Why concrete cracks and how to minimize and control it.

Concrete is and always has been a durable and affordable surface for residential construction of pathways, patios, driveways and as a base for masonry overlay. Concrete is simply a combination of:

  • Sand
  • Cement
  • Aggregate rock
  • Water

The size, cleanliness, and ratio of each is critical in assuring quality of the finished concrete product. Reputable concrete suppliers within reasonable driving distance should be utilized to insure maximum quality. There are numerous site conditions as well as techniques in the forming, placement and finishing of the product in the field that affect the aesthetic quality of the finished product.

 

SITE CONDITIONS

Type of Soil: San Diego has several distinctly different soil types:

  • Sandy: (example: Solana Beach & Del Mar) Coastal soils, which compact and percolate well and are well suited as a base for concrete construction.
  • Clay Soils: (Tierrasanta & Rancho Santa Fe) Are difficult soils to pour concrete atop. The soil expands and contracts with the addition and subtraction of water. Thus, concrete atop this soil also shifts, moves and cracks.
  • Decomposed Granite: (Poway & Escondido) Is a soil that percolates water and compacts well. It makes a suitable base for concrete construction.
  • Rock & Clay: (Scripps Ranch) Is the worst of both worlds. A nightmare to pour concrete atop. Poor percolation and expansion of soil as water is added makes this a tough soil type to contend with. The rock, whether cobble or shatter rock, make grading and forming difficult as well.
  • Compaction of Soil: All trenches regardless of soil conditions that lie underneath concrete need to be back-filled, moistened and compacted. All base soil should be compacted well and graded for even slab thickness. A compaction of 90% or more is desired.
  • Weather: Conditions the day of a concrete pour can greatly affect the finished aesthetic quality of concrete.
    - Rain: Best to avoid pouring on rainy days as excessive moisture atop new concrete may not only affect its color, but also its strength.
    - Santa Ana Conditions: It is critical to wet the sub-grades surface substantially in these conditions to insure that the concrete retains moisture as much as possible. Best to avoid pouring during Santa Ana conditions. Slab moisture is lost too quickly. This leads to a slab that is not only difficult to finish, but will most likely crack substantially.
    - Extreme Heat: Similar to Santa Ana conditions. Better to pour at a time of day when the temperature is substantially cooler, also adding time for finishing. In fact, pouring in the afternoon as the temperature is dropping into early evening hours affords the most time for a quality finish. Retardants that slow the drying time of concrete can be used, but it is better to pour only as much concrete as can be finished by the manpower on hand effectively.

Beyond the site conditions for which we have no control, there is technique in ordering, forming, placing, and finishing, which can greatly influence the final product.

  • Reinforcement Methods: Although concrete may be poured without reinforcement, it is strongly advised to use one or more of the following reinforcement methods:
  • Welded Wire Mesh: A minimal approach. Will hold slab together when it cracks. Provides minimal tensile strength.
  • Fiber Mesh: Mixed into the concrete at the plant. Similar to fiberglass hairs”, fiber mesh is used to keep small surface cracks from spreading.
  • Steel Rebar: Typically used in 3/8” and 1/2” diameter for residential construction. Steel rebar tied in a grid at each interval and propped into the middle of the slab offers the greatest tensile strength and reinforcement. Size and spacing of rebar are in accordance to soil conditions and the weight being supported atop the slab.

MIX DESIGN

Concrete Pumping: Most residential concrete is pumped from the truck at the street through a hose into the forms. Typically, these “pump mixes” are designed to be pumped through one of several types of concrete pumps.

 

Pea Gravel Pump Mixes: Are pumped from smaller pumps that must have a higher water content to facilitate ease of pumping. These mix designs also use a small 3/8” size rock. The high water content creates surface shrinkage in the slab as it evaporates from within the mix. Most pea gravel pump mixes are sold for use in concrete wall footings or grout within a block wall where the propensity for cracking will not affect aesthetic quality.

 

Trailer Pump Mixes: Are pumped through larger pumps which have the capacity to pump concrete without an exceedingly high
water content. Trailer pump mixes have a low water content and use 3/4” aggregate rock at a 50% ratio to volume as opposed to
only 30% of pea gravel in pea gravel mixes. The larger aggregate and increased amount create a better overlap of rock to rock within the mix. This overlap creates added resistance to cracking. The lower water content leads to less shrinkage at the surface and thus less cracking. Avoid adding excessive water to the mix upon arrival of concrete.


Control Jointing: The water content loss in concrete as it dries creates shrinkage and surface tension. Pea gravel mixes because of their high water content undergo greater shrinkage at the surface than trailer pump mixes. Both types of concrete require periodic jointing to actually weaken the surface tension and control the cracks by getting them started. Thus, by jointing correctly, one can hope to “control” cracking (but not eliminate it ). Remember, all concrete cracks. Control jointing is both art and science as the placement of them can enhance or detract from aesthetic quality. The function of control jointing should be paramount.

