In the design of concrete structures, where concrete slabs and walls are used as the environmental barrier, water leakage is a concern. Monolithic concrete elements of compressive strength around 3000 psi, and as-thin-as one inch, are generally water-tight except at discontinuities.
Locations where concrete elements are not generally monolithic include:
- Temperature, shrinkage, and structural cracks
- Construction (cold), isolation and expansion joints
- Interfaces of (M/E/P), mechanical, electrical, and plumbing penetrations
- Honeycombs, and similar locations of poor consolidation
In well consolidated, normal weight concrete, high pH components within the cement/gel paste, accumulate around and react with iron on the steel surface to resist corrosion from actively occurring. This is called “passivation”. However, concrete cracking due to high stress and associated strain defects in the element allows leakage and lateral migration of water and effectively introduce all of the necessary components — oxygen, water, and an electrolyte — needed for uncontrolled corrosion of steel reinforcement. Groundwater leakage through concrete continues to bring water with dissolved oxygen and enhances the electrolytic environment; further saltwater exacerbates the problem dramatically.
When tasked with solving a below-grade water leakage problem, either around a building foundation or civil engineering structure, there are five classic positive side, exterior, and negative side, interior, approaches for consideration.
To help control leakage into a concrete element, it is important to understand the best water control solution is generally to provide a new, exterior membrane to a structure. However, initial cost, accessibility, public nuisance, or other circumstances, may dictate that such a solution is prohibitive. In such cases, other methods of water control must be explored.
When solving a water leakage problem around a building foundation or civil structure (tunnels, tanks, vaults, pipes, bridges and docks), approaches can be applied either to the exterior (positive side) or the interior (negative side).
Positive side solutions are in direct contact with hydrostatic water, and therefore resist water penetration at the interface, while negative side solutions are not in direct contact with the source of water. It is common to use a combination of solutions, or perhaps different solutions on various parts of a structure, to control water migration.
After a leakage problem has been diagnosed, five classic approaches to an acceptable resolution can be explored:
- Crack/Joint Routing, Caulking, and/or Dry-packing
- Crack/Joint Injection, Chemical Grouting
- Water Management & Drainage
- Coatings, Sealers, Reactants, Sheet Liners
- Electro-Osmotic Pulse (EOP) Technology
There is not always one right answer to the problem, and there are exceptions to any water control solution, so blended approaches are very common.
Crack/Joint Routing, Caulking, and/or Dry-packing
This technique involves an interior surface seal on the leaking concrete element. Water is controlled on the interior concrete element face; however, the void remains filled with water. A simplistic, first attempt is to paste over a leaking crack/joint with a high viscosity, elastomeric, compound. This paste should typically extend over the crack/joint one inch and the paste should be about 1/8-inch thick, sometimes it is reinforced with a scrim or mesh. On some occasions this may resolve the leakage problem long term, provided cracks are static; however, in most situations, this solution is generally short term in performance. Costs vary, $5.00/LF is common, plus mobilization.
A slightly more robust solution would be to grind-out or rout-out the crack in a “U” or “V” shape, and fill with a high viscosity, low modulus filler. The performance of this solution depends upon slot depth and width, use of bond breaker, and adhesion to concrete sidewalls. Slots ranging from 3/8 x 3/8 inches, up to 3/4 x 3/4 inches, are common. Fillers ranging from elastomeric (low modulus) sealants, to high modulus epoxies are used. Costs vary, $25.00/LF is common, plus mobilization.
Similar to using polymeric compounds in saw-cut or routed-out concrete slots, would be to fill the slot with a Portland cement-based compound/mortar. The most common technique is to use a Portland cement, chemically reactive, crystalline growth mortar. Similar to using high modulus, epoxy fillers, the crack needs to be relatively static. Routing out slots in concrete for crystalline mortar dry-packing generally requires a 1 x 1 inch slot, up to 1.5 x 1.5 slot; larger ones are also common. Costs vary; $45.00/LF is common, plus mobilization.
