Frequently Asked Questions

Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

Question: Curing Cylinders for Acceptance Testing

Question: Rebound hammer and in-place strength testing

Question: Why is a spherically seated bearing block required in testing cylinders?

Question: Size of test cylinder

Question: ACI code for evaluating existing buildings prior to repair

Question: Low-strength cylinder test results

Question: Estimating in-place concrete strength

Question: What methods of capping are recommended for compression specimens?

Question: Air tests for nonair-entrained concrete

Question: Using core strength to calculate compressive strength of an existing structure

Question: Standards for 7-day and 28-day strength test results

Question: Maximum temperature limits for hot-weather concreting

Question: Strength of concrete

Question: How can it be determined that an admixture will serve its intended purpose?

Question: How is the required strength selected, measured, and obtained?

Question: Are there any tests that may be made directly on the structure to determine if the concrete has developed adequate strength without risk of overloading or causing damage to the structure?

Question: What test conditions affect the results of compressive strength tests of concrete?

Question: What procedure should be followed for standard curing of specimens for strength tests?

Question: Are load tests on completed structures ever employed?

Question: How are properties of concrete in the structure measured?

Question: What are the ultrasonic pulse velocity and the pulse echo tests?

Question: Is pulse velocity related to strength?

Question: What other uses are there for the pulse velocity?

Question: What are rebound, penetration resistance, pullout, and break-off tests?

Question: Why are many non-destructive tests used as “estimators” of concrete strength?

Question: How is strength estimated by the maturity method?

Question: How is the shrinkage of concrete determined?

Question: Using cube tests in lieu of cylinder tests

Question: Consistency, plastic consistency, and workability

Question: Factors governing the strength of concrete

Question: The effect of air content on concrete strength

Question: Resistance to cycles of freezing and thawing

Question: Unsoundness in portland cement

Question: How can durable aggregates be ensured?

Question: The effect of maximum size of aggregate on concrete strength

Question: Why is the workability of concrete important?

Question: How is workability measured and specified?

Question: Precautions against sulfate attack

Question: Improving the abrasion resistance of concrete

Question: Protecting against excessive expansion due to alkali-aggregate reaction (AAR)

Question: The most important single factor affecting compressive strength

Question: How is the water-cementitious material ratio (w/cm) expressed?

Question: The effect of changes in proportions of the fine and coarse aggregate or the quantity of cementitious material on the strength

Question: If time is not available for preliminary tests, how can the desired strengths be ensured?

Question: Are there data relating w/cm to flexural strength that are as reliable as those for compressive strength?

Question: Can tests be conducted by anyone?

Question: What tests are necessary or desirable in making a series of trial mixtures to arrive at a suitable mixture for a particular job?

Question: Can trial mixtures be made with full-size batches at the start of a job?

Question: How should samples of freshly mixed concrete be taken in the field to provide information on concrete in the structure?

Question: How can the surface or free water carried by the aggregates be determined?

Question: Is the slump test of value as a measure of consistency or workability?

Question: How is the slump test made?

Question: Why is the slump test not an absolute measure of consistency or workability?

Question: When is the slump test useful in detecting changes in the concrete?

Question: When does the slump test provide a useful indication of workability?

Question: What test is available to evaluate the behavior of no-slump concrete as it is vibrated?

Question: How is the air content of freshly mixed concrete determined?

Question: Is density a good indicator of air content?

Question: How is the temperature of concrete measured?

Question: Does the method of molding test specimens affect the test results?

Question: What irregularities are to be avoided in the test specimen?

Question: Does a wet specimen test higher than a dry one?

Question: Is storing the specimens on the structure always a satisfactory procedure for estimating in-place strength?

Question: Is the cutting of cores or other shaped specimens directly from a completed structure ever used for determining the strength of the concrete?

Question: How is the air content of hardened concrete determined?

Question: How is the resistance to freezing and thawing determined?

Question: How is the permeability of concrete determined?

Question: How is the resistance to sulfate determined?

Question: How is the resistance to alkali-aggregate reaction (AAR) determined?

Question: How is the abrasion resistance of concrete determined?

Question: Alternative methods for measuring crack width

Question: Mixture design and strength submittals

Question: Curing Test Specimen at High Temperatures

Question: Addition of lime in curing tanks

Question: Definition of setting time

Question: The difference between setting and hardening

Question: Measurement of setting time

Question: False and flash set

Question: Definition of portland cement

Question: The cause of false set

Question: Important requirements of a concrete aggregate

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