Moisture Intrusion into Concrete Slabs-on-Grade


Expertise Includes:

    • Building Damage Assessment
    • Construction Defect Evaluation
    • Building Foundation Issues
    • Building Envelope/Water Intrusion
    • Building Codes and Standards
    • Exterior Wall & Roofing Systems
    • Structural Design - Collapse/Failure Analysis

Scenario:  A young couple is excited about buying their first home.  They pick it out from a catalog of house plans from the developer of a new neighborhood in town.  It will be a brand new construction starter type bungalow home on a slab-on-grade foundation.  For a cost adder, which is considerable to them, they decide to upgrade the finished flooring from carpeting to hardwood laminate.  Unfortunately, after only a few weeks of residency, the hardwood planks begin to buckle and separate from the slab throughout the house.  They do not know what is causing it, or what to do to remedy it, so they hire a Structural Engineer to evaluate the problem.  After careful examination, the Engineer concludes that the flooring damage is being caused by moisture emanating from the slab-on-grade subflooring.  The culprit:  The omission or improper installation of a vapor barrier on the soil below the slab.  Without a proper vapor barrier, moisture from the soil below is soaking into and permeating through the entire slab, ultimately leaking out onto the top of the slab, and breaking the adhesive bond between the slab and the wood planking.  The only remedy the Engineer can think of to recommend to the homeowners to resolve the issue is to pull up all the hardwood laminate flooring, install a sealant or vapor barrier over the top of the slab, and then reinstall the floor planking using sleeper boards as nailers.

A bungalow with slab-on-grade foundation

As demonstrated in this scenario, moisture intrusion into reinforced concrete can cause a whole host of detrimental effects.  This is the first in a series of blogs in which I will discuss the consequences of moisture intrusion into reinforced concrete and masonry.  In this first blog, I will discuss slabs-on-grade (SOGs).  In a future blog, I will discuss methods to prevent moisture problems with slabs-on-grade and slabs-on-metal-deck (SOMDs).  In another future blog, moisture intrusion into structural slabs and other structural members will be discussed.  For the purposes of these articles, “concrete” will refer to both cast-in-place (CIP) or precast (PC) reinforced concrete (RC), or reinforced concrete masonry blockwork (CMU).  For information, the basic, standard ingredients in a concrete mixture are:  Portland cement, sand, crushed stone/gravel, water, and fly ash.  Various other additives such as water reducers, etc. are typically added to the mix.

Concrete is a porous material that is very brittle.  It is extremely strong in compression but has virtually no strength in tension.  Reinforcing steel bars “rebar” are relied upon to provide concrete with strength in shear and bending, and therefore tension.  Rebar consists of incrementally sized ribbed or “deformed” mild carbon steel bars, typically conforming to ASTM A615 or ASTM A706, and of various strength grades.

Due to its porosity, concrete permits moisture to soak in and permeate into its surface.  The moisture will then propagate or seep through the slab or member due to capillary action.  Because of its brittle nature, concrete is very susceptible to hairline or spider web cracking. This can occur during curing (thermal or drying shrinkage cracking), or in-service due to aging and weathering, or high bending and/or shear loading.  Hairline shrinkage cracks in CMU mortar joints are also very common.  Larger cracks can occur from causes such as impact, extreme expansion or compression.  “Spalling” or breakage of the concrete can occur due to the corrosion of the internal reinforcing steel.  It should also be noted that concrete or masonry exposed to moisture can promote the growth of mold and cause protective coatings/sealants/water repellants to blister or peel.  “Efflorescence”, or white crusty chloride residue on the surface of concrete caused by wetting and drying is also common.

Slabs-on-grade are slabs supported by the soil underneath them.  There are two basic types of slabs-on-grade:

  • Those subjected to relatively light loading such as the type which comprise the floors of houses, basements, commercial buildings, and garages.
  • (2) Structural slabs which support heavier loading such as airport runways, helipads, or industrial facilities and warehouses with high payload forktruck or heavy equipment traffic.

Floor slabs-on-grade are typically on the order of 4” to 6” thick and are normally reinforced with sheets of galvanized steel welded wire fabric (WWF) or fibermesh, in lieu of rebar.  Concrete sidewalks, driveways, and patios are very similar to floor slabs, but they are exposed to the weather and therefore do not have a vapor barrier, and they may have less thickness.  Moisture intrusion into slabs-on-grade can lead to corrosion of the WWF, which can lead to concrete spalling and/or rust stain “bleed out”.  See photograph below of a concrete slab with extremely rusted WWF.  Slabs-on-grade for structural applications will be discussed in a future blog.

Concrete slab with severe rust staining

Moisture intrusion into slabs-on-grade is typically caused by one or both of the following issues: (1) Rising moisture from the soil beneath; or (2) “Free water” in the concrete present after casting and during curing.  Moisture rising from the soil beneath the slab is a source of perpetual moisture exposure. Free water occurs within the concrete when there is an excess of water in the mix, beyond that which is required to hydrate the dry Portland cement particles and produce a workable mix.  Factors which can lead to moisture accumulation in slabs and the subsequent problems it causes include:

  • “Fast-track” construction schedules (which do not allow the free water to evaporate naturally)
  • Inadequate sub-slab moisture protection (vapor barriers)
  • Late mix changes
  • Wet construction sites
  • Inadequate water drainage off structures

In the next blog, I will discuss methods to prevent moisture problems with slabs-on-grade, as well as with slabs-on-metal-deck.

George Sanford, PE, holds a Bachelor of Science in Mechanical Engineering from North Carolina State University in Raleigh, North Carolina. George has more than 20 years of applied structural engineering experience specializing in residential, commercial, and industrial structures and foundations. Throughout his career, George has designed and analyzed structures, supervised engineers, prepared construction documents (drawings and specifications).  He has an in-depth knowledge of many building codes, standards, rules, and regulations including the agencies that govern and provide guidance to building designers such as the International Code Council (ICC) American Society of Civil Engineers (ASCE), Steel Joist Institute (SJI) and the American Iron and Steel Institute (AISI).

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