Previously we discussed acceptance testing of concrete according to ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field and ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. We focused on how errors in making, curing, handling, and testing yield artificially low results. That is, poor-quality testing increases the producer’s risk of rejecting good concrete. Now let’s look at the cost of bad testing.
Meeting the strength requirement—or not
ACI 301 has two requirements for concrete strength tests:
- Every average of three consecutive tests equals or exceeds the specified strength, fc’.
- No test falls below fc’ by more than 500 psi if fc’ is 5000 psi or less, or by more than 0.10fc’ if fc’ exceeds 5000 psi.
If the concrete doesn’t meet these criteria, the Engineer of Record will want to determine its strength in situ. Nondestructive tests can survey a large area and show where the concrete is weaker or nonuniform. Normally the Engineer will take cores as well.
All of these tests are more complex than compressive strength tests of cylinders. A lab that isn’t producing reliable cylinder strengths will be even less reliable when it comes to additional testing.
The Engineer may analyze the structure to determine what strength is necessary in each location. Often they won’t specify more than one or two different concrete strengths for simplicity. It may be that a particular member can perform satisfactorily with a lower strength than specified. If necessary, the Engineer can also perform a load test on the structure.
If the strength is not acceptable, the Engineer must decide whether reinforcement or repair is in order. Removal and replacement of substandard concrete would be a last resort.
Naturally everyone wants to avoid having to stop the project while the Engineer figures out what to do. Mobilizing technicians for nondestructive testing takes a few days. Because cores must be at a consistent moisture condition before testing, obtaining the strength from them takes about a week. Load tests are both time consuming and risky. And remedial measures take time and money.
Bad testing penalizes the concrete producer
Construction contracts typically provide for liquidated damages for late completion. That is, there’s an explicit equivalence between time and money. The ready-mix concrete producer is liable for delays due to substandard concrete, as well as the direct costs of additional testing and any remedial measures.
Even with good quality control, the concrete strength will vary. To meet the criteria of ACI 301, the mean strength fcr’ must be higher than the specified strength fc’. To simplify the discussion, let’s look at the case of 6000 psi where we have at least 30 previous tests for analysis.
In this situation, ACI 301 requires the mean strength fcr’ to be the larger of:
- 6000 psi + 1.34ss
- 5400 psi + 2.33ss
where ss is the standard deviation. The more variable the strength data, the higher the mean strength the concrete producer must provide. The question is how much variability is due to the concrete production and how much to the testing.
The producer’s risk of rejection of good concrete is increased when the precision of the test method cannot match the uniformity of the production. The economic advantages of high-quality uniform concrete can be negated by imprecise testing. The producer is compelled to design for even higher strengths than would be required by the normal variation of the concrete alone—Kennedy et al., 1995
As we pointed out previously, just about any deviation from the standard testing procedure yields artificially low strength results. Both erratic testing and consistently improper testing will force the concrete producer to increase the mean strength of the concrete just to pass the test.
The cost of bad testing
Unless the reproducibility is good, concrete mixes need to be overdesigned, with resulting higher costs. In addition, poor reproducibility…increases the probability that some test cylinders will fail to meet specified strength requirements. Even if the concrete in the structure…proves to be adequately strong, disruption and economic loss result from the publication of erroneous results.—Detwiler and Bickley, 1993.
How does the concrete producer increase the mean strength of the concrete? Usually that means increasing the portland cement content and reducing the aggregate content. Of course that adds to the direct material cost. However, there’s a lot more to it than that.
Portland cement is the component of concrete with the largest carbon footprint. The cement industry has pledged to achieve net zero carbon by 2050, but it still has a long way to go. For now, adding portland cement increases the carbon footprint of the concrete.
Adding more cement can also adversely affect the performance of the concrete. In many projects, the strength of the concrete isn’t the only—or even the most important—property. In mass concrete, a higher portland cement content means more heat generation and greater tendency to crack. More cement also means greater creep and shrinkage. Depending on the application, that can mean increased deflections, loss of prestress, or greater tendency to crack. If the service conditions include severe exposure, the concrete will be less durable.
The preceding discussion assumes that the concrete producer can obtain the portland cement to add to the concrete. However, that isn’t always the case. Even before the pandemic disrupted supply chains, the construction industry experienced occasional shortages of cement. Usually they would occur near the end of the summer after several months of high demand had depleted cement stockpiles.
Now, though, the problem is more severe. Even during the lockdown due to the pandemic, the construction industry was deemed essential. Demand for cement and other construction materials increased, and cement plants kept their kilns going. Normally plants schedule an annual shutdown for maintenance. However, some plants postponed maintenance, with the result that the kiln linings started to fail. Refractory tiles come from China, and delays in production and shipping mean the cement plants can’t repair their kilns. With a reduced production capacity, cement plants can’t keep up with demand. Instead, concrete producers receive reduced allocations, limiting their capacity to supply concrete.
Initially, the concrete producer bears the cost of bad testing in terms of higher cement content with the consequent increase in material cost. But both producer and contractor suffer the consequences of construction delays, possible legal action, and loss of reputation. Ultimately the owner will also bear the cost of delays and higher prices for the work. Unnecessary remedial measures, including removal and replacement of the concrete, only add to the cost of bad testing.
Even though vetting the qualifications of the testing lab and going through NRMCA’s preconstruction checklist may seem expensive, it’s cheap at the price. The cost of bad testing is much higher.