TECH BULLETIN: Cracking Of Concrete Slabs On Grade (March 2017)


Most modern Florida homes are constructed with concrete slabs on compacted soil commonly known as a slab on grade. The concrete floor slabs are typically 4” thick with welded wire fabric for crack control. In many homes, which have ceramic, porcelain or natural stone floor tile installed over a concrete slab on grade, cracking of the floor tile is a concern. Typically, cracks in floor treatments caused by cracks in the slab below are relatively straight, cross grout lines and the tiles are bonded to the concrete floor slab pictured to the right.

Causes of Cracking of Concrete Slabs

Cracking of concrete slabs on grade is typically caused by restraint of movement brought about by shrinkage, thermal contraction and expansion. The outer portion of the slab restrains the middle portion of the slab. Structural members, such as, columns, footings, walls and other slabs, also restrain slabs. Cracks due to settlement may also occur.

These types of cracks are often the result of improper construction practices, such as:

  • The lack of control joints and or joints too far apart or nonexistent.
  • Isolation joints not provided around columns and walls.
  • Concrete is of a low strength mix with too little cement, too much water, or both.
  • Inadequate or lack of moist curing, especially in hot weather.
  • Omission of control joints at re-entrant corners.
  • Control joints not tooled to a depth of 1/4 of the slab thickness or greater.
  • Inadequate slab thickness.
  • Inadequate compaction and or preparation of the subgrade.

The Portland Concrete Association, Concrete Floors on Ground, which provides specifications and design guidelines for concrete slab on grade, recommends that control joints be provided every 10 to 15 feet. The actual spacing of the control joints is primarily a function of the layout of the room, slab thickness, size of aggregate in the mix, and the concrete slump.

Control joints (a.k.a. contraction joints) do not prevent the concrete slab from cracking; however, when properly spaced and constructed the slab will crack at the joint with minimal cracking at other locations. Control joints are not typically placed in residential interior floor slabs and for this reason this office has found many cases of shrinkage cracks in Florida homes.

Reinforcing steel or wire mesh, which is typically placed in concrete slabs on grade, does not prevent cracking but serves to control the crack widths by keeping the crack edges close together. Additionally, it is our experience that the wire mesh often ends up at the bottom of the slab instead of in the middle or upper middle where it can serve its purpose. Mesh in the bottom of the slab is useless for crack control.

Wider cracks are typically caused by settlement, both long and short term. Both short and long-term settlement occurs for a combination of reasons related to the preparation and compaction of the soil, which may include the presence of organic materials, the presence of debris, the natural raising and lowering of the ground water table or the compaction of soil from an accumulation of water runoff over the years.


In summary, it is concrete’s nature to crack and cracking cannot be totally prevented. However, with proper construction procedures, adequate jointing, and the use of quality materials cracking can be minimized and controlled.


TECH BULLETIN: Swimming Pool Failure (June 2013)


Hydrostatic forces from elevated ground water have the potential to cause an in-ground swimming pool to pop out of the ground and create significant damage. When soils are saturated with water, in-ground pools are susceptible to float out of the ground after they are fully or partially emptied of water. This is because of the potential for the hydrostatic force of water pushing from below to overcome the weight of the buried pool and other restraining forces, such as friction between the pool walls and the ground. If the hydrostatic uplift force exceeds the restraining forces, the pool will move upward, causing damage. This is commonly called “pool popping”, which happens every year somewhere in Florida, mostly during the rainy season.


When an object is submerged in a liquid, the upward or buoyancy force on the object is equal to the weight of the liquid displaced by the object. The buoyancy force is opposed by the total weight of the object. The same principle applies to a swimming pool built in soil saturated with water. Under normal conditions, the pool is filled with water. The weight of the water, weight of the pool, and the restraining effects, such as friction of the soil on the pool walls, counteract the buoyancy force on the pool, and the pool remains in place. However, if the pool is empty or even partially empty when the surrounding soil is saturated, the hydrostatic uplift force can cause the pool to float or “pop” out of the ground. This usually results in significant cracks and other damage to the pool and the surrounding components, which may include concrete walkways, wood decks, supply and drain lines, lighting and other finishes. Pool pop failures are often more severe at the deep end of the pool.

Fiberglass Pools Shells

Fiberglass shells are susceptible to popping from hydrostatic uplift due to their relatively low weight. When the pool is full, water in the pool opposes the lateral forces of the soil and water surrounding the pool shell. When the pool is emptied, this opposing hydrostatic force is not present, and the weight of soil and water outside the shell may cause the side walls of a fiberglass or vinyl pool liner to deflect inward. Concrete pool walls are typically 4” to 6” thick and reinforced with steel. This is obviously a much heavier structure than a fiberglass pool. However, concrete pools are still susceptible to significant upward hydrostatic thrust. This office has inspected many hydrostatic failures of concrete pools. Lateral deflection of the side walls of concrete pools is not common due to the strength and stiffness of the reinforced concrete walls. Hydrostatic valves and drain pipes are sometimes used to help prevent a hydrostatic failure. A hydrostatic relief or check valve is often placed in the main pool drain line. The purpose of this valve is to equalize the pressure between the water beneath the pool and the water at the bottom of the pool. Should the water pressure beneath the pool substantially exceed the water pressure at the bottom of the pool, the valve is designed to open, allowing water beneath the pool to flow into the pool bottom. A murky or dark cloud near the pool drain may indicate recent relief valve activity. Some problems with these valves include the valve being stuck open or failure of the valve to open to relieve high water pressure beneath the pool. In the case of the latter, the high water pressure may cause the pool to pop.

