Note: Descriptions are shown in the official language in which they were submitted.
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VEHICLE ARRESTING BED SYSTEMS
This invention relates to systems for slowing travel of vehicles and, more
particularly, to cellular concrete arresting bed systems to safely decelerate
an aircraft
which runs of;Fthe end of a runway.
BACKGROUND OF THE INVENTION
Aircraft can and do overrun the ends of runways raising the possibility of
injury to passengers and destruction of or severe damage to the aircraft. Such
overruns have occurred during aborted take-offs or while landing, with the
aircraft
traveling at speeds to 80 knots. In order to minimize the hazards of overruns,
the
Federal Aviation Administration (FAA) generally requires a safety area of
1,000 feet
in length beyond the end of the runway. Although this safety area is now an
FAA
standard, many runways across the country were constructed prior to its
adoption
and are situated such that water, roadways or other obstacles prevent
economical
compliance with the one thousand foot overrun requirement.
Several materials, including existing soil surfaces beyond the runway have
been assessed for their ability to decelerate aircraft. Soil surfaces are very
unpredictable in their arresting capability because their properties are
unpredictable.
For example, very dry clay can be hard and nearly impenetrable, but wet clay
can
cause aircraft to mire down quickly, cause the landing gear to collapse, and
provide a
potential for passenger and crew injury as well as greater aircraft damage.
A 1988 report addresses an investigation by the Port Authority of New York
and New Jersey on the feasibility of developing a plastic foam arrestor for a
runway
at JFK International Airport. In the report, it is stated that analyses
indicated that
such an arrestor design is feasible and could safely stop a 100,000 pound
aircraft
overrunning the runway at an exit velocity up to 80 knots and an 820,000 pound
aircraft overrunning at an exit velocity up to 60 knots. The report states
that
performance of an appropriate plastic foam arrestor configuration was shown to
be
potentially "superior to a paved 1,000 foot overrun area, particularly when
braking is
not effective arid reverse thrust is not available." As is well known,
effectiveness of
braking may be limited under wet or icy surface conditions. (University of
Dayton
report UDR-TR-88-07, January 1988.)
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More recently, an aircraft arresting system has been described in U.S. Patent
No. 5,193,764 to Larrett et al. In accordance with the disclosure of that
patent, an
aircraft arresting area is formed by adhering a plurality of stacked thin
layers of rigid,
friable, fire resistant phenolic foam to each other, with the lower-most layer
of foam
being adhered to a support surface. The stacked layers are designed so that
the
compressive resistance of the combined layers of rigid plastic foam is less
than the
force exerted by the landing gear of any aircraft of the type intended to be
arrested
when moving into the arresting area from a runway so that the foam is crushed
when
contacted by the aircraft. The preferred material is phenolic foam used with a
compatible adhesive, such as a latex adhesive.
Tests of phenolic foam based arrestor systems indicate that while such
systems can function to bring aircraft to a stop, the use of the foam material
has
disadvantages. Major among the disadvantages is the fact that foam, depending
upon
its properties, can typically exhibit a rebound property. Thus, it was noted
in
phenolic foam arresting bed testing that some forward thrust was delivered to
the
wheels of the aircraft as it moved through the foamed material as a result of
the
rebound of the foam material itself.
Foamed or cellular concrete as a material for use in arresting bed systems has
been suggested and undergone limited field testing in the prior art. Such
testing has
indicated that cellular concrete has good potential for use in arresting bed
systems,
based on providing many of the same advantages as phenolic foam while avoiding
some of phenolic foam's disadvantages. However, the requirements for an
accurately
controlled crushing strength and material uniformity throughout the arresting
bed are
critical and, so far as is known, the production of cellular concrete of
appropriate
characteristics and uniformity has not previously been achieved or described.
