ACI Field Testing I common mistakes
ACI Field Testing I · Understand the traps and how to avoid them.
Collecting the sample only from the first portion of the discharge or scooping from the top of the wheelbarrow, rather than combining increments across the middle of the discharge.
The first and last concrete out of a mixer or truck is richer or leaner in mortar and aggregate; a single-point sample cannot represent the batch, and top scooping biases the sample toward paste, shifting slump, air, and strength results.
What to do instead
Take at least three increments at approximately equal intervals over the middle 1/2 to 2/3 of the discharge, catch them in a clean, dampened wheelbarrow, and combine; do not sample from the very beginning or end unless the specification requires it.
Beginning slump, air, or density tests on a sample that has not been remixed, or allowing the sample to sit beyond the allowed time before testing.
Concrete segregates and loses workability after sampling; testing an undisturbed sample gives a non-representative slump and air content, and delayed testing lets absorption and evaporation change results.
What to do instead
Remix the sample with a shovel for about 30 s before each test, start tests promptly (within 15 min of sampling per C172), complete them within the allowed interval, and discard and resample if the sample has stiffened or segregated.
Inserting the thermometer only about 25 mm (1 in.) into the concrete and reading it immediately after insertion.
A shallow or short immersion measures the surface temperature, which is affected by air and container, and the thermometer needs time to equilibrate; immediate readings are biased high or low.
What to do instead
Insert the thermometer at least 75 mm (3 in.) into the concrete, close the concrete around it, leave it for at least 2 min and no more than 5 min, and read while the stem is still immersed; measure within 15 min of sampling.
Rodding the slump cone with uneven strokes or through the full depth of each layer, and lifting the cone with a twist or in less than 5 s.
Uneven or excessive rodding overconsolidates some zones and leaves others honeycombed; through-rodding the upper layers over-compacts lower concrete; twisting or fast lifting drags the concrete and changes the slump.
What to do instead
Fill in three equal layers; rod each layer 25 strokes uniformly, with the bottom layer rodded through its depth and upper layers just into the previous layer; lift the cone vertically in 5 ± 2 s without lateral or twisting motion.
Using a unit-weight measure whose volume is assumed rather than calibrated, and filling it without the specified three layers and 25 strokes per layer.
A dented or uncalibrated measure introduces a fixed volume error into density and yield; poor consolidation leaves voids that lower unit weight and falsely raise yield.
What to do instead
Calibrate the measure by filling with water at a known temperature and calculating volume from the mass of water; fill in three equal layers, rod 25 strokes per layer for a 0.5 ft³ measure, tap the sides, strike off, and determine mass to the required precision.
Calculating yield from design batch weights or from an assumed air content instead of the actual batch weights and measured air content.
Yield is the actual volume produced per batch; using design weights hides over- or under-batching, and ignoring measured air changes the calculated density and volume.
What to do instead
Use the actual batch weights from the batch ticket, compute total mass, divide by the measured fresh density to get yield, and compare with the design volume; use the air content measured on the same sample.
Reading the pressure-meter gauge directly as total air content without applying an aggregate correction factor, or using a correction factor from a different aggregate source.
The pressure method measures air in the concrete plus air trapped in pores of the aggregate; without the source-specific correction, the gauge overstates the air content.
What to do instead
Determine the aggregate correction factor for the actual aggregate combination per C231, subtract it from the gauge reading, and re-determine it when aggregate sources or proportions change.
In the volumetric air test, reading the liquid level as soon as the meter is inverted a few times, before bubbles have stopped rising, or adding an arbitrary amount of alcohol.
Entrained air is released slowly; an early reading includes bubbles still in the liquid and underreads true air, while incorrect alcohol volume changes the liquid level and meniscus.
What to do instead
Add the specified amount of isopropyl alcohol, cap, invert and roll the meter for the required time, then let it stand until bubbles stop, tap the gauge, and read the bottom of the meniscus at eye level.
Leaving field cylinders exposed to sun, wind, or freezing temperatures during initial curing, or rodding them with the wrong layer and stroke pattern.
Rapid moisture loss or temperature extremes during the first 24 h cause surface cracking, incomplete hydration, and low strength; improper rodding creates voids or segregation.
What to do instead
Rod 6×12 in. cylinders in two layers with 25 strokes per layer (4×8 in. also two layers, 25 strokes), strike off, cover to prevent evaporation, cure initially at 60–80°F, and protect from freezing and vibration until demolding.
Testing a cylinder with a cap that is not perpendicular to the axis or is thicker than allowed, and applying load at a rate outside the specified range.
A tilted or thick cap introduces eccentric loading or a soft layer, causing premature failure; too-fast loading raises apparent strength and too-slow loading lowers it.
What to do instead
Cap with a flat plate and guide so the cap is perpendicular within tolerance and no thicker than 3 mm (1/8 in.) for sulfur mortar, then test at 0.25 ± 0.05 MPa/s (35 ± 7 psi/s) and center the specimen on the platen.