A slab-on-grade foundation is typical for a variety of homes, garages, and buildings that do not have deep foundation walls. This would mean the slab on grade would be at ground level, and subsequently, there are also slabs on grade that are well below the foundation line. Concrete slabs that are not installed or poured on grade could be known as suspended slabs, cantilevered slabs, or even overhang balcony slabs.
The reinforcement(rebar) requirements between slabs-on-grade and suspended slabs(elevated) are significantly different. For instance a suspended slab will typically have two layers (top & bottom) for rebar, and might have closer O/C(on center) spacing than a slab-on-grade. Slab on grade foundation is installed on grade, as name implies, can be below main floor elevation or in a basement.
Water intrusion into concrete and pooling around foundations are concerns with concrete slab construction, so systems are typically utilized to avoid this.
Photo Above: Slab-on-grade construction and concrete foundation walls are typical for residential concrete work. If the building/home has below-grade basements, drainage from the property should be assessed.
Weeping Tile System – Concrete Foundations
When built with foundation walls, homes may be provided weeping tile systems to ensure the proper drainage of the surrounding rainwater accumulation. A weeping tile system is installed surrounding the foundation walls of the building, home, or property to pull rainwater away from the exterior footings and foundation walls of the building, either directing this runoff to a sump pump, or to the exterior at a designated area away from the home, somewhere off property or away from the building.
The role of the weeping tile system is to reduce hydrostatic pressure that can build up after heavy rainfall events.
Likewise, the utilization of crushed gravel, washed rock, and other free draining materials obviously do not absorb water like loose-fill soils and/or do not trap water like impermeable materials like clays. Granular soils may also drain water well but not to the extent to which washed rock materials or even a crushed gravel mixture will.
Photo Above: Concrete backfill to be completed for detailed excavation around pilasters and pile-caps for piles. Slab-on-grade may be poured on top, tying with these elements, such as grade beams too, with entire structure receiving engineered fill.
How Backfill Affects Building Drainage
Part of the reason you might find a slab-on-grade foundation saturated is the construction process utilized initially with the concrete structure. If the proper structural engineered fill was not utilized correctly for the placement of the slab-on-grade construction, this could lead to excess water pooling at below-grade elevations and increases in hydrostatic pressure surrounding the concrete foundations and below the slab-on-grade.
Backfill selection and weeping-tile systems are common for larger buildings, like multi-residential and commercial buildings.
In some scenarios where below-grade drainage measures are utilized, the reliance on proper free-draining materials is essential to avoid saturation of slab-on-grade concrete construction.
Photo Above: Backfill around structures like this can be draining-fill to work with a drainage system like a weeping-tile system.
Water can Penetrate into Concrete
Concrete is a porous material, so with time, as water builds up around concrete foundations, or below concrete structures, it can migrate into and through concrete structures. This is natural, and in some cases, water can even get trapped within the concrete. The utilization of air-entrained concrete mitigates some of the risks of water-logged concrete, avoiding damage during the freeze-thaw cycle for concrete structural elements. A concrete air-entrained mixture, once cured, effectively has enough pores to allow for water to migrate effectively, and when the freeze cycle occurs, water has the chance to escape, and not cause significant damage to the structural concrete structure via cracking or other means.
This is especially important with concrete foundations due to experiencing significant hydrostatic pressure.
Air entrainment can negatively affect the finish of an interior slab though.
Traditional Waterproofing Methods are not 100% Reliable
Waterproofing means and methods for traditional residential concrete construction may not be 100% effective in their capacity at completely stopping water from entering a concrete structural element(e.g. slab-on-grade). That means that the condition for hard rain to saturate a concrete slab is possible even if concrete structures are completely waterproofed.
Spray-applied waterproofing systems, roll-on waterproofing systems, or membrane-applied waterproofing systems can never be 100% effective all the time.
Sub-surface Conditions
Subsurface conditions below the footings and bottoms of foundation walls can also lead to the migration of property drainage underneath the slab. Excess pooling underneath a slab, as with conditions where the property drainage is not sufficient, can cause saturation of a slab on grade.
What about new concrete pours?
