Once in a while, especially in cold-weather states, joints in concrete pavements have shown signs of deterioration relatively soon after pavement construction (five to 10 years).
The phenomenon, known as premature joint distress, usually presents as shadowing, microcracking and flaking. These tell-tale signs can progress to joint spalling and significant loss of material along the joint. Unless the damage is repaired and the underlying causes are addressed, premature joint distress can reduce a pavement’s service life.
“The occurrences are limited,” said Gordon Smith, president of the Iowa Concrete Paving Association (ICPA), “but concrete associations and roadway agencies in northern states are eager to know what’s causing them and how to prevent them in the future.”
Through pooled-fund projects and other sponsored projects, state highway agencies from Indiana, Iowa, Michigan, Minnesota, New York, South Dakota and Wisconsin, along with the Federal Highway Administration (FHWA), have invested in finding answers. Iowa State University’s National Concrete Pavement Technology Center (the CP Tech Center) has led the research efforts, along with partners Michigan Technological University, Oklahoma State University, Oregon State University and Purdue University.
They are finding answers and suggesting preventive practices.
As a result, when the city of West Des Moines, Iowa, constructed several pavement sections to serve the new Microsoft Alluvion data center development in fall 2015, the city was eager to incorporate lessons learned from recent research. The city hired a team of consultants including H.R. Green, MSA Professional Services, Bolton & Menk, McClure Engineering Co., and Shive-Hattery to design the series of projects. Concrete Technologies Inc. (CTI) was ultimately contracted for paving the street sections.
Throughout the project, these organizations worked closely with each other, the Iowa Department of Transportation (IDOT), the CP Tech Center and its partners, and the ICPA.
A VKelly test is performed on the project-designated mix.
Researchers have determined that, although no single mechanism can account for every occurrence of premature joint distress, there are two primary causes:
- Freeze-thaw damage in saturated concrete pavements. The resulting damage appears as layers of thin flakes; and
- Salt-related chemical reactions in pavements treated with anti-icing and/or deicing salts during the winter months. The damage appears as cracking at regular intervals parallel to the saw cut and/or as loss of paste, exposing clean aggregate.
The freeze-thaw phenomenon and its potential to damage concrete pavements is well understood. Basically, freeze-thaw damage occurs when water trapped in cement paste freezes and expands, exerting force on the paste and aggregate in the concrete.
According to work conducted by Oregon State University’s Jason Weiss, Ph.D., the expansive force of trapped water-turned-ice is one of the most destructive internal forces that concrete experiences.
To reduce or mitigate freeze-thaw pressures, it is common practice to specify that concrete mixtures include strong, durable aggregates, as well as a well-distributed system of air bubbles, or air voids. The air-void system is introduced via a special chemical, or admixture, and provides space into which expanding water/ice can move, relieving some of the pressure on the hardened paste and aggregate.
In projects displaying premature joint distress, however, freeze-thaw damage has occurred even in mixtures with durable aggregates and adequate air-void systems. Researchers discovered a common factor in several of the projects: saturated concrete.
Saturated concrete is highly susceptible to freeze-thaw damage. In fact, when concrete exceeds a critical degree of saturation—approximately 85% of the void space—freeze-thaw cycles will likely result in damage.
Concrete saturation can be exacerbated by a number of factors, including a wet, poorly draining subgrade/sub-base, water trapped in the joints, and high concrete permeability. In addition, joint damage at the bottom of a saw cut (caused, for example, by sawing machines with worn bearings or an inappropriate blade) can increase the rate of concrete pavement saturation. Finally, some products of cement-salt reactions can reduce the space available for fluid movement into and out of air voids, increasing the rate of saturation.
Instances of premature joint deterioration are primarily occurring in northern states where deicing and anti-icing chemicals are used
High concentrations of salt can react with the cements in concrete. One problem reaction occurs between salt (calcium chloride, CaCl2) and one of the initial products of cement-water hydration, calcium hydroxide (CH), which results in a compound called calcium oxychloride (CaCl2 · 3Ca(OH)2 · 12H2O). (Note: Magnesium chloride (MgCl2) salt also can produce calcium oxychloride.)
Calcium oxychloride is expansive and can damage the cement paste. This can have a significant impact on durability.
