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Methods of Impact Attenuation & Types of Fall Safety Surfacing
Playground safety surfaces and fall safety
There are a number of playground safety surfaces, all of which carry different levels of fall height safety,
functional characteristics, initial and ongoing maintenance requirements & costs, lifespans, as well as certain pros and cons.
The purpose and goal here is to provide a basic overview of the methods of impact absorption and each type of safety surface, while certain
specific products may have slightly different attributes.
The animated graphic diagrams accessible below within the Methods of Impact Attenuation... section
provide for most people a much easier way to understand what happens during playground fall testing of different types of safety surfaces,
as well as how they commonly react during real-life playground fall situations, and how the particulate size can play a significant role
in the actual fall safety and reduction of serious injuries.
NOTE: Simply click on the name of the Method, Animated Diagrams or Type of Surface to expand
for more detailed information. Clicking again will compress it.
Methods of impact absorption for playground fall safety surfaces
In general, there are two primary methods of impact absorption, or attenutation, and each playground safety surface
provides one or both methods, in varying degrees. The elements of kinetic energy involved during impact attenuation include the size of impact area,
the force and duration of impact. The force depends on the weight of the item impacting the area, as well as the speed, which accelerates and
increases based on the fall height (duration of time falling). The duration of impact is determined more by the surfacing material and method of
impact attenuation, whether dispersion, compression, or a combination of both.
Click on the Impact Attenuation Method below to expand it
for more detailed information. Clicking on it again will compress it.
Dispersion: These are commonly loose-fill surfaces such as sand or pea gravel, which are not comprised of
rubber, wood chips or Engineered Wood Fiber (EWF).
Dispersion surfaces scatter for impact attenuation, allowing whatever impacts the surface
to travel into or through the surfacing material. Because these types of surfaces lack any element which could stretch, expand or compress,
it's solely up to how well the material disperses, which can vary quite a bit depending on the overall size of the impact area, the amount of
kinetic energy upon impact, the depth of application and the average size & weight of the surfacing particulate. Proper installation depth,
periodic maintenance and replacement are key factors.
Compression: These types of surfaces are normally referred
to as unitary or monolithic surfaces, which include Poured-In-Place (PIP), playground mats and tiles, and more recently synthetic turf with
a padding layer underneath it.
Compression surfaces do just what the method describes, they provide compression upon impact,
absorbing the kinetic energy and if compression is the only impact attenuation method they often return to their original position quickly.
All too often, whatever impacts the surfacing initially becomes trapped at that location during the compression and the kinetic energy
initially absorbed by these types of surfaces is then immediately transferred directly back to the source while still falling or impact
continues, often increasing the impact duration and amplifying the nature and severity of injury. Key factors involve the proper
compressive strength and elasticity of the materials and manufacturing methods used, proper installation depth and ongoing maintenance
and/or replacement.
In some cases, wood chips or Engineered Wood Fiber (EWF) and playground
sand can become a monolithic or unitary surface, which is all too common when either are not properly maintained and/or replaced as
recommended. Wood chips or EWF can effectively become a more solid surface due to mold, mildew or fungus which knits the particulates
together. Playground sand can become almost solid over time due to compaction along with moisture and organic materials. At the point
where playground sand, wood chips or EWF no longer disperse well, they really lose a great deal of their fall height protection
characteristics because they can no longer function as intended.
Combined Dispersion & Compression: These types of surfacing
products commonly include recycled rubber mulch, wood chips and Engineered Wood Fiber (EWF) products.
Combined Dispersion & Compression surfacing products often provide the "best of both worlds," by
displacing and absorbing a large part of the kinetic energy at initial impact, allowing whatever is impacting it to travel into or through it,
then providing compression as a secondary form of impact attenuation, without transferring it back to the source. The key factors here involve
the average size and weight of particulate, proper installation depth, as well as regularly scheduled maintenance and replacement as recommended
by the manufacturer.
