EOP – The Driving Force of Water
Escalating land costs, parking requirements, urban heat island effects, the desire to be “green” and various requirements to store runoff water on site have led many design professionals to place buildings below grade and cover parts of them with vegetation. These energy and environmental conservation techniques have brought a heightened awareness to the thermal and moisture protection envelope of a building. Coupled with these “green” concepts, the government is requiring tighter building envelopes to reduce air transport, thus reducing heating and cooling costs. Placement of air barriers and waterproofing layers need to be carefully integrated with insulation and vapor retarders to ensure good long term performance because condensation issues can occur when placement is incorrect.
Water is frequently referred to as the universal solvent. Given enough time, it breaks down most substances and man-made environments. Water enters buildings in either or both its liquid or gaseous state. Liquid water entry above or below grade is usually through envelope construction defects. Water vapor enters through building envelope defects in the air barrier and through planned areas, such as (doors) and unrealized openings, such as internal joints, cracks, gaps and breathable building materials such as concrete, masonry, or wood.
To complicate the demands on the design professional, buildings today are built faster, tend to be lighter, have less envelope layers, and are constructed with materials more sensitive to moisture absorption. The days of thick stone wall sections, multiple layers of brick, clay tile or masonry withes are generally past. Less structure, more insulation and an air/vapor barrier are common requirements for residential, commercial, institutional and other habitable and storage space requirements. These factors, in large part, contribute to making water leakage claims high on the list of frequent litigation issues by homeowners, condominium associations, governmental agencies, and building owners.
All below-grade spaces are potentially vulnerable to humidity, moisture, and water leakage problems originating from two sources: 1) rainfall, irrigation, and melting snow from the ground surface, and 2) the subsurface water table. Three sets of techniques or lines-of-defense can be used to alleviate below-grade water problems. First, surface drainage and roof water collection coupled with site drainage must be designed to direct water away from the building foundation.
Second, a subsurface drainage system can be used to collect and drain away water percolating down from the surface; and in some cases, to drain away groundwater when the water table rises to a level above the building foundation. Third, a waterproofing system can be applied to the foundation walls and floor, if necessary, to prevent moisture from penetrating into the building envelope.
The focus of this discussion is on the waterproofing system. This system may be a positive or negative side technique. Many types of waterproofing systems have been developed for the commercial building market, particularly for application on horizontal deck surfaces. In the light commercial and residential building market, waterproofing systems are not prevalent on slab-on-grade or crawl space foundations. They are, however, appropriate for basement walls and sometimes floors.
While water problems in commercial and residential basements are well-known, opinions differ about the source of the problems and the appropriate solutions. Building codes generally have not set high standards for subdrainage and waterproofing, in part because some degree of leakage in basements has been considered tolerable. It is also clear, however, that standard or industry accepted practices often fail to some degree. Although high-quality commercial-grade waterproofing systems can be effective in both commercial and residential applications, the guidelines for when to apply these systems are sometimes unclear.
There are many building material types used for basements and below grade foundations. If untreated with any waterproofing system, in terms inherent resistance to water intrusion, these methods would be rated as follows:
- Cast-in-place (formed) concrete (reinforced or post tensioned);
- Solid precast panels;
- Wet or dry mix shotcrete;
- Hollow core precast panels;
- Solid (core filled) masonry (reinforced);
- Solid core, insulated concrete panel systems (ICF’s);
- Unreinforced cast-in-place (formed) concrete;
- Hollow core precast panels;
- Open cell masonry walls (unreinforced);
- Waffle or grid screen ICF walls;
- Stacked stone with mortar joints;
- Rubble or stone mortar walls
- Steel panels, wood foundation, and other hybrid system
- Selecting and rating foundation wall systems is subjective; however, rating waterproofing techniques’ and systems is even more subjective.
Formed or cast-in-place concrete can be thought of as a breathable, watertight membrane. When mix design, crack control, and exceptional placement techniques are all achieved in the field, concrete can be watertight to liquids (i.e. water) and water vapor permeability very low. However, as simple as it may seem, this is difficult to achieve. Consequently, concrete structures may leak at cracks, joints, penetrations and at internal honeycombs. There are many techniques available to correct formed concrete leakage areas. These techniques generally consist of the following:
- sealers, coatings, mortars, reactive penetrates
- crack injections
- electro-osmotic pulse technology
- drainage via water management
- sheet overlayments.
