Success and growth in the landscape, commercial and transportation markets brought big production demands to the industry but also introduced a relatively new consideration for SRWs, freeze-thaw durability, to an otherwise proven product. The aggressive actions of expanding water during freeze-cycles and chemical attack has provided a challenge to all concrete products. Research introduced new mix designs, additives, and quality control programs that work collectively to increase the durability of our products. The R&D in wet-cast concrete and drycast has improved their performance in freeze-thaw conditions though the issue has not been fully solved for all conditions and considerations. This has helped to meet the strict requirements set forth by transportation departments. Today’s products perform better than even just a decade ago.
Photo from our article, “Versatility is the Point in Highway Construction”
Development in Durability with Moisture Conditions
In 2004, the estimated value of concrete production for highway construction and maintenance was over 9 billion dollars but 34 percent of the United States’ major roads were still in poor to mediocre condition. While the most persistent problem for concrete in cold climate regions is the concrete deterioration caused by freezing and thawing; preventative solutions are not universally accepted. While air-entrainment has been used to improve freeze-thaw resistance of wet-cast concrete products since the 1930s, more research is still underway to produce durable wet-cast concrete products to provide more acceptable performance under freeze-thaw conditions and chemical reactions to deicing chemicals.
While zero-slump concrete products have been used since the early 1900s, most of the applications were not subject to saturated conditions and exposed to the extreme freezing and thawing environments. Most walls performed well for many years before experiencing any kind of degradation. In 1997 the Minnesota Department of Transportation (MnDOT) issued a memorandum that SRW products should not be used for retaining wall structures along roads and highways due to freeze-thaw durability concerns in the presence of deicing salts. This memorandum came out about 10 years after the introduction of SRWs and millions of square feet of SRWs had been installed. The questionable performance referenced by Minnesota Department of Transportation represented only a small percentage of the total installed product used for DOT applications.
In a historical perspective, the timing of this action was relevant because freeze-thaw durability is a function of exposure time in addition to other variables such as the number of freeze-thaw (F/T) cycles, the degree of saturation, and the concentration and exposure to deicing chemicals. The majority of walls have performed perfectly with no observed distress. In the last 15 years, the performance of SRW units has improved significantly. Problems viewed in the field today may not reflect performance characteristics of current production, but rather that of units produced more than 15 years ago before the improvements were understood and put into place.
Causes of Freeze-Thaw Durability Issues
Below is a brief summary of freeze-thaw degradation process:
- Water enters the pore spaces of the concrete
- Upon freezing the water expands up to 9%
- The pressure from the ice causes fracturing of the internal structure of the product causing cracking and spalling of the concrete. Freeze-thaw damage is generally seen to the top exposed surfaces where water and moisture can collect and stand.
Research into wetcast showed that improved performance can be achieved through:
- Lowering the permeability of the concrete (keep the moisture out of the product)
- Increasing the tensile resistance (strength) of the concrete to resist the internal expanding pressures
- Provide air entrainment (microscopic air voids) to allow movement of water if freezing should occur.
These techniques have worked well for traditional freeze-thaw actions with water, but the highway departments also apply aggressive deicing chemicals to reduce the freezing temperature of water and prevent freezing on the roadways. This now introduces a chemical attack on concrete. We know what causes freeze-thaw degradation: ice and chemicals. We know what can reduce degradation: increased strength, and reduced permeability.
Research Addressing Freeze-Thaw Degradation
Research has shown that for water expansion to cause detrimental effects, the pores need to be about 91.7% filled. This is termed ‘critical saturation’. This is important since in the lower Midwest areas of the U.S., there is not a sufficient moisture supply (snowpack) to keep the units saturated during the F/T cycles and there shows very little damage. In the northern states, where a snowpack is more readily present, there is more moisture available and more damage has been noted.
Deicing chemicals have caused the largest amount of damage to pavements and wet-cast concrete structures. Chemicals used for pavement deicing include sodium chloride, calcium chloride, magnesium chloride, and potassium chloride. Deterioration of concrete by deicers is related to complex processes associated with physical and chemical alteration in cement paste and aggregates. Deicing salts also allow a deeper penetration of moisture into the concrete matrix and greater water saturation into the unit thus increasing the potential of achieving critical saturation.
The National Concrete Masonry Association (NCMA) began a research program to identify the causes of degradation and provide recommendations on manufacturing durable units. A great deal of research followed over the following decades. See our extensive article for details on the research of Freeze-Thaw durability and SRWs.
With prior testing demonstrating that there is no reliable F-T durability indicator, the need for a standard test became clear. After similarly evaluating and discounting the efficacy of other existing methods, the NCMA supported the development and standardization of ASTM C1262, Standard Test Method for Evaluating the Freeze-Thaw Durability of Dry-Cast Segmental Retaining Wall Units and Related Concrete Units in 1998. When ASTM C1372, Standard Specification for Dry-Cast Segmental Retaining Wall Units, was printed, it suggested that in areas of repeated freezing and thawing under saturated conditions, freeze-thaw durability shall be demonstrated by test or proven field. When testing is required, then the units should have less than 1% weight loss for 100 freeze-thaw cycles in water. For transportation work, many states with freeze-thaw conditions specify that SRW units be tested using ASTM C1262 with a 3% saline solution when those units may come into repeated contact with of deicing chemicals. Many states have adopted ASTM C1262 and require less than 1% weight loss after 40 cycles in a saline solution. However, criteria adoption is inconsistent between states with some having both more and less stringent requirements.
With standardized test methods in place (ASTM C1262) and criteria for both commercial and DOT applications for acceptable products (ASTM C1372 and various DOT requirements) in place, NCMA focused on performing research to assist manufacturers in producing durable products that would repeatedly comply with those requirements. As expected, research concluded SRWs required more cement, required durable aggregates, and needed more compaction resulting in accompanying industry recommendations:
- Susceptible aggregates adversely affect the freeze-thaw resistance of manufactured products. Recommendation – use DOT-approved aggregate sources or demonstrate adequate performance of units using freeze-thaw testing.
- Increased amounts of cement paste improve freeze-thaw resistance. Recommendation – increase cement content in mix design as needed to achieve specified performance.
- Improved levels of compaction during manufacturing improved freeze-thaw resistance. Recommendation – increase cycle time and, if needed, increase mix water content. Optimize aggregate gradation. Institute quality control measures to ensure manufacturing repeatability.
NCMA research demonstrated improved performance of units with higher cement contents and with more durable aggregates. In a normal masonry products some of the cement remains un-hydrated, perhaps as a result of the low levels of mix water used to manufacture zero-slump products. Newer generation admixtures provide for better dispersion of the cement and, therefore, better hydration.
In conclusion, the last twenty+ years has brought focus to the durability of SRWs in the face of the freeze-thaw cycle and de-icers. Research and testing has improved quality control and standards. This has especially served to meet the needs of Department of Transportation requirements in Northern states where the presence of snowpack creates the most challenges to the concrete unit durability.
SRW Best Practices Manual
To help designers selecting the right properties for their SRW projects, NCMA published in 2016 new recommendations for SRW units subject to freeze-thaw conditions for different exposure and weather conditions and can be seen in the SRW Best Practices Guide. The Guide is available for a free download here.
This article is an excerpt from the article, “The Durability of Segmental Retaining Walls“.