Comparing and Explaining Polymers in Concrete Construction, Protection and Repair

Posted by John Bors on Thu, Aug 8, 2019 @ 18:08 PM

Examples of polymer uses:  

                         Joint materials                      Waterstops

                         Membranes                            Bearing pads

                         Conduits, pipe                      Coatings, linings

                          Panels, carpets                    Windows (shatter proof)

                          Flooring materials              Overlays

                          Bonding agents                   Grouts (including injection materials)

                          Penetrants                             Void fillers

 

Types of polymers used:

I. Preformed, factory made, finished products.

                          Examples:

                                   Polyethylene, polypropylene, polycarbonate,                                                                                             SBR, ABS, PVC, polystyrene, acrylics.

 

II. Reactive single or multi-component liquids and pastes, finished at the job site.

                          Examples:

                                      Epoxies, polyurethanes, polyureas, polyesters,                                                                                          silicones, acrylics.

In concrete protection and repair, Type II polymers are often selected or specified because they have the ability to "wet" surfaces when applied and to bond to the substrate when cured. In the application and cure process the reactive single or multi-component liquids (or pastes) are converted into solid polymers. Traditional building materials such as cement or lime use a similar hardening process.

 

                Uncured State                                                                            →           Cured State

 

                Cement (trisilicate) + water (not a polymer)                 →          Hydrated cement 

                Lime (calcium oxide) + water + CO2 (not polymer)    →          Calcium carbonate

                Epoxy resin + amine curing agent                                     →           Epoxy polymer

                Urethane prepolymer + polyol                                           →           Polyurethane

                Urethane prepolymer + amine curing agent                →           Polyurea

                Poly(or vinyl)ester resin + styrene+ catalyst                 →            Polyester

                Acrylic monomer + catalyst                                                 →            Acrylic polymer

                Silicone resin prepolymer - acetic acid                          →            Silicone polymer

                Silane + water                                                                            →           Silicone polymer

                Polymer latex or emulsion (Acrylic, SBR) - water       →            Polymer

                Epoxy water emulsion + amine curing

                agent - water                                                                              →            Polymer

 

 How did construction polymers get their names?

Epoxies                                  From the epoxy groups in these resins which are capable of                                                                establishing links to the hardener.

Polyurethanes                    From the name of the link (urethane) that is formed when                                                                    isocyanate group containing resins and resins containing                                                                      hydroxyl  groups and polyols react.

Polyureas                              From the name of the link (urea) that is formed when                                                                               isocyanate group containing resins and resins containing amine                                                       groups react.

Acrylics                                  From the name of the acid (acrylic acid) which is common in all                                                        of  these products.

Silanes                                   From the chemical name of these products.

Siloxanes                               From the name of the partial hydrolysis products of silanes.

Silicones                                From the name of the hydrolysis end-product of silanes and                                                               siloxanes.

Latex polymers                    From the name of the chemical used to make the polymer. A                                                                latex is a water-based emulsion of such polymers.

 

What determines the characteristic properties of construction polymers?

 The bulk of the product and not the reactive sites determine properties. On a molecular scale, the bulk of the resin (and hardener) can be compared to more or less cross-linked chains made out of flexible or hard links which have additional carabiner hooks attached. These hooks establish links to other chains in the curing step. The end-products can range from totally linear, endlessly long chains to completely cross-linked, short chain, three dimensional networks. Linearity of the chains in these products brings about elastic behavior, while a three-dimensional array of the links is hard.

 

Key advantages and disadvantages of certain construction polymers

Polymer type

Advantage

Disadvantage

Epoxy (100% solids)

High strength, HDT

Bonds to wet surface

Excellent chemical resistance

Low cure shrinkage

Yellows

Brittle

Slow cure

Epoxy (water dispersions)

Low viscosity

Environmentally friendly

Easy clean-up

Yellows

Limited chemical resistance

Slow cure

Polyurethane

Flexible

Tough

Abrasion resistant

Poor bond

Moisture induced gas (bubble) formation

Yellows

Polyurea

Flexible

Tough

Abrasion resistant

Fast cure

Limited bond

Needs primer in critical use

Limited chemical resistance

Yellows

Polyester (Vinyl ester)

Tough

Best chemical resistance

Good overlay material

Shrinks

Need primers

Styrene odor

Acrylics (reactive)

Don't yellow

Low viscosity

Fast cure

Reactive concrete consolidants when epoxy injection is too expensive

Odor

Some are brittle

Expensive

Cure can be inhibited

Acrylic latex

Best cement product modifier

Permeability

Environmental resistance

Limited freeze-thaw

Silicone resins

Best long-life sealant

Color stability

No age hardening

Expensive

May need primer

Silanes

Best sealer penetrants

Low viscosity

Expensive

 

Material Properties and Behavior

Many construction polymers have "thermoset" character, especially if the use is load bearing. Exceptions are very flexible epoxies, some polyurethanes, polyureas, hybrids of these materials, silicones and some latex polymers. The latter group is generally "thermoplastic" or elastomeric.

Thermoset polymers products are made up of highly cross-linked molecular elements which maintain their shape when heated, but do not melt. Above about 240 to 300° F, they will lose strength and begin to decompose. Thermoset polymers are rigid (high modulus of elasticity), somewhat brittle at low temperature and have limited flexibility at higher temperatures. The mid-point of the temperature range at which the transition from rigid to flexible takes place is called the glass transition temperature. The Heat Deflection Temperature (HDT) is a simple indicator of the glass transition temperature. The HDT is referred to in the ASTM C881 specification and other specifications used in construction adhesive applications. Many construction polymers have an HDT in the range between 65 and 120° F. At temperatures in and above the glass transition region, construction polymers subjected to high loads will deform which leads to loss of bond and ultimate failure of the application.

