Plastic shrinkage (shrinkage) cracks (fissures) form on the surface of fresh concrete immediately after pouring and while it remains in a plastic state, mainly on horizontal surfaces.
They are usually parallel to each other at a distance of about 0.3 to 0.9m, relatively shallow, and generally do not intersect the perimeter of the slab. When high evaporation rates cause the concrete surface to dry out before it has set, plastic shrinkage cracking is likely to occur.
While plastic shrinkage cracks are unsightly, they rarely affect the durability or strength of floors or pavements. The development of these cracks can be minimized if appropriate measures are taken before, during and after the concrete pour.
Plastic shrinkage cracks are caused by a rapid loss of water from the surface of the concrete before it has set. The critical condition exists when the rate or rate of evaporation of surface moisture exceeds the rate at which rising exudate (bleed) water can replace it.
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Polyamideimide has a high resistance to elevated temperatures and is capable of performing under severe conditions. PAI machined parts provide higher compressive strength and higher impact resistance than most advanced engineering plastics.
Polybutylene terephthalate, a thermoplastic of the polyester polymer family, is characterized by high strength, stiffness and resistance to thermal deformation, as well as very high dimensional stability and low tendency to creep.PBT polymer is especially suitable for food industry applications due to its excellent resistance to chlorine and caustic cleaning solutions.
Chemically resistant and insoluble in common solvents including acids, salts and oils. Good dimensional stability at high temperatures (important due to its industrial applications). Its modulus of elasticity is strongly influenced by its degree of crystallinity; because of its viscoelastic nature, depending on temperature different tensile strength values are obtained, polyetheretheretheretherketone (PEEK) is lighter than steel, aluminum and titanium. PEEK™ is sterilizable in autoclave, by application of ethylene oxide or by radiation and presents a remarkable resistance to radiation.
What is stress cracking in plastics? del momento
When stress is applied to the plastic, it is transmitted along the polymer chains causing the molecules to strain to remain in contact. The polymer chains eventually drag into tension and deform. This stage is called creep. With force and time, stress cracking can occur. Some chemical additives or temperature variations can accelerate the process.
Some plastics such as ABS, acrylics, polycarbonates and polystyrenes are more susceptible to stress cracking. These are amorphous plastics, and they bond easily. Other plastics such as polyethylene, polypropylene and polytetrafluoroethylene (Teflon) are semi-crystalline thermoplastics and are more difficult to bond.
Check for other stress-inducing chemicals. What does that mean? Some chemicals can travel (leach) from one substrate to another. The adhesive is not always a barrier to that leaching. When bonding PVC to polystyrene, plasticizers in the PVC can leach directly through the adhesive and attack the polystyrene. Add heat, and it will attack even faster!
What is stress cracking in plastics? online
The stiffness of a polymer is described by a modulus of elasticity which measures the ability of a material to bend or deform elastically or non-permanently when a specific force is applied. It is measured as a ratio of pressure to strain. Specifying how pressure and strain are to be measured, including directions, allows different types of modulus of elasticity to be defined. The most commonly used for plastics are Young’s modulus and flexural modulus (ASTM D790). The result is generally expressed in Pa or psi. The higher the result, the stiffer the material. This property is also sometimes expressed as flexibility, but is measured in the same way. The lower the result, the more flexible the polymer.
Environmental stress cracking is a form of degradation that breaks the secondary bond between polymers and is caused by the simultaneous action of stress and chemicals. Resistance to environmental stress cracking is tested using ASTM D 543 or ISO 220088-3: 2008 standards. The tests check for any change in weight, appearance or tensile strength of the samples compared to control samples. Results are not measured by a unit of measurement, but by a series of observations such as swelling, decay, cracks, fissures and/or changes in physical properties.