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  • Users: JConc
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  1. JConc

    Plastic shrinkage cracks

    In the future, providing your letter to the contractor with an NRMCA "Concrete In Practice" Technical Info sheet on Plastic Shrinkage [CIP 5]would easily back you up. These bulletins are easy to read, to the point and provide preventative measures for all [educational]. In my experience these...
  2. JConc

    25 x 25 Concrete slab with crazing cracks

    Crazing cracking tends to be unsightly only, the service life of the slab should not really suffer. Crazing [ is somewhat common] is a finishing issue and is more noticeable when an accelerator [Daraccel] is used. As far as hiding it, most people don't. A concrete stain may work from...
  3. JConc

    Multiple Truck Concrete Test.

    CSA states that a strength test should be done on each class of concrete every 100 cu. metres. There should be at least one strength test for each class of concrete placed that day, regardless of size. Today the rule is that a 30 litre concrete grab sample is to be taken between 10 and 90% of...
  4. JConc

    Predicting 28 day strength from 7 day tests

    Typical 7-day strengths are 70-75% of 28 days. Percentage goes up if using mainly portland cement and decreases when using higher [20-25%] flyash or slag replacement.
  5. JConc

    Air entraining increase with water

    Air increases % can vary with the slump of concrete, the speed of mixing and the admixtures present in mix. One load of concrete may be an isolated case but I do not consider it too unusual though a little high.
  6. JConc

    Concrete cylinders do not gain strength as expected

    Your comment about the concrete not mixed in the truck would most likely only be true if water or AEA was added on site and then the load not mixed a minimum of 30 turns. Cores vary due to the destructive nature of drilling and possibly varying consolidation/no consolidation in structure...
  7. JConc

    concrete curing

    I would ask the concrete supplier's Tech Services to give you their recommendations. They will have the best estimate of what strength you will achieve under the ambient conditions and your curing practices.
  8. JConc

    Hairline Cracks after 3 days

    If that means using a soft cut saw -yes - otherwise in cool/cold weather the concrete is usually not strong to make a clean cut with a typical saw until the next morning sometime. Prematurely cutting the floor results in joints with broken out aggregate along the edges so the cut time has to be...
  9. JConc

    Hairline Cracks after 3 days

    Russellwh Your cracks are unsightly only, it should be fine structurally. Ideally the contractor should have cured with a typical black industrial blanket on the concrete to keep in the heat [and moisture] for 3-4 days where after that the concrete would have reached ambient temperature and the...

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