  • Pea Gravel Mixes: Need very close spaced joints typically 8’ – 10’
    Apart to control a high degree of shrinkage and surface tension.
  • Trailer Pump Mixes: Require control jointing at 12’ – 15’ intervals.
  • Joint Location: It is not always possible to space joints this close, but adherence to these standards is recommended. Some general rules are:
    - Remember to use a tamper as the concrete is placed to push rock deep enough to joint over.
    - Keep joints spaced in accordance with mix design.
    - Joint off all corners of objects within the area of the pour, such as pools, bbq’s, firepits, drains, etc.
    - Joint to prevent angles from exceeding 180 degrees as measured on the inside of the concrete forms.
    - Attempt to keep interior angles at 90 degrees or less for greater crack control.
    - Always continue a joint to the outside edge of the pour or to an expansion joint.
    - Joint along ridges and valleys: Concrete outdoors is always poured with a slope of minimum 2% (1/8” per ft.) so that water
    may flow across its surface. Drains must be placed to collect water and the high points and low points used in forming create ridges and valley. These points undergo more stress and need jointing along them. The worse the soil conditions, the tighter the spacing between joints.
    - Concrete Finishing Tools: Discard old worn out jointers as the penetration into the concrete is not enough to control cracking.

Expansion Joint: All concrete and masonry components built atop soil are subject to stresses. In California, the ground shifts and shakes, soils expand and contract. On sites where soil expansion is most likely to occur, it makes good sense to periodically sever the slab and leave a caulked joint in its place. Divide the slab area into more manageable sized pieces and then control joint them
further if needed. A foam strip 1/2” wide may be placed and caulked atop between “floating” pieces of slab. This caulking is typically a mastic product which has an elastic nature and can expand and contract with limitation. Use of these expansion joints against all foundations, masonry walls and swimming pools will insure that the slab has the capacity to absorb added stresses.

 

Thickness of Concrete: Standard forming lumber for residential concrete construction is 2” x 4” and 2” x 6” for straight line forming. Patios and entry ways are usually poured at 3 ½” thickness or the width of a 2” x 4” form. This is considered an industry standard.

 

It is strongly advised to consider increasing the thickness (especially in expansive soil conditions) to 5 ½”, thereby using 2” x 6” lumber for forming. Increased thickness 12” wide along the outer edges of concret where it abuts planter or lawn area, will help keep water from quickly migrating under the slab. It is water that expands certain soil conditions that lead to cracking. Drop an additional horizontal rebar in the thickened edge to create an edge that’s highly resistant to cracking.

 

Drainage: Large expanses of concrete will have a considerable volume of water flow atop them during storms. It is imperative that proper drainage techniques be applied.

  • Deck Drains: Where possible collect water in drains within the decking area. Adding to the flow of water into lawn and planter beds adjacent to decking will only result in super saturating that soil. That in turn may encourage migration of water under the slab which results in soil expansion and concrete cracking. It is practical to drain most pathways, entries, and small use areas, depending on soil conditions.
  • Lawn & Planter Drains: All areas near concrete require drainage as well. It makes good sense to drain nearby planters and lawn areas or grade them to encourage water flow out and away from the slab area.
  • Roof Gutters: The volume of water collected in rain gutters is considerable. Always tie downspouts from the gutter system either into the drain system or daylight them a considerable distance away where water can flow into a storm drain system or approved natural run-off location.
  • Base Material: If soil conditions are highly expansive, where economically practical, a road base may be installed underneath concrete. Sand and gravel combined make an easy compactable base that is firm and non expansive. Poorly compacted, it may actually aid in channeling water under the slab.
  • Pool Plumbing: Shallow and poorly compacted pool plumbing can actually wick water through the trench and under the slab where it will expand soil, shift and crack concrete. The diameter of trench required for pool and spa plumbing is typically several feet wide but often only inches deep. Pool plumbing should be buried and compacted a minimum of 18” underneath the slab. Where plumbing lines enter the concrete area, it is critical to have excellent drainage. Consider French draining across the plumbing line in highly expansive soil.
  • Hot Load: It’s important to have a fresh batched load of concrete arrive on time at the job site. Concrete can dry considerably on a long delivery or stand by. Ordering from a plant that sells a quality product within reasonable distance is imperative. Timing the trucks for arrival and unloading fresh is equally important.

As one can clearly see, there are a multitude of factors that have an effect on concrete cracking. There are no standardized specifications that work for every site and every condition. It makes good sense to use all the available means to insure the desired results. Anticipating a high degree of success when using the best possible standards is as reasonable as assuring a certain degree of failure when using minimum standards.

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