Crack/Joint Injection, Chemical Grouting
This technique, when properly performed, may fill the void within the leaking concrete element. Water and air are controlled from freely interacting at the steel reinforcing interface. There are three basic approaches for sealing water leaks at concrete wall/floor discontinuities (cracks/ joints and interfaces). These approaches are:
- Backside Injection (curtain grouting); whereby a grout material is deposited behind the wall/floor crack/joint, or into the earth backfill (permeation grouting or soil solidification). The grout material solidifies soil and plugs leaking point of water origin. This process can be done by drilling down alongside the wall from above, or directly through the wall or floor.
- Internal Injection (interception grouting); whereby a grout material is deposited into the internal concrete crack/joint at section mid-depth. The injected grout fills the crack/joint from mid-depth interior to both external faces.
- Surface Mounted Injection; similar to interception grouting, whereby a grout material is applied to the surface of the leaking crack/joint; or is partially routed and plugged with a water impedance material prior to pressure injection from one of the exterior faces.
Typical generic grout types for water control available in the industry include:
- Epoxies
- Bentonites
- Hydrophobic Urethanes
- Hydrophilic Urethanes
- Two-Component Structural Urethanes,
- Acrylamides
- Acrylic Resins
- Rubberized Mastic or Gels
- Micro-Fine/Ultra-fine Portland cement
- Type I-or-III Portland cement
All these injection techniques involve placement of a reactive or non-reactive resinous polymer, reactive solution, and/or reactive/non-reactive gel or paste into a crack/joint zone or behind it. Most chemical grouting within the crack/joint zone involves placement of a low viscosity, reactive, epoxy, urethane, or acrylic-based polymer.
These reactive polymers fill the cracked/jointed section and theoretically plug the void. Grout viscosity must be properly considered in conjunction with crack/joint width. Further, they may bond to the crack/joint sidewalls in various degrees, depending upon crack/joint cleanliness. These various reactive polymers exhibit different degrees of elongation. Other considerations include: adjacent crack/joint widths, movement, orientation, age, and sequence.The cost of injection grouting varies considerably based on mobilization and access, quantity of work, security requirements, time duration per shift, and material used.
Current rough cost ranges may be as follows:
- Backside Injection (curtain grouting); variable cost range = $35.00/SF to $100.00/SF
- Internal Injection (interception grouting); variable cost range = $45.00/LF to $450.00/LF
- Surface Mounted Injection; variable cost range = $20.00/LF to $70.00/LF
Water Management and Drainage
This concept is not considered a waterproofing solution by definition, it is a water control technique; it continues to allow flow from exterior to interior. It can be very simple to complex.
Simple water management solutions may involve cutting a slot/opening into a floor slab, adjacent to footing or wall leakage area(s), drilling relief holes into the composite wall structure, and channeling captured water to a nearby drain. Slot/opening depths and widths vary based on water flow rates and technique used. Varying costs from $20.00/LF to $50.00/LF plus mobilization is common.
A more comprehensive solution may involve cutting/chipping out the floor slab, drilling relief holes into the composite wall structure, and excavation of earth fill adjacent to the footing. After the concrete debris and existing soil is removed, installing flexible or rigid, perforated drain pipe, or a manufactured drainage mechanism, that collects water and channels it to a sump collection area. The depth and width of excavation, pipe slope, use of filter fabric, and type of granular fills vary. Costs range from $20.00/LF to $100.00/LF plus mobilization.
The most complete water management system involves placement of a drainage media completely up the wall and/or over the floor slab to move water from the leakage area to a collection point. Collected water is either pumped to the surface and/or gravity drained. Water management materials usually consist of some type of deformed plastic sheeting, attached to wet floor and/or wall substrates that collect, channels and deposits water to centralized locations. Concrete, masonry, or gypsum sheeting, often cover these systems for aesthetics and fire protection reasons. Costs vary and range from $75.00/LF to $400.00/LF plus mobilization.
