By Don Greive, PE and CEO of Pinnacle Structural Engineers

In the last couple of years, our perception is that more concrete than usual is not reaching its specified 28-day design compressive strength in testing. We do not know the reason for this, but there may be some possibilities. In Texas, there has been a cement shortage that resulted in many projects having allocations of concrete. As a result, perhaps contractors felt added pressure to place concrete on schedule, due to project requirements and deadlines, which may have resulted in concrete being placed in less-than-ideal conditions. Also, Type IL cement was rolled out in many regions, and there was a learning curve with mixes utilizing Type IL cement. 

Whatever the reason, low breaks on test cylinders have occurred frequently enough that a plan for evaluating the results and determining further actions is needed.

Test Results

Your project is under construction, and you just received concrete test results showing the tested strength to be less than specified. How does this happen when the average 28-day compressive strength from the historical data submitted with the mix design is usually well over the required strength? Now what? There are numerous thoughts on how to proceed.

The first recommended action is to look at the test reports for possible explanations for low breaks. For example, was there significant water added to the truck? Was the temperature such that either hot or cold weather concreting practices should have been followed? Did a pump break down? Was there a delay in placing the concrete? These items, and any others that could factor into the test results, should be reviewed and evaluated.

Also, when looking at the test results, we suggest checking that the mix used matches the mix that was submitted and approved. Especially in areas with concrete allocations, we have seen multiple concrete mixes submitted for approval because this allows the contractor to have options for obtaining concrete when they are ready. It is always possible that the wrong mix was ordered. With the transition away from Type I/II cement, it may be possible that the batch plant  switched the type of cement during the project. We suggest confirming that the cement type used in the approved mix design is what was used in the concrete. If suspicion exists regarding the type of cement, petrographic analysis can be performed.

Frequently, the first thing everyone does is look at the testing procedures. Those on the construction side will focus on how the cylinders were made, how they were stored, and how they were transferred to the lab to be tested. There is absolutely a possibility that there may be issues with the cylinders being tested. I was on a call where low test results were discussed, and the first thing mentioned was how cylinders were mishandled on another project. Again, that is just one possibility to explain what went wrong. 

Everyone knows that the cylinder test results determine how the concrete that was placed is judged on every project. If there are issues on-site regarding the making of cylinders, storage of cylinders, and/or transportation of cylinders, the GC and concrete contractor should note this at the time and find solutions at that time with the testing lab. The owner/developer and the design team should be notified as soon as items of concern are identified. It makes no sense for the GC and concrete contractor to ignore concerns until there is an issue. A pre-concrete conference, which includes the testing lab, is the ideal time to discuss procedures to eliminate this as a future concern. If anyone at the pre-concrete conference is proposing items regarding testing that are a concern to anyone, they should be discussed at that time.

The next consideration should be time. Suppose your 7-day breaks suggest that you may have issues at 28 days. I would suggest that you consider the following questions when making decisions:

• What is the element with low test results?
• What are the impacts if you proceed with construction and have issues with the 28-day breaks?
• Are there items that may affect the anticipated strength gain?
• Are there any issues with testing?

The first item listed is the element with low test results. For example, suppose foundation members test low at 7 days, and let’s say the foundation consists of piers. Since pier design is often controlled by soils rather than concrete strength, it may be possible that the structural design will still work with a lower compressive strength. However, if you do need the required strength noted in the drawings, the decision becomes pressing. What if you wait for the 28-day breaks, which are still substantially low? If you have placed additional concrete on top of the piers, you must potentially remove that concrete in addition to remedying the low-strength piers. Also, your schedule is adversely impacted. I have seen a contractor take down concrete columns on a project at two days because they were still green and the GC was concerned that they would not reach the required strength, and the construction schedule required two additional concrete floors to be placed on top of the columns before they would have 28-day breaks. In their view, waiting for future test results for those two columns was not worth the risk that they would not have adequate strength and that the columns would have to be removed later. Tough decisions must be made that consider risks.

On a related note, there may be elements that carry a low risk if waiting for test results. Slab-on-grade would be one of those elements. If adequate strength is not obtained in future testing, the removal and replacement of slab-on-grade would not be made materially different by waiting.

