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Concrete Tips

Tip #1: The Addition of Water


Remember that if you add only one gallon of water to a yard of properly designed 3000 PSI concrete mix:

  1. YOU increase the slump about one inch.

  2. YOU reduce the compressive strength by as much as 200 PSI.

  3. YOU waste the effect of ¼ bag of cement.

  4. YOU increase the shrinkage potential about 10%. This leads to cracking.

  5. YOU decrease the freeze-thaw resistance of the concrete by 20%.

  6. YOU decrease the resistance to attack by de-icing salts.

  7. YOU increase the possibility of scaling.

  8. YOU delay your set time.

This is but a partial listing of the ways in which you lower the quality of a yard of concrete by the addition of one gallon of water.


Tip #2: Control Joints

  1. Calculate your joint spacing by multiplying 2.5 times the depth of the slab in feet. (example, a 4" slab would require 10' joint spacing).

  2. The length of un-jointed area should not exceed the width over 1.5 times. In other words, vertical joints are necessary as well.

  3. If you are using saw cuts for your control joint, make sure the saw cut is ¼ the depth of the slab but not less than 1.0".

  4. Avoid re-entrant corners. Position your control joints as squaring off these corners.


Tip #3: Plastic Shrinkage Cracks


Plastic shrinkage cracks are cracks that appear on the surface of freshly placed concrete during the furnishing process or soon after. This problem usually occurs during hot weather. These cracks are generally 1 to 2 inches deep and do not impair the strength of concrete, however they leave an undesirable appearance. What causes plastic shrinkage cracks? Simply put, the rapid evaporation of water from the surface of the concrete. The main causes are: wind, hot temperatures and dry conditions.


Simple Preventive Tips:

  1. Thoroughly wet the sub-grade.

  2. Erect wind barriers or breaks.

  3. Fog the surface on hot days.

  4. Apply evaporation retardants such as Eucobar or Confilm to the surface during the finishing process.


Tip #4: Curing Concrete


What is Meant by the "curing" of concrete? Curing is primarily the maintaining of satisfactory moisture content and temperature in concrete after the placement and finishing process is complete. An adequate supply of moisture is essential to "HYDRATION", which is the chemical process which provides for concrete to develop the desired strength and hardness.


Three Common Methods of Curing Concrete:

  1. Burlap: Spread burlap or a like material onto the finished concrete surface. Soak the material, and keep it wet for a minimum of 3 consecutive days.

  2. Sprinkling: Place a water sprinkler or a soaker hose on the slab and sprinkler continuously for several days. Only use these water curing methods providing the temperature is above freezing.

  3. Liquid Membrane-Forming Compounds: These are commonly known as curing compounds or cure and seals. It is important to cure all concrete both interior & exterior.


Tip #5: Cold Weather Concreting Practices


Never pour on frozen subgrade: Sub-grade Temperature = Concrete Temperature Use Accelerating Admixtures: Types are Cholordie and Non-Choloride Request concrete to be batched using hot water where possible. Pour with the lowest slump practical or your project will allow. Upon completion, never leave fresh concrete unprotected.

  1. Remember: The heat of hydration will usually keep the concrete from freezing the night it is poured. However, the rest of the week the concrete will need protection.

  2. Methods of Blanketing the Concrete: Thermal curing blankets or layer concrete with Poly, Straw, and top with Poly. Always weigh in place. Enclose area if possible and heat the space.

  3. Caution should be taken to never exceed the accelerator maximum dosage rate. Remember: Each 20 degree drop in Concrete Temperature will double set time!

Temperature         Approx. Set Time 100                       1 to 2 hours 90                         2 to 3 hours 80                         4 hours 70 (Ideal)              6 hours 60                         8 hours 50                         10 to 12 hours 40                         14 to 16 hours


Tip #6: Choosing Exterior Strengths to Fit Climate


Did you know that Virginia, North Carolina and Western South Carolina are in a climate ranked by the Weathering Index of the U.S.A. as SEVERE? In our case, this term indicates the number of "freezing-cycle" days in a year our state goes through. The word severe indicates the weathering exposed concrete is typically subjected to as indicated in the Manual of Concrete Practice. It is the recommendation of ACI 332-R (residential construction) that all concrete, which is exposed to weather in a severe region, should be an air-entrained, 3500 Psi mix. The table further states that the 3500 Psi mix should be placed at a slump 5.0" +/- 1.0". The air content should be 6.0% +/- 1.5%.


Tip #7: Factors to Consider When Placing an Order

  • Name of person placing order

  • Contact phone number

  • Customer account charged to

  • Project name

  • P.O. # (if applicable) or Lot #

  • Job address

  • Good directions to job

  • Concrete mix needed and/or Mix ID# if possible

  • Specify Air or Non-Air mix

  • Slump (if less than 4.0" is desired)

  • Number of yards

  • Designate the "holding load" or callback

  • Time desired on job

  • Make the order "FIRM" if at all possible

  • Desired spacing of the trucks

  • Are any extra admixtures needed (ex. Retarder)

  • Did you include FIBER in your order?

  • Would plasticizer help facilitate your pour?

  • Purpose of pour (i.e., Slab, Footing, Stairs, Curb, etc.)

  • Manner of placement (i.e., Trailer pump, Boom pump, Georgia buggy, Wheelbarrow, or Truck poured?)

  • Conditions of pour (i.e., Pouring down a slope, Pouring on an incline, Pouring into a pan or form, is it overly muddy? Basically any variables which we should be aware of in the execution of your pour.


Tip #8: Hot Weather Concreting


With the onset of Hot Weather, everyone needs to be aware of potential problems the heat can bring.

  1. Increased water demand

  2. Higher rate of slump loss

  3. Accelerated rate of cement hydration resulting in faster setting times

  4. Greater temptation for re-tempering

  5. Reduced time to Place and Finish the concrete

  6. Increased potential for plastic shrinkage and drying shrinkage cracking

  7. Potential for thermal cracking

In summary, high concrete temperatures and rapid evaporation of water are the main culprits of Hot Weather concreting difficulties. However proper project planning and management can overcome these obstacles.


Tip #9: Set-Retarding Admixtures


What is Retarder?  A Set-retarding admixture, commonly known as Retarder, is a chemical admixture added to concrete during the batching process to offset the accelerating effect of high ambient temperatures associated with hot weather. In general, retarders extend the time in which concrete can be transported, placed, and finished. During extreme heat, retarder also helps in the preventing of cold joints. This admixture works on concrete by slowing the set of the cement paste in the concrete mix itself. It is important to note that a set-retarding admixture does not decrease the initial temperature of concrete. Original Source: ohioreadymix.com


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