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We know the more you learn about concrete, the better informed decisions you will be able to make for your home, business, or community. Below are resources for you to learn more about concrete and why it is the most versatile, affordable, and green material for your construction needs.

 

 

Beyond The Basics - Ready Tips

  1. The Addition of Water
  2. Control Joints
  3. Plastic Shrinkage Cracks
  4. Curing Concrete
  5. Cold Weather Concreting Practices
  6. Choosing Exterior Strengths to Fit Climate
  7. Factors to Consider When Placing an Order
  8. Hot Weather Concreting
  9. Set-Retarding Admixtures
 

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 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.

Refer to NRMCA CIP 26 for detailed information on Jobsite Addition of Water (Return to Top)

Tip #2: Control Joints
It is Hot! Remember those 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 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.
  5. Above all in this heat, cut your joints the day of the concrete pour. Preferably about 2 hours after initial set (When the concrete is hard enough to walk on without indentation)
Refer to NRMCA CIP 6 for detailed information on Joints in Concrete Slabs on Grade (Return to Top)

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.

Refer to NRMCA CIP 5 for detailed information on Plastic Shrinkage Cracking. (Return to Top)

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.

Refer to NRMCA CIP 11 for detailed information on Curing of In-Place Concrete  (Return to Top)

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

Refer to NRMCA CIP 27 for detailed information on Cold Weather Concreting  (Return to Top)

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%.

Refer to ACI 318, Chapter 4 for detailed information on durability requirements for concrete   (Return to Top)

Tip #7: Factors to Consider When Placing an Order
  1. Name of person placing order
  2. Contact phone number
  3. Customer account charged to
  4. Project name
  5. P.O. # (if applicable) or Lot #
  6. Job address
  7. Good directions to job
  8. Concrete mix needed and/or Mix ID# if possible
  9. Specify Air or Non-Air mix
  10. Slump (if less than 4.0" is desired)
  11. Number of yards
  12. Designate the "holding load" or callback
  13. Time desired on job
  14. Make the order "FIRM" if at all possible
  15. Desired spacing of the trucks
  16. Are any extra admixtures needed (ex. Retarder)
  17. Did you include FIBER in your order?
  18. Would plasticizer help facilitate your pour?
  19. Purpose of pour (i.e., Slab, Footing, Stairs, Curb, etc.)
  20. Manner of placement (i.e.,  Trailer pump, Boom pump, Georgia buggy, Wheelbarrow, or Truck poured?)
  21. Conditions of pour (i.e., Pouring down a slow, 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.
These are a list of the most common variables when ordering. Refer to NRMCA CIP 31 for detailed information on Ordering Ready Mixed Concrete  (Return to Top)

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

Refer to NRMCA CIP 12 for detailed information on Hot Weather Concreting  (Return to Top)

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.

The dosage rate of the retarder determines the approximate delay in the setting time of the concrete mix. The time of the delay will vary according the outside / ambient temperature. (Return to Top)

 



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