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It is called poured in place rubber for the fact that the system consists of rubber granules that are mixed with a polyurethane binder and mixed and poured on site. Could the industry have come up with a better name? absolutely. But this is what we are working with now. Popular since the late eighties, poured in place rubber is a unitary surface that is very well suited for ADA accessibility and as a surfacing solution around equipment that is already installed since it is a troweled product, whereby installers can easily work around existing posts and structures. It is also used to create 3D shapes around surface and can be used on hills and mounds. Finally, the surface has permeated other industries as well for its look, skid resistance and fall height protection. So, we are seeing more installations around pool decks, horse stalls, boat decks, nursing home walk ways and stairs…. Etc.
The system is made up of two layers:
A base layer: is made with SBR. SBR (styrene-butadiene rubber) is a synthetic rubber widely used in making car and truck tires. To be specific, the base layer is made up of buffings: these are created when tires are retreaded. The tire must be buffed before the new tread is melted on top. The shavings that are created in this process are recycled and sold to the playground industry as buffings. Buffings are elongated strands of rubber that contain a lot of air pockets when stacked together making it ideal as a cushion layer.
Buffings are of course not the only base layer material used. SBR nuggets and other forms of rubber have been used as well. For example, in Europe they tend to use larger granules of SBR as base. Whatever base you choose to use, make sure it passes the impact attenuation tests, and make sure you are using the right binder concentration with it.
The top layer: otherwise known as wear layer or the wear course is typically made up of EPDM (Ethlyn Propylene Diene Monomer) rubber, although recently TPV (thermoplastic vulcanizates) and other polymers have been introduced to make this layer. The granules come in different sizes but on average they range from 1 mm to 4 mm in any particular batch based on the sieve used to grind the rubber sheets. Both layers are mixed with a binder and troweled on the spot and left to cure to make one continuous surface. Usually, the curing takes 24 to 48 hours depending on ambient humidity and temperature.
I will focus here on EPDM rubber because it is by far the most used type of rubber granules in poured in installations.
Components of a PIP granule:
· EPDM (Synthetic rubber)
· Sulfur (Cross linking agent.
· Paraffinic oil
· Selective additives (UV Stabilizers, fire retardants. Etc.)
· Color pigments
· Fillers (Calcium carbonate or chalks)
These components are mixed in like a cake dough in the right proportions, extruded into a sheet that is then baked and granulated.
The following are the types of binders used in a poured in rubber installation:
Base layer Binders: Base layer binders are a recent introduction to the poured in place rubber ecosystem. When I started in this business twenty years ago, there were no base layer binders. We used aromatic binders for both buffings and EPDM granules. As a matter of fact, even the mixing percentages for base layer binders were different then. We used to mix at a 12% ratio. We now do it at a 16% concentration. This is due to the increased dust content in buffings. This concentration of course differs with crumb rubber or different types of base layer rubber used out there.
Base layer binder is less refined binder. It is almost black compared to the amber aromatic binder and the often-clear aliphatic binder. I use base layer binder strictly as a cost cutting measure. On a 10,000-sf job at a 4-inch-deep base layer, the savings are substantial. The base layer binder does not bind as much as other binders, you will find that out easily when you get it on your skin. It is a lot easier to remove than other binders. Can you use base layer binder with the wear layer? Absolutely not. Can you use it with bonded rubber at the right mixing proportions? Absolutely not. Base layer binder can only be mixed with base layer material.
Aromatic binders: These binders are the most frequently used binders in wear layer applications. They are usually amber in color so they tend to have a yellowing effect on EPDM granules. Some colors are more affected than others by Aromatic binders, such as some shades of blue, light grey and some light beiges. The ambering/yellowing effect is temporary. As the surface is used, foot traffic will get rid of the top varnish over the granules and the EPDM granules will go back to their original color. In the repair above, the actual mix with blue and black, but because of the yellowing effect, the granules tuned into green and black temporarily.
Aromatic binders are also formulated to work in different temperature thresholds. You might hear of a winter Aromatic mix designed to cure quicker than a summer mix. In any case, the installer has to be well versed with the cure times each binder manufacturer allows for.
Aliphatic binders: Aliphatic binders are clear binders. They are generally double the price of aromatic binders which makes them non affordable in some situations. Because they are clear they tend to be more UV resistant and will not change the color of the EPDM granules. They are also more resistant to pool chemicals and chlorine, which makes them the premiere choice over aromatic binders in those applications. Just like Aromatic binders, Aliphatic binder are made in different flavors, winter or summer mixes based on the cure time.
So which binder should you use? Aromatic or Aliphatic?
If you are on a budget and your safety surface uses a mix of black and one of the darker colors, and you are nowhere near a pool or splash pad, the answer is easy. Go with the cheaper aromatic binder. If the surface is a splash pad, a pool deck or if you have intricate designs with lighter colors, go with aliphatic binders.
The thickness of the base layer is what determines the impact attenuation properties of the system. For example, a 4’ fall height is typically achieved with an inch and a half base layer. Please note this number will greatly vary based on the following factors:
- How compacted the buffings are when troweled.
- The weather the day of the install.
- The proportion of binder to granules used.
- Introduction to foreign objects into the mix.
Keep in mind that the thickness above only refers to the base layer. The thickness of the system as a whole will be a half inch more as you add the wear layer. Also, it is noteworthy to add that there is a limit to how thick the base layer can be without leading to cracks in the wear layer I would not install a base layer thicker than 6 inches. Doing so might lead to cracking in the wear layer as the base layer depresses more than the tensile strength of the wear layer can bear.
