When it comes to freeway test specimens, nothing is as frustrating as the Modulus of Rupture (MOR) flexural beams. Do you find yourself asking more questions and feeling a sense of anxiousness right after the specimens are fabricated? Will they be picked up tomorrow? How will they be transported? If cured onsite, will they stay safe?
These are all legitimate questions, especially when the quality of your concrete pavement is represented by these sensitive test specimens. In most publications describing how to make and handle flexural beams, experts will constantly remind the reader that extra care should be taken in the early ages of a beam specimen to prevent damage and from drying out. At G3 Quality, we have developed answers to these questions, and our team is proud to take the extra cautionary steps necessary to protect and retain the integrity of every MOR test specimen during fabrication, initial curing, transportation, and final curing of the beam specimens.
Fabrication and Curing
During the fabrication and initial curing of the beam specimen, a 6” wide, 6” height, and 21” long beam is subject to the extreme ambient conditions of the project site. It is very important to assure the complete protection of the beam specimen by applying curing compound immediately after fabrication and surrounding the mold with dampened sand as per CTM 523. This is especially critical during very warm and windy ambient conditions.
When the beam specimens are transported, they must be carefully removed from the sand and placed in the transporting vehicle without jarring the samples. Although the beams are in a mold and the concrete has hardened, they are still very vulnerable at the beginning of the hydration and strength process.
When transporting test specimens even the shortest of distance, it is important to strap the beam molds and secure them properly so they do not move or bounce around in a vehicle. During transportation, G3 Quality employs a wind cover over the bed of the vehicle to ensure the specimens do not dry out due to excessive wind as an extra measure of protection.
It is at this most crucial point, after the specimens are delivered to the laboratory for the final curing process, that due diligence is required. According to the July 2012 version of CTM 523, “Method of Test for flexural Strength of Concrete”, once the beam specimens are at the final curing location, they are to be placed in a moist sand pit until 24 hours +/- 6 hours prior to testing. It is during this time frame in the sand pit that it becomes very critical for a beam specimen to gain the majority of its strength up to 26 days prior to being placed in a water lime bath.
It has been noted by many publications and especially in CTM 524 to, “Keep the beams thoroughly damp throughout the curing period. Even a few minutes exposure of beams to drying can seriously affect the test strength.”
G3 Quality takes this time frame with the utmost care to assure the beam specimens are kept in a moistened state and in an ambient curing range, which prevents them from drying out. In our Cerritos and Redlands laboratories, the curing is conducted with our sand pit inside the building, therefore keeping the sand moist and ambient temperature range at 60° F to 80° F.
When a laboratory does not have floor space inside the lab, the sand pit is most likely outside in what can be considered very extreme ambient conditions. In some climatic regions of California, the lows can be 30° F in the winters and the highs at 115° F in the summer, with more extreme ranges during the day and night as temperatures fluctuate. These ambient conditions can create a challenge to maintain the curing specification.
This challenge can be mitigated in colder temperatures by protecting the specimens in the pit with a thermal blanket covering of the whole area and by using airflow heaters that turn on and off when the temperature falls below 60° F. This gives the best chances of hydration of the test specimens from low strength due to poor hydration during a cold curing period.
The summertime poses a greater challenge for curing. Certain climatic regions are subject to high heat and winds, which will quickly dry out the surface of the sand pit area. Utilizing a tarp covering and diffusers over the area will protect it from direct sun exposure heat. Incorporating water atomizers to keep humidity in the sand pit area is an additional measure to maintain the proper moisture. Practicing due diligence in keeping the sand pit area moistened several times a day will help to reduce extreme moisture loss during the summer heat.
As engineers, suppliers and contractors know, nothing can be more frustrating than having a 10 day beam break at 80% of f’c to only have it test lower at 28 days due to the specimens drying out in an unprotected sand pit from extreme ambient conditions. There are many contributing factors to the variability of low beam strengths, but this type of strength loss can be attributed to the poor curing conditions of the sand pit during the 26 days of curing.
When you put your projects’ Q/A or Q/C in the hands of G3 Quality, these are the types of measures we take to assure the best quality of inspection and testing. If you would like to have a further discussion regarding the care we take during the curing and testing of your Q/A or Q/C test specimens, we invite you and your team to explore our labs and meet our team!
At G3 Quality, we are an elite team of engineers and professionals who provide world-class quality management, pavement engineering, materials design, construction inspection, and testing on infrastructure projects throughout California and the western United States.
To learn more about how G3 Quality can contribute knowledge, expertise, consulting, and professional services to your project, contact us.