LATEX MODIFIED CONCRETE AS REPAIR MATERIALUpadhyay Tejas J. SD 1507

INTRODUCTION :Latex Modified Concrete, primarily used as a thin bonded overlay to protect bridge decks and parking structures, has been described as the "ideal repair material."

Latex modified concrete is also known as polymer modified concrete. Conventional concrete made by replacing part of mixing water with a latex.

The materials and the production technology for concrete in LMC are the same as those used in normal Portland cement concrete except that latex, which is a colloidal suspension of polymer in water, is used as an admixture.

INTRODUCTION CONTINUEDSynthetic latexes are made by dispersing polymer particles in water to form a polymer emulsion. When these emulsions are added to Portland cement concrete, the spheres of polymer will coalesce or come together to form a film that coats the aggregate particles and the hydrating cement grains and seals off voids.

The resulting mixture develops higher strength, bonds better to existing concrete, has a higher resistance to chloride penetration and is more resistance to chemical attack than plain concrete.

Both PC and LMC have been in commercial use since the 1950s.

LATEXES USED IN LMC :Earlier latexes were based on polyvinyl acetate or polyvinylidene chloride, but these are seldom used now because of the risk of corrosion of steel in concrete in the latter case, and low wet strengths in the former.

Three basic polymers used as latex modified concrete for concrete or mortar are acrylics, styrene-butadiene rubbers (SBR) and polyvinyl acetates (PVA).

LATEXES USED IN LMC CONTINUED PVA are not generally recommended for use in wet environments because some types may hydrolyze or break down chemically.

Elastomeric or rubber like polymers based on styrene- butadiene and polyacrylat copolymers are more commonly used now.

DOSAGE AND MIXTURE PROPORTION :Latex modified mortar, recommended for sections to inch deep should have proportion of 1.0 part cement to 3.0 to 3.5 part fine aggregate by weight.

The water-cement ratio ( including water in the latex) should be 0.3 to 0.4 by weight and the latex solids-cement ratio 0.1 to 0.2 by weight.

Because the concentration of latex solids in commercial products varies, the dosage in gallons of latex per 100 pounds of cement will vary with the brand of latex.

DOSAGE CONTINUED. For instance, assume you want a mix with 16 percent latex solids by weight and a maximum water-cement ratio of 0.40. the calculations of the quantities would be:0.16 X 100 = 16 pound of latex solids per 100 pound of cement0.48 X 8.4 = 4.0 pound of latex solids per gallons of latex (the rest is 4.4 pound of water)16 / 4 = 4.0 gallons of latex per 100 pound of cement.Weight of water in 4 gallons of latex = 4 X 4.4 =17.6 pounds

DOSAGE CONTINUED.Minimum cement content 390 kg/m

Latex emulsion admixture 121.30 l / m

Water/cement ratio (by weight) 0.30 to 0.40

Air content 3 to 6.5 percent of plastic mix

Coarse aggregate 50 percent of total aggregate by weight

OVERLAYING THE DECKS WITH LMC :There are five stage of good overlaying:

1. Preparation of surface .

2. Batching and mixing of LMC.

3. Placement of LMC.

4. Finishing.

5. curing

1. Preparation of surface :Resurfacing an old deck requires much more preparation than overlaying a new deck.

One must remove the top inch of concrete ( because it usually contains oil and dirt) and any weak concrete below the surface.

Use of hydro demolition or scarifying equipment to remove the top surface and power driven tool to remove the deteriorated concrete underneath.

1. Preparation of surface continued.Also wait 48 hours before scarifying or chipping concrete within 6 feet of previously placed LMC.

Splice new rebar to any reinforcement bar that have lost or more of their original diameter.

If concrete is removed mechanically, sand blast or water blast is applied on the surface within 24 hours before concrete placement.

At least 1 hour before placement of concrete surface should be wetted but not flooded with water.

2. Batching and mixing of LMC :For small volume work, 5 to 10 cubic foot batch mixers may be used. For large areas such as bridge decks requiring more than 4 cubic yards of mixed material, mobile batcher mixers are recommended.

Because LMC should not be mixed for more than 5 minutes, it should made on site.

Because of the relatively short working life for latex modified mixes, the amount mixed will be limited by the placing and finishing time - about 20 minutes.

2. Batching continuedWith the drum rotating at minimum speed (16 to 18 rpm), aggregate and cement are next introduced, followed by the remainder of the mixing water needed to achieve desired consistency.

