Bonding Fresh Concrete to Existing Concrete Surfaces
Overlays or patching materials of PCC, terrazzo, mortar, grout and extruded
curb based on Type I through Type V cements or White Cement may be structurally
bonded with liquid epoxy adhesives to existing concrete surfaces. Most
cementitious admixtures in Types A through E defined in ASTM C-494 are
compatible with epoxy when employed in normal concentrations. The existing
surface may be lightweight or dense portland cement concrete (PCC),
asphaltic concrete (AC), steel or wood. The bond line may be vertical,
oblique or horizontal and can be on or below grade.
Using liquid epoxies in the viscosity range of 2000-6000
cps, apply 15 mils minimum adhesive thickness over entire area of the
hardened concrete to be bonded. Lightweight concrete or other porous
substrates may require a second coat. A one-gallon liquid epoxy mix
yields approximately 100 square feet on a smooth surface and 50-75 square
feet on a rough or chipped surface. As the temperature drops below 55°F,
the material becomes thicker and more difficult to spread with a resulting
decrease in the yield.
Ambient curing epoxies have short working times and
cure quickly in hot weather and slowly in cold weather. Cure almost
ceases at freezing temperatures. Regular grades of ChemCo concrete bonding
agents should not be used at temperatures below 40°F and "long pot
life" grades should not be used below 50°F. The workhorse products
for this application are CCS Bonder
Paste LWL, followed by CCS Bonder
Paste SWL (for colder uses).
In cold weather, store unmixed containers of Part A and Part B inside at 60°F or above. DO NOT ADD SOLVENTS OR WATER TO THIN THE MIX. The containers may be heated by electric pail or drum heaters to reduce viscosity. If unmixed components have been exposed to the sun and are hot, working life (pot life) will be shortened. Immerse but do not submerge sealed containers in cold or ice water to cool or keep them in the shade.
Epoxy bonding adhesives must be tacky or wet prior to placement of plastic concrete. Use long pot life grades on surfaces over 90°F or scale down the area to be covered to insure retention of tack prior to concrete placement.
Contaminants, such as oil, grease, tar, asphalt, paint, wax,
curing compounds and surface impregnants, such as linseed oil, should
be removed; so should dust, laitance and weak or loose concrete. When
bonding to asphalt, the surface should be roughened so that clean aggregate
is exposed. Do not apply epoxy adhesives in the rain or in
the presence of standing water.
Two methods of surface preparation in order of their preference are:
- Removal of surface by grinding
- Heavy mechanical scarification (for example, by Tennant machine).
- Acid etching with 15% by weight hydrochloric acid solution followed by thorough flushing with a high pressure stream of water.
The use of epoxy adhesives will allow feather edging of the concrete mix for areas other than joints, edges or corners. A preferred edge should be 1/4" minimum in thickness. The edges of the deteriorated area should be saw cut to eliminate edge spalling. As an alternate, the sides of the spall should be chipped as steeply as possible. Cut well back into sound concrete.
Proportioning and mixing
Both A and B components should be poured into a disposable container for mixing. Calibrate two containers in the volume mix ratio given on the data sheet. Scrape the containers used for proportioning to insure accuracy. Removing the rims of gallon cans with a twist-type can opener greatly assists this operation.
Mix the contents with a paint stirrer (e.g., a Jiffy®
paddle, Jiffy Mixer Co.) attached to a low speed (400-600 rpm) electric
or pneumatic drill for about 3 minutes. Move the stirrer up and down
and around the sides until an even streak-free color is obtained. Avoid
making the mix frothy by whipping in an excessive amount of air.
If power is not available, use a steel pallet knife cut off at the tip to give a flat end to reach into corners of the can and mix vigorously by hand for at least 5 minutes. A stick will not provide good mixing.
Do not mix more epoxy concrete bonding agent
than can be used within the pot life of the product. An end
to the pot life is indicated by heat build up in the mixture, followed
by solidification (gelation). Spreading the material in a flat shallow
container will extend the working life.
ChemCo Systems epoxy adhesives for bonding new concrete
to existing surfaces may be applied and distributed by broom, brush,
roller, notched trowel, squeegee or spray gun depending upon the nature
of the surface, the size of the area, and the scheduling of the concrete
Patches and Vertical Joints
Medium stiff nylon bristle paint brushes.
Overlays on Rough Surfaces
Spread with rubber, PVC or polyethylene squeegee or 1/16" notched trowels. Work into surface with stiff bristle street brooms.
Overlays on Flat Smooth Areas
Short nap paint roller fitted with long handle or squeegee.
Sandblasted Dry Steel
Medium stiff nylon bristle paint brushes.
Spray gun with selective brooming in rough spots. Spray apparatus may be two-component equipment that meters, mixes and applies or conventional single component where large batches may be premixed (long pot life types). Airless type spray equipment minimizes overspray and wasted material. Equipment providing internal heating of A and B components is most useful in very cold weather since solvents not be employed for thinning. Manufacturers of suitable spray equipment are Graco, Inc., Binks Mfg. Co., Pyles Industries, Inc., and Spee Spray.
Where the bond coat has lost its tack but not become hard, a second coat of epoxy must be applied immediately prior to placement of concrete. Should the epoxy have cured hard, the first coat must be roughened with a wire brush, abrasive wheel or preferably light sandblasting and a second coat of epoxy applied prior to concrete placement. Fresh epoxy will not bond to unroughened cured epoxy. Tack free or cured epoxy will not provide a structural bond to fresh PCC.
Placement and curing of concrete
The plastic PCC should be a relatively dry mix with a standard slump of between 0-1/2". Where feather edging is required, the aggregate grading should be adjusted accordingly. Best results are achieved with a minimum 1" overlay.
When vibrating epoxy bonded concrete, care should be taken to keep the head of the vibrator sufficiently away from the bond line to avoid wiping the epoxy from the old surface.
Normal finishing methods are employed for epoxy bonded concrete.
Curing Concrete Overlays
Thin overlays, such as terrazzo or cement mortar, must be kept moist with burlap, polyethylene sheeting or a good curing compound for 5-7 days. Under normal temperature conditions, Type III concrete bonded with epoxy should not receive heavy traffic for at least two days.
The moderate use of most admixtures for air-entrainment,
water reduction or acceleration and retardation of hardening rate do
not appear to affect compatibility of PCC with epoxy bonding agents.
Synthetic latex admixtures based on polymers other than epoxy (e.g.,
acrylic, PVA, styrene-butadiene) should be tested first for compatibility.
Old-to-new concrete bonding specifications
include ASTM C881 Type II and ASTM C881 Type V. For applicable products,
see our ASTM C881 product listing.
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