Coatings, Sealers, Mortar Reactants
This process involves placement of a bonded layer or chemically reactive mortar (crystalline growth) agent, on the concrete interior surface experiencing water leakage (negative side treatment). All coatings and reactive agents require the substrate to be free of permeating water prior to installation. Thus, chemical grouting or crack/joint routing, caulking, and/or dry-packing may be first performed to alleviant active water leakage prior to coating or reactive mortar agent installation.
Interior coatings generally come from the same generic family of polymers as used for pressure injection (i.e., epoxy, urethane, acrylics and/or latex). Interior coatings must resist both the effects of negative side water and water-vapor pressure. These coatings can be “breathable” (vapor permeable) or “non-breathable” (vapor retarders/barriers).
Interior coatings depend upon adhesion to the concrete substrate exceeding any liquid and/or water vapor pressures. Further, they must be flexible to accommodate substrate movement. Costs vary and range from $5.00/SF to $20.00/SF plus mobilization.
Some coatings/treatments may be chemical reactive agents with the existing concrete.
These surface reactive treatments applied to a damp concrete substrate can be formulated to react with un-hydrated cement, free calcium oxide (CaO) and calcium hydroxide (Ca(OH)2), creating non-soluble, silicate based crystal growth in pore spaces and microvoids within the concrete gel matrix. These crystal growth by-products can plug micro voids and pores, and very fine cracks in the concrete matrix, creating a water-resistant barrier. Costs vary and range from $4.00/SF to $12.00/SF plus mobilization.
Electro-Osmotic Pulse (EOP) Technology
This technology is an electrical solution for drying out concrete, and capable of protecting reinforcing steel in concrete. EOP systems fundamentally consist of a power supply and two oppositely charged electrodes. The power supply charges an anode (+) terminal at one end of a concrete element, and a cathode (-) terminal at the other end. Low voltage (24 to 28 volts) output from the power supply across the terminals is created through the concrete element via water containing micro-voids. Current flows from cathode to anode and electrons flow from anode to cathode. EOP installations are designed to create low intensity, electric field(s) within wet concrete element(s) and adjacent soil.
Fundamentally, the design places oppositely charged electrodes on each side of a wet concrete element or area. If the concrete element and adjacent soil is conductive (wet), and the micro-voids are partially filled with water, ions can move from electrode terminal to electrode terminal because an electric field was created in the damp concrete between terminals. Over time, water moves from anode to cathode and dries the concrete out.
Wet concrete is more conductive than dry concrete, thus high internal moisture content concrete is more conductive than dry concrete. Lower strength concrete is usually more conductive as well. The highest electrical conductivity is at water leakage locations because the concrete micro-structure is close to saturation.
EOP system design depends on many factors. Placement of anodes is either in a grid system or at wet locations where defects in the concrete element exist (at or near cracks and joints). Cathodes are spaced in a manner whereby the electric fields are relatively uniform across the concrete element. Because this is an electrical solution, the designer needs to ensure that outside electric field(s) will not interfere with the EOP low voltage system and otherwise. The cost of EOP installations varies considerably, however, a range of $400.00/LF to $1,200.00/LF of wall length is common. Slabs may be in the range of $25.00/SF to $80.00/SF.
Conclusions
Facility owners often approach water leakage problems in a systematic way. They tend to explore solutions on a lowest-to-most-costly basis; coupled by spot location repair rather than broad-based scope repair. Engineers and contractors, with input from a facility owner, generally will take a similar approach.
Consider further, that each water control approach can use a vast array of products with material properties that may seem similar, however, can have quite different results. Be mindful that the contractor’s experience may be more important than the design approach selected.
When engineers and consultants are involved with the water control process, they typically want to determine leakage root cause prior to design. A systematic engineering approach for identifying and solving moisture problems could be outlined as follows:
- Gather history of problem
- Understand the existing Design
- Search for the Leak Path(s)
- Perform Proper Test(s)
- Discover the Root Cause
- Determine Solution and Approach
After the water leakage problem root cause is determined, the engineer, consultant, and/or contractor must match a cost-effective solution with the budgetary criteria that provides the longest service life. This requirement involves considerable experience and knowledge about the concrete structure, materials and tradesmen capabilities.