Production of structural concrete for building purposes is an old art
involving
relatively simple process steps. Production of cellular concrete, while
generally
involving simple ingredients, is complicated by the nature and effect of
aeration,
mixing and hydration aspects, which must be closely specified and accurately
controlled if a uniform end product, which is neither too weak nor too strong,
is to be
provided for present purposes. Discontinuities, including areas of weaker and
stronger cellular concrete, may actually cause damage to the vehicle that is
being
decelerated if, for example, deceleration forces exceed wheel support
structure
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strength. Such nonuniformity also results in an inability to accurately
predict
deceleration performance and total stopping distance. In one recent
feasibility test
utilizing commercial grade cellular concrete, an aircraft instrumented for
recording of
test data taxied through a bed section and load data was acquired. Even though
steps
had been taken to try to provide production uniformity, samples taken and
aircraft
load data from the test arresting bed showed significant variations between
areas
where the crush strength was excessively high and areas where it was
excessively
low. Obviously, the potential benefit of an arrestor system is compromised, if
the
aircraft is exposed to forces that could damage or collapse the main landing
gear.
A 1995 report prepared for the Federal Aviation Administration entitled
"Preliminary Soft Ground Arrestor Design for JFK International Airport"
describes a
proposed aircraft arrestor. This report discusses the potential for use of
either
phenolic foam or cellular concrete. As to phenolic foam, reference is made to
the
disadvantage of a "rebound" characteristic resulting in return of some energy
i 5 following compression. As to cellular concrete, termed "foamcrete", it is
noted that
"a constant density (strength parameter) of foamcrete is difficult to
maintain" in
production. It is indicated that foamcrete appears to be a good candidate for
arrestor
construction, if it can be produced in large quantities with constant density
and
compressive strengths. Flat plate testing is illustrated and uniform
compressive
strength values of 60 and 80 psi over a five to eighty percent deformation
range are
described as objectives based on the level of information then available in
the art.
The report thus indicates the unavailability of both existing materials having
acceptable characteristics and methods for production of such material, and
suggests
on a somewhat hypothetical basis possible characteristics and testing of such
materials should they become available.
Thus, while arresting bed systems have been considered and some actual
testing of various materials therefor has been explored, practical production
and
implementation of an arresting bed system which, within specified distances,
will
safely decelerate aircraft of known size and weight moving at a projected rate
of
speed off of a runway, has not been achieved. The particular material to be
used, as
well as the configuration and fabrication of an arresting bed, are all
critical to the
provision of an effective arresting bed system. To provide an effective
arresting bed
for vehicles of a range of sizes, weights and bed entry speeds, requires use
of bed
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designs, materials and fabrication techniques capable of providing predictable
drag
effects and rates of vehicle deceleration. Computer program models or other
techniques may be employed to develop drag or deceleration objectives for
arresting
beds, based on calculated forces and energy absorption for aircraft of
particular size
and weight, in view of corresponding landing gear strength specifications for
such
aircraft. However, such objectives remain merely an abstract goal in the
absence of
effective bed configurations, materials and fabrication techniques appropriate
to
convert arresting bed objectives into reality to achieve the desired results.
As a
result, prior information as to potential arresting bed materials and
deceleration
objectives has been inadequate to enable fabrication of a practical arresting
bed
suitable for use by commercial passenger aircraft and other vehicles.
Objects of the invention are, therefore, to provide new and improved vehicle
arresting bed systems and such systems having one or more of the following
advantages and capabilities:
- assembly from pre-cast cellular concrete which has been acceptance tested;
- block or aggregate assembly enabling progressive variation of both depth and
compressive strength characteristics;
- predetermined arresting characteristics, substantially independent of
weather
conditions;
- long-life weather resistant construction;
- hardcoat covering to support pedestrian access;
- ability of crash/fire/rescue vehicles to fully maneuver on an arresting bed;
- ease of exit by passengers from a vehicle which has entered an arresting
bed;
and
- ease of repair by block or aggregate replacement following use by an
overrunning vehicle.
SUMMARY OF THE INVENTION
In accordance with the invention, a vehicle arresting bed system includes an
initial section including first and second lateral rows of blocks of cellular
concrete
having a first dry density in the lower portion of a range of 12 to 22 pcf
Blocks of
the second row have a height incrementally greater than the height of blocks
of the
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first row. Also, the blocks of the first and second rows have a first
compressive
gradient strength to provide vehicle deceleration.
The bed system has a further section including third and fourth lateral rows
of
blocks of cellular concrete having a second dry density greater than the first
dry
density. Blocks of the fourth row have a height incrementally greater than the
height
of blocks of the third row and the blocks of the third and fourth rows have a
second
compressive gradient strength, greater than the first compressive gradient
strength, to
provide greater vehicle deceleration.