Waterproofing consultants and most design professionals would generally agree that if economically feasible, the best way to repair a leaking structure is to remove the overburden and place a new positive side waterproofing system. The cost of positive side waterproofing is quite variable. First, many times it is not an option because adjacent property owners will not allow it. If allowed, positive side waterproofing programs can vary from $50 (US) per linear foot to $100 (US) per linear foot on a one-level foundation with minimal landscaping requirements; to $150 (US) per linear foot to $400 (US) per lineal foot if landscaping, hardscapes and building projections (decks) are present.
Negative side waterproofing techniques are very common if positive side systems are not economical. Interior coatings and water management (drainage) systems are popular options. These systems vary in cost greatly. Simple systems may cost $20 (US) per linear foot to $50 (US) per linear foot. Complex systems may cost $100 (US) per linear foot to $300 (US) per linear foot, depending upon interior conditions.
It is important to distinguish between water-management and waterproofing. Water-management systems allow external water to enter the structural unit; and then they drain or re-direct the water to a location (sump) for removal. Waterproofing systems or treatments seal the external water from entry, and do not collect water at drains, sumps, or with mechanical devices. There are many hybrid systems that do some of each. Water-management systems tend to focus on liquid water only, waterproofing techniques may solve both liquid and excessive vapor drive problems. Waterproofing systems tend to be more costly. Long term reliability of both water management and waterproofing techniques is consistent with cost, i.e. higher cost techniques tend to be more reliable.
A relatively new concept in cast-in-place concrete for liquid and gaseous water control is electro osomotic pulse (EOP). This system works by creating an electric field across the section of damp or wet concrete, via placement of a positive terminal on one side of the element and a negative terminal on the other. A low voltage electric field moves ions and water within the concrete micro pores from the positive side of the circuit to the oppositely charged negative terminals outside the structure.
EOP operates on the principal of a double charge layer that forms naturally in the siliceous capillary structure of concrete. Due to the makeup of the capillary network of pore walls, which carry a slight negative charge, cations (Ca++) are absorbed or attracted to the pore walls.
These adsorbed Ca++ ions line the pore walls and capillaries. Due to the slight polar charge on the water molecules, these weakly attach (or attract) themselves to the cations via weak hydrogen bonding.
Upon the application of an electric field, the cations are repelled from the positive charge at the positively charged terminal (the anode) because like charges repel. As a result, the weakly bonded water molecules are dragged along with the cations, resulting in the development of water flow from positive to negative terminals. This flow, therefore, results in a microstructure pore pressure that resists groundwater intrusion.
As the ions concentrate within the microstructure, on the wet side of the concrete element, osmotic pressure builds up within the pores and resists groundwater intrusion from the exterior. A negative side system becomes a positive side membrane.
While most of these waterproofing techniques are well documented, electro osmotic pulse (EOP) technology may need additional clarity. Review of the basic definitions helps explain the concept.
- Solvent – the dissolving medium.
- Solute – the substance being dissolved.
- Osmosis – the process of a solvent moving from the dilute solute concentration side through a semi-permeable membrane, to the side of a higher concentration of solute.
- Semi-permeable membrane – a membrane that allows small solvent molecules to pass back-and-forth, but does not allow larger solute molecules to pass.
- Cation – a net positive charged atom (ion).
- Anion – a net negative charged atom (ion).
- Anode – the positive electrode (terminal) that supplies current to the circuit and attracts anions.
- Cathode – the negative electrode (terminal) that receives current from the circuit and attracts cations.
- EOP, electro osmotic pulse technology is a process whereby the following events occur:
A power supply is connected to a damp or wet concrete element at an anode location (the interior surface where it leaks) and a cathode is placed out, beyond the anode surface of the concrete element (usually in the soil backfill).
The power supply produces 24 to 28 volts of pulsed, variable length, reversing amplitude, direct current at a repeated duration that creates a potential between the anode and cathode, i.e. an electric field. The power supply is programmed to limit amperage output based on concrete resistance to electric field generation.
