Electro-Chemical Dewatering System
For building owners and managers, one of the biggest issues is a damp or moldy basement area resulting from water intrusion through cracked concrete slabs and foundation walls. This is evidenced by the fact that structural engineers are often asked by architects and building owners about the cost to design crack-free concrete structures. Those with experience in concrete repair have found that structural and concrete mix designs must be coupled with exceptional field curing and placement practices in order to achieve “crack-free” or “watertight” concrete.
Structural engineers often state that concrete has two very important and distinctive features: it gets hard and it cracks. One cannot place enough steel in concrete to stop random cracking. However, concrete repair specialists who remediate water leakage in cracked concrete profess that it is better and more reliable to place a positive side membrane, rather than negative-side crack filler. If cost was not an issue, the most reliable approach for long-term water tightness in a leaking concrete structure is to place a new positive side membrane to ensure against future cracks and subsequent leakage. Most agree that keeping oxygen, water and potentially damaging ions at the concrete crack or steel reinforcing bar interface would be preferable.
The technology exists today for concrete to be crack-free, however there is a catch: cost must not be an issue, temperature variation should not be greater than approximately 20-degrees Fahrenheit from concrete placement time until the concrete is in service, and the concrete structure must stay at a constant service temperature. Obviously, these factors complicate concrete performance. Concrete in excess of 3,500 psi compressive strength is considered watertight except at joints, cracks, tie-holes and honeycombs. Advancements in both powdered and liquid admixtures have helped reduce the shrinkage effects of water, and improved flow and placement characteristics. When low water-to-cement ratio mix designs are used in heavily reinforced sections, coupled with complex forming configurations, the probability of subsequent cracks is almost certain. Even when the most recent ACI code structural design parameters are followed, state-of-the-art admixtures are used, coupled with best practice concrete placement and curing techniques, cracks can still be probable.
A New, But Proven Solution
In the search for remediation techniques, a relatively new approach has been developed by the U.S. Army Corps of Engineers with funding from the U.S. Department of Defense. This technique involves electro-chemical dewatering that not only stops water leakage through concrete cracks, but reduces the humidity within the concrete as well.
Electro-Osmotic Pulse (EOP) is based on the principles of electro-osmosis that were first published by physicist F.F. Reuss in 1809. Back then, Reuss discovered that by applying an electric current, he could move water uphill through a porous outlet. In the modern version of electro-osmosis, EOP provides pulses of electricity to effectively reverse the flow of moisture intrusion, driving it out of concrete and masonry structures affected by infiltration. In essence, the entire treated surface of the material becomes an electro-chemical and osmotic barrier that prevents water from entering a structure. Ancillary benefits of keeping the concrete free of moisture include minimizing corrosion of the reinforcement and eliminating the humid environments that can lead to fungi growth, mold, mildew and noxious odors effecting air quality.
EOP waterproofing technology can be considered a viable solution for a variety of structures including parking facilities, hospitals, residential buildings, commercial, libraries, museums, industrial plants, stadiums, storage facilities, tunnels, roadways and more. Essentially, EOP can be used in any structure where leakage through concrete is unacceptable and other physical barrier techniques such as coatings, penetrating sealers or waterproofing membranes have proven ineffective. In fact, where moisture proofing is critical, such as in military installations, or where moisture penetration leads to unhealthy environments within a structure, EOP has proven to be the most reliable and fail-proof option in preventing moisture intrusion.
How EOP Works
An EOP system is fairly simple and basic. Each system uses only three components: anodes, cathodes and controllers to provide a reliable moisture proofing solution. For a typical EOP installation, anodes, such as mixed metal oxide (MMO) on titanium substrates of various geometries, are strategically placed along the inside face of the concrete or masonry structure that is being treated. Cathodes, made of copper-coated rods, are placed through the floor or wall outside and below the edge of the foundation. A controller or power supply, located in a convenient, accessible location, applies a safe, low-voltage DC electric field between the anodes and cathodes, causing the anodes to develop a positive charge and the cathodes a negative charge. This generates a flow of charged ions, or broken molecules in the fluid that is within the structure to flow towards the anodes and cathodes (positively charged ions flow towards the cathodes and negatively charged ions flow towards the anodes.) This net flow, further enhanced by pulsing the power supply, drags water out of the structure and prevents it from re-entering. Implanted within the structure, reference cells and probes continuously monitor moisture levels, temperature, voltage, current and rebar corrosion potentials, as well as regulate the operation of the EOP system for optimum performance.
EOP systems can be designed with varying degrees of redundancy depending on the criticality of the application. Redundant anodes and cathodes can be specified to ensure a fail-safe installation. Connections are welded and encased in epoxy, making it difficult for any air or water to enter into the connection and damage it over time. In case of a power outage, solar-powered or battery back-up equipment can be incorporated into the EOP design to keep the system up and running. The entire installation can be monitored constantly through internet communications which provides owners real-time data for remote system management.
With roots in the United States military, EOP seeks a failsafe dewatering and humidity control system for mission-critical defense structures, such as missile silos and launch facilities. Since 1992, the Department of Defense (DOD) has invested significantly into EOP research and development. Currently, the technology has evolved to a state where it has become a standard for mission critical applications. Continuing research by the Army Corps of Engineers and others is exploring applications where EOP technology may prove beneficial for related issues such as Alkali Silica Reaction (ASR) and other property improvements of concrete structures.