A hardcoat layer overlays blocks of the first, second, third and fourth rows.
The hardcoat layer comprises cellular concrete having a dry density greater
than the
first and second dry densities and a thickness not exceeding ten percent of
the
average height of the blocks.
A vehicle arresting bed system may desirably have additional characteristics,
such as the following. The blocks are preferably formed of cellular concrete
having a
wet density in a range of 1 S to 23 pcf and cured in forms of predetermined
sizes. By
way of example, all of the blocks in a preferred embodiment are of the same
length
and width, but some are of different predetermined heights, with a first
section of
blocks having a 60/80 compressive gradient strength and a second section of
blocks
having an 80/100 compressive gradient strength. The hardcoat layer may be
formed
of cured-in-place cellular concrete having greater strength to provide overall
protection of the arresting bed and permit maintenance personnel to walk on
the bed
without damaging it.
For a better understanding of the invention, together with other and further
objects, reference is made to the accompanying drawings and the scope of the
invention will be pointed out in the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1 A, 1 B and 1 C are respectively a plan view, and longitudinal and
transverse cross-sectional views, of a vehicle arresting bed system using
block
construction in accordance with the invention.
Figs. 2A and 2B are respectively portions of similar plan and longitudinal
views of a vehicle arresting bed system using aggregate fabrication in
accordance
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with the invention.
Fig. 3 shows dimensions of a typical block of cellular concrete suitable for
use
in an arresting bed system.
Figs. 4, S and 6 show alternative constructions of cellular concrete blocks.
Figs. 7 and 8 show test results in terms of compressive force versus
percentage of penetration for samples of cellular concrete of two dii~erent
strengths.
DETAILED DESCRIPTION OF THE INVENTION
The use of cellular concrete in arresting bed applications requires the
material
to be generally uniform in its resistance to deformation since it is the
predictability of
forces acting on the surface of contacting members of the vehicle which is
being
stopped that allows the bed to be designed, sized and constructed in a manner
which
will ensure acceptable performance. In order to obtain such uniformity, there
must
be careful selection and control of the ingredients used to prepare the
cellular
concrete, the conditions under which it is processed, and its curing regime.
The ingredients of cellular concrete are generally a cement, preferably
Portland cement, a foaming agent, and water. Very fine sand or other materials
can
also find application in some circumstances, but are not used in presently
preferred
embodiments. The currently preferred type of cement for arresting bed
application is
Type III Portland cement. For present purposes, the term "cellular concrete"
is used
as a generic term covering concrete with relatively small internal cells or
bubbles of a
fluid, such as air, and which may include sand or other material, as well as
formulations not including such sand or other material.
Construction of the arresting bed system can be accomplished by producing
the cellular concrete at a central production facility or at the site of the
bed and
pouring the concrete into forms of appropriate dimensions to achieve the
desired
geometry for the system. However, in the interests of uniformity of material
characteristics and overall quality control, it has currently been found
preferable to
cast sections of the overall bed using forms of appropriate size and then
transport the
sections to the site and install them to provide the overall configuration of
the bed.
In the latter case, such units or sections, in the form of blocks of
predetermined sizes,
can be produced and held until completion of quality control testing. The
blocks can
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then be placed at the site and adhered to the runway safety area using
asphalt, cement
grout, or other suitable adhesive material, depending on the construction
materials of
the safety area itself.
In either case, in accordance with the invention a hardcoat is applied to the
outer surface of the assembled arresting bed to provide a stronger surface
that is not
as easily deformed as the major structure of the bed itself. This allows
maintenance
to be performed without serious deformation damage to the main structure. A
preferred hardcoat consists of foamed concrete wherein the wet densities are
somewhat higher, for example in the range of about 22 to about 26 lbs./cu. ft.
Finally
a weather resistant protective film or paint can be applied to give the
structure a
desired visual appearance and act as protection against weather degradation.
Preferred coatings include water based silicone materials.