The hardened cement paste (“the hard sponge as it is commonly referred to”) and aggregate in concrete has a net negative charge. This hard sponge absorbs water and positive particles in the micro-voids to help neutralize the internal negative charge imbalance. Ions of Ca++, Mg++, K+, Na+, (known as hardness in water), readily exist in the absorbed water.
Thus, water molecules form concentrated “clusters of water” around each individual cation+ in solution (solvated) and to a much lesser extent around the anionsˉ (unsolvated) in the micro-voids pore fluid. For each individual cation+, “tens” of water molecules closely orient and surround it (solvated). This orientation of the oxygen atoms towards the cation+ is like an ionic bond or attraction (hydrogen bond).
Cations+ with their clusters of attached water molecules in the center of the interconnecting macro-and micro-voids, and in addition, water molecules attracted to cations in the diffuse layers, near the pose wall surface, move in the direction of their oppositely charged electrodes. The cations+ and attached water molecules slowly move in the electric field and concentrate on the cathode side of the concrete element in the micro-voids.
This concentration of cations+ (an increase in molarity) causes additional osmotic flow towards the concentration cell. This creates very large osmotic pressure gradients within the outer third of the concrete elements microstructure. It is important not to oversupply voltage to the electric field. The voltage should only create enough electric field to concentrate the cations+.
The general osmotic pressure may have internal gauge pressures of 10 to 20 psi. Remember that 10 to 20 psi osmotic pressure can resist 20 to 40 feet of hydrostatic pressure.
At the cathode side of the concrete surface, amperage and voltage control must be maintained, so that electrically stimulated osmotic pressure limits discharges of some small fraction of cations+ with water molecules, beyond the concrete substrate. The concrete element thus dries out slightly. The relative humidity on the anode side of the concrete element is reduced and the cathode side is typically, unchanged. This osmotic pressure release counteracts hydrostatic pressure and creates a water repellant force that liquid water under hydrostatic pressure to re-enter. A negative side treatment creates a positive side membrane.
Further, EOP offers cathodic protection to the steel reinforcing bar; which must be connected to the power supply negative terminal. The power supply sends electrons to the steel (impressed current) and cathodically protects it.
EOP systems are state-of-the-art technology that can control liquid and gaseous water in residential, commercial and institutional structures. Costs of installed systems vary based on accessibility to leaking joints and cracks, embedded objects and the nature of leakage. EOP system costs vary from $175 (US) to upwards of $300 (US) per linear foot of anode placed. These systems use very little energy to operate (10 watts per 1000 linear feet of anode) and/or have design lives of more than 40 years.
In conclusion, anodes are placed in wet or at risk concrete elements (cracks, joints, and honeycombed concrete). Cathodes are placed out beyond the concrete element in the soil backfill or under the floor. An external power supply creates an electric field within the concrete. As the concrete dries, the concrete resistance increases and the power demand lessons. Remember that solid concrete structures work best for this technology.
A military base in the United States has several earth covered magazines (ECMs) used for storage of a wide variety of explosive ordnance. The magazines consist of a reinforced concrete floor with reinforced concrete head walls at each end. The side walls and ceiling consist of a reinforced knee wall approximately 15 inches high with a galvanized corrugated steel-panel arch bolted on top and at various points into the head walls.
Because of failures of the waterproofing membranes and french drains, large amounts of water was seeping through the concrete walls, floors, wall/ceiling joints and ceiling panel joints of the ECMs. Water intrusion through theses structures was corroding ammunition and equipment stored inside the magazines, and also starting to corrode the reinforcement steel embedded in the concrete floors and walls.
Conventional methods for preventing water intrusion were expensive, labor intensive, and time consuming with a high probability of failure once completed. They also failed to address the difficult problem of water intrusion through the bunker floor. Electro-Osmotic Pulse (EOP) systems were selected to safely apply low-voltage DC pulses from inside the concrete to create an electric shield against water intrusion. These transparent energy walls effectively sealed off the below-grade ECM’s. Extensive field and laboratory testing was performed to validate the system as a long-term solution. Then, EOP systems were installed in 11 magazines and proved to be the ideal solution and the repair is expected to last the life of the structure.