A reported 4.1 billion metric tons of concrete structures are poured annually, with the United States accounting for upwards of 90 million. Considered one of the strongest and most cost-effective building materials, concrete production and usage are expected only to increase in the coming years.
But the criticism of this building material revolves around the significant amount of carbon dioxide it releases annually. If the carbon dioxide released from concrete each year were compared to that of the world’s largest nations, it would reportedly rank third. The factors affecting the carbonation of concrete and the release of this greenhouse gas put construction workers and the environment at risk. By minimizing the carbonation of concrete and ensuring workers have respirators and other personal protective equipment, hazardous effects can be avoided.
What is Carbon Dioxide?
Commonly referred to as CO2, carbon dioxide ranks as the fourth most prevalent gas in the Earth’s atmosphere. Comprised of one carbon atom and two oxygen atoms, carbon dioxide remains odorless, colorless, and non-flammable at room temperature.
It can be converted into the solid known as “dry ice” and is responsible for the positive environmental function of trapping heat. This ensures the planet does not decline into an uninhabitable deep freeze. Excessive CO2 production through human activity and natural systems has emerged as a concern. Many scientists indicate an overabundance of CO2 has become a “greenhouse gas,” causing global temperatures to rise.
Why is Carbon Dioxide so Dangerous?
It’s important to understand that carbon dioxide is not necessarily a poisonous gas. It continues to be a vital aspect of the environment and human breathing mechanisms. People rely on CO2 for every breath they take. That being said, too much of a good thing can prove dangerous.
Perhaps the greatest risk carbon dioxide poses stems from its asphyxiant nature. High CO2 levels in a closed environment can result in people inhaling carbon dioxide that replaces oxygen in the body. This typically causes the body to slow down and systems to shutter.
Among the first hazard that occurs is that this asphyxiant negatively impacts brain functions. Even at seemingly moderate CO2 levels, the ability to focus on a task or concentrate becomes impeded. People that inhale high levels of carbon dioxide often suffer significant cognitive reductions. Advanced brain functions falter, and people suffer symptoms such as fatigue or headaches. What’s particularly concerning about carbon dioxide inhalation is that workers too often attribute symptoms to lack of sleep and believe the air is just “stuffy.” The following outlines the anticipated impact of carbon dioxide inhalation at parts per million (PPM).
- 1,000 ppm: Minor to no negative health risks.
- 1,000 ppm to 2,500 ppm: People experience fatigue and difficulty concentrating.
- 2,500 ppm to 5,000 ppm: Headaches, pervasive fatigue, and drowsiness typically result at this level of CO2 inhalation.
- 5,000 ppm to 40,000 ppm: Work environments with this heightened concentration of CO2 may be in violation of OSHA regulations. Workers experience a sense of intoxication and excruciating headaches.
When CO2 levels run between 40,000 ppm and 100,000 ppm, they create a clear and present danger to workers’ health and lives. Symptoms from this level of exposure involve dizziness, rapid heart rates, trouble breathing, seizures, and loss of consciousness. When exposure thresholds exceed 100,000, people usually pass out within minutes. The result of prolonged carbon dioxide at or above this concentration may cause workers to fall into a coma or die.
Understanding Natural vs Man-Made CO2 Emissions
A great deal of media reporting regarding CO2 revolves around human activity and its relation to climate change. While human beings are a key driver in carbon dioxide production, it’s also important to understand it remains a naturally occurring gas.
Carbon is stored in wide-reaching natural items such as the oceans and living organisms. It gets released into the atmosphere in the form of carbon dioxide when living things decompose. Humans and other living creatures breathe in oxygen to sustain life and exhale carbon dioxide. Other conditions that cause carbon to transition to carbon dioxide in the environment include forest fires, volcanic activity, and when carbonate rocks decay due to weather, among others.
On the human activity side of carbon dioxide releases, burning fossil fuels and cement production rank among the most prevalent ways the greenhouse gas occurs. Carbon and carbon dioxide are necessary for sustaining life. However, excessive CO2 and high levels in the workplace put everyday people and the environment in harm’s way.
