Every single house and business creates waste. This waste needs to be disposed of and treated. Generally in cities and towns there is a public waste system that channels the wastewater into treatment facilities. Concrete tanks are widely used to contain the wastewater during treatment. Coatings and Linings help extend the life of wastewater tanks and linings in Arizona.
Harsh Environmental Conditions
Cement is an exceptional material for construction. We can build just about anything out of concrete with the right mix. Man has been using cement for centuries because it is very strong in compression and very durable. Cement is made stronger with internal reinforcement. Steel rods are tied together in the concrete forms so when it dries it has a stronger metal “skeleton” inside of it. The addition of these rods, or rebar, increased the tensile strength of concrete. This advancement made it possible to build pipes and tanks from cement.
The only drawback to adding a metal skeleton to the concrete is the tendency of metal to corrode. Corroding metal inside concrete causes problems with the durability and structural integrity of the tank or pipe. Corroding steel rods in concrete cause what is called spalling, or cracking. This causes the surface of the concrete to crack open, and sometimes fall off completely. Any water getting into the concrete accelerates the degradation of the structure, and allows more water to get to the rebar, which causes even more spalling.
Wastewater tanks aren’t generally kept in climate controlled areas. This means in areas where winter gets cold there is a freeze and thaw cycle in the tank. Repeated expansion and contraction causes surface tension and stress which can crack the concrete. In addition the chemicals used to treat wastewater can be harsh and wear at the surface of the concrete. With debris and materials in the wastewater there is also a abrasive affect on the structure.
Variables Affecting Longevity
Given enough time all concrete deteriorates. The quality of the concrete is the first variable that dictates how long a structure will last. The second variable is the environment that structure is put in.
Quality Of Tank & Pipe Construction – It goes without saying that the better things are made, the longer they last. This is true for wastewater tanks and pipes like anything else. Concrete quality is a function of the properties of the original mix design. The water to cement ratio can dictate the durability, and the specific PSI rating of the concrete. Placement of rebar inside the concrete is also a major factor in the overall longevity of the structure. If it is placed too close to the surface the structure will degrade faster. Curing and covering after the pour also contributes to the overall quality of the structure.
Tank & Pipe Operating Conditions – Environment is the second factor in the overall longevity of a wastewater tank or pipe. Wastewater treatment plants provide especially harsh environment conditions for concrete. Chemical attack, freeze-thaw & wet-dry cycling, and abrasion are just a few of the environmental factors that wear on these concrete structures. The best concrete will eventually deteriorate over time in these conditions. Poor quality concrete will obviously degrade faster. Protection of the concrete surfaces is clearly the key in increasing the longevity of both good and substandard concrete.
Protective Concrete Coating & Linings
The harsh chemicals and abrasive qualities of waste water erode and corrode the concrete we use for our wastewater tanks. The primary way we can protect these structures is coatings and linings. Coatings and linings prevent with water from making direct contact with the concrete. Wastewater tanks have especially harsh conditions and care must be used in choosing which linings and coatings to use. Professional coating installation is also advantageous, as correct application is critical for the lining to be effective.
The Best Time To Protect Your Tank
Clearly having your concrete tanks coated before using them is going to give you the longest durability. This is before harsh chemicals come in contact with and begin to degrade the tanks. Acids, salt, and sulfates are common in concrete tanks and the only real barrier you have to extend the life of your tanks is to use a good coating.
Many of the tanks we use today were built in the 1970’s and 1980’s. This was a time where materials science was not as advanced as today, and these tanks are already into their 3 or 4th decade of use. Inspection of these tanks and proper maintenance will help avoid having to completely rebuild wastewater treatment plants. The spalls, cracks, and leaks are evidence of these older tanks decay. Upon discovery of spalling, cracks or leaks immediate action must be taken, because the harsh chemicals have already found a way into the concretes inner material.
Tank Rehabilitation Process
The first step in repairing tanks that are compromised is to correctly diagnose why the tanks are deteriorating. This is a critical step in the tank rehabilitation process as it avoids a never ending repair cycle. Concrete testing firms and consulting engineers typically are the experts consulted to do the inspections and evaluations of these concrete structures. With correct professional analysis an effective rehabilitation strategy can be developed. This ensures the root cause of the concrete tank deterioration is addressed.
