Material Properties

MATERIAL PROPERTIES

 

Products are only as good as their weakest link.  In order to make a wise product selection it is necessary to understand which product is best for your application.  You can do this by getting a basic understanding of how different products perform under given conditions.  Their material properties and construction determine this performance.  New products are emerging every day and not all of these will be discussed here, however, the same issues will plague all products and they should each be evaluated for their merit.  A product cannot simply be evaluated by it's material properties as installation ease and cost are always real life factors that also guide our buying decisions.  The more critical the need for a quality product the more weight that should be put on the evaluation of their material properties.

 

With that said there are several different properties that govern the longevity and performance of any trench drain.  The trench body, rails, grates, and lock down devices need to be evaluated separately and weighed accordingly.  The following aspects should be studied:


Compressive strength

Tensile strength

Flexural strength

Impact resistance

Water absorption / freeze thaw / thermal expansion

Bond

Bearing area

Fire resistance

Abrasion resistance

Chemical resistance

UV resistance

 

COMPRESSIVE STRENGTH

The compressive strength is technically defined as the ability of a material to withstand a force that is pushing the molecules together.  Typically, trench drains transfer loads from the grate through a rail, into a channel, and then into the concrete encapsulating the channel.  In this transfer if one of the materials is not capable of handling this load then the material will fail and usually begin a downhill process of destroying the trench drain.  Grates made of cast iron, metals, and some plastics have high compressive strengths.  Avoid rails, grates, and channels made of soft plastics when loads will be transferred through them.

Material comparison based on typical trench drain construction (IE: material thickness, fabric orientation, resin content, etc).

MATERIALRELATIVE COMPRESSIVE STRENGTH

Cast Iron

High (gauge dependent)
Ductile IronHigh (gauge dependent)
SteelMedium (gauge dependent)
FRP (Fiber Reinforced Plastic)Medium (fabric orientation dependent)
PolyethyleneLow (resin content dependent)
Polymer concreteMedium (resin and aggregate content dependent)
ConcreteMedium (water / cement ratio dependent)
TENSILE STRENGTH

Tensile strength is defined as the ability of a material to withstand a force that is pulling the molecules apart.  Ex:  When you push down on a board the top of the board is being compressed as it bends but on the under side of the board the molecules are being stretched and pulled apart testing the tensile strength.  Tensile strength is especially important for grating materials as they can easily be flexed putting large tensile loads on the materials.  Metals typically have very high tensile strength.

Material comparisons based on typical trench drain construction (IE: material thickness, fabric orientation, resin content, etc).

MATERIALRELATIVE TENSILE STRENGTH
Cast IronMedium (gauge dependent)
Ductile IronHigh (gauge dependent)
SteelHigh (gauge dependent)
FRP (Fiber Reinforced Plastic)High (fabric orientation & fabric strength dependent)
PolyethyleneMedium (resin content and void dependent)
Polymer ConcreteLow (resin/aggregate/void dependent)
ConcreteLow (water/cement ratio dependent)
FLEXURAL STRENGTH

Flexural Strength is the amount that you can bend an object without it breaking.  Flexural strength varies with material type but is also very dependent on the thickness of the cross section.  This property is most important to grating materials as they take loads and span a distance allowing them to flex.  Typically a channel should not be experiencing these stresses, however, many channels improperly installed do see these stresses and then the quality of the channel material can bridge these gaps or simply fail.  For gratings look to thicker materials and choose ductile irons over cast irons if this type of loading is expected to be severe.

Material comparisons based on typical trench drain construction (IE: material thickness, fabric orientation, resin, and aggregate content, etc).

MATERIALRELATIVE FLEXURAL STRENGTH
Cast IronLow (gauge dependent)
Ductile IronMedium (gauge dependent)
SteelHigh (gauge dependent)
FRP (Fiber Reinforced Plastic)High (fabric orientation dependent)
PolyethyleneHigh (resin content dependent)
Polymer ConcreteLow (resin and aggregate dependent)
ConcreteLow (water/cement ratio dependent)
IMPACT RESISTANCE

This property is specially important for the grating material and the rails as they are constantly in contact with hard items such as carts, fork truck tires, and items being dropped.  The cyclic nature of these dynamic forces can instantly or over time cause cracks to propagate until finally the material fails.  During the design process it should be determined if this is going to be a problem and if there will be a high frequency of impact or very heavy impacts then heavier gage materials should be chosen and ductile irons should be selected over cast irons.  Steel typically has high impact resistance but be ware that if welding is done improperly that the failure may come at the welds even thought the gauge and material are sufficient to handle the impact loads.

WATER ABSORPTION / FREEZE THAW / THERMAL EXPANSION

Trench bodies (the channel material) should be made of a material that does not absorb large amounts of fluids.  When materials go through a freeze thaw cycle if there is water absorbed inside the material it can cause failure within.  Joints should be sealed between channels if freeze thaw is considered to be a problem in your climate.  

MATERIALRELATIVE ABSORPTION RESISTANCE
Cast Iron

High (non porous material)

Ductile Iron

High (non porous material)

Steel

High (non porous material)

FRP (Fiber Reinforced Plastic)High (non porous material)
PolyethyleneHigh (non porous material)
Polymer ConcreteHigh (non porous material)
ConcreteMedium (porous material)

Many regions of the world can experience temperature differences of 20 to 40 degrees in a single day from the heat of the day to the coolest part of the night.  Seasonal changes can cause temperatures to have a difference of 100 degrees.  During cool temperatures the materials shrink and during warmer temperatures materials expand.  The rate and amount of expansion and contraction can cause materials to fail especially if they are mechanically fastened to other materials that are expanding and contracting at different rates.  Also, consider the grate locking mechanisms when looking at thermal expansion.  Many lock down methods hold the grate rigidly in place by bolting the grates into the rail assemblies or straps welded to the rail assemblies.  These rigid connections can cause problems when the concrete expands and contracts.  The rails are held rigidly together by the grates and bolts while the concrete is contracting.  The build up of stress has been known to cause the concrete near the anchors to fail and if the rails were not properly anchored the rails have been known to release from the concrete leaving a failed trench.  Thermal expansion is a very serious issue that cause numerous failures each season.  Choose materials wisely when you are looking for products that will perform over wide temperature differentials.

