Super Structures LearningA Super Structures company
← Lessons

Testing, Common Concrete Defects & Production Rates

Testing, Common Concrete Defects & Production Rates

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Testing, Common Concrete Defects & Production Rates. Don't let the plain title fool you. Here's the part that actually matters on the job: The tests prove what was delivered, the defects tell you what the crew or the mix did wrong, and the production numbers tell you whether the next bid is right. Learn it well and you can defend your concrete when a cylinder breaks low, diagnose a bad surface from across the slab, and estimate the next job from your own crews' real rates.

The intro course listed the field tests and how often ACI 318 requires them. This lesson is about what the results mean, what to do when they are bad, how to read the defects that show up anyway, and how to turn all of it into numbers an estimator can use.

Acceptance testing and low breaks

Field tests are run by the owner's testing agency under the ASTM methods: sampling (C172), slump (C143), air (C231 or C173), temperature (C1064), and cylinders made and cured (C31) and broken (C39). Under ACI 318, concrete is acceptable when every average of three consecutive strength tests equals or exceeds f'c and no single test falls below f'c by more than 500 psi (for f'c up to 5,000 psi; a percentage applies above that).

When a test breaks low, the first question is the cylinders, not the concrete. Cylinders left in a truck bed overnight, cured in the sun, frozen, or bounced to the lab on a dashboard are the leading cause of low breaks. ASTM C31 requires initial curing in a controlled range, typically 60 to 80 degrees F, and delivery to the lab within about 48 hours. If the cylinders were handled right, the engineer may accept the concrete on the structural margin, ask for in-place tests (rebound hammer or pulse velocity give relative comparisons only), or require cores under ASTM C42: three cores from the area in question, acceptable when the average reaches 85 percent of f'c and no single core is below 75 percent. Beyond that lie load tests, strengthening, or removal. Maturity testing (ASTM C1074) tracks in-place strength from the concrete's temperature history using a calibration for the specific mix; it is the standard tool for stripping, post-tensioning stressing, and opening slabs to traffic.

Reading the common defects

Most defects have a short list of causes, and the pattern tells you which:

ACI 301 defines the surface defects the contractor must repair, how, and within what time after stripping.

Measuring the work for the estimate

A concrete estimate is built from quantities the drawings give and production rates the company's own history gives:

Rates vary by crew, equipment, and region, so the honest ones come from your own job cost reports. As a frame of reference, a hand-screeded residential flatwork crew of three or four might place and finish a few thousand square feet in a day, a commercial crew with a truss screed and ride-on trowels several times that, and a laser screed crew tens of thousands. A pump moves roughly 40 to 100 yards an hour. Formwork labor runs from well under 100 sfca per carpenter-day on hand-set complex work to several hundred with gang forms. Treat all of those as ranges to check against your own numbers.

Going Deeper (Intermediate)

Job cost is where estimating improves. Set up cost codes that match the way the work is actually done, form, strip, rebar, place, finish, cure and saw, patch, and track labor hours against quantities on every pour. After a few jobs the company knows its real production for a wall in gang forms versus hand-set plywood, and the next bid uses those numbers instead of a guess.

Advanced / Pro-Level

Pros run concrete as a quality system: testing reports are read the day they arrive, trends in slump, air, and seven-day breaks are watched so a drifting plant is caught before a low 28-day result, defects are photographed and traced to a cause, and the production data drives both the estimate and the crew plan. When a result is bad, they bring the engineer the pour log, the batch tickets, and a proposed investigation rather than an argument.

Practice Challenge

A 28-day set from a column line breaks at 3,650 and 3,700 psi against a 4,000 psi specification. The previous two tests averaged 4,300. The technician's log shows the cylinders sat on the deck for two days in October before pickup. What is the situation and what do you do? (Answer: the test falls short of f'c by less than 500 psi and the three-test running average still exceeds 4,000, so the concrete meets the ACI 318 acceptance criteria; even so, notify the engineer, document the improper initial curing of the cylinders in your pour log, and have the testing agency correct its field curing so the next set is not in question.)

In Practice

A foreman who reads every test report and walks the slab the morning after with the finisher learns what the crew did right and wrong while it is still fresh. A foreman who files the reports unread finds out from a letter that three sets of cylinders have been low for a month.

Common Mistakes to Avoid

Takeaway: The tests prove what was delivered, the defects tell you what the crew or the mix did wrong, and the production numbers tell you whether the next bid is right.

Educational content only — not structural design, engineering, or safety advice, and not a substitute for hands-on training or a licensed engineer. Follow ACI 318, ACI 301, ACI 117, and the building code as adopted in your jurisdiction, the project drawings and specifications, the formwork and shoring drawings for your project, and the manufacturer's instructions for every product you use.

Sign in to track your progress