ACI Strength Testing key terms

The maximum stress a hardened concrete specimen can carry in axial compression before failure, calculated by dividing the peak load by the cross-sectional area. It is the primary measure used for acceptance and quality control.

A molded or drilled cylindrical concrete test specimen, commonly 150 by 300 mm (6 by 12 in.), cast in reusable molds and cured under controlled conditions before compression testing.

A rigid, nonabsorbent container that holds fresh concrete in the required shape and dimensions while it sets, typically with a removable base plate and a leak-resistant joint.

Compacting fresh concrete in a mold by pushing a standard tamping rod through the lift in evenly spaced strokes, used for normal slump mixtures to remove voids.

Compacting fresh concrete in a mold by inserting a mechanical vibrator at defined points and depths, required for low-slump or stiff mixtures where rodding is not adequate.

One of the equal-depth portions of concrete placed and compacted in sequence to fill a mold; the number and depth of lifts depend on specimen size and method of consolidation.

A round, straight, smooth steel rod of specified diameter and length with a rounded tip, used to compact concrete in molds and to strike off the surface.

The final operation of leveling the top surface of a molded specimen by moving a straightedge or rod across the mold rim to remove excess concrete and produce a flat finish.

A measure of the consistency of fresh concrete equal to the vertical drop of a molded cone when the form is lifted; it indicates workability and helps select the consolidation method.

The relative ease with which fresh concrete can be mixed, transported, placed, consolidated, and finished without segregation; it is judged by tests such as slump.

The process of removing entrapped air from fresh concrete in a mold or form by rodding or vibration so the specimen is dense and free of large voids.

The separation of coarse aggregate from mortar or paste in a concrete mixture, often caused by overvibration, excessive water, or rough handling; it produces nonrepresentative test specimens.

The procedure of taking a representative portion of fresh concrete from a batch or delivery, following prescribed methods, so test results reflect the material actually placed.

The period immediately after molding during which specimens are kept undisturbed in a moist environment or protected from moisture loss at controlled temperature before further handling.

Storing hardened test specimens in a saturated environment, such as a moist room or water tank, at a controlled temperature so cement hydration continues and strength develops normally.

The prescribed moist storage conditions of temperature and humidity applied to specimens intended for acceptance testing, so results can be compared with specified strength at a given age.

Maintaining adequate moisture and temperature in concrete after placement so cement continues to hydrate; for test specimens it is controlled to produce reproducible strength results.

Applying a thin layer of material, such as sulfur mortar, cement paste, or a neoprene pad, to the ends of a cylinder so they are flat, parallel, and perpendicular to the axis.

A resilient elastomeric pad placed between the specimen end and the testing machine platen to distribute load uniformly, used as an alternative to a bonded cap.

A metal ring that holds a neoprene pad centered on the end of a cylinder during testing, keeping the pad aligned with the specimen axis.

The contact area between the specimen end and the testing machine platen; it must be flat and perpendicular to the load axis to avoid eccentric loading and low results.

Application of compressive load not aligned with the specimen's longitudinal axis, which produces uneven stress, premature failure, and erroneously low strength values.

The correct positioning of the specimen, caps, and platens so the load acts along the specimen axis; it is verified with a calibrated device or careful centering.

A compression testing apparatus with platens, a load-measuring device, and a means of applying load at a controlled rate; it must be calibrated and meet accuracy requirements.

A curved bearing block on the upper platen that tilts to seat uniformly on the specimen end, helping to distribute load evenly despite small surface irregularities.

The speed at which compressive stress is applied to a specimen during testing, specified as a stress per unit time so results are comparable between laboratories.

The maximum force recorded by the testing machine during a compression test, occurring just before the specimen can no longer sustain increasing load.

The characteristic shape of the break in a tested cylinder, such as a cone or shear plane; an unusual pattern may indicate capping, alignment, or specimen problems.

The process of comparing a testing machine's load readings against known reference values and adjusting or verifying them so measurements meet required accuracy.

A periodic check of a testing machine's indicated loads against a proving device or reference standard to confirm it remains within specified accuracy limits between calibrations.

The compressive strength of concrete required by the project documents at a stated age, used as the basis for mixture design and acceptance of test results.

The rules in a specification that judge whether strength test results comply, often based on averages of sets of cylinders and limits on individual low results.

The strength value obtained from a single specimen or the average of a set of specimens tested at a given age, reported to the required precision for comparison with criteria.

The elapsed time between molding a specimen and testing it, commonly 7 or 28 days, recorded so strength can be compared at a consistent maturity.

A cast cap made of melted sulfur compound applied to a cylinder end, allowed to harden, and trimmed so the bearing surface is flat and perpendicular to the axis.

A reusable pad-type end treatment, such as neoprene, that is not attached to the specimen and relies on confinement to distribute load across the bearing surface.

The change in specimen length under load divided by its original length; in compression testing it relates to deformation, though strength is the primary reported property.

A calibrated elastic ring used as a reference load-measuring device to verify or calibrate testing machine readings through its measured deflection under load.