Causes of concrete deterioration in wastewater facilities

water treatment

Causes of concrete deterioration in wastewater facilities

Despite the well-known hazards to concrete structures posed at wastewater facilities, concrete continues to be the most popular material used to build them.  To understand how to best protect concrete at these facilities, it is important to understand how wastewater can impact exposed concrete structures at these sites.

Understanding concrete in wastewater facilities

Concrete is a strong alkaline product. While it is externally dense, it contains many pores through a channel of capillaries formed from the hydration phase of concrete hardening. These capillaries are the main cause of concrete permeability and act as channels for chemical deterioration.

A common reaction that results from unprotected concrete being exposed to wastewater is the breakdown of the cement paste that holds aggregate in place. This happens when the chemicals from wastewater enter the capillaries in the concrete structure. Another common form of concrete deterioration in wastewater facilities occurs when the chemicals found in wastewater react with the concrete to reduce its pH, which can result in a weakening or breakdown of the adherent sand matrix at the surface of concrete.

This explains why the concrete industry has focused on the mix design of concrete to reduce corrosion risk. A proper mix design can reduce the number of capillaries found in the concrete structure. That reduction leads to a decrease in the amount of moisture that enters the concrete, which is the main villain involved in concrete deterioration.

While a good concrete mix design can reduce the porosity of a concrete structure, applying high-performance protective coatings is a more effective approach to ensuring a longer service life for a wastewater facility.

Causes of concrete deterioration

When determining how to protect the concrete in a wastewater facility, you must first consider the corrosive environment the wastewater creates. The most common corrosive exposures in a wastewater facility include, but are not limited to, chemical attack, abrasion erosion, chloride-induced corrosion and freeze-thaw conditions.

Chemical attacks on concrete – Chemical attacks can occur both naturally and synthetically in wastewater facilities. A synthetic chemical attack happens when manufactured acids are mixed with the wastewater, lowering the pH and initiating an acid attack.

A natural chemical attack commonly found in wastewater facilities includes sewage that forms a layer of sludge. The sludge contains sulfate-reducing bacteria known as SRB. The bacteria react with oxygen in the sulfate ions in sewage to form sulfide ions. The sulfide ions will then react with hydrogen to form hydrogen sulfide, which will create a hydrogen sulfide gas that reduces the pH of concrete.

Once the pH of concrete is reduced from roughly 12 (which is normal) to around 9.5, sulfuric acid can be formed that attacks Portland cement concrete. In addition to the atmosphere in wastewater facilities harvesting or creating an abundance of moisture and oxygen, those elements combine with the lower pH to create sulfur oxidizing bacteria (SOB) that colonize on the concrete substrate. The bacteria use the oxygen- and hydrogen-rich atmosphere to again form sulfuric acid that attacks the concrete.

Abrasion erosion – Wastewater can contain foreign materials such as sand, rocks, silt, or ice. These materials impact the substrate during unstable water flow conditions. This repetitive contact causes the concrete to break down and produces a smooth wear pattern on the substrate.

Chloride-induced corrosion – Chloride-induced corrosion can happen in any environment, not just wastewater facilities. Moisture often enters the pores of a concrete surface and penetrates the protective film of the steel reinforcement (rebar). The chloride ions found in the moisture can break down the protective film and cause the steel to rust. The buildup of rust causes the concrete to expand, potentially leading to cracking or spalling.

Carbonation – Carbonation occurs naturally in all concrete exposed to the atmosphere. It involves the reaction of atmospheric carbon dioxide with the hydrated components of Portland cement paste, especially carbon dioxide. The reaction of the carbon dioxide with concrete gradually reduces the cement paste’s pH. As the pH is reduced, the rebar no longer has alkaline protection and begins to corrode.

Freeze-thaw deterioration – Regulating thermal conditions in a wastewater facility is nearly impossible. Concrete will deteriorate over time when exposed to varying thermal conditions and humidity cycles on opposite sides of the structure. Irregular water immersion along with freezing temperatures can result in concrete deterioration.

It’s clear that concrete structures at wastewater facilities need to be protected. While choosing concretes with improved mix designs have a role to play in the protection of these assets, protective coatings are critical. For an introduction to coating systems well-suited for service at wastewater facilities see “Protective coatings commonly used in wastewater facilities.”