Concrete joints can be “collector areas” for highly concentrated salt solutions, making the joints susceptible to damage from the formation of calcium oxychloride. In addition, salts tend to hold water, potentially increasing the risk of concrete saturation and thus for freeze-thaw damage.
According to Dan King of the ICPA, who was part of the advisory team for the West Des Moines project, significant energy was devoted to developing an appropriate mixture and following construction practices that would yield a durable pavement resistant to premature distress.
For this project, a performance-based specification for a modified quality management concrete (MQMC) mixture (an IDOT mixture) was developed. The goal was a workable mix for a durable pavement with long-term resistance to freeze-thaw pressures and problems associated with concentrated salts.
Based on recommendations by Weiss, cement was replaced with Class C fly ash at a rate of 30-35% (a standard IDOT mix is 20%) to reduce risk of chemical attack by deicing salts. Fly ash converts calcium hydroxide (CH), a product of hydration, into the more desirable product calcium silicate hydrate (C-S-H); a higher flash ash content reduces the formation of calcium oxychloride. A higher fly ash content, therefore, results in more durable concrete, with less permeability (potentially lowering saturation rates) and greater long-term strength (increasing resistance to freeze-thaw damage).
Other mix modifications were based on a Minnesota specification that has resulted in durable, long-lasting pavements. They included a minimum of 6% air behind the paver and a target water-to-cementitious-materials (w/cm) ratio of 0.40 (maximum 0.42; the standard IDOT mix is about 0.45). The high air volume and low w/cm ratio can contribute to reduced permeability in concrete (reducing saturation rates and the potential for freeze-thaw damage) and greater long-term strength.
Because construction was scheduled for mid to late fall, the mixture was further modified to counter potential challenges inherent when paving during cold weather. Such challenges include delayed set times, which significantly increase the risk of early-age cracking. To counteract these impacts, 400 lb minimum cement per cubic yard of concrete was specified.
Conventionally, locally available materials were used in the mixture. These included well-graded, durable aggregates as well as admixtures such as air-entraining admixtures, all specified in accordance with special provisions developed from IDOT, Minnesota DOT and Iowa’s Standard Urban Design and Specifications (SUDAS) guidelines, as well as with advice from local contractors and the ICPA, according to Jeremy Huntsman, project manager from H.R. Green.
Design and construction: the West Des Moines project was designed and constructed using best practices, not only in placing the concrete, but in providing good drainage and protecting the concrete from water and chemical infiltration.
For example, a 12-in. subgrade and 6-in. sub-base were prepared, and a subdrain was installed for all sections of new pavement. The pavement was thoroughly cured, and blankets were available to cover the pavement if temperatures dipped below 40°F.
Also, the contractor experimented with more aggressive use of pavement sealers. The joints were thoroughly sealed after sawing, and the slab was sealed after a period of time.
A super air meter (SAM) test is employed to measure air-void spacing and air content of the concrete.
Tests and more tests
Laboratory tests were conducted on samples cast in the field to investigate freeze-thaw damage resistivity, air structure and potential for joint deterioration. Field tests were conducted to investigate the robustness and consistency of the modified mix proportions. Core tests were conducted to compare field samples with lab test results.
Three new tests, which are being investigated as part of the FHWA’s effort to develop more effective performance-based specifications based on desired critical pavement properties, were included in the field-testing protocol to provide an extra layer of information:
A VKelly test, in which the rate that a Kelly ball sinks into concrete is determined while an attached vibrator is running, measures the rate of a mixture’s movement under vibration as well as the initial yield stress. The VKelly test is an inexpensive, portable test that provides reasonable measurements for determining the suitability of a mixture for slip-formed paving applications.
A box test, in which the surface of a cubic sample is observed and evaluated after six seconds of vibration, indicates the response of the mixture to vibration. According to studies by Oklahoma State University’s Tyler Ley, Ph.D., this test is a simple and economical test method for evaluating the suitability for slip-formed concrete paving. It is useful in prequalification of mixtures, but not for quality acceptance.
The super air meter (SAM) measures both air-void spacing and air content of plastic concrete in about 10 minutes. Also based on Ley’s work, air-void spacing has been shown to be a better indicator of a mixtures’s freeze-thaw durability than total air content, but until now it has been challenging to measure this value in fresh concrete. The SAM allows for better prediction of freeze-thaw durability as the concrete is being placed.