DEFINITION OF "COMPRESSIVE GRADIENT STRENGTH" OR "CGS"
The term "compressive strength" (not CGS) is normally understood to mean
the amount of force (conventionally measured in pounds per square inch) which,
when applied at a vector normal to the surface of a standardized sample, will
cause
the sample to fail. Most conventional test methods specify test apparatus,
sampling
procedures, test specimen requirements (including size, molding, and curing
requirements) rates of loading and record keeping requirements. An example is
ASTM C 495-86 "Standard Method for Compressive Strength of Lightweight
Insulating Concrete." While such conventional test methods are usefial when
designing structures that are required to maintain structural integrity under
predicted
load conditions (i.e., have at least a minimum strength), the object of
arresting bed
systems is to fail in predictable specified manner, thereby providing
controlled,
predictable resistive force as the vehicle deforms the cellular concrete
(i.e., a specific
compressive gradient strength). Thus, such conventional testing focuses on
determining strength up to point of failure, not strength during compressive
failure.
Stated more simply, knowing what amount of force will shatter a specimen of
cellular
concrete material does not answer the critical question of what amount of drag
or
deceleration will be experienced by a vehicle moving through an arresting bed
system.
In contrast to a "one time" fracture strength as in the prior art, for present
purposes
testing must evaluate a continuous compressive failure mode as a portion of a
specimen is continuously compressed to about twenty percent of its original
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thickness. Equipment and methods suitable for such continuous testing as
appropriate for present purposes have generally not been previously available.
Because of the wide range of variables available in materials and processing
of cellular concretes, and the size and cost of constructing arresting beds
for testing,
it is imperative that accurate test information be available to predict the
amount of
resistive force a particular variety of cellular concrete, processed and cured
in a
certain way, will provide when used in an arresting bed system. By developing
new
test methodology to focus the resulting data on measurement of the resistive
force
occurring during continuous compressive failure of a sample, instead of simple
one-
time "compressive strength", new test methods and apparatus have been
developed
to enable reliable testing and confirmation of appropriate cellular concrete
materials
and process variables.
As a result it has been determined that the compressive force needed to crush
cellular concrete to 20 percent of its original thickness varies with the
depth of
penetration. This characteristic, which the present inventors term
"compressive
gradient strength" or "CGS" must be accurately specified in order to construct
a
cellular concrete vehicle arresting bed having known deceleration
characteristics to
safely decelerate an aircraft. Thus, a penetration type test method where the
compressive strength of a sample of cellular concrete is gauged not by
applying a
force that will fracture a sample, but rather will continuously provide data
on resistive
forces generated as a test probe head having a specified compressive contact
surface
is moved through a volume of cellular concrete, is key to obtaining the data
necessary
to formulate and use cellular concrete in arresting bed applications. As thus
measured, CGS will vary over a range with penetration depth, resulting in a
gradient
value (such as 60/80 CGS with an average CGS of 70 psi over the penetration
range)
rather than a simple singular fracture value as in prior tests.
For present purposes, the term "compressive gradient strength" (or "CGS")
is used to refer to the compressive strength of a section of cellular concrete
from a
surface and continuing to an internal depth of penetration, which may
typically be 66
percent of the thickness of the section. As defined, CGS does not correspond
to
compressive strength as determined by standard ASTM test methods. Test methods
and apparatus suitable for determining CGS are disclosed in application serial
No.
08/796,968 filed concurrently herewith, having a common assignee, and hereby
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incorporated herein by reference.
ARRESTING BED OF FIGS. 1 A, 1 B and 1 C
With reference to Fig. 1 (collectively including Figs. 1 A, 1 B and 1 C),
there is
illustrated an embodiment of a vehicle arresting bed system in accordance with
the
S invention. As shown in Fig. 1 A, the bed has a length and width and also a
thickness
as shown in Figs. 1B and 1C. The bed is configured to decelerate an aircraft
entering
the bed from the left in Fig. 1A. Basically, the Fig. 1 system is constructed
of pre-
cast blocks of cellular concrete having two different compressive gradient
strengths
and a variety of different thicknesses, with intended installation at the end
of an
airport runway. Subsurface SO supporting the system should typically be
relatively
flat, smooth and level (subject to having a slope appropriate for water runoff
requirements) and capable of supporting aircraft which leave the runway.
