Concrete with polymers added during mixing to modify properties of hardened concrete are polymer modified cement concrete (PMC).
Polymers are added to the concrete mixes either in the form of an aqueous emulsion or in a dispersed form. This is to improve the following properties of concrete:
- The extensibility and the tensile strength of the concrete structure
- The impact resistance
- The Abrasion resistance
- The durability and the resistance to the aggressive fluids
- The bond between the old and the new concrete
The polymer modified cement concrete is a composite that is obtained by the incorporation of a polymeric material into the concrete. This is carried out during the mixing stage. The polymers that are incorporated at this stage should in no way interfere with the hydration process.
Since many polymers are insoluble in water, their addition can only be carried out in the form of emulsion or dispersion or in the form of latex. The composite formed is then cast into the required shape in the conventional manner of construction. Later the curing is carried out similar to normal concrete curing.
The hydrated cement and the polymer film is formed due to the curing of the polymeric material constitute an interpenetrating matrix that binds the aggregate. The polymeric materials in the form of lattice and prepolymers may be added to modify the cement concretes.
The quantities of polymers required for polymer modified concrete are relatively small. This range from 1 to 4% by mass of the composite. Polymer modified concrete is least expensive. The processing of PMC is found to be very simple.
In terms of mechanical properties of PMC concrete, it is not up to the mark. The polymer impregnated concrete is found better in terms of mechanical properties compared to polymer modified concrete.
Types of Polymer Modified Concrete (PMC)
Based on the type of the modifier used, polymer modified concrete can be classified into the following types:
- Latex-Modified Cement Concrete
- Pre-Polymer Modified Cement Concrete
Latex-Modified Cement Concrete
The lattices are white milk like suspensions that consist of very small sized polymer particles, that are suspended in water with the help of emulsifiers and stabilizing agents. It contains about 50% of polymer solid by mass.
Both elastomeric and glassy polymers have been employed in the lattices for modifying cement concrete. These impart high modulus of elasticity, higher strength and low rate of corrosion of the reinforcement.
The latex admixtures can be stored in an adequate exposure that helps the mixture to be protected against the freezing and temperature exposures.
If the mixture stored drum is exposed to sunlight, it has to be properly covered with a suitable insulating blanket. The enclosed temperature to be maintained must be less than 85F.
The natural rubber latex, acrylonitrile-butadiene, neoprene and rubber latex are some of the commonly used elastomeric latex. The glass polymers have a greater modulus of elasticity, strength and they undergo a brittle type of failure.
The common examples of the same are polyvinylidene chloride, acrylic polymers, polyvinyl acetate and styrene butadiene copolymer latex. The polyvinylidene copolymer is used in unreinforced concrete applications due to the presence of residual chlorine that causes the corrosion of reinforcement.
Moist curing of composite for a period of one to seven days forms the optimum curing temperature. This is later followed by a dry curing. At the 28 days of curing, the latex modified concrete would have gained its final strength.
Prepolymers-Modified Cement Concrete
The polyester -styrene based system, epoxy system and the furan system etc. are used for this. The strength improvement of this type of PMC is of the order of 50-100% over the conventional concrete.
Its adhesion property is good and has the improved durability properties. The tensile strength and the modulus of rupture are more than twice those of the conventional concrete. The formation of the micro-cracking is less in the case of pre-polymer-modified cement concrete.
There is lower water cement ratio and the filling of pores within the pre polymer modified concrete which will help in improving the durability of the concrete compared with the conventional concrete. But the cost of the polymer is not accounted in the strength gain of the polymer modified concrete.
Properties of Polymer Modified Concrete
The properties of Polymer modified concrete are explained below:
- The addition of polymers makes the concrete mix to become more workable. This can hence reduce the amount of water that is added to the concrete mix.
- The crushing strength of the concrete is increased using the polymer in concrete. This is because the polymer reduces the water cement ratio which in turn increase the crushing strength.
- The bond between the aggregate and the matrix is improved.
- The polymer modification increases the flexural strength of the concrete.
- The polymer modified concrete consist of at least 3 % more amount of entrained air than the plain concrete. This additional amount of entrained air will reduce the modulus of elasticity of the concrete (PMC).
- The polymer addition increases the setting time of concrete.
- The resistance of the concrete against abrasion is increased using the polymer.
- Freezing and thawing resistance of the concrete structure is improved by polymer modification.
- The penetration of chlorine ions and other deleterious materials is restricted. The PMC gain higher resistance against such undesirable effects.
- The PMC gains superior shear bond strength.
- The ductility property of the polymer modified concrete is more compared with the conventional concrete.
- These gain a superior tensile and flexural strength compared to the conventional concrete.
Applications of Polymer Modified Concrete
- The polymer modified concrete can be used in the repair and the rehabilitation of old damaged concrete.
- The floor construction in frozen – food factories gains great application. This is because of the higher freeze and thaw resistance of PMCs.
- For floor construction of factories were chances of the splitting of chemicals and oils more prone to happen.
- For the preparation of steel bridge and ship decks surfaces.
- For the concrete structure that is more subjected to large doses of de-icing salts.
- For the cementing ceramic tiles to concrete.