Polymers have been used as additives in cement mortars and concrete since the 1920s when natural rubber latex was added to road paving materials. Since then there has been considerable development of polymer modification for cement and concrete.
Polymer modification of cement mortar and concrete noticeably improves application and performance characteristics. These property advancements include easier handling, better finishing, higher strength and adhesion, and increased durability.
The properties of flexible concrete overlay depend significantly on the type of polymer utilized and the polymer-to-cement ratio, which is defined as the mass ratio of the number of polymer solids to the amount of cement.
The polymer-to-cement ratio can vary from 0 to 1/3 depending on the type of polymer used in the properties required for the application.
There are two theories for why the properties of concrete are improved with the addition of polymers. In the first theory, there is no chemical interaction between the polymer and the cement.
During the hydration of the cement, the hydrophilic part of the polymer is oriented toward the water phase, whereas the hydrophobic part is directed toward the air phase (pores and capillaries that are not filled with water). On drying, the water is removed and the hydrophobic particles coalesce together and form a film.
The second theory is that the polymer interacts with the components of the Portland cement hydration products and forms new complexes. This creates a type of reinforcement in the concrete and produces semipermeable membranes.
Chemical reactions have been noticed to take place between the particle surface of reactive polymers such as polyacrylic esters and calcium ions in the hydrating cement. Such reactions can improve the bond between the cement hydrates and aggregates and improve the final properties of the modified cement.