Fall Impact Animated Diagrams: There are well defined
testing parameters under ASTM F1292 Standards for playground fall safety testing, where a projectile is dropped from a specified height onto
the safety surface being tested and data collected from the equipment inside the projectile. This test can be performed on-site at a playground and
manufacturers typically conduct independent laboratory tests over a series of three drops at each of three different temperature ranges on a small
sample supplied by the manufacturer.
Click on "Fall Impact Animated Diagrams" above to expand and view the animations.
For loose-fill surfaces, the size, shape, uniformity and composition of loose-fill particulate plays a significant role in the relative dispersion during
impact, as the projectile travels through the loose-fill material. Only particulates with elastic properties such as rubber will provide
significant compression properties as a secondary method of impact attenuation. Other loose-fill surfaces such as sand will have no compression, and
wood chips or EWF (Engineered Wood Fiber) will have varying degrees of compression, which tends to degrade or get eliminated if they absorb moisture
and develop into a more unitary surface. At sych a point there tends to be very little fall height protection left (see photos below under Wood Chips &
EWF).
Monolithic surfaces tend to keep the projectile at the
point of impact, with the only impact attenuation being compression. The kinetic energy absorbed during impact is immediately released and
tends to push back against the projectile while it's still falling, sometimes at a different angle than the actual fall direction, because
monolithic or unitary surfaces always return to their original shape and position.
Simple drop test animated diagrams - Below are 4 animated GIF files which provide a very basic graphical
depiction of a typical drop test as may be performed under ASTM F1292 Standards, on two primary different types of playground safety surfaces,
loose-fill and monolithic or unitary surfacing.
Clicking on any of the animations below will open up a pop-up window with a larger animation.
Loose-Fill Medium
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Loose-Fill Small
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Loose-Fill Large
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Monotlithic or Unitary
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More realistic animated diagrams of fall impact - Below are 4 animated GIF files which provide a very basic graphical
depiction of a more realistic fall impact on a playground, where children can often fall at an angle away from the equipment, twisting and turning as they fall.
Clicking on any of the animations below will open up a pop-up window with a larger animation.
Loose-Fill Medium
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Loose-Fill Small
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Loose-Fill Large
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Monotlithic or Unitary
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Types of playground safety surfaces
Perhaps the most important element during the selection of a proper playground safety surface, is making sure
the product  meets or exceeds your maximum fall height
requirements and the manufacturer provides a qualified report for ASTM F1292-09 or F1292-13
testing with all applicable test data (Head Injury Criteria and G-max. scores) for all three test drops, over all temperature ranges and fall heights tested.
In addition, all product manufacturers should provide installation and maintenance recommendations, which always need to be followed. If this information
cannot be provided, it may be best to not consider using that particular product for your specific application.
Every customer should know, up front, the level of fall safety provided, all applicable installation and maintenance requirements & costs, as
well as life expectancy and expected deterioration of fall safety provided over its lifespan. That way, a customer may fully consider all applicable data
and make a fully educated buying decision.
Click on the Type of Playground Surfacing to expand it
for more detailed information, and click it again to compress it.
Unitary or Monolithic Surfaces: These types of surfaces are usually the most dangerous and most costly surfacing to install and maintain.
Click on the Type of Playground Surfacing below to expand it
for more detailed information.
- Poured-In-Place (PIP): Easily one of the most costly surfaces initially and can be negatively
affected by temperature, over time it often gets more expensive, pretty unsanitary, less effective, and in many cases downright dangerous for higher fall heights later in it's lifespan.
Data has shown a child can be 444% to 700% more likely to sustain an injury on poured-in-place than rubber mulch, and higher temperatures
can play a role in limiting impact safety of PIP. When additional studies showed that a majority of PIP tested at playgrounds failed the fall height testing
and were improperly maintained, it's perhaps not really a very good choice for injury protection for playground fall heights of higher elevations or
involving playground fall heights for younger children.
