expended the effort to
obtain test results for concrete mixtures of either aggregate size (357 and
57), so that it could get approval for using either mix on the job.
[0046] It was expected that for similar dosage of expansive
component, both concrete mixtures would expand the same amount. For
the first seven days - the normal test duration - this was so. However,
long-term test results show the concrete with the larger, #357 sized
coarse aggregate expanded less than the concrete made with only #57
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stone. After more than 90 days, it was evident the difference was
approx. 100%. Arguably, only the concrete paste expands.
[0047] For the purpose of this discussion only the paste volume
enlarges while the aggregates remain their original size. Also, for the
purposes of this discussion the following terms are defined.
[0048] a) Paste = cement mixed with water, including any
pozzolans, liquid admixtures, and expansive components.
[0049] b) Mortar = Paste mixed with fine aggregate, where fine
aggregate is defined by ASTM.
[0050] c) Concrete = Mortar mixed with Coarse aggregate, where
coarse aggregate is defined by ASTM.
[0051] For the purposes of this discussion, the following concepts
are employed as factual:
[0052] a) There is more surface area on small aggregates than
on equal volumes of large aggregates.
[0053] b) There is more mortar in a concrete containing small
coarse aggregate than in a concrete with an equai concrete volume of
large coarse aggregate. Therefore, there is more paste in a concrete
containing small aggregate than an equal volume of concrete made with
large aggregate.
[0054] 1. No literature on, or installation of, concrete floor slabs
exists that is based on adjusting the aggregate size, shape, or quantity as
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a means of adjusting expansion of a shrinkage-compensating concrete
floors.
[0055] 2. It is novel to consider the surface area of the coarse
aggregate as a measurable factor involved in the amount of expansion (as
a restraint mechanism) for shrinkage-compensating concrete floors.
[0056] 3. Paste volume as a percentage of concrete volume has
not been explored as a mechanism for determining the volumetric
expansion of shrinkage-compensating concrete.
[0057] 4. That the amount of external or internal restraint of
concrete can be reduced based on using the coarse aggregate as a
restraint mechanism.
[0058] 5. The amount of expansive material can be adjusted
based on the size, shape, and quantity of coarse aggregate employed.
[0059] 6. Shrinkage-compensating concrete floor slabs can be
constructed without non-concrete restraint mechanisms provided that the
volumetric expansion occurs within the elastic limit of the mortar, given
sufficiently small subgrade friction, sufficiently small other elastic
restraints, if any, and elimination of rigid restraints. Other elastic
restraints might be storage racks mounted on the floor, not intended to
be part of managing the volumetric expansion.
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Externally Restrained Shrinkage-Compensating Concrete
[0060] The invention involves:
[0061] 1. a procedure for elastically restraining volumetric
changes of concrete which eliminates all forms of internal elastic restraint
other than the concrete materials (sand, stone, pozzolan, cement,
admixtures, etc.), by eiiminating all non-concrete material forms of
reinforcement, such as reinforcing bar, all forms of fibers (for example,
steel, polypropylene, and many others) and any other device embedded
into or integrally added into the concrete for the purposes of reinforcing
for crack control; elastic restraint; inelastic restraint; and moderating the
effect of drying shrinkage; of a: shrinkage compensating concrete; low
shrink concrete; or non-shrink concrete.
[0062] 2. a device which encircles a shrinkage compensating
concrete floor siab with sufficient strength to provide some elastic
restraint.
[0063] a. device made of steel bar, channel, plastic which
can provide perimeter elastic constraint to a concrete slab and also
provide protection of the perimeter slab edge from damage from traffic.
[0064] i. That while serving as a perimeter elastic
restraint, its dual purpose is to protect the slab edge.
[0065] b. devices which can be composed of intermediately
sized pieces and welded or fastened together to make a continuous elastic
band.
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[0066] c. devices which can be composed of intermediately
sized pieces not welded or fastened together, fashioned to integrally
attach to the concrete slab edge.
[0067] d. a device made of steel cable which can be post-
tensioned (tensioned after the concrete has hardened to a certain
strength) and used separately from or ill addition to that device in item
a.
[0068] i. the cable when enveloped by the concrete
slab substantially mimics the procedure claimed herein, provided it: forms
an ellipse concentric with the centroid and with the same aspect ratio as
the rectangular slab length and width; is in a slab that is rectangular or
square; provided it is nominally located at the perimeter of the slab at the
slab edge mid Points.
[0069] ii. the cable can surround the floor slab
placement.
[0070] 1. The cable may be attached to a
temporary bulkhead. When the bulkhead is removed the visible cable is
flush with the perimeter slab face.
[0071] 2. The cable may be oriented to minimize
upward slab edge curling. For instance, higher at the corners of the slab
and always above the slab mid-depth elevation.
[0072] 3. Corner plates may be mounted into the
slab corners to minimize concrete destruction by the cable force.
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[0073] e. devices which may be designed to provide varying
restraint across the slab section. For instance, thicker or stronger at the
top of the slab edge than at the bottom of the slab edge.
[0074] 3. the use of gussets to improve the perimeter
restraint
[0075] a. Where such gussets are elastic
[0076] b. Where such gussets may be connected to the
perimeter restraint
[0077] c. Where such gussets may not be connected to the
perimeter restraint but located within the slab with or without end points
enlarged to provide anchorage
[0078] d. Where such gussets are installed tangentially to an
imaginary circle about the centroid when located within the concrete slab
[0079] e. Where such gussets may not be located within the
slab, but anchored to the slab at their end points or their end points are
fastened to the perimeter restraint.
[0080] f. Where such gussets may be round, square, or
rectangular in cross-section.
Despite That External Restraint Is An Idea That
Exists, This External Restraint Is Novel Because:
[0081] 1. It is now defined and geometrically related to the
volumetric enlargement of the slab.
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[0082] 2. All forms of internal restraint, except the concrete
material, are eliminated. Internal restraint is the only approach examined
by the ACI 223 document. The only form of external restraint examined
is subgrade friction.
[0083] 3. Issues of indeterminate restraint are addressed by
gussets and increasing or decreasing the elasticity across the vertical
section of the device.
[0084] 4. It has not been done before. It hasn't been done
because until 1997, no one was using slip sheets under shrinkage-
compensating concrete floor slabs when Kalman introduced the concept to
the industry. It wasn't until then that subgrade friction was reduced
enough to be able to examine associated reductions in slab
reinforcement. It was prohibitively risky and costly to experiment on a
image scale, as well. Furthermore, without slip sheets, the industry was
accustomed to unexplained random cracking of shrinkage compensating
concrete floors. Once freed from the subgrade, each crack became
traceable to rigid restraints or imperfections in construction causing
discontinuities in the slab cross-sectional stresses.
[0085] 5. Its composition is now defined as related to the practical
production of an exterior elastic members. Further, its construction can
include edge protection. Please note that customary edge protection
devices include the embedment of, for example, a 3" x 3" x 1/4" x 10'
long steel angle with 1/2" dia. x 6" long headed NelsonTM studs into the
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top floor edge at bulkhead construction joints. More commonly used today
is a 3/8" x 2" x 10' long steel plate with studs on the interior face plate
embedded into the concrete slab. A back-to-back steel edge condition
occurs when adjacent slabs are lined with these angles/plates. Traffic
across the joints impacts the steel device rather than the brittle concrete,
protecting the edge, providing long term durability. The elastic members
can incorporate that traditional approach to edge protection while also
serving as the mechanism for restraint.
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