When looking at early breaks, consideration should also be given to items that can affect strength gain. There might be clues in the mix designs or the test reports. For example, the cement type or the use of fly ash in a mix may retard the rate of strength gain. Perhaps the temperature or the use of admixtures could have some effect. In addition, did the concrete sit in the truck for an extended amount of time? Was a significant amount of water added to the truck? Various items can affect the rate of strength gain, as well as the 28-day compressive strength.

As noted above, there is always a possibility of testing issues. Once again, we suggest that if anyone sees something happening that could result in compromised testing results, that should be brought up immediately when it occurs. If someone on-site has issues with how cylinders are being stored on-site before being picked up for transport to the lab, they should say so immediately. Also, by having a pre-concrete conference with the testing lab and concrete contractor, these potential issues could be avoided.

Next Steps

Let’s say the compressive test results are in and you have low breaks. Assuming you either don’t have time constraints for the concrete in question or you are willing to roll the dice, what do you do next?

We have discussed looking at the test reports to see if there is information regarding why cylinder tests did not reach the required compressive strength. Perhaps a cause can be found, but if the test results are indicative of the actual concrete strength, further action is required.

As noted above, the typical first response is to question the test procedures. Unless the lab knows there were issues with the testing, other items should be reviewed.

Assuming the 28-day test results are lower than specified, the recommended action is to look at the statistical analysis of the compressive test results of the mix. ACI has a procedure for the investigation of low compressive strength test results. For compressive strengths of 5000 psi and less, if no test is less than 500 psi below the required compressive strength and the arithmetic average of any three consecutive tests exceeds the required strength, the concrete placed is considered acceptable. Often, the testing lab can help provide the information for the engineer to review and analyze.

If the statistical analysis determines the concrete is not acceptable, often other testing is considered. Frequently, testing utilizing a rebound hammer (Swiss hammer) is proposed. This device essentially uses a plunger pressed against concrete to obtain a reading. While this test may be useful, there are several limitations. The rebound hammer typically comes with a chart that correlates test results to a compressive strength. If the rebound hammer is used, we do not recommend using the chart—instead, the rebound hammer should be used on concrete with adequate strength to compare results. An additional concern with using the rebound hammer is that the device essentially measures hardness. Therefore, if you hit a rock with the rebound hammer you can get an inflated reading, which is why, in our opinion, the rebound hammer has limited use in the assessment of concrete with low test results. We definitely do not recommend its use on hard troweled surfaces such as slabs-on-grade, and there are few conditions where we would consider its use. Other nondestructive tests like the Windsor probe also have limited applications in reviewing potentially low strength concrete.

If your 28-day breaks are lower than specified, and if the statistical analysis does not work out, the next step would be to assess if you can wait for 56-day breaks. We typically specify a hold cylinder to be tested if the 28-day compressive strength requirements are not met. In our opinion, if the tested 56-day cylinders meet or exceed the specified strength, the concrete is adequate, but this is a case of engineering judgment. For most applications, the concrete design assumes a required strength. The 28-day requirement is somewhat arbitrary because, in most cases, the required strength is only necessary when the element is loaded, which may not be related to time.

What happens if the 56-day breaks still do not reach the specified strength? There are some options. One option depends on what the concrete element is. For example, the low strength results may be for slab-on-grade. In warehouses, the most common loads are post loads for racking. Using common design procedures, it is possible to determine the post load capacity of a slab for a given post spacing and base plate size at both the design strength and the tested strength. Based upon the results, the structural engineer could provide information to the owner to evaluate whether the reduction in slab capacity would affect their ability to lease or sell the building, which may help determine if further action is required. For example, the engineer might determine, at the tested strength of the concrete, the post load capacity is 97% of what it would be at the required compressive strength, which might be satisfactory for the owner. Another way of looking at slab-on-grade test results would be to examine how much the factor of safety is reduced, in regards to cracking, if the slab capacity for a given post load is the same. We typically use 1.4 as the factor of safety for cracking in slabs-on-grade. What is the impact of the factor of safety being 1.3? This is a discussion to have with the owner. If looking at concrete tilt-up panels, the structural engineer could review the panels to confirm that the panel designs work with a reduced strength. Perhaps the owner might choose to accept the concrete with lower strength. All parties should keep in mind that the analysis with lower concrete strength takes time.

