Friday, 15 November 2013

Repairing a Damaged Concrete Slab - Spalling

Concrete spalling occurs mainly due to 2 reasons: 

1. Carbonation. Iron/steel rods corrode due to carbonation (penetration of gases such as sulphur dioxide, carbon dioxide etc) and increase in volume leading to cracks in concrete and ultimately spalling.
2. Corrosion of rods due to moisture penetration into concrete. Porous concrete/lack of water tightness.

Repairs can be carried out as follows. 

Materials Required

. Cement
. Concrete aggregates such as sand and jelly.


. Hack the surface to obtain a sound concrete surface.
. Remove all debris.
. With a rust removing agent clean the iron/steel rebars.
. Prepare a mix/bond slurry by mixing Latex Bonding Agent : Water : Cement in the ratio 1:4:8 by weight.
. Apply over the surface to be repaired using a hard broom or brush. 
. Prepare a concrete mix with the Latex Bonding Agent as an admixture in the concentration of 5-10% by weight.
. While the bond slurry is still in the wet condition, apply the concrete mix using a trowel.
. Plaster over the concrete to get an even surface.

Voila... repaired concrete!

How to protect and finish Interior and Exterior Woods with PU Coatings

Polyurethane(PU) is an excellent choice when you want to protect expensive Teak, Walnut, Wenge or other exotic woods from the elements. It has good UV resistance and excellent sealing properties against water, moisture and humidity. PU retains it finish over prolonged exposures to UV. PU Matt and Glossy finishes for wood.
Let us see how to apply PU coatings to wood.

African Wenge treated with PU Sealer and PU Glossy top coat.

Materials Required

. 2 Part Solvent Based PU Sealer.
. 2 Part Solvent Based PU Top coat - Matt or Glossy.
. Wood stains, if required.
. PU thinner.
. Compressor for spray.
. Paint Gun. (250ml or 500ml Cup is sufficient)
. Dry Sanding Aloxite Paper 80, 120, 180
. Wet Sanding Paper 320, 600,  800, 1000
. Cotton Waste.
. Filter Cloth.
. Masking Tape.
. Face Mask to prevent inhalation of spray.
. Gloves and eye glasses.


. Prepare the wood by dry sanding with emery grit 80 to smooth the grains and undulations on the surface. Sanding should be done along the grain.
. Once major imperfections are removed, dry sand with emery grit 120 to get a smoother finish.
. Switch on the compressor to get the required PSI. (Here is an excellent article on how to spray paint using a paint gun and a compressor). Connect the spray gun to the compressor.
. Wipe clean the surface to be sprayed using a dry cotton cloth.
. The Sealer can be tinted or transparent depending on the desired final finish.

. Pre-preparation For Porous woods

. Mix the Sealer base and hardner in the given proportion without adding thinner. (All Mixes must be consumed within the prescribed pot life)
. With a muslin/cotton cloth apply sealer mix along the grain on to the entire surface maintaining uniform thickness.
. After the sealer has completely cured and hardened, sand the flat surfaces with 180 grit free cut/ open cut emery using an emery trowel. For uneven, round, carved etc surfaces, 3M hand pads/scotch brite hand pads equivalent to 180 grit can be used. All excess sealer must be sanded and removed completely to get a smooth uniform surface. Wipe the thus surface clean.
. The rest of the procedure is the same as for regular woods.

. For Regular woods

. Mix Sealer base and hardner along with 10% - 15% thinner. Pour mix into the spray gun.
. Spray the sealer onto the surface.
. Clean the spray gun.
. Wait for sealer to cure and harden.
. Sand with emery/hand pad grit 600. Wipe clean.
. Additional coats of sealer can be applied for a smoother finish.
. Mix Top coat (Matt or Glossy or Mix a of the two) base and hardner in the given proportion along with 10% - 15% thinner. Pour mix into the spray gun.
. Spray mix onto the entire surface. 
. Wait for top coat to cure and harden. 
. Sand with emery/hand pad grit 800. Wipe clean.
. Spray a second coat of the top coat.
. Wait for top coat to cure and harden.
. In case of gloss finish, additional top coats can be added by sanding with 1000 grit emery between coats.
. Gloss top coats can be further buffed using rubbing compounds to get a high gloss finish.


. If the wood is previously coated, remove the coating by scraping and a paint remover. Ensure the paint remover has completely evaporated before coating. 
. Ensure exterior woods are coated from all sides.

A Tough and beautiful finish is thus achieved ... and the wood grains stand out!!!