The final problem with thermal cycling is with thermal plastic materials.  Thermal plastics are heated to be formed into a shape.  They can also be heated and bent into a new shape at any time.  In processes where high temperature fluids are used thermal plastic materials such as Polyethylene should be avoided as they can lose their shape and strength.  Other resins like FRP and Polymer concrete and epoxies are thermal set resins that have a chemical reaction to take their shape and are not generally affected by higher temperatures.

 BOND

Bond strength is a load transfer mechanism that occurs when a material chemically bonds to the surrounding concrete allowing for a transfer of  stress to the surrounding material.  Unfortunately there is no chemical bonding of materials currently in the trench drain business unless a poured in place material is used and the surface is properly prepared.  This means that loads must be transferred through mechanical bonds or keyways.  When choosing a channel make sure that it has sufficient keyways on the side that will be encapsulated to transfer any loading that the system may experience.

 

BEARING AREA

Bearing area is the cross sectional area under the grate seat that mechanically transfers the load from the grate to the trench drain body and surrounding concrete.  If the area is insufficient to carry the load or the material that the load is transferred to is too weak a system failure will occur.  There are two common problems.  The first is that the area is too small usually caused by a system with a small rail design that provides little bearing area.  The other problem commonly seen is that plastics or other weak (in compression) materials are placed between a rail and the concrete.  If the sandwiched material has low compressive strength it allows a rigid rail assembly to flex and causes concrete to fail around the anchors and edges.  If the loading is simply too severe for the bearing area then the concrete will sheer off leaving the rail to rock or twist causing the same type of failure.  This is a very important issue for heavy loads and where high speed traffic is likely.  High speed traffic, even if technically considered light, can put much higher dynamic loads on a system than a much heavier force at slower speeds.

 

FIRE RESISTANCE

Fire resistance is important in industrial applications where welding occurs, hangars where fuels can ignite, and many other places where added flammability has the potential to spread fire to areas where the problem can become more severe.  

MATERIALFLAMMABILITY
Cast Iron

Low (not flammable)

Ductile IronLow (not flammable)
SteelLow (not flammable)
FRP (Fiber Reinforced Plastic)Medium (does not sustain a flame)
PolyethyleneHigh (flammable material)
Polymer ConcreteMedium (does not sustain a flame)
ConcreteLow (not flammable)

 

ABRASION RESISTANCE

Trench bodies are subjected to a large amount of grit that over time can cause the trench liner to be abraded away.  Grit in the bottom of the trench acts as sand paper removing material from the trench body over time.  The thicker the material the longer it will last, but more importantly the hardness or resistance to being worn away will ensure a long life.

Below is a relative scale of the abrasion resistance of typical trench body materials.  It is based off the material type and thickness of typical cross sections of trench bodies.

MATERIALABRASION RESISTANCE
Cast IronHigh (thick cross section with hard/dense material)
Ductile IronHigh (thick cross section with hard/dense material)
SteelMedium (hard material but typically thin cross section)
FRP (Fiber Reinforced Plastic)Medium (hard material but typically thin cross section)
PolyethyleneLow (soft material)
Polymer ConcreteHigh (thick cross section with hard material)
ConcreteHigh (thick cross section)

 

CHEMICAL RESISTANCE

Many manufacturing plants need to consider chemical resistance before choosing a material.  For specific chemicals and how they affect different materials see the chemical resistance charts.  The biggest problem we see in the industry is that a good material is used but the joints in the materials are not treated properly.  Any connections also need to be properly sealed.  If chemicals are severe a test should be done before concrete is poured around the trenches.  The outlet can be plugged, fill the trench with water, and then look for leaks.  Make sure all are repaired before concrete is poured.  The other critical thing is making sure that the proper sealant is used.  For steel you should weld joints, for fiberglass and polymer concrete the joints should be roughed up and a proper chemical resistant epoxy should be used, for polyethylene the joints should be welded (nothing sticks long term to PE), and for concrete the whole trench may need to be coated with an epoxy system.

MATERIALCHEMICAL RESISTANCE
Cast IronMedium (strong acids and hard to seal joints cause problems)
Ductile IronMedium (strong acids and hard to seal joints cause problems)
SteelLow/High (carbon steels are typically coated but the coatings abrade then they are chemically reactive and deterioration is rapid - stainless steels are great if seams are welded)
FRP (Fiber Reinforced Plastic)High (Resistant to broad range of chemicals)
PolyethyleneHigh (Resistant to broad range of chemicals)
Polymer ConcreteLow (Good resistance but typically short channels difficult to seal)
ConcreteMedium (reacts with acids & sugars)

 

UV (ULTRAVIOLET LIGHT) RESISTANCE

Gratings and trench bodies exposed to the sunlight should be made of UV resistant materials.  The best UV inhibitor is carbon black which you can generally tell when the plastic is black.  Many colored plastics also have a UV inhibiting feature.  Beware of materials that have little or no color as they likely do not have a UV package incorporated.  Ask the manufacturer to certify that their product does have a UV inhibitor in all materials exposed to sunlight.  In exterior applications we have seen plastics turn brittle from UV and fail under much smaller loads than they were designed to handle.