Subsurface
SO should be in good condition and cleaned satisfactorily for placement and
bonding
of the arresting bed system. To show vertical details, the vertical dimensions
of Figs.
1 S 1B and 1 C are expanded relative to the dimensions of Fig. 1 A (e.g., the
width of the
bed in Fig. 1 A may typically be 1 SO feet, while the maximum thickness of the
bed in
Figs. 1B and 1C may typically be 30 inches). Also, certain dimensions, such as
block
size, are distorted for clarity of illustration (e.g., rather than show the
thousands of
blocks actually included in a typical arresting bed).
A typical block suitable for use in the Fig. 1 system is illustrated in Fig.
3. As
shown, block 70 may be fabricated by placing wet cellular concrete in curing
forms of
uniform width 74 {typically 4 feet) and length 76 (typically 8 feet). Block
thickness
72 may be varied in a range of 8 to 30 inches, for example, to provide blocks
having
heights varying in increments (typically of from 3/4 inch increments of height
for a
2S fine taper to increments of 3 inches) in order to enable provision of front
to rear
tapered bed configurations able to provide predetermined incremental increases
in
drag forces. In the block embodiment shown in Fig. 3, there are included
transverse
lifting slots 78 and 80. Slots 78 and 80, suitable for use with a fork lift
type of lifting
mechanism, are formed by placing lightweight rectangular plastic sleeves in
the
bottom of a form when casting the block. Other block features and embodiments
usable in arresting beds constructed in accordance with the invention will be
discussed with reference to Figs. 4, S and 6.
As shown, the Fig. 1 vehicle arresting bed system has a bed area of cellular
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concrete which includes a first section 52, comprising an assembly of blocks
having a
first CGS and a first dry density, and a second section 54, comprising an
assembly of
blocks having a second CGS and a second dry density. As shown in the side
sectional view of Fig. 1B, sections 52 and 54 partially overlap (in what might
be
considered section 52/54), with a darkened line indicating the juncture
wherein
certain blocks of section 52 overlie blocks of section 54 in a transition
region. In a
particular embodiment, the section 52/54 blocks may actually be composite
blocks
(i.e., single blocks including a 52 portion having a first CGS and also a 54
portion
having a second CGS). In other embodiments separate blocks of different CGS
may
be stacked for section 52/54.
More particularly, vehicle arresting bed systems of the type illustrated in
Fig.
1 include a first lateral row of blocks (e.g., row 52a) of cellular concrete
having a
first CGS and a first dry density in a range of 13 to 18.5 pounds per cubic
foot (pcfj.
Each of the blocks in first row 52a has a first height and is fabricated to be
vertically
compressible to a compressed height (e.g., typically compressible to about 20
percent
of initial thickness). These blocks may be fabricated to exhibit a 60/80 CGS
characteristic as represented in Fig. 7, which will be discussed below. As
shown in
Figs. 1A and 1B, the first section 52 includes a second row 52b and a
plurality of
additional lateral rows illustrated as rows 52c through 52n, formed of
cellular
concrete having the same basic characteristics as in the blocks of row SZa,
but some
of which differ row-to-row by an incremental height differential. Also, as
discussed
with reference to overlap section 52/54 certain rows of blocks, such as row
52n,
overlay blocks of row 54d on a composite block or stacked block basis. In this
embodiment successive 3/4 inch changes in thickness were utilized in section
52 to
provide tapered or sloping characteristics resulting in gradually increasing
vehicle
arresting capabilities. Corresponding 3 inch changes in thickness were
utilized in
Section 54, in this particular design.
Arresting bed systems of the type illustrated also include a third lateral row
54g of blocks of cellular concrete having a second dry density which may be at
a
higher level in the same range as the first blocks in section 52. As shown,
lateral row
54g is positioned parallel to and to the rear of the first lateral row 52a.
Row 54g is in
turn followed by a lateral row 54h of incrementally greater height. The blocks
of
section 54 are fabricated to be vertically compressible subject to a second
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compressive gradient strength, which will generally be specified to exceed the
CGS
of the blocks of section 52. These blocks may be fabricated to exhibit a
80/100 CGS
characteristic, as represented in Fig. 8 which will be discussed below, and a
dry
density in a range of 16 to 21.5 pcf. In the illustrated embodiment the first
row of
blocks 54a of section 54 includes only a single course or layer of the second
CGS.