Other EOP Benefits
EOP offers further benefits not found in other waterproofing systems. A by-product of the application of EOP to a reinforced concrete structure is that cathodic protection is provided to steel rebar and other embedded metals within the concrete. The circuitry used for EOP is the same used for cathodic protection. A well engineered EOP design will incorporate connections to the reinforcing steel to optimize operation of the EOP system. This, in turn, makes the steel electrically continuous with the cathodes, resulting in corrosion protection of the steel, prevention of damage and premature structural degradation. The system also prevents corrosive agents, such as sulphates and salt water, from penetrating the concrete and causing deterioration.
A structure experiencing wetness and water leakage problems is probably also experiencing excess humidity, musty odors and mold build-up, factors generally categorized as indoor air quality (IAQ) issues. These problems are not only unsightly and damaging to a structure, but they can result in increased healthcare costs for occupants, decreased worker productivity, and possible litigation issues. EOP is known to reduce relative humidity within the concrete or masonry to levels that do not sustain growth of mold, mildew and other contaminants that can affect the environmental quality within a structure. Concrete’s relative humidity below 55 percent has been measured on structures being treated with EOP. Controlling humidity is important for applications where humidity can corrode equipment, such as computer and data centers. For the military, controlling humidity in applications such as underground launch facilities, where high humidity can be unsafe and can corrode critical equipment, is essential.
After an EOP system is installed, it is possible that new cracks can develop in the concrete due to the drying effect, also known as shrinkage. This can occur as soil settles around a structure and as the concrete dries out. With conventional waterproofing systems, the entire system has to be replaced in order to find the source of the leak or cracks have to be injected repeatedly with filler. When EOP is installed on a structure that develops new cracks caused by joint movements, drying, seismic events or settling, repairing the cracks topically from the negative side reinstates the waterproofing properties originally established with EOP.
Considering EOP as a Waterproofing Solution
It is important for structural engineers to understand the pros and cons of both positive and negative-side waterproofing systems, as well as how EOP can be compared as an option based on the structure and repair situation.
Conventional negative-side systems have two principal disadvantages compared to positive-side applications. The first is that negative-side systems allow groundwater to enter the concrete substrate. This could cause corrosion of reinforcing steel, accelerated deterioration of the substrate itself, and even growth of unhealthy organic substances. The second disadvantage of negative-side systems, particularly cementitious coating and grouts, is a relatively short service life. While a negative-side applied system, EOP does not allow groundwater to enter the concrete substrate and the components of the system are designed for unlimited operation with periodic preventive maintenance.
Positive-side systems do prevent ground water to enter concrete and normally have a longer service life than most negative-side systems, but they also have two disadvantages. The first disadvantage deals with inaccessibility of exterior walls due to zero-lot line construction, high water tables, excessive depth, or hardscape. The second involves higher relative cost due to excavation, shoring and replacement of the soil and hardscape (when access is not an issue.) EOP provides the same benefits of most positive-side systems without the need for excavation and hardscape removal as it is applied on the interior side of the structure and is almost completely unaffected by exterior conditions.
A structure may be a good candidate for EOP when there is low leak-risk tolerance, indoor relative humidity must be controlled and there needs to be a high sensitivity to organic growth. Other factors that make EOP a viable solution include a scenario in which the exterior wall/foundation is not economically accessible, the structure is threatened by corrosive soil chemicals or salt water, negative- and positive-side systems have been previously tried without success, or the building owner is looking for a permanent solution to water intrusion.
Case studies using EOP
U.S. Treasury Building, Washington, D.C.
The vaults in the Treasury Building were so degraded from wet, moldy conditions that they became unusable. Dangerous mold, which had accumulated under the tile floor, made it impossible to use the rooms to store valuable documents and the issue also posed a health problem for personnel. Instead of using a solid, copper rod cathode, the Corps of Engineers decided to use a perforated, copper tube that they installed about a-foot-and-a-half above the vaulted ceiling. The positively-charged anodes drove the water up the perforated tube, where the water drained to a sump pump. With the installation of the EOP system, the vaults are now completely dry and watertight enough to be used as computer rooms.
Lock & Dam No. 27, Alton, Illinois
Another example is locks, which are used to transfer ships and boats from high water to low water and vice versa by filling and emptying a series of locks. However, there are places in a lock’s structure where water is not acceptable – as was the case for this 1,200-foot lock in Alton, Ill.. The lock operates by electricity which is housed within galleys located 60-feet under the water’s surface.
Water was seeping into the galleys, causing a hazardous condition for maintenance workers who had to walk along the galleys when they needed to repair the lock’s pumps. Because the walls and floors were wet, if a worker accidentally touched one of the electric cables running along the wall, they would be electrocuted. After the EOP waterproofing system was installed, the galleys are completely dry and water seepage is no longer a problem, making it safe for the maintenance workers to do their job.
Metro Tunnel, Washington, D.C.
A transportation system, especially in a bustling environment such as Washington, D.C., is expected to operate with no unscheduled stoppages. Unfortunately, the Metro system was plagued by continuous outages due to water that was coming into the substation, flowing into the counterspace and shorting-out the equipment. To stop the water problems, ElectroTech, along with the Corps of Engineers, installed anodes on the inner surface of the tunnel, and located the cathodes along the backfill. Some of the cathodes were actually embedded in the bedrock. Further complicating the project was the fact that the contractor could only work two hours a night, when the transportation system was not operating. After the EOP waterproofing system installation, the water leakage problems were solved and the tunnel is now dry.