Concrete Tank Corrosion Protection
Corrosion of the reinforcing steel rebar is one of the most common causes of tank deterioration. Corrosion will occur if it is exposed to oxygen and moisture. New concrete has a property called a passivating layer. This is due to the inherently high alkalinity of concrete. A Passivating layer provides a natural barrier that protects the rebar inside the concrete. This layer does not last as most of the water in wastewater applications has a measure of dissolved sodium. This sodium erodes the passivating layer and allows breaches in the surface of the concrete. Lower qualities of initial concrete have a greater permeation level and will allow water, oxygen and salts into the surface of the concrete much sooner. If the steel is allowed to rust it will expand and cause internal tensile forces on the concrete structure. These internal forces are what causes the cracks in the first place and allow the elements under the surface of the concrete.
Corrosion Inhibiting Sprays
Penetrating corrosion inhibitors is one of the strategies that can help slow down the decay of concrete wastewater tanks. These inhibitors can come in the form of a liquid or a powder. The definition of what elements qualify as an inhibitor is any element that effectively slows the corrosion of the reinforcing steel rebar. A liquid amino alcohol based corrosion inhibitor is used on existing concrete structures. It penetrates the surface of the concrete and is designed to make it all the way to the rebar. Once the chemical reaches the rebar it coats it, and creates a protective barrier. Independent laboratory testing has shown that this reduces corrosion by as much as 60-70%. It is also possible to start with this inhibitor already in the concrete mixture as it is poured. This is a great way to create a lasting structure.
Additional Environmental Issues
Apart from simple oxidation of the steel there are a number of other threats to concrete tanks. Some of these include the thaw-freeze cycle, chemical attack, and abrasion. Abrasion is simply caused by waterborne particulates. Things like silt, sand, dirt, and gravel are carried in the water and wear on the surfaces of the concrete. Chemical attacks on the concrete come in a couple of forms. A common form of chemical attack is simply when the pH is too low or acids are present in the water. Acidic water causes the cement matrix to dissolve loosening the aggregate in the concrete. Additionally the presence of sulfates can react to the tricalcium aluminate in the concrete. This causes the material to expand which causes internal stress and cracking or crumbling. In the cases of tanks that have frequent wet-dry cycles this affect is increased and more aggressive.
Protective Coatings For Water Tanks
Concrete tanks can be protected from these effects. This is done by preventing the contents of the tanks from ever making direct contact with the surface of the concrete. The most common way of doing this is protecting coatings. There are different requirements for wastewater to water tanks. When developing a protection plan it is critical to know the difference between the two as the chemicals present in either are vastly different.
Protective Water Tank Coating
For tanks that contain potable water a polymer-modified cementitious coating has been extensively used, and with great success. While a polymer-modified cementious coating is flexible it is also possible to be an extremely thick and dense protective coating. The flexible property of this coating has the benefit of effectively sealing hairline cracks without spending the man hours to find each of them. It is thick and flexible enough to shore up these issues and effectively provide a protective barrier for the tank. Styrene-acrylic and acrylic based polymers are the foundation for a flexible protective coating. This is what will help protect the concrete even if there is minor expansion behind the coating.
Protective Wastewater Tank Coating
Clearly in wastewater there is a lot more going on chemically. This means that the chemical resistance of these coatings is a lot more important. The only way that these coatings can be effective is to be specifically designed for the particular presence of various chemicals and concentrations of those chemicals in the wastewater tanks. Clearly specific laboratory analysis is the best way to determine each locations individual needs and requirements, yet it can be a costly expenditure. If the funds are not available coating companies can be consulted that have similar tanks and comparable chemical operating parameters.
Epoxy coatings are extremely common in wastewater tanks as they offer somewhat easy application and a higher degree of chemical resistance. One very durable option is bisphenol A epoxy. With the addition of a polyamine hardener this solution is fairly easy to apply and has shown great chemical resistance. Other options for coatings include urethanes, vinyl, and polyureas. These alternative coatings also offer high levels of chemical protection, but are not as easily applied.
Professional Coatings & Linings by All Kote Lining Inc.
All Kote is Arizona’s preferred lining and coating contractor for water tanks, pipes, and cooling tower coatings. If there is a project that our skills can be used on please feel free to contact us. We know the industries concrete tanks are used in, and also work on a variety of other materials to help extend longevity and prevent spills