Successive rows of section 54 include increasing thickness of the second CGS
material, until the section 54 blocks reach the full height of the arresting
bed beyond
section 52. Successive rows of section 54 then increase in thickness by 3 inch
increments in advance of reaching maximum height in a rear level portion
comprising
rows of the same thickness continuing to final rear row 54n. Rows of increased
height, such as row 54n, may be formed of two or three superimposed blocks of
reduced thickness or of rows of single relatively thick blocks, depending upon
fabrication, handling and site delivery considerations.
Fig. 7 illustrates the CGS characteristics of a cellular concrete sample
representative of a block from section 52 of Fig. l, as determined by test. In
Fig. 7,
the bottom scale represents percentage of test probe penetration expressed in
tenths
of sample thickness or height. The vertical scale represents test probe
compressive
force expressed in pounds per square inch (psi). The test data of principal
interest is
typically within the range of penetration from 10 to 60 percent of sample
thickness.
Data outside this range may be less reliable, with crushed material build-up
effects
occurnng beyond about 70 percent penetration.
As illustrated in Fig. 7, the failure strength of cellular concrete exhibits a
gradient with resistance to compression increasing with depth of penetration.
For a
particular design of an arresting bed as illustrated in Fig. 1, the line
through points A
and B in Fig. 7 represents a generalized 60/80 CGS, l. e., a CGS characterized
by a
compression strength changing from approximately 60 psi to approximately 80
psi
over a 10 to 66 percent penetration range. The average, over this range is
thus
nominally equal to 70 psi at mid-point C. In Fig. 7, the line joining points A
and B
represents a typical generalized compressive strength gradient line for blocks
of
section 52 of Fig. 1. Lines D and E represent quality control limits and line
F
represents actual test data as recorded for a specific test sample of cellular
concrete.
In this example, a test sample for which test data over a 10 to 60 percent
penetration
range remains within quality control limit lines D and E, represents an
arresting block
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fabricated within acceptable tolerances. Fig. 8 is a similar illustration of
CGS
characteristics of a test sample of a block suitable for use in section 54 of
Fig. 1,
having an 80/100 CGS which is nominally equal to 90 psi, when averaged over a
selected depth of penetration (e.g., a 10 to 66 percent penetration range).
For
present purposes, "nominal" or "nominally" is defined as referring to a value
or
relationship which is within about plus or minus 15 percent of a stated value
or
relationship.
As shown, the Fig. 1 system further includes an inclined entrance ramp 56
positioned across the vehicle entrance front side of the first lateral row
52a. The
ramp, which may be formed of asphalt mix or other permanent type material,
tapers
up to a height adjacent the blocks of row 52a, which is typically greater than
the
compressed height of the blocks of row 52a. In a particular embodiment, a ramp
height of about 3 inches was used adjacent to 8 inch blocks having an
estimated
minimum compressed height of 1.8 inches. Ramp 56 is thus effective to
gradually
raise an aircraft above general runway level, so that the aircraft can enter
the
arresting bed on a relatively smooth basis as the wheels leave ramp 56 and
begin
compressing the blocks of row 52a.
Also included in the Fig. 1 system is a hardcoat layer 62, in the form of a
relatively thin protective layer of cellular concrete material, overlaying the
blocks of
both section 52 and section 54 (represented by the uppermost boundary line of
the
bed in Fig. 1B). In Fig. 1A the hardcoat layer is represented as being
transparent in
order to show underlying details, even though the hardcoat layer will
typically not be
transparent. In a preferred embodiment, hardcoat layer 62 comprises a
relatively thin
layer of cellular concrete having a strength to support a pedestrian (e.g.,
sufficient to
support a maintenance person walking on the arresting bed) and may be covered
by a
weather resistant paint or similar coating. Layer 62 is applied over the
arresting bed
after all blocks of sections 52 and 54 are positioned and appropriately
adhered to
supporting surface 50. Hardcoat layer 26 may typically be formed of 22 to 26
pcf
dry density cellular concrete with an average thickness of about one inch. In
an
arresting bed which may include blocks ranging in thickness from 8 to 20
inches or
more, the thickness of hardcoat layer 62 will typically not exceed 10 percent
of
average thickness or height of the blocks and may be closer to 5 percent.
