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Information about concrete.

Tips on concrete work.

DeFInition of concrete

mixture of cement, aggregates, sand, water and other ingredients with various properties. Reaches full strength after 28 days.

Resistance

- PSI: Pound per Square Inch
- MPA: Mega-PAscal (1 MPA = 145,1 PSI)

Recipes / resistrance in PSI

- 1 cement, 2 sand, 2 gravel = +/- 4200 PSI
- 1 cement, 2 sand, 3 gravel = +/- 3800 PSI
- 1 cement, 3 sand, 3 gravel = +/- 3500 PSI

1 cubic yard of 3500 PSI (25 MPA):
- 6 Portland 40kg
- 0,75 ton of sand
- 0,75 ton of gravel

Concrete work

1- Forms: Make sure your forms are solid and strong.
2- Pouring and spreading: Start spreading concrete from the furthest point of pouring source.
3- Leveling: With a rake, slightly above final wanted level. Make sure it is done before water comes up on the surface of concrete.
4- Finishing work: Has to be done once water has evaporated and concrete has hardened enough so your foot leaves a +/- 2 inch mark in the concrete.

Precautions

- Hot and/or windy weather, it is recommended to cover concrete with wet jute for 2 or 3 days to avoid cracking and hasty evaporation.
- Use of de-icing salt is harmful to all concrete surfaces weaker than 30 MPA (4000 PSI).
- Protect concrete with water repellant sealer. However, you have to wait 28 days (curing time) before applying on new concrete.
- Expansion joints are recommended for sidewalks and driveways. For better results, to avoid water in ltration, premature aging and deterioration, protect brous trimming with plastic cover (snap cap). The height of trimming should be 1/2” lower than concrete slab’s thickness.

Main causes of problems

- Overhandling.
- The use of a metal trowel on concrete with air entrained kills the bubble network so leads to fragile and flaky surface. Should use tools made of either aluminum or magnesium alloy. Metal tools can be used for concrete without air entrained.
- Finishing work before evaporation of shiny thin layer of water on the surface of the concrete.
* These conditions all lead to flaky and fragile surfaces.

Accelarating hardening time with calcium chloride

- Dilute 0,73kg of calcium chloride in 1 liter of water.
- Use 0,33 to 1 liter (max.) of this mixture per 30kg of pure cement or per 100kg of pre-mixed concrete. The more you mix in, the faster your concrete will harden.

Approximate costs for concrete work


- raised slab, supported by 12” sonotubes and reinforcing mesh every 12 square inch...................................................+/- $40 / sq.ft.
- ground level concrete slab...................................................................................................................................+/- $20 / sq.ft.
- ground level slab not a affected by frost, meaning supported by sonotubes every 8 linear feet......................................+/- $27 / sq.ft.