Well Points

Well points are sometimes used for groundwater control. These consist of a plumbing pipe installed in cohesionless soil (sand) or gravel beneath or beside the pool shell. The well point is used to draw ground water out from beneath the pool before it is emptied, reducing the potential hydrostatic uplift pressure to prevent the pool from popping.


Hydrostatic forces may cause in-ground pools to float or pop out of the ground and cause damage to the shell and surrounding components. A pool that has “popped” out of the ground from hydrostatic failure should not be refilled with water. If this is done the pool will likely crack further and or crack across the bottom. If this occurs the potential for salvaging the pool will likely be lost. A pool that is only slightly popped and not otherwise significantly damaged may be salvageable by cutting the top walls, pressure grouting under the pool to fill voids, and re-backfilling around the pool. An experienced pool maintenance contractor should be consulted before emptying an in-ground swimming pool. A Professional Engineer, structural discipline, should be consulted if a problem occurs and a pool “pops” out of the ground before further action is taken in an attempt to correct the structural problem. The pool should not be refilled with water until this is done.

Materials Uncategorized

TECH BULLETIN: Moisture Problems In Floors (January 2013)


Moisture damage is a common problem to flooring and floor structures in Florida. Vapor drive through concrete slabs often has an adverse effect on interior wood flooring. Inadequate crawlspace ventilation may lead to damage to floor structures beneath a home.

Concrete Floor Slabs on Grade

Most Florida homes are constructed with concrete slabs on backfilled soil which is also known as a slab on grade. The concrete floor slabs are typically 4″ thick with welded wire fabric for crack control. In most new homes and other buildings, the concrete floor slab on grade is placed over plastic vapor retarder. However in older buildings, this vapor retarder may not exist or is improperly placed.

In the absence of a properly installed vapor retarder, the finish flooring may be subject to moisture accumulation and moisture damage as a result of vapor drive. Vapor drive is the diffusion of moisture in the form of a gas or vapor from the soil beneath the slab through the concrete slab. If unimpeded, this moisture vapor is dispersed to the interior of the home where it is removed by the AC system. However, if an impermeable finish floor is installed on the concrete slab, the floor finish will act as a vapor retarder and trap the moisture within the floor system.

A common problem exists with the installation of modern hardwood or engineered wood flooring over older concrete slabs on grade. Modern wood flooring typically has a polyurethane finish for protection and wear, but this coating has a low permeability and can act as a vapor retarder at the top surface of the wood flooring. The polyurethane finish traps moisture from vapor drive within the wood, which can cause moisture damage to the wood flooring over a relatively short period of time.

Because of vapor drive, many flooring manufacturers require that concrete slabs be tested for moisture vapor transmission rates to determine whether or not the slab is suitable for their product. Some manufacturers state in writing that all concrete slabs to receive their flooring have a minimum 6-mil poly film between the ground and the concrete to act as a vapor retarder. Unfortunately in many cases, proper testing of the concrete slab is not performed before the flooring is installed and problems with elevated vapor drive are not discovered until after the floor is installed.

A 6-mil poly film is often called a vapor barrier; however, the plastic only serves to slow or retard the moisture vapor drive and does not actually create a barrier from all moisture vapor. In floors that function well, vapor may passes through the floor and is not retained. That is why wood boardwalks along the ocean or wood balconies in the mountains may perform well for years, but wood floors inside a home may rot in a short period of time and why an unfinished wood floor over a slab on grade will perform better than one with a polyurethane coating.

Other modern floor systems are also susceptible to moisture problems due to vapor drive. These include vinyl flooring, rubber-type flooring used in medical facilities, resilient flooring used in commercial applications and gymnasiums, and area mats under chairs, which may also act as vapor barriers. Chair mats over a wood floor can leave those portions of the floor subject to elevated moisture levels and localized discoloration and rot.

Floor Structures Over Crawlspaces

A common problem with crawlspaces is that over time, the vents in the stem walls are blocked by additions, vegetation or covered up for other reasons. The blocked vents reduce direct ventilation and cross ventilation, which leads to elevated moisture within the crawlspace. Long term exposure to high moisture can adversely affect wood, concrete, and steel structural members of the floor system. Wood may rot and/or be more susceptible to insect damage, metal trusses may corrode, and reinforced concrete may experience accelerated corrosion resulting in concrete spalling.

Over time as the affected floor sheathing, beams, joists, trusses, and connections deteriorate due to high moisture conditions they begin to deflect under load. This deflection may be the first noticeable manifestation of damage or a problem. If not visibly deflected, the floor may have a ‘spongy feeling’. Elevated moisture also tends to cause discoloration or stains in the wood. Other manifestations of deflection of deteriorated floor framing may include cracks or gaps in floor finish, walls, and ceilings.

Moisture damage to floor systems can also occur from condensation caused by inadequate crawlspace ventilation. Particularly during summer months, significant differentials in temperature and humidity exist between the relatively cool, dry interior air and the warm, moist exterior or crawlspace air.

Inadequate crawlspace ventilation combined with modern finish flooring can result in significant moisture problems and ultimately failure of the floor system. As with vapor drive through concrete slabs on grade, a polyurethane finish installed on modern wood flooring in an older home built over crawlspaces can act as a vapor retarder and result in condensation within the flooring system.

Condensation occurs as the relatively warm moist air in the crawlspace comes into contact with the colder underside surface of the wood floor structure, which is cooled by the air conditioning inside the home. The cold air within or near the floor system cannot hold as much moisture vapor as the warm air in the crawlspace and the moisture condenses on or within the layers of the floor structure, causing moisture damage and decay.


Water in the form of vapor drive or condensation may cause damage to finish flooring systems and floor structures. This problem is often caused or exacerbated by an inadequate moisture retarder beneath a slab on grade or insufficient crawlspace ventilation.