ICRI Carolinas Chapter Student Scholarship Program
The Chapter is pleased to have been able to award a $2000 and a $1000 scholarship to deserving students for 2020. We received several quality applications and the judging was very tight. Each applicant was asked to write a brief essay on this topic:
One challenge with using concrete in construction is the potential for cracks. Consider a concrete balcony constructed in the coastal Carolinas. Briefly explain the long-term effects of cracks in the concrete balcony and provide a recommendation on how to repair the concrete balcony.
Our two winners and their essays are listed below.
$2000 SCHOLARSHIP WINNER:
University of North Carolina at Charlotte
B.S. Construction Management & Engineering Technology
M.S. Construction & Facilities Management
Cracking in concrete occurs from many different variables that can occur because of poor construction, using unfit materials, or environmental aspects. An example of environmental area that can pose negative effects on reinforced concrete structures are coastal areas, especially those in the Carolinas. In these areas the structures are subjected to increased levels of sulfates from either being emerged in salt water, increased amounts of moisture and salt in the air, or both. These factors can cause degradation and possible failure of the structure if the proper types of cement and aggregates are not used that have a high resistance to sulfates. However, even if the necessary materials are used, the structure can still be subject to failing due to poor construction techniques that can lead to cracking.
Cracking can occur for a variety of reasons during the placement and curing of a concrete structure, but the most common is due to shrinkage when the concrete goes through the curing process. Ways to ensure that shrinkage does not happen is through using the proper amount of water to limit evaporation and using proper curing techniques depending on the environmental conditions such as wind or heat. When the concrete cracks it exposes the interior of the structure and any reinforcing steel to the elements, which in the case of a coastal structure would be salt and fresh water, and salt from the moisture in the air. Over time this can corrode the interior steel or cause sulfate attack on the aggregates, overall resulting in the failure of the structure. Which, in the case of a balcony, the results of a failure could be detrimental. Another long term effect can be the crack increasing in size and depth due to freeze thaw cycles. The coastal Carolinas experience a wide range of temperatures that can vary from relatively warm temperatures to freezing temperatures sometimes within the same week during the winter months.
When dealing with cracking on a balcony it is best to obtain a core sample of the structure so that the depth of the cracking and the severity of the internal damage can be examined prior to recommending a repair technique. The simplest answer to repair the structure would be to patch the cracks, but this should only be done if the crack has not been exposed to the elements for a extended amount of time. Simply patching the crack if it has been exposed to sulfates or freeze-thaw does not solve the interior issues that can occur such as sulfate attack on the aggregates and steel or more internal cracks forming. This is why it is important to have good practice when placing the structure in the first place and also important to inspect the structure periodically during its lifespan. A variety of sealants are available to repair cracks in concrete that range in material type. For repairs in this area it is important to ensure that the material selected will not react with the elements from the surrounding environment such as sulfates, can withstand freeze-thaw cycles, and is compatible with the mix design that was used during construction. It is also important to make sure that they crack is clean and dry before repairing so that it is less likely that interior cracking or corrosion does not continue.
$1000 SCHOLARSHIP WINNER:
University of North Carolina at Charlotte
Construction Management Graduate Student
Cracks in the concrete will inevitably speed up the infiltration of harmful agents into the concrete. With the balcony being located in the coastal area, corrosion of the reinforcing steel is the paramount worry when it comes to infiltration of harmful agents. The salt within the sea spray can corrode the reinforcing steel which will cause the steel to expand and separate from the concrete causing even larger cracks for salt and water to enter through. If the risk of cracking is addressed in the design phase, I would suggest a medium high percentage of approximately 30% Class F Fly Ash replacement for the Portland cement. This would not only lessen the initial micro-cracks but would also decrease the permeability of the concrete balcony once fully cured. If not already in the mix design, I would suggest adding in a corrosion inhibiting admixture to the concrete in order to mitigate the damage from salt penetrating into the balcony. If the fix is happening after the balcony has been placed and formed its cracks, I would suggest hydro-demolishing the balcony to remove all of the compromised concrete. This would also have the benefit of showing the state of the reinforcing steel to determine if it needs replacement as well. Once the balcony’s top cover has been removed a rapid set overlay could be placed on top to limit the steels exposure time to the open air.