How Does “Carbonation” Relate to Concrete?
The carbonation of the concrete process begins the moment that building material comes in contact with the atmosphere. Depending on the grade of concrete’s permeability and porosity, carbonation can move forward at a rate of 1-5 mm per year. Carbonation results in steel reinforcement elements corroding while the hardened material weakens and cracks over time. The carbonation of concrete ranks as a primary cause for otherwise robustly reinforced slabs to degrade.
On a chemical level, the carbonation of concrete involves a reaction between the moisture and calcium hydroxide present in concrete mixtures and carbon dioxide. The interaction generally results in the production of calcium carbonate, which is known to lower the pH level. As the pH declines, the inherent protections or rebar and other steel reinforcements start to break down.
Why is this Process Dangerous?
The primary danger associated with the carbonation of concrete involves the weakening of slabs, foundations, and other building elements. Although structural concerns have largely been the focus of building safety, the concrete also results in significant CO2 exposure in the workplace. According to sources such as the National Ready Mixed Concrete Association, 0.93 pounds of carbon dioxide are released for every pound of concrete produced.
Given the high level of carbon dioxide release, consider the risk that masons, laborers, and other construction sector workers face when pouring slab floors for often enclosed, big-box retail outlets and warehouses. The potential to exceed OSHA breathable air standards and the weakening of aged slabs make carbonation a dual threat to workers pouring or demolishing concrete.
Factors Affecting Carbonation of Concrete
It’s essential to understand that the carbonation of concrete is not necessarily a foregone conclusion. There is a range of factors affecting the carbonation of concrete that can be identified and curbed in many cases. The following are commonly cited reasons for accelerated carbonation.
- The grade of the concrete
- The water to cement ratio
- The concrete’s permeability
- Surface protection measures
- The age of a slab or structure
The initial rate of carbonation largely depends on the porosity and the amount of moisture included in the material. Therefore, high water-to-cement ratios are more likely to increase the carbonation cycle when exposed to naturally occurring carbon dioxide.
How to Prevent Carbonation of Concrete
Because carbonation is something of a normal reaction when concrete elements come in contact with carbon dioxide, it may not be entirely preventable. However, there are widely adopted strategies to significantly reduce the process, improve concrete strength, and minimize workplace health and safety risks.
Lower water contents and extended concrete curing periods usually reduce carbonation and improve workplace safety. The use of protective coatings has also emerged as a proactive measure that may reduce the carbonation of concrete. But it’s crucial for workers to take appropriate safety measures regardless of concrete grades, low-water content, or when protective coatings are used.
Concrete Safety Tips and Best Practices
The release of carbon dioxide when pouring concrete slabs and floors in enclosed spaces presents a clear and present danger to workers. OSHA regulations require employers to limit CO2 exposure to no more than 5,000 ppm an hour averaged over an 8-hour shift. This time-weighted exposure limit also comes with a maximum threshold of 30,000 ppm over a 10-minute stint. According to OSHA regulations, 40,000 ppm is considered inherently dangerous.
That’s largely why the agency tasks employers with ensuring proper ventilation methods are observed during concrete pours, curing, finishing, coating, and demolition as well. The use of CO2 gas monitoring devices is widely recommended when enclosed concrete work is being performed and is advisable until the material has fully cured and been carbon protected with a coating.
Personal Protective Clothing when Working with Concrete
Construction sector outfits are tasked with providing workers and other personnel with respirators capable of filtering out carbon dioxide and other contaminants. Deploying the following personal protective clothing and equipment is also considered a standard safety practice.
- Comfortably fitting waterproof gloves
- Knee pads
- Splash-resistant coveralls
- Waterproof footwear and coverings
- Secure eye protection, such as goggles
- Head protections or hooded coveralls
It’s essential for employers to maintain a reasonable stockpile of these and other personal protective clothing and equipment to ensure workplace safety. Project supervisors are also tasked with ensuring the space offers adequate fresh air and crews do not conduct work without putting on protective wear first. Concrete not only creates a carbon dioxide hazard, but it also may possess chemicals that burn the skin or cause other health problems.