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  1. concreteguru

    ASTM updates

    ASTM has updated the following tests: C231-04 Pressure Meter C172-04 Sampling C143-05 Slump C1064-05 Temperature I'm wondering if there are any significant changes in these tests. Usually the changes are minor,but in 2001 they radically changed the procedure for the roller meter. Any...
  2. concreteguru

    Cement Additives

    There are a number of admixtures available with classification of HRWR -High Range Water Reducers (Superplasticizers) that have NSF approved status. NSF certifies most concrete components such as cement, flyash, slag or admixtures can be used in potable water applications. NSF is a U.S. based...
  3. concreteguru

    Reliable Concrete test

    The truth of course is that neither accurately reflects the strength of insitu concrete. Both standards for acceptance or rejection purposes require that the specimens be cured in a moist condition under ideal temperature i.e. 70 degrees F. This procedure will indicate the potential strength...
  4. concreteguru

    7 Days Compressive Strength of Concrete

    All response posters are correct but I think Stephen is right on the money, especially when looking at in-situ strengths. Mass traps heat and heat cures concrete, hence strength develops much quicker in a massive concrete beam than it might in a 6" cube or cylinder. An excellent tool to...
  5. concreteguru

    ACI-318 and interesting core results

    It is my opinion that taking one core and cutting it into three specimens does not best represent an average of the in-situ strength of the concrete member. Your objective would be better served by random sampling core locations and then averaging the results. As to why the actual strengths...
  6. concreteguru

    Question about concrete strength

    In Contrast with techmaximus, I have seen plenty of low strengths that were attributed to poor sampling technique or poor initial curing or early cylinder handling. One can avoid the marginal quality readymixed concrete suppliers by specifying that the concrete supplier must ship from a NRMCA...
  7. concreteguru

    Question about concrete strength

    In my experience, with numerous DOT's, they usually have a list of approved suppliers which would include cement, admixtures, aggregates and concrete. The materials are tested well in advance and testing is usually done by AASHTO approved labs. Minimum Cement contents are generally higher than...
  8. concreteguru

    General concrete reference

    A excellent resource for concrete knowledge is available from your local portland cement manufacturer. Typically this is a free service available to users of portland cement. Given that ready mixed concrete and/or precast concrete contains high volumes of portland cement, this service is there...
  9. concreteguru

    About snow on freshly poured concrete...

    If you are placing concrete and weather reports indicate snow is coming or if the temperatures at at or below 32F, you need to protect the concrete from freezing. The water in the concrete mix will freeze and thaw and expand and cause cracking and disruption of the cement aggregate paste. Not...
  10. concreteguru

    ACI-318 and interesting core results

    To try to explain why three parts of the same coring may exhibit wide variation, I would suggest that in-situ strength is a function of time and temperature. Since mass traps heat and heat cures concrete, one would expect higher peak heat closer to the core of the concrete member and this would...
  11. concreteguru

    Concrete Mix Water Chemistry

    In addition to Ron's advice, I would suggest that Type V Portland is in very short supply throughout the U.S. An alternative would be to use a Type II plus enough slag cement to give you the chemical composition of a Type V cement. Typically a mix design with Type II Cement and 50% slag cement...
  12. concreteguru

    concrete curing

    Using a percentage of design strength allows for the use of performance based concrete mix designs, which is the better alternative. Performance based mix designs allow suppliers to design mixes that will achieve higher ultimate strengths, lower maintenance costs and extend the life cylcle of...
  13. concreteguru

    Post tensioned slab blow out

    The members have provided a great deal of insight as to what might have contributed to your "blow out" problem. I have seen PT contractors use 'field cured cylinders' to determine when PT operations should commence. There can be huge differences between the strength of field cylinders and the...
  14. concreteguru

    Concrete Mix Design

    What I want to see is a letter from the concrete supplier with the proportions of each raw material, giving both weights and volume occupied. The weights should be based on materials in a S.S.D. (Surface Saturated Condition). It should also list sources of raw materials. Each mix design being...
  15. concreteguru

    Swiss Hammer in Iraq

    A swiss hammer is quite useful as a paper weight on my desk. It is a useful tool to measure uniformity of strength on a single concrete member. The test is subjective at best and almost any result you want to get, you can get. That makes it the least expensive, all time favorite of the...
  16. concreteguru

    Swiss Hammer in Iraq

    John has the right idea and anytime you can avoid a 'post mortem' by doing things right the first time, you are miles ahead. NIST and OSHA funded a great deal of research on Maturity and their findings were that Maturity Testing of concrete represented the most accurate, non-destructive test...
  17. concreteguru

    Concrete testing interval

    CSA and ASTM specifications spell out frequency of testing fairly well. When I am having verification cylinders done to insure that nothing has changed from the original trial batch on the project, I would require that cylinders, slump, air, unit weight be done every 60 cubic yards for vertical...
  18. concreteguru

    Help with quality of testing equipment...

    Gilson is a distributor of testing equipment and a manufacturer of some specific, specialized test equipment such as a "Gilson Shaker" used to perform sieve analysis of fine and or coarse aggregates. They most likely also make other similar equipment. They do not make compression machines but...
  19. concreteguru

    What makes a good Concrete Engineering Tech?

    Boy, what a complex question. ACI Certifications are great and I love to see project specifications require testing be carried out by ACI Field Certified technicians. Better yet, who on the job ever asks a tech to produce his certification. I have seen techs with hardhats with an ACI...

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