ACI CLTT-2 common mistakes
ACI CLTT-2 · Understand the traps and how to avoid them.
Reporting a compressive-strength result without first confirming the cylinder's measured diameter and cap planeness against the tolerance, then averaging it in with the other specimens.
Candidates focus on the load at failure and the arithmetic, treating specimen preparation and geometry as setup trivia rather than as conditions that invalidate the result.
What to do instead
Before testing, verify and record diameter to the nearest 0.01 in (or 0.25 mm), check cap planeness and perpendicularity against the standard's tolerance, and note any specimen that fails these checks so the result is flagged rather than silently averaged.
Applying the wrong strength correction or none at all when cylinders are not at the standard 28-day age or when field-cured versus standard-cured specimens are compared.
The exam presents ages and curing histories that look routine, so candidates apply the default 28-day assumption and skip the age/curing adjustment logic.
What to do instead
Read the specimen's age and curing condition before computing strength; state explicitly which correction or acceptance criterion applies and show the corrected value, not just the raw break load divided by area.
Calculating slump to the nearest whole inch or eyeballing the drop instead of measuring to the specified precision and reporting the type of slump.
Slump looks like a coarse field test, so candidates round aggressively or record only a number without noting shear, collapse, or true slump behavior.
What to do instead
Measure to the nearest 1/4 in (5 mm), record the slump type observed, and note whether the sample was a true slump before writing the value on the report.
Treating air content and unit weight as independent readings, failing to account for the aggregate correction factor or the calibration of the meter.
Candidates memorize the pressure-meter procedure but forget that the aggregate correction factor must be subtracted, and that meter calibration and aggregate absorption affect unit weight.
What to do instead
Work through the full calculation with the aggregate correction factor applied, confirm the meter's calibration status, and state the basis (SSD vs. oven-dry) for unit weight before reporting.
Reporting a sieve analysis gradation using the total sample mass rather than the mass retained after washing, or omitting the wash loss from the percent-passing calculation.
The wash step feels like preparation, so candidates compute percentages from the original dry mass and lose the material finer than the No. 200 sieve.
What to do instead
Compute percent passing on the washed, oven-dry mass basis, report the wash loss separately, and check that the cumulative percentages sum correctly to 100 percent at the finest sieve.
Accepting sieve results without verifying that the sieves are clean, undamaged, and that the sample was split to a representative mass for the top size.
Candidates treat sieving as mechanical and skip the pre-check of sieve condition and sample representativeness, which are the main sources of gradation error.
What to do instead
Confirm sieve certification and physical condition, verify the sample mass meets the minimum for the nominal maximum size, and use a properly reduced split rather than a grab sample.
Confusing bulk SSD, bulk oven-dry, and apparent specific gravity, and using the wrong denominator in the specific gravity formula.
The three specific gravity definitions differ only by which mass appears in the denominator, so candidates mix them under exam time pressure.
What to do instead
Write out each formula with labeled masses (oven-dry, SSD, submerged) before substituting, and state which specific gravity the question asks for before computing.
Reporting absorption or moisture content without distinguishing SSD from oven-dry basis, producing values that are off by the absorption amount.
Candidates use whichever mass is handy rather than the defined reference state, so the reported moisture is on the wrong basis.
What to do instead
Identify the reference state required, dry or SSD the sample accordingly, and label the reported value with its basis so the number is unambiguous.
Using a single test result as the reported value when the standard requires averaging multiple determinations and checking them against an allowable range.
Candidates report the first acceptable number and skip the replicate-and-compare step that the test method mandates.
What to do instead
Run the required number of determinations, check the spread against the method's allowable difference, and report the average only when the replicates agree within tolerance; otherwise retest.
Failing to distinguish which test method or standard governs a given property, so procedures from one method are blended into another.
Many test names sound similar across concrete and aggregate testing, and candidates rely on remembered procedure fragments rather than matching method to property.
What to do instead
For each question, name the governing standard and the property it measures before answering, and reject any step that belongs to a different method even if it seems familiar.