Likewise, hard rains are also a threat to new concrete pours. Curing concrete needs consistent temperatures and weather conditions to ensure a strong cure. While some methods for curing concrete include wet curing, hard rain is typically not factored into the process. Rather, the process consists of burlap, tarps, and light water application to ensure the slab is kept wet for the curing. This process can vary from slab to slab, with mix design and super-plasticizers playing a vital role in the overall curing period of concrete, as hard rain is a detriment to curing concrete if the concrete is not sheltered from the rain.
On-site / Property Drainage
The other factor here that you need to ensure is carried out properly, is on-site property drainage. Just as with the subsurface conditions, such as weeping tile systems, drainage on the surface conditions surrounding the building are vitally important with at-grade slab-on-grade construction. At-grade slab construction significantly relies upon the total depth of build-up of a compacted sub-base such as crushed gravel; and/or utilization of sub-slab insulation, but needs to be protected from erosion and/or pooling of waters surrounding the slab.
Building elements like roof slopes, flashings, downspouts, eavestroughs, and trench drains are vital to ensure that water moves away from a slab on grade construction.
This applies to foundation walls, subgrade slabs, basement slabs, and other structural concrete components. Proper drainage is vital to avoid water pooling near structural elements and prevent concrete saturation.
Crystalline Waterproofing
Other preventative measures that can be taken include crystalline waterproofing methods, which are actually admixtures provided within the concrete to help protect it from water intrusion. They act like a surface-applied waterproofing membrane but work through crystallization through the curing process, affecting the entire concrete structure rather than just the surface. The crystalizing is inherent in the concrete, penetrating deep into slabs and foundation walls.
Slabs can Saturate Over Time
Concrete slabs can become saturated if drainage is not sufficient, waterproofing fails, or other methods are not employed. This process can take some time for accumulated moisture/water to affect the slab. One rainfall may not be the issue, but long rainy seasons can be an issue.
Drainage methods should also include for proper material selection like the right weeping tile pipes, geotextile fabrics, and engineered fill, along with proper back-fill installation, compaction methods, and where needed, mechanical system integration with sump-pump and stormwater systems, mechanically designed to purge a building and home of excess water build-up.
If there is a sudden water issue in a basement or with a slab-on-grade following a hard rainfall chances are it is a mechanical issue, like a failed sump pump or drainage system. The result for concrete slab saturation would be a more long term consequence where-as sump pump or drainage blockage will show immediate water back-up and pooling issues.
Concrete Waterstops
Another method that could be used in mitigating water migration through concrete structural elements or potential water-logging are concrete waterstop joints. Due to the nature of concrete pours, often times concrete placement for foundation walls will require pour-breaks. A pour break can be a natural point to which water intrusion could occur. This is as the break in the concrete placement allows for a natural crack to be formed between the old and the new concrete.
As such, concrete waterstop inserts should be installed to further combat water intrusion into buildings, through concrete, and prevent excess seepage to a slab from hard rain from the surrounding building footprint. Concrete keyways are installed in concrete foundation walls to install waterstops. Vertical concrete stops and horizontal concrete stops are both important, as concrete pour breaks can be both vertical and horizontal in nature, depending on the height of the concrete foundation wall.
Trench Backfilling & other sub-slab conditions
Other factors that can be considered are the utilization of proper materials in backfilling mechanical/electrical trenching underneath slabs used to house the subsurface plumbing, pipes, and conduit. Alternatively, subslab insulation placed below the slab would also come into play, obviously creating a large barrier between the concrete itself and the actual ground.
Where radon mitigation is an issue that needs to be addressed, poly vapor barriers can be applied under rebar to further prevent gas migration. If they can prevent gas/air-vapor migration, they can also stop moisture.
Photo Above: This slab has a poly-barrier under the rebar mat.
Conclusion
Slab-on-grade concrete foundations can show saturation with water under hard rain conditions. Many preventative measures are used to avoid this and cracks from freeze-thaw, but if these measures are not utilized, saturation can occur. Old slabs can accumulate moisture and water over time naturally, but with best construction practices, some of the issues can be mitigated to a degree. The effects of hard rain on concrete slabs are not immediate, but over time, accumulated water is a concern in and around foundations and with the slab.
For all your concrete foundation repair and concrete foundation construction in the Calgary area, turn to Deluxe Concrete Services. They are experts in all manner of structural foundation concrete services for residential, multi-residential, and commercial construction projects. Their team of Calgary concrete placers will ensure a neat and structurally sound installation for all manner of structural concrete elements including slabs.
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