ACI Field Testing I key terms

A test sample created by combining two or more portions of concrete taken at different times or locations during discharge of the same batch, so the result represents the whole batch rather than one spot.

The minimum quantity of fresh concrete collected for testing. For strength specimens and most field tests, standards require at least 1 cubic foot (about 28 liters) so enough material exists for every required test.

Collecting concrete from a stationary mixer by discharging the middle portion of the batch into a clean container, setting aside the very first and very last material that may be unrepresentative.

Reducing a large sample to a smaller one without changing its proportions, typically by remixing it on a clean, flat, nonabsorbent surface and quartering or splitting so each portion stays representative.

Rules governing how long sampling and testing may take: the composite sample is gathered over no more than 15 minutes, and testing begins within 5 minutes after the sample is obtained.

Thoroughly mixing a sample with a shovel or scoop before each test portion is taken, so aggregate and paste stay evenly distributed after transport and the sample does not segregate.

Checking a field thermometer against a reference device or known temperatures at regular intervals to confirm its readings are within 1 degree F, since unverified readings make temperature records unreliable.

The distance the thermometer is pushed below the concrete surface, at least 3 inches (75 mm), ensuring the sensing element is surrounded by fresh concrete rather than air or surface material.

The measurement of freshly mixed concrete temperature, taken after the thermometer sits in place at least 2 but no more than 5 minutes and read to the nearest 1 degree F.

A measure of the consistency or workability of fresh concrete, expressed as the vertical distance the freshly molded sample settles after the cone is lifted away.

The metal mold used to form the slump specimen, 12 inches tall with an 8-inch bottom and 4-inch top diameter, fitted with a base plate and foot pieces.

A round, smooth steel rod 5/8 inch in diameter and about 24 inches long with a rounded tip, used to consolidate concrete in the slump cone, unit weight measure, and cylinder molds.

The drop from the top of the cone to the displaced original center of the slumped specimen, measured to the nearest 1/4 inch (5 mm) after the concrete comes to rest.

A slump result where one side of the specimen slides down more than the other, indicating the concrete is not uniform; the test may need to be repeated on a new sample.

The mass of fresh concrete per unit of volume, determined by filling a calibrated measure, consolidating it, striking off the top, and dividing the mass of concrete by the measure's volume.

The actual volume of concrete produced by a batch, calculated as the total mass of all materials batched divided by the measured unit weight of the fresh concrete.

Air content computed by comparing the measured density of fresh concrete with its theoretical air-free density; the difference divided by the theoretical density, times 100, gives percent air.

Determining the exact volume of a container by filling it with water at a known temperature and weighing it, so the volume used in unit weight calculations is accurate.

The density concrete would have with no air voids, computed from batch proportions by summing the absolute volumes of cement, water, aggregates, and admixtures.

Removing excess concrete above the rim of the measure or mold with a straightedge, using a sawing motion and then a flat finish, so the top surface is level with the rim.

The ratio of the actual yield of a batch to its designed volume, showing whether the batch produced more or less concrete than planned for the given materials.

A device that measures air content by sealing a concrete sample in a vessel, applying air pressure above the water, and reading the gauge change that corresponds to compressible air in the concrete.

A small percentage subtracted from the meter reading to correct for air trapped in the pores of the aggregate, established beforehand by testing representative aggregate samples with the same meter.

The process of verifying a pressure air meter using known volumes or the manufacturer's procedure, so gauge readings correctly translate pressure change into percent air content.

An apparatus that measures air content by filling a calibrated vessel with concrete and water, adding alcohol, and agitating until the water level shows the volume of air removed from the sample.

A liquid added to the volumetric air meter to break surface tension and reduce foaming, so the water level can be read clearly and the air volume measured accurately.

Inverting and rolling the volumetric meter on a flat surface after adding alcohol, repeating the agitation until the water level stops changing and all released air has collected at the top.

A cylindrical test specimen of concrete, commonly 6 by 12 inches or 4 by 8 inches, cast in a mold, cured under controlled conditions, and later tested in compression.

Consolidating concrete in a mold by pushing the tamping rod a set number of times per layer, commonly 25 strokes in each of three layers for 6-by-12-inch cylinders.

A single-use or reusable cylindrical form made of material that does not absorb water or react with concrete, oiled lightly before filling and sealed so it holds its shape.

Storage of freshly cast specimens at 60 to 80 degrees F in a moist environment for the first day, protecting them from freezing, drying, and vibration until the molds are removed.

Moist curing of hardened specimens in limewater or a moist room at about 73 degrees F until the age of test, so strength results are comparable to reference laboratory conditions.

A stiff material such as sulfur mortar or gypsum paste applied to a cylinder end to create a flat, smooth bearing surface; it must reach adequate strength before the specimen is tested.

The flatness of a capped cylinder end, required to be within about 0.002 inch across the cap so the load is applied uniformly and the specimen is not loaded eccentrically.

The maximum compressive load a concrete specimen carries per unit of cross-sectional area before failure, usually reported in psi or MPa at a specified age.

The speed at which the testing machine applies load, held within a specified range such as 35 psi per second, because faster or slower loading changes the measured strength.

The shape of a cylinder's break after compression testing, such as cone, cone and shear, or columnar, recorded because unusual patterns may point to capping or machine problems.

The machine component that aligns with the top of the specimen so load is applied evenly, seated properly and kept clean to avoid eccentric loading during the test.

The cross-sectional area of a cylinder, computed from a diameter measured near mid-height rather than the nominal size, used to convert failure load into compressive stress.