PIP may not be a good surfacing choice when you take into consideration: a) the initial and ongoing maintenance requirement costs;
b) the fact that the real thresholds at which it is likely that children suffer permanent injury, Traumatic Brain Injury (TBI) or death,
are actually 39% to 70% of what the industry standards currently allow; coupled with, c) the higher potential for serious injuries or deaths due to the
nature of PIP's compression impact attenuation; and not really safe for fall heights for younger children or at higher fall heights for older kids.
PIP typically consists of a base layer thickness of between 1" and 4" of SBR (Styrene Butadiene Rubber) "pin buffings," which are really long strands
of recycled tire rubber shavings. They have a great deal of elasticity and are mixed together commonly with a liquid polyurethane binding agent, then troweled into
the permeable base thickness intended to correspond to the maximum fall height. This base layer is normally topped with a uniform 3/8" (or thicker)
upper wear course layer of EPDM (Ethylene Propylene Diene Monomer) or TPV (Thermo Plastic Vulcanized Rubber) mixed with the same liquid binding agent.
EPDM has greater elasticity than TPV, and is also more expensive.
Often costing $12 to $15 per square foot (or more), PIP can require patching every 2-3 years, at a cost of around $2-$3 per sf, losing up to 50%
of the impact attenuation each time it's patched, unless the entire base and wear course are replaced properly. Even then the patched seam areas can have problems.
PIP commonly only lasts for between 7 and 10 years at most, and at that point you could have spent upwards of $20 or more per sf and have very little to no
effective fall height injury protection.
On top of all of this, because it's permeable and kids tend to spill things, cough, sneeze, spit, and PIP often maintains heat, air & moisture,
this product can become a giant Petri dish, breeding such lovely stuff as Salmonella, Streptococcus, and Staphylococcus (among other things),
unless it's regularly cleaned and disinfected (which can also deteriorate the binding agent properties and fall safety, if not done properly).
Damage to PIP can be seen wherever cracking, peeling and delamination occurs, often in as little as 2-3 years in frequency in higher use areas.
Sometimes installers mix black SBR granules with EPDM or TPV in the upper wear course to save money on installation costs, which can lead to
faster deterioration or discoloring of the upper wear course layer and costing the playground more in the long run.
Taking into account all of the aspects of PIP, it is probably best utilized for ADA accessibility, safer pathways or jogging tracks,
or for permeable splash pads at water recreation areas, and not injury protection from higher fall heights on playgrounds.
- Playground Mats and Tiles: Similar to PIP, but comes in rolls or tiles
which can be affixed into position.
Playground mats and tiles have many of the same drawbacks of PIP due to being a unitary or monolithic method of impact attenuation. The
difference is it can often be installed without a qualified installer, which can also be a drawback. These products are often molded or cut out of
sheets of material, and their impact attenuation can also be affected by temperature shifts. The additional problem comes from what happens in between
tiles or underneath the material, where heat, air and moisture becomes trapped. Just like PIP, lots of not so lovely stuff can start to grow and breed.
At a cost of normally between $10 to more than $14 per square foot, it's pretty expensive, and still lacks the fall height protection of loose-fill
surfaces. You can't really patch it, so it's into the landfill and replacement time when it becomes damaged.
- Synthetic Turf with Padding: Perhaps the most expensive option,
over time, that really doesn't provide good fall height protection.
A typical quality synthetic turf installation can run upwards of $8 to $10 per square foot, or more, without any impact protection padding.
With the padding necessary to provide any level of proper playground fall safety, it ends up costing upward of $14 per sf.
The biggest problem is the padding is normally guaranteed for fall height protection for 2 years, while the synthetic turf is often
guaranteed for up to 8 years. Once you remove the turf to replace the padding, it normally cannot be re-installed into the same area
due to the need to have excess material to stretch and secure it prior to trimming off the excess material during installation.
So you essentially can get a mediocre safety surface, complete with all of the problems with impact attenuation of PIP, with an effective
lifespan of about 2 years.