Since the
thin hardcoat layer has relatively little effect on aircraft deceleration,
test samples
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typically need not be subjected to testing as described above.
As illustrated, the arresting bed system also has associated with it a debris
shield 58 and service vehicle entrance ramps 60. Shield 58 may be formed of
relatively light weight aluminum sheet stock adequate to deflect particles
blown by jet
exhaust, etc., but fragile enough to readily yield to the tires of an
aircraft. Ramps 60
are proportioned and constructed to enable airport fire or rescue vehicles to
drive up
onto the arresting bed in order to provide assistance to passengers of an
aircraft
which has come to a stop within the boundaries of the arresting bed. Ramps 60
may
be constructed in a stepped form, of elongated blocks of cellular concrete of
appropriate strength or other suitable material. As shown, ramps 60 are
constructed
of blocks of square cross-sectional dimensions able to be accommodated by a
fire or
rescue vehicle driving onto the bed.
In a typical arresting bed installation, appropriate for arresting travel of a
variety of types of aircraft, the blocks of section 52 may typically have
thicknesses
varying in 3/4 inch increments from 9 inches to 24 inches, a dry density of
about 17
pcf, and provide a 60/80 CGS as described above. The blocks of section 54 may
correspondingly have thicknesses varying in three inch increments from 24
inches to
30 inches, a dry density of about 19 pcf, and provide an 80/100 CGS. In
fabrication
of the blocks, the blocks of section 52 may be formulated from cellular
concrete
having a wet density toward the lower portion of a range of about 14 to 23
pcf, with
the blocks of section 54 fabricated from cellular concrete having a wet
density toward
the upper portion of such range. The composite blocks in section 52/54 would
correspondingly consist partially of 60/80 CGS material and partially of
80/100 CGS
material. Overall, sections 52 and 54 may have an aggregate length of 400
feet, a
width of 150 feet and front end and rear end thicknesses of 9 inches and 30
inches,
respectively. It will be appreciated that for any particular implementation of
the
invention, performance achieved will be dependent upon the characteristics of
the
materials and arresting system design as specified and fabricated in order to
meet
identified site-specific performance objectives. Parameters relating to
materials or
systems for any specific implementation are beyond the scope of present
purposes
and specific values are discussed only as general examples of possible
parameter
magnitudes.
As described, the two major sections 52 and 54 can be constructed by
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contiguous assembly of preformed blocks which are then grouted or otherwise
adhered to the support surface. Alternatively, other forms of construction may
be
employed in accordance with the invention. For example, with appropriate
process
control, an arresting bed similar to that illustrated can be poured and cured
in place
on a unitary or sectioned basis. Another form of construction is illustrated
in Fig. 2
(comprising Figs. 2A and 2B).
Referring now to Figs. 2A and 2B, there is shown a portion of a vehicle
arresting bed system in accordance with the invention, which includes a bed 90
formed of an aggregate including pieces of cellular concrete. For present
purposes,
and consistent with its dictionary definitions, "aggregate" is defined as a
mass or
volume of material formed of homogeneous or non-homogeneous units, pieces or
fragments of the same or different sizes and of regular or irregular shapes.
Pursuant
to the invention, an aggregate as used in bed 90 may consist entirely of
pieces of
cellular concrete, typically having dimensions smaller than one-quarter of the
average
bed thickness, or may comprise pieces of cellular concrete with other material
mixed
in. Such other material may include pieces of phenolic foam or other
compressible
material, hollow glass spheres, hollow ceramic spheres, or other crushable
items of
selected material and shape. As shown, bed 90 has length, width and thickness
and is
configured to decelerate a vehicle, such as an aircraft, entering the bed from
the left.
More particularly, as represented in Fig. 2B, the aggregate of bed 90 is
arranged to
increase in thickness from left to right, so that some portions have different
thickness
than other portions. In addition, at 90a there is indicated a sloping portion
of
aggregate which may have a higher compressibility than the partially overlying
aggregate portion to the left in Fig. 2B. The bed may thus include portions
having
different compressibility so that vehicle drag or deceleration increases as a
vehicle
travels through the bed.
