ACI Aggregate Level 2 common mistakes
ACI Aggregate Level 2 · Understand the traps and how to avoid them.
Recording the mass of a sample to the nearest gram or dropping trailing zeros when the balance reads to 0.1 g or 0.01 g, so the reported mass does not match the balance's readability.
Candidates copy the habit of rounding from field work and assume 'close enough' is acceptable, when the test method requires recording to the sensitivity of the balance used.
What to do instead
Before each test, note the balance readability printed on the instrument and write every mass with that many decimal places; re-read the display and confirm the last digit before recording.
Using the wrong number of significant figures or rounding intermediate values and then carrying the rounded number into the final calculation, producing a result that differs from the reference answer.
Candidates round at each step out of habit from hand calculation instead of carrying full precision and rounding only the final reported value.
What to do instead
Carry unrounded values (or one extra digit) through all intermediate steps and round only the final result to the required precision stated by the test method.
Reporting a moisture content or absorption value without subtracting the tare mass of the container, or subtracting it twice.
The container mass is recorded on a separate line and candidates lose track of which masses are gross and which are net when working quickly.
What to do instead
Label every mass as container, container plus wet sample, or container plus dry sample before calculating; write the subtraction explicitly in the calculation line.
Selecting the wrong sieve set or reporting percent passing on sieves that are not in the specified series, and mis-ordering the stack so the coarsest sieve is not on top.
Candidates memorize a single sieve stack and apply it to all gradation work regardless of the specification or aggregate size being tested.
What to do instead
Read the specification for the material first, list the required sieves in descending order, and verify the stack order and pan placement before shaking.
Failing to verify that the total mass retained on all sieves plus the pan equals the original sample mass within the allowed tolerance, and not investigating when it does not.
Candidates treat the check as a formality and skip it under time pressure, so material loss or a misweighed fraction goes undetected.
What to do instead
Perform the mass-balance check as a required step, record it, and if the difference exceeds the method's tolerance, re-sieve or discard and repeat the test rather than reporting the result.
Oven-drying samples at the wrong temperature or for an insufficient period, or using a sample that is still warm when weighed.
Candidates assume 'dry' means surface-dry or leave samples in a cooling period too short to reach room temperature, and warm samples give low mass readings.
What to do instead
Confirm the oven set point against the method, dry to constant mass, cool in a desiccator, and weigh only after the sample reaches room temperature.
Performing a compaction or density test with the wrong number of layers, wrong blows per layer, or a drop height that does not match the specified effort.
Candidates conflate different compaction efforts (for example standard versus modified) and reproduce the procedure they practiced most recently.
What to do instead
Before starting, state aloud the required layers, blows per layer, and drop height from the method being used, and have a second person confirm the hammer and mold match that effort.
Determining maximum dry density and optimum moisture content from too few points or from points that do not bracket the peak of the curve.
Candidates take the minimum number of points and stop as soon as they see a curve, without ensuring points fall on both sides of the peak.
What to do instead
Plan moisture points to bracket the expected optimum, plot as you go, and add points until the curve clearly rises and falls around a defined peak.
Failing to verify calibration or check the field density apparatus (sand cone, balloon, or nuclear gauge) against a known standard before use, or ignoring the gauge's daily standard count.
Candidates treat calibration as the technician's job and skip the pre-test check when the equipment appears to be working.
What to do instead
Build the calibration verification or standard count into the start of every density test, record the result, and stop and escalate if it falls outside the permitted range.
Miscalculating field dry density by using the wrong moisture basis, or dividing by the wet mass instead of the dry mass when converting.
The relationship between wet density, moisture content, and dry density is easy to invert, and candidates plug numbers into a remembered formula without checking units and basis.
What to do instead
Write the formula with defined symbols before substituting, track whether moisture is on a dry-mass basis, and sanity-check that dry density is always lower than wet density for a moist soil.