The fact that many construction polymers are used in the glass transition temperature range should always be considered when recommending a material. Know the HDT of the product you use. Many failures can be traced to a disregard of the material behavior changes that occur in this temperature region.

Significantly below the HDT (~20° F) thermoset polymers can withstand substantial loads without deformation (creep) which is the basis for their use in construction (crack repair, bolt and dowel grouting, bonding applications).

Except for truly elastomeric products, all construction polymers have a more or less pronounced visco-elastic character. When stressed they do not entirely respond in the manner an elastic body(spring) would and they don't flow in the manner of a liquid (response like a shock absorber to a bump).

The response of a construction polymer to a stress (load) is partly elastic and partly of viscous nature. The prevalence of each of the components in response to a stress (load) depends on the composition of the polymer, the stress level, the stress exposure duration, and temperature. Short term loads generate a quick elastic response. The response to long term loads is seen in creep which is a form of viscous flow.

As a consequence of their visco-elastic nature construction polymers have time and temperature dependent properties (see graph below). Typical rigid stress-strain behavior at low temperature changes to elastomeric behavior at high temperature, while increased in strain rate at a given temperature produce a increasingly rigid response. All visco-elastic construction polymers have temperature ranges where they are rigid or flexible. The glass transition temperature separates the two ranges.concrete repair epoxy stress-strain

In structural applications it is critical that the construction polymer does not creep under sustained load.  The tendency to creep depends on the load level and the prevailing temperature. Each material has an envelope where creep does not produce failure at a given temperature and load level. Exposure to high temperatures such as those of a building fire should be considered as well.

Construction polymers used in "load bearing" applications should have a high HDT. When such polymers are subjected to permanent loads, particularly when these loads have a high shear component, the surrounding structure element should always remain at least 20° F below the HDT. In critical applications and as a safety measure in case of a fire thermal protection should be provided. We suggest that specifiers for construction polymers should limit the use of most structural polymers to transmit loads and not act as a point of load concentration.

In recognition of the material performance needs in load bearing applications ChemCo Systems products were deliberately designed to have higher HDTs than are offered by the industry.

Thermoplastic polymers melt when heated and return to their original state when cooled again, unless they were heated to a point above their decomposition temperature. Their glass transition temperature is often very low (< -40° F).

Thermoplastic and slightly cross-linked thermoplastics have largely predictable properties in normal construction polymer use temperatures because they are used above their glass transition temperature. Exceptions are polymers with crystalline segments within their molecular structure which melt in this region. These are more or less elastic (low modulus of elasticity) and get largely employed for this property (elastomer membranes, joint materials, roofing compositions, seals).

Liquid/Solid Transition during Cure - Cure shrinkage

When a liquid solidifies (except water) a volume contraction ensues. This contraction is called shrinkage and is inevitable unless counteracted (which is difficult and very expensive). Cure shrinkage varies among types of polymers:

Type

Volume Shrinkage, %

Linear Shrinkage, %

Epoxy

4 - 5

0.1

Acrylic

6 - 9

0.2

Polyester

9 - 14

1.0

Polyurea

7 - 10

0.3

Volume shrinkage during cure is of significance when the curing construction polymer is in a confined space (void, gap, crack, joint, patch) and documents itself by a meniscus formation on the surface.

Linear shrinkage is an indication of the true shrinkage which takes place after the curing liquid has no longer enough mobility to flow to compensate for the shrinking volume. This form of shrinkage stresses the bond between the substrate and the construction polymer and can lead to bond or in extreme cases to substrate surface failure. To compensate for cure shrinkage many systems are extended with fillers which reduce the volume fraction of the polymer.

Flexible systems can respond to cure shrinkage by getting into a partial state of extension which in time relieves itself by viscous flow ("stress relief"). Rigid, high modulus systems respond by placing significant stress on the bond line or in the substrate surface which can lead to bond-line or substrate failure.

Exotherm

The cure reaction of construction polymers is normally associated with heat formation (exotherm). In cross-linked, thermoset systems, this phenomenon places an additional stress on the bond-line/substrate surface, because the product sets in an elevated temperature expanded state which contributes to cure-related shrinkage stress when the product contracts as it cools to the normal ambient temperature.

Expansion Coefficient

Construction polymers have an expansion coefficient which is 4 - 10 times larger than that of concrete. When bonded to a concrete substrate this difference can cause failures when significant temperature drops are encountered. Rigid, high HDT construction polymer installations are most prone fail under these circumstances, unless heat formation during cure, the bond-line thickness and the polymer volume fraction are properly controlled.

Combined Effects of Polymer Volume Fraction, Expansion Coefficient, Exotherm, Cure Shrinkage and Modulus.

In specifying use of a particular construction polymer for critical application, the potential for significant interaction of the material's expansion coefficient, the mass involved, the exotherm on cure, the cure shrinkage and its modulus (and change with temperature) must be considered.

Physical Property Comparison

Property

Concrete

Construction Polymer (by flexibility)

 

 

Structural

Flexible

Elastomeric

Modulus of Elasticity

     4,000,000

     400,000

     60,000

            600

Expansion Coefficient

     .000,004

     .000,02

     .000,04

      .000,05

Tensile Strength

         400

       8,000

       1,500

         1,500

Elongation

        nil

           1-2

      35-100

            400

Shear Strength

         800

       1,500

       1,200

           NA

Flexural Strength

      1,200

      4,000

          NA

           NA

Compressive Strength

      4,000

     14,000

       5,000

           NA

 

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