Loose-Fill Surfaces: These have been recognized as providing the best fall height protection, although they vary quite a bit when it comes
to respective levels of fall safety, longevity, installation and ongoing maintenance costs.
Click on the Type of Playground Surfacing to expand it
for more detailed information.
- Playground Sand:
A lot of people like playgrounds with sand in them, although it's not really as safe as people think it is when it comes to playground fall height injury protection
and other dangers which commonly exist within it.
Playground sand has issues with silicosis exposure, which is a pretty minor issue, when you consider what most animals like to use the sand at playgrounds for.
This is one of the reasons the National Institutes of Health (NIH) recommend removing and replacing playground sand (and wood chips) every 2 years. This is due to the pathogens
found in fecal matter, not only from feral animals, but from most pets as well. Pathogens can easily pass through the skin instantly with momentary contact, and
lots of information is available about zoonotic parasites associated with this. When it is not properly maintained, sand can eventually get to be a solid mass that's as
hard as packed dirt, due to attrition and integration of dirt, other organic materials and moisture. At that point, there's really no fall height protection.
If the cost of playground sand sounds inexpensive, remember that a 12" thickness is usually required to be installed and maintained, add to this the cost of topping it
off at least every year, removing and replacing it every two years, plus doing what the Consumer Product Safety Commission (CPSC) recommends, keeping animals
and birds from accessing it, 24 hours a day, 7 days a week, 365 days a year. I have yet to see any playground that properly maintains their playground sand.
Once it all shakes out, sand can be pretty expensive for what you get, and what you get is not always what you might want.
- Pea Gravel:
Pea gravel used to be an acceptable type of playground safety surface, but no longer.
Pea gravel was a commonly used playground safety surface, until the rates of injuries started to be compiled and analyzed. Pea gravel is not recommended as a playground safety surface.
- Wood Chips or EWF:
Wood chips or Engineered Wood Fiber is another common playground safety surface, but it usually doesn't even provide safety as good as sand, at greater expense,
and it often deteriorates rapidly with a high rate of attrition due to wind and rain (or runoff).
Wood chips or Engineered Wood Fiber (EWF)
require maintenance for re-topping typically as often as every 6 months to 12 months, and the National Institutes of Health (NIH) recommends
removal and replacement every 2 years, just like playground sand, for sanitary reasons. In addition to some animals using the wood chips or EWF for things we wished they wouldn't, there
are also the other lovely things that are attracted to them, like termites, rats, mice, slime mold, and lots more. Many playgrounds have some level of moisture which
accumulates within the wood chips or EWF, which causes slime mold on the surface, as well as other forms of fungus to "knit" together the material underneath, effectively
creating a monolithic or unitary surface, which can no longer provide the level of fall height protection intended. At this point, it has to be completely removed
and replaced, usually at a hefty cost.
A 2009 study found that falls onto EWF versus sand were between 287% and 496% more likely to result in arm fractures.
- Recycled Rubber Mulch: Commonly regarded as the safest and
longest lasting playground fall safety surfacing. It costs far less than other products over it's lifespan, providing the highest level of
fall height protection over a much longer lifespan, without the rapid deterioration in fall safety or high maintenance requirements common in many other surfaces.
Recycled rubber mulch is made from recycled tire rubber, just like the kind used for PIP, but in either shredded or nugget form. Here, the shape and
size of the particulate plays a huge role in fall safety. The shredded material really doesn't work nearly as well as the nuggets, and can often blow or
wash away easier than wood chips or EWF in wind or rain. Whereas the recycled rubber mulch nuggets will sink to the bottom of a glass of water, and
resist wind and rain far better. A lot of people have stated that most rubber mulches require a lot of maintenance to maintain the proper level. This
is because most manufacturer's products exhibit up to +/-4" in variance during playtime. With a 6" normal application thickness, that means that it could
vary anywhere from 2" (or really no fall safety) up to 10" (which means it's spilling out all over the place and has
to be raked back into the playground).