If the 56-day breaks do not reach the specified compressive strength, and additional investigation does not result in acceptance of the concrete, more testing is required. Most likely, the next step would be taking cores to test and evaluate the compressive strength. The “failed” cylinder tests typically identify where, in the placement, the cylinders were taken. Since concrete cylinders are generally taken every 100 to 150 cubic yards, the first step would be deciding where cores should be taken. We suggest that the previous and next “passing” tests in a placement sequence should be set as the range for possible inadequate concrete, as the area requiring testing should not be limited to only the area attributed to the failed cylinders. At 100 cubic yards between tests (about 10 trucks), a lot of concrete can be placed between good tests. While the location of the failed test is known, it is unknown if the concrete that was not tested has adequate strength or not. This is why we recommend using the passing tests, that bookend the failing tests, to establish the boundaries for additional testing. We do not recommend that the area being tested is limited to only the location identified in the “failed” test.

As an example, suppose there are some low breaks in a slab-on-grade placement. Let’s say that the placement started at grid 1 and proceeded to grid 10. The cylinder test results indicate that the cylinders had adequate strength tested at grids 1, 3, 7, 9, and 10, but had low breaks at grid 5. In determining where to take cores, we would recommend taking three sets of cores for testing, considering where the passing results were on each side of the failed tests. For this case, we suggest taking one set near grid 3.5, one set at grid 5, and one set near grid 6.5. In our opinion, the cores in a set do not have to be taken very close to each other, but should represent concrete placed at a similar time. If the failed test results are at a tilt-wall panel, a similar procedure could be applied in selecting which panels to core.

The testing of cores should comply with ASTM requirements. We also recommend having coring and the testing of cores done by a third party so there is no possibility of a conflict of interest, which may exist if this work is done by the contractor or the concrete supplier. With ASTM requirements, three cores make up a test. However, as stated above, the three cores do not need to be taken near each other as long as they represent concrete from the same pour placed in a similar time frame. In addition, test locations should be chosen by determining which locations will have the least impact on the element being tested—including how the performance of the element may be affected as well as the difficulty in obtaining cores—and how the appearance of patched cores will be viewed. For example, if the low breaks are in panels, it is preferable to select solid panels (or those with minimal openings) for testing. Also, cores should be located away from panel connections to the foundation or roof structure. In addition, any concrete where cores are taken from should be scanned to prevent cores from cutting rebar.

After the cores are tested, ACI provides information to determine if the test results meet the required concrete compressive strength. If the average compressive strengths of the core tests are at least 85% of the required concrete compressive strength, and no tests are less than 75% of the required concrete compressive strength, the concrete is considered acceptable. ACI recognizes that there are differences in the sizes of test specimens, conditions with obtaining samples, and procedures for curing between core tests and cylinder sampling, which is why only 85% of the required strength is considered acceptable with core testing.

If the cores do not result in passing test results, more considerations are required. One obvious option would be to remove and replace the concrete that failed to reach its design strength. This may be easier for items such as slab-on-grade. However, this can be more difficult for other concrete elements, especially if they are supporting other construction elements. We will discuss a couple of different elements.

Slab-on-grade concrete has been somewhat discussed previously. If the cores do not result in acceptable concrete for the slab-on-grade, evaluations to those similarly discussed can be repeated. If those evaluations do not result in the acceptance of the concrete, the concrete will need to be removed and replaced. Special attention should be given to the construction joints in the concrete placement, and to the installation of contraction joints when replacing concrete at slabs-on-grade.