Total mixing time shouldnt exceed 4 minutes at slow speed, longer mixing time may incorporate excess air in the mix.

The slump shall be measured 4 to 5 minutes after discharge from the mixer.

3. Placement and finishingFor small areas or parking decks obstructed the LMC can be placed with a screed and spud vibrator. For bridge deck overlays use a self propelled finishing machine that can move forward and backward.

But now a days roller finishing machines are more common.

Edge tooling shall be required at joints, except next to metal expansion dams, curbs and previously placed lanes.

Placing an LMC overlay with conventional equipment. The concrete is batched and mixed in a mobile concrete mixer and struck off with a vibrating screed. A fog spray is applied, as shown, to increase the relative humidity and help prevent plastic shrinkage cracking.

4. Curing The hardening of a latex takes place by drying or loss of water.

LMC forms a plastic film at the surface upon drying, usually within 25 minutes in hot, dry weather.

To prevent the surface from cracking, cover it with pre-saturated burlap as soon as it will support the burlap without significant deformation. Overlap the burlap a minimum of 6 inches. Cover it with second layer of wet burlap or with 4 mil white polyethylene. Hold down the edges of the polyethylene with weights.

4. Curing continuedAfter 24 hours remove the burlap and polyethylene and allow the LMC to dry cure for 72 hours before opening the deck to traffic.

If rain wets the overlay for more than one hour or more during this dry cure period, do not count those hours as part of the dry curing.

LMC overlay is quickly covered with wet burlap and polyethylene to provide a moist cure.

PROPERTIES OF LMC :The most impressive characteristics of LMC are its ability to bond strongly with old concrete, and to resist the entry of water and aggressive solutions.

The internal plastic films reduce penetration of moisture and chloride ions, improving freeze-thaw resistance and protecting reinforcing steel from corrosion.

The abrasion tests revealed that LMC outperformed conventional concrete by 19-32% depending upon the particular test performed

PROPERTIES CONTINUEDThe modulus of elasticity of LMC is 15% lower than conventional concrete, making it less brittle and more flexible. This allows it to withstand stresses that can cause premature cracking and delamination.

These characteristics have made the LMC a popular material for rehabilitation of deteriorated floors, pavements, and bridge decks.

Points to be kept in MindDont place an LMC overlay

Less than 1 inches thickNext to a parallel strip of overlay that is less than 72 hours old.When rain is expected.At temperature lower than 40 F.When the surface evaporation rate is more than 0.15 pound per square foot per hour.

MEASURE AND PAYMENT :The unit of measure for the various types of Bridge Deck Concrete Overlay will be the square yard, complete in place.

The number of square yards will be the actual surface area computed from measurements taken in the field.

Payment for the specified type of Bridge Deck Concrete Overlay will be made at the contract unit price per square yard, which payment will include removal and disposal of existing asphalt surface and membrane waterproofing, scarifying if specified, sandblasting, air cleaning, grouting, and proportioning, mixing, placing, finishing, and curing concrete.

MEASURE CONTINUEDPayment will also include the cost of furnishing all materials, tools, equipment, incidentals and labor necessary to complete the item as shown on the plans and specified .

Payment for removal and disposal of deteriorated concrete beyond the prescribed thickness will be paid for at the contract unit price per cubic yard for Structure Hard Surface Excavation.

Case study of bridges overlay with Latex modified concrete

Name of bridge : The Marquam Bridge

Location : Portland

Year of completion : 1966

Daily traffic on bridge : 86000 vehicles

Problem and the solution:

After 17 yr of service, the bridge, which was opened to traffic in 1966, had a badly worn deck and numerous deck expansion joints in need of repair.

To correct the deck and joint problems, a contract was given in May 1983 for a latex-modified-concrete overlay and joint repair.

Problem continuedThree years after completion of the overlay, a check on the wear was made.

This found wide variation in the projected life due to traffic patterns and irregular wear in the individual wheel tracks.

From the survey, a general conclusion can be made that the latex-modified-concrete has equivalent or possibly better wear resistance than the original concrete surface had.

Reference :Concrete repair volume 1 , 2, 3.

Standard Specification for Latex- Modified Concrete (LMC) Overlays (ACI 548.4-93).

Concrete: Microstructure, Properties, and Materials P.K. Mehta and P.J.M. Monteiro.

Evaluation, Repair and strengthening of structures.