The arresting bed system of Figs. 2A and 2B includes edge members 92 and
94 along the perimeter of bed 90 to constrain the aggregate from spreading
beyond
the desired length and width of the bed. As illustrated, the edge members are
blocks
of cellular concrete similar to those described above and each having a
suitable CGS.
In Fig. 2A, each edge member 92 and 94 includes a row of blocks and the
complete
bed system would have a suitable overall length, with an additional row of
blocks
across the right hand end of the bed. The arresting bed system, as
illustrated, also
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includes a stabilizing layer, represented by line 96 in Fig. 2B, overlaying
the bed 90 to
limit movement of the aggregate within the bed. Stabilizing layer 96 may
typically be
a relatively thin hardcoat layer of cellular concrete as described above. In
Fig. 2A the
stabilizing layer is represented as being transparent in order to show
underlying
details.
Figs. 4, 5 and 6 illustrate particular embodiments of cellular concrete blocks
usable in arresting bed systems pursuant to the invention. The block of Fig. 4
is a
composite block including an upper portion 100 of cellular concrete having a
desired
CGS and a thin lower layer 102 of stronger cellular concrete or other material
to
provide added strength, particularly during block transport and installation.
Fig. 5
shows a block of cellular concrete 104 which includes within its lower portion
reinforcing members, illustrated as grid of suitable fiber, metal or other
material. Fig.
6 illustrates a block 108 of cellular concrete containing within it crushable
pieces or
forms of other material. As represented in somewhat idealized form, such
material
may comprise one or more of regular or irregular pieces of compressible
material;
glass or ceramic spheres; hollow items of selected material and shape; or
other
suitable pieces. Such items or materials will typically be positioned near the
bottom
or distributed through out the block and have minor effect in decelerating a
vehicle,
or be taken into account in determining CGS, or both.
The nature of a cellular concrete arresting bed system is such that its
construction will inherently be relatively time consuming and expensive.
Therefore, it
is important that the method and information used to design the system be
reliable
enough to correlate with and predict performance under actual conditions of
use.
The use of a computerized modeling program, data obtained from appropriate
test
methodology, or both, can provide the necessary correlation between prediction
and
field results.
In general, to be effective a computer modeling program must be arranged to
accept data as to aircraft weight, center of gravity, moment of inertia,
landing gear
structure and stress capacity and projected speeds at entry into the bed. The
specifics
of a selected bed geometry and material strength relative to the crushing of
the
arresting bed as the vehicle moves through are typically also inputs into the
program.
The program would then be configured to use that information to provide output
data regarding deceleration versus distance and resulting loads on nose and
main
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landing gear at different speeds.
The necessary material strength information for the program can be provided
in one of two ways. First, actual test information using test methodology for
samples
of cellular concrete, can be used in the program. In this manner, the program
accepts
the material characteristics of a selected formulation of cellular concrete as
fixed
information and determines results based on that information. Alternatively,
it can be
assumed that the cellular concrete to be used will exhibit a certain
characteristic drag
force. Then, the designers of the arresting bed can use the above described
testing
methodology to identify cellular concrete formulas, processing techniques, and
curing
regimes that will result in materials that match the requirements for the
design.
As an alternative to a comprehensive computer modeling program, arresting
bed design can be more closely based on pro forma testing. Bed sections can be
constructed for testing using cellular concrete of one or more compressive
failure
strengths. Aircraft, instrumented wheel structures or other compressive
structures
can then be driven into sample bed sections and resulting bed performance can
then
be determined and utilized in design of a complete arresting bed. Many other
alternatives and variations will become apparent to skilled persons having an
understanding of the invention. For example, beds or sections thereof may be
of
uniform or varying thickness, may have gradual or stepped thickness variation,
may
be of uniform or multiple CGS, may be of unitary or stacked blocks or
aggregate, and
may be of selected width and overall length, as suited for particular
applications and
use by particular aircraft or other vehicles.
While there have been described the currently preferred embodiments of the
invention, those skilled in the art will recognize that other and further
modifications
may be made without departing from the invention and it is intended to claim
all
modifications and variations as fall within the scope of the invention.
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