For tilt-wall panels, there are other considerations. First, the structural engineer can analyze the deficient panels with a lower concrete strength. Everyone should recognize the effort required to complete this analysis, especially if numerous panels are affected. If the panels work with a reduced concrete strength, there should be discussions with the owner to determine if they are willing to accept the concrete with a lower strength. If not, the structural engineer can determine methods of strengthening the panels. This may include adding structural steel adjacent to the panels. However, we have found that very few owners are willing to accept this option as they believe this type of solution will adversely affect the value of their new building. Also, many believe that solutions of this type will leave prospective tenants questioning the condition of the building and make leasing more difficult. In these cases, replacement of panels will become necessary.

Foundation elements are unique in that there are two different materials that need to be reviewed: soils and concrete. Many times, the limiting factor in design is the interaction between the foundation element and the soils. For example, let’s consider spread footings. The size of the footings would likely be determined by the allowable bearing pressures. Generally, most engineers specify a concrete compressive strength of 3000 psi for spread footings, but frequently that strength is not required as the footing design is controlled by the soils. Therefore, the footing design might be acceptable with lower strength concrete. Ultimately, if the footings do not work with lower strength concrete, augmentation or other methods are available to resolve the issue.

As noted above, when dealing with concrete-strength issues, building owners are typically concerned with their impacts on leasing or selling the building. Anything that potentially indicates a problem is given special consideration. For example, we have had multiple projects where a lift or truck hit a steel column during construction, damaging the columns to various extents. In our experience, even if the column could be repaired by augmentation with steel plates, the owner decided to remove and replace the damaged portions of the columns because they did not want visible, obvious repairs on a new structure. Owners may also request extended warranties or rebates if they do accept concrete with less strength than specified.

Prevention

The best solution to dealing with low concrete test results is to not have them. As such, we have provided some recommendations to reduce the possibility of having low breaks.

Our first recommendations are to place more emphasis on the mix designs. It is imperative that submitted mix designs have test data that represents the materials incorporated into the concrete. If aggregate sources change, new testing should be conducted instead of using a previous mix design. With the current transition from type I/II cement to type IL cement, new mix designs are necessary. The use of old mix designs that do not accurately reflect the materials in the concrete can overstate the actual 28-day compressive strengths, and be misleading. Contractors and engineers should insist on having accurate test data for proposed mixes and not allow a mix design to switch out cement types and use previous test data.

The second recommendation is to have a pre-concrete conference on every project. This conference should include the general contractor, the concrete sub, the concrete supplier, the testing lab, the structural and civil engineers, the architect, the owner (and representatives), and the concrete consultant (if applicable). Procedures should be discussed regarding mix designs, placement on-site, and testing. Agreement should be reached on how cylinders will be made, stored, transported, tested, and reports should be distributed at that conference. All parties should recognize that the cylinders are how the concrete is judged, and immediately discuss any issues when observed instead of waiting for low breaks from test results. This helps everyone, as well as the project.

Another consideration is to evaluate conditions when determining whether or not to place concrete on a particular day. Obviously, rain during a placement can have an adverse effect on the concrete. Also, the temperature can result in the need to implement special procedures. Scheduling concerns are always an issue, and if an evaluation determines that placement should occur, the contractor must prepare for possible impacts that result from that decision.

Other Resources

This article suggests some possible procedures for evaluating low concrete breaks and reducing the possibility of having low breaks on projects. There are other resources available for consideration. One is a session in the Tilt-Up Academy available on the Tilt-Up Concrete Association’s (TCA) website. The session is titled “What’s Next When a Bad Break Happens” and is presented by Kim Basham. Another is an article titled “Type IL Cement and Tilt-Up” that I wrote with David Buzzelli and is published in Volume 31, Issue 4, Winter 2024 of Tilt-Up Today; it is also available on the website at www.tilt-up.org.

Summary

Our perception is that there have been more concrete low strength test results recently than in the past. The intent of this article is to provide some thoughts on possible procedures for evaluating low breaks and to provide a possible framework for further investigation of the concrete to determine the appropriate actions for remedying the issues. In addition, we have suggested some actions for future projects to help reduce the possibility of having low strength concrete.

Ultimately, everyone wants concrete test results to be acceptable. All parties on the project have ways to help make this happen. Following the suggestions in this article will not eliminate low concrete compressive strength test results, but hopefully they will reduce the occurrences.