The real key here is that most manufacturers make a "one-size-fits-all" product of about 3/4" in avg. size for both landscaping and playgrounds, made
from common steel belted radials. They state they're 99.9% wire-free, which simply means they can have between 20-30 exposed rigid steel wires per pound
of material. With an average residential playground often requiring a couple tons of rubber mulch, or quite a bit more for public playgrounds (that's a
whole lot of exposed wires). These wires can cause punctures and lacerations.
Another issue is the marking up of skin and clothing, sometimes within as little as 12 months from installation. This has everything to do with the type
of paint used, the method of application and curing. When the National Football League (NFL) tested all of the recycled rubber mulches manufactured in
North America in independent lab tests, what they found was only one manufacturer still maintained their color, without color loss, at the end of the test.
All other competitors lost their colors in 25 days or less. That same company whose color lasted is now in over 93% of the NFL fields witgh synthetic turf,
in the form of a finer particulate painted in-fill product.
One particular company (the same one as tested by the NFL) manufactures separate products for landscaping from their playground safety surfacing product,
which is made from bias-ply tires, so there's no steel wires to begin with. The smaller particulate tends to maintain better density, more uniform thickness
without constant raking and leveling maintenance, and provides up to 100% better fall height protection. All of this results in a product that tends to vary
only about 1/2 what other rubber mulches do during playtime activities, or +/-2" variance vs up to +/-4". At a cost of roughly $7 per sf, with a safety guarantee of 25 years,
with the best fall height protection of any playground safety surfacing. There's really no viable competition to it.
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Playground fall safety surfacing selection recommendations
All manufacturers of playground fall safety surfacing products would love to say their product(s) provide the best playground fall safety,
but they all can't. The only way anyone has to determine if they're safe or not for a specific application is to compare data from uniform testing for all products
being considered. Make sure the lab tests were performed under the applicable standards of ASTM F1292-09 or F1292-13 at an IPEMA approved testing lab and,
all data is included for all three drops, over each of three temperature ranges, hopefully at three different fall heights, with all applicable HIC and G-max.
scores for every drop being shown.
If any manufacturer of IPEMA approved playground safety surfacing is not willing to provide you the full and complete test data and information you need to make a proper comparison and assessment,
then that manufacturer may be trying to hide something not too favorable about their product fall safety testing. When you encounter this, it may be best to remove such products from your consideration.
NOTE: The revision year within ASTM standards are designated in the two numeric digits following the hyphen after the ASTM numbered standard.
What changed significantly between the 2004 and 2009 revisions of ASTM F1292 were minor in the number of words, but the effect was a more stringent and accurate
wording of the testing standards which defined certain testing procedures as mandatory or required, where they were merely recommended & suggested previously,
which had left them open to potential variations in interpretation and therefore could result in inconsistent and inaccurate test data from various testing labs.
Know the age range of the children who will be playing on the playset being considered, make sure the playset is manufactured for the
same age range, and all applicable safety use zones, surfacing installation depths and maintenance requirements are budgeted for, established and maintained.
Know what the real HIC score is where it is likely a child suffers permanent injury or death (390 HIC for ages 1-3, 570 HIC for ages 4 & 5, and 700 HIC for ages 6
through adult), so that you can select a safety surface which provides impact attenuation at numbers lower than these thresholds for the maximum fall height of
your specific play structure and age range of child(ren) intended to be using the playground.
Find out and know up front about: a) the costs associated with not only the proper initial installation of the safety surfacing; but also, b) the
recommended maintenance, frequency and associated costs; c) whether there is any impact attenuation deterioration over different temperature ranges or over time;
as well as, d) the life expectancy and what is involved in removing and replacing the safety surfacing at the end of that functional lifespan.
This way, you can rest easier knowing you properly researched and utilized all available information to make a truly educated decision, instead of merely going
with something that someone recommends, whether or not it's really going to provide you the fall safety you really require. From there, it's important to plan and budget for
proper maintenance and replacement in accordance with the manufacturer recommendations to insure it always provides that same level of injury protection or better.
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