Site Preparation
Poured in place rubber is best installed on concrete or asphalt. A quick note of caution. Never install PIP on freshly poured concrete slab. Give it 28 days before you even think about it. The gases that the fresh concrete emits as it is curing breaks the binders down. Your concrete or asphalt surface should also be in pretty good shape and not have big cracks and hole. Make sure you fill these before starting the PIP job. Equally important is for these surfaces to have a pitch for drainage.
Having said that, we don’t always have concrete or asphalt surfaces to work with. It is typical for our PIP surfacing to go in a grassy area. So, we follow the following steps to create a surface that is as close in hardness and consistency as asphalt.
1) Remove all existing mulch, grass and all organics. You might have to dig 6 inches or more here into the soil. Any organic material you leave here will decompose and create pesky air pockets later.
2) Install ¾ inch stone with fines or road base and compact at 95% compaction rate. Make sure you base has a pitch for drainage.
3) Optional but helpful sometimes is to install a layer of geo fabric on top of the stone.
When the site prep is done, the next step is to install the base layer of the poured in place system. This involves mixing the buffing bags (usually 50 pounds) with a base layer binder and troweling them on site using a gauge depending on the desired thickness. A trowel is used to gently tap the mix after its spread to keep the pockets in the buffings. The base layer is usually mixed at a 16% ration to the rubber by weight. Meaning for every 100 pounds of rubber buffings you will need 16 pounds of base layer binder. Back in the days we used to do 12% but today’s buffings for some reason have a lot more dust than they used to and that dust does eat up binder. So, the percentage increased.
Mixing both base layer and wear layer is done using a mortar mixer. Never use a concrete mixer for this as it does not mix the granules and binder evenly. There are also specialty mixers that are made specifically for poured in applications, but these tend to be prohibitively expensive. A good mortar mixer than can handle at least two bags at a time will do the job just fine.
One important thing to remember if you are installing your base or wear layer on freshly installed concrete: wait 28 days for the concrete to cure before attempting to install your rubber. While curing, the concrete is emitting heat and gases that affect the poured in place binders.
Sometimes, the base layer is not needed at all. For example, if you have a hard surface such as asphalt or concrete, and you just want a running surface that is skid resistant, or you just want to add some color and designs to the surface, the wear layer is enough. You cannot however install a wear layer by itself on top of stone aggregate. Your base will eat up too much wear layer during the install. So when you are installing over aggregate, we recommend you install at least an inch of base layer. You install base layer when you are trying to achieve impact attenuation, or a critical fall height. Which in turn is related to the fall height of your equipment. How do we know if the base layer needs to be exactly that thickness? Well. Because that is what the impact attenuation tests done by the manufacturers of the materials tell us. Should you be conservative and go with a thicker system just in case. I would err on the side of conservatism. Remember that PIP is a hand troweled product. You can have one installer with a heavy hand compact the buffings mix more than needed on one and another one tapping them gently on the other and you end up with two different impact attenuations on the same surface.
It is important to prime the base layer and whatever surface the wear layer will touch. That can be the railroad ties or the concrete borders. I am a huge fan of priming. It ensures a much better bond over time. Priming is accomplished by using a ready-made binder primer such as the Rebinder B, or diluting aromatic or aliphatic binders with xylene. You can use a cheap brush or roller to prime. Don’t use expensive brushes to do this as you will be throwing them away after use. You can dilute to a 1 to 4 ratio. As long as the resulting liquid solution is easy to spread with a brush or roller, you are good to go. When priming, be careful not to prime any surfaces that don’t need to be primed as you will leave a yellowish residue on them. You can always mask those surfaces to avoid stains.
Installing the wear layer of the poured in system
The wear layer of the poured in system is the capping layer. It simply encloses or protects the base layer. Without a wear course, the base layer will simply disintegrate with time. The wear layer is typically a half inch in thickness, although it is typical to go thicker in high traffic areas such as slide exists and swings. The wear layer is either mixed with an aromatic binder or aliphatic binder based on the application. The proportion at which it is mixed is 22% by weight. Meaning, for every 100 pounds of EPDM rubber, you will need 22 pounds of binder. The average pail of binder contains 44 pounds. The average bag of EPDM weighs 55 pounds.
We highly recommend at least one electronic fish scale onsite to accurately weigh the binder. Once the correct binder amount is determined, it is ok to drill a screw at the right height in the pail to measure against so you are not weighing every batch.
Mixing EPDM granules requires a little bit more attention than mixing buffings, especially when both black and color granules are involved, which is mostly the case. Black granules are really recycled SBR granules from car tires. This means they have a lot of dust content. Mixing those with a color EPDM needs to be done at a consistent rate. Meaning if you mix a bag of green and a bag of black for 30 seconds, and then mix another batch for one minute you will have discoloration on your surface in the form of black waves. So, it is very important that whoever is in charge of the mixer understands this.
Troweling the wear layer is also very different and more time consuming than troweling base layer. Because the binder content is more concentrated and because this is the finishing layer, imperfections in the surface are not acceptable. Unlike the base layer, when working with the wear layer the trowel will have to be lubricated more often. Soapy water is adequate to do so. More attention must be paid to how the mixer puts down the mixes. Depending on the size of the job, there is usually a person whose job is to screed the mixture for the person troweling. Their job is to spread the mix in such a consistent way that the troweler has the right thickness to work with.
Closing the granules is an industry term that literally means flattening them as the EPDM granule is jagged and has many sides. This is a result of the grinding process. The many edges of the granules make it easy for them to "interlock". However, you cannot leave a spiky edge on the surface as it will become a weak link and foot traffic will dislodge it. Hence, closing the granules. The process involves lifting one end of the lubricated trowel, and applying pressure as you are sliding back and forth on the surface.
