Polymer-modified concrete

Polymer-modified concrete

Reported by ACI Committee 548

ACI Committee 548 report covers concrete made with organic polymers in combination with hydraulic cement and discusses the polymer systems used to produce polymer-modified concrete, including their composition and physical properties.

Polymer-modified

cementitious mixtures (PMC) are defined as hydraulic cement combined, at the time of mixing, with organic polymers that are dispersed or redispersed in water, with or without aggregates, an organic polymer being a substance composed of thousands of simple molecules combined into large molecules. The improvements from adding polymer modifiers to concrete include increased bond strength, freeze/thaw resistance, flexural and tensile strength, abrasion resistance and reduced permeability and elastic modulus.

As for specifications for polymer-modified mixtures, the ASTM C 1439 test method was issued in 1999, prescribing that PMC specimens are to be cured by covering them with plastic sheeting for 24 hours followed by air curing at 23C and 50% relative humidity until the time of the test. The organic polymers are supplied in three different forms: as a dispersion in water (called a latex), as a redispersible powder, or as a liquid that is dispersible or soluble in water.

Styrene-butadiene latexes

These are useful for a variety of applications, the most important being bond to substrate and low permeability. Compared with conventional mortar and concrete, these materials experience improved workability, while setting time is longer than conventional concrete. Furthermore, their use involves reductions in drying shrinkage and increases in plastic shrinkage cracking. Additionally, the permeability value is lowered and resistance to freeze/thaw cycles is outstanding.

As for construction techniques, these modifiers are to be mixed in a mobile mixer and, if pumping is required, no change in mixture proportioning will be needed. Moreover, because latex-modified systems achieve their potential properties by air curing, placement of such materials underwater is not recommended.

Acrylic latexes

These are designed to improve specific properties of cement mixtures such as adhesion, abrasion resistance, impact and flexural strength, durability and permeability. The physical properties of a latex-modified cement mortar are affected to an extent by the same variables that affect unmodified ones, but in order to obtain the most desirable physical properties, these materials should be air cured, which is a procedure in contrast to that for unmodified mortar. The reason for this difference is that for the latex to beneficially modify the properties of the mixture, it must be allowed to coalesce and form a film; in this practice, the removal of water is the key step in the film formation process. Care should be taken to avoid rapid dehydration during the first 24 hours to avoid plastic shrinkage cracking. These latexes are used for the repair of industrial and commercial floors subjected to deterioration from abrasion, vibration, spillage and aggressive conditions. Furthermore, they can be employed for marine decks to provide skid resistance and as a protective surface to steel.

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Epoxy polymer modifiers

The incorporation of epoxy components involves improvements from the standpoint of adhesion, flexural and tensile strength increase (by as much as 100%), with reductions in modulus of elasticity and permeability, compared with plain materials. An epoxy-modified concrete mixture requires less mixing water for the same slump as a comparable unmodified mixture and is easier to consolidate.

These materials are generally used in applications where adhesion and low permeability are required, such as in grouts, stuccoes, liners, protective and skid-resistant coatings and the repair of concrete structures, including overlays for bridges and parking decks.

Redispersible polymer powders

These are referred to as redispersible in that they convert to latex on mixing with water. They impart comparable properties to hydraulic cement mixtures and are used for similar applications as their latex counterparts, but give the convenience and accuracy of premixing with cement aggregates and other possible powder components. Properties of the hardened powder-polymer-modified mortars are marginally reduced compared with those obtained with latexes of similar composition. The three major end uses are ceramic tile adhesives and grouts, underlayments and industrial floor toppings and concrete repair and patching mortars.

In conclusion, this report is a fundamental means for both users and scientists wishing to use polymer systems to improve the overall qualities of mortars and concrete in such situations where high performances are required. Last but not least, it gives indications on procedures and both types and quantities which are to be used from the standpoint of cost-effectiveness.

"This report is a fundamental means for both users and scientists wishing to use polymer systems to improve the overall qualities of mortars and concrete in such situations where high performances are required."