Adhere Concrete to Concrete

Concrete is usually made of cement, water, gravel and sand. This combination creates a very hard, durable surface. However, cracks and damage can occur. Fixing concrete takes a number of materials because it does not readily adhere to other concrete. The mixture and formation relies on a chemical reaction that stops once dried. If you need to patch a concrete surface or pour new concrete on top of old concrete, you must invest in a strong bonding agent and a concrete patching mix. If all precautions are taken, you can create a repair that will last a few decades.

Steps

Patching a concrete surface

1. Wait until a cool, overcast day to do your concrete repairs. This is the best time to adhere the concrete because it takes longer for the water to dry out, and it has more time to react with the cement.
2. Buy concrete patching material. It is available in a pre-mixed form or by buying the separate ingredients in bulk. If you haven't adhered concrete to concrete before, it is a good idea to go with the pre-mixed version, which only requires that you add water.
   • Buying the raw materials, gravel, Portland cement and sand is considerably cheaper than buying pre-mixed concrete patching. If you are patching a deep hole, you can use 1-inch (2.54-cm) gravel, otherwise, use very fine gravel.
   • Mix the dry materials in a ratio of 3 parts gravel to 2 parts sand to 1.5 parts cement in a bucket. This is sometimes listed as 3 to 2 to 1. A higher amount of cement will create a stronger material. There will be more of chemical reaction between the cement and water, which will create more crystals and a harder structure.
3. Sweep the concrete surface carefully. You must remove all loose stones, or the bonding agent and cement will not reach the surface of the concrete.
4. Dust the surface thoroughly after you have swept. You can use a blower or a soft brush; wipe away the dirt that is stuck to the surface.
5. Rinse the concrete. Sprinkle an even amount of water onto the surface with a hose attachment. Stop adding water before you create standing water on the surface.
   • This will stop the porous concrete from sucking the moisture out of the bonding agent and concrete bonding material.
6. Create cement paint. Mix Portland cement, which is available at most hardware stores, with water. Stir the 2 ingredients together until they form the consistency of wet paint.
   • You can also buy an acrylic bonding agent to use in place of the homemade cement paint. They are made of resin and can be either added to your patching concrete or applied like the cement paint. Follow the instructions on the can or bottle very carefully, as many of the products have different application instructions and drying times.
7. Apply a thin layer of the cement paint to the old, damp concrete with a paint brush. Do this right before you plan to pour the new concrete patch onto your old concrete surface.
8. Add the water to the homemade or pre-mixed concrete patch just before applying. Mix it well. Pour the patch into holes and cracks or pour a 3/8-inch (1-cm) layer to flat surfaces.
9. Wipe the surface of the concrete with a wooden float. Apply it to the surface with a back and forth motion until the pieces of gravel have sunk below the surface. The sand and cement should rise to the surface.
10. Allow water to bead and rise to the surface. It will evaporate on its own. For a smoother finish, wait until the concrete is hardening and almost plastic in texture, then apply a smooth back and forth motion with a metal trowel.
11. Cover your patch with plastic sheeting while it dries. This will keep as much water as possible within the concrete mix and it will bond better.
12. Spray the new concrete with a coat of water every day for 4 to 7 days. This will keep the chemical reaction going and make the new concrete stronger.

Pouring a new slab over an existing one

This method works best when you have someone to help you with the mixing and brushing. Do not attempt large areas until you have practiced and achieved success on smaller learning areas.

1. Mix one part polybond with 4 parts water.
2. Add the mixture to clean dry cement powder.
3. Mix until a slurry like consistency is achieved.
4. Brush this slurry mix onto old concrete slab.
5. Pour new concrete while mixture is still wet.
6. Keep applying slurry mix ahead of new concrete as you go.
7. Finish concrete in your usual manner.

Tips

• If possible, shade your new concrete from hot sun. This can leach out the water and make the bond weaker.
• To fix hairline cracks in concrete, you do not need to use a cement paint and a concrete patching compound. You can fill in the cracks with a thick paste of Portland cement and water.
• Creating a smooth finish on concrete is difficult. It takes practice to achieve a professional look. For larger jobs, consider hiring a professional.
• If you are trying to repair a step or the corner of a sidewalk or driveway, you should use reinforcing steel bars, or rebars, to reinforce the new concrete and adhere the 2 materials together. Consult your local hardware store on the best size of pins for your project.
• Always wear clothing that can be damaged when working with wet concrete. The materials and mixing process can be messy.
• The scope of work described above will not provide a long term durable repair. The methods do not follow industry best practices set forth by the International Concrete Repair Institute (ICRI). For example, concrete is a porous material that requires mechanical surface preparation, opening the pour structure, enabling adequate surface bonding and intimate contact between the substrate and Repair material. Bonding agents are effective. However, if guidelines are not strictly followed, they can actually perform as bond breakers.

Things You'll Need

• Portland cement
• Water
• Paint brush
• Stirring stick
• Water hose
• Pre-mixed concrete patching compound
• Sand
• Gravel
• Bucket
• Metal trowel
• Plastic drop cloth
• Wood float
• Broom
• Blower or soft brush

Related Articles

• Decorate Concrete
• Finish Concrete
• Choose Concrete for a Project
• Break Up Concrete
• Use Prepackaged Concrete Mix
• Waterproof a Concrete House

Sources and Citations

• http://www.askthebuilder.com/B123_Long_Lasting_Concrete_Repair.shtml
• http://www.askthebuilder.com/123_Concrete_Repairs_101.shtml

See also

• Remove Rust from Cement
• Drill Into Concrete
• Tie Rebar
• Build a Concrete Sidewalk
• Break Up Concrete
• Pour a Concrete Patio
• Pour a Concrete Foundation
• Paint a Concrete Floor
• Figure Concrete Yardage
• Acid Stain Concrete

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