A Deep Dive Into Structural Cracks 

Cracks cannot be ignored. 

Small cracks can ruin any beautiful, well-painted wall. Medium cracks pose a threat to the durability of the concrete element. Bigger ones, or commonly called structural cracks, can cause many problems, 

Cracks are the most common concrete defect. When left unrepaired for a long time, cracks can cause a lot of trouble. 

What is Structural Integrity?

Structural integrity determines if a structure was built for its purpose. 

Concrete elements, such as slabs, columns, beams, and retaining walls, are designed to carry a definite amount of load or traffic. Some concrete structures are also designed with special features to fulfill specific requirements. With these specified strengths and capabilities, concrete elements are expected to provide a safe structure for many years. 

A home with good structural integrity has three basic characteristics:

  1. Safe – pertains to its load-carrying capacity 
  2. Reliable – refers to its ability to prevent structural failure
  3. Performing – tells if it is functional and can last its designed lifespan

Concrete structures can easily meet these criteria. In fact, newly-built structures can easily be considered safe, reliable, and well-performing. But, even if the concrete has good structural integrity, it can still suffer from cracks for so many reasons. 

Once that happens, you’re back to square one. 

Possible Causes of Crack Formation 

Cracks don’t just appear on a wall or floor. There’s a reason behind every crack you see in a structure. 

Basically, a crack’s origin or cause could fall into any of concrete’s three states, starting when it was still fresh. 

1. Plastic State 

The plastic state pertains to wet or fresh concrete. There are two types of cracks that may occur in this state

  • Plastic Settlement

Plastic settlement cracks are caused by excessive bleeding, which is worsened by restraining elements (rebars, ducts, and  inserts) preventing the concrete from settling properly.

When fresh concrete is poured over the surface, only the “unobstructed’ surrounding concrete settles. However, the concrete on top of the restraining element fails to settle, creating an unleveled surface that leads to cracking. 
Plastic settlement cracks mirror the pattern of their restraining elements. They are usually repaired by reworking the surface during its plastic state.

  • Plastic Shrinkage 

Plastic shrinkage cracks usually happen a few hours after concrete placement. It is caused by the rapid loss of water from the surface before the concrete can even set.

There are two possible scenarios for how plastic shrinkage cracks develop:

    • Concrete drying from the top occurs when moisture rises to the top surface and the rate of evaporation exceeds the rate of bleeding. As a result, the concrete shrinks as it dries out, and cracks tend to form.
    • Concrete drying from the base takes place when the ground below absorbs the water in a slab and affects the bleeding. This will then lead to increased concrete settlement and cracking.

Often, exposure to sunlight or heat causes these cracks. That’s why they can be easily prevented by using proper curing practices during the plastic state. 

2. Curing State

Cracks can form during the curing stage after the concrete has already hardened. At this stage, there are two types of cracks you should be concerned about.

  • Crazing Cracks 

When concrete begins to set, it’s the stage when it starts to gain strength. Therefore, a complete curing process is required, which entails maintaining adequate moisture content within a proper temperature range.

Crazing cracks happen in the curing state due to improper curing or intermittent curing.

    • Improper Curing happens when the evaporation rate is higher than the moisture gain due to direct sunlight, low humidity, or drying winds.
    • Intermittent Curing happens when the concrete dries for some time and the surface tries to absorb water. 

For critical applications, the use of a curing compound is recommended. Membrane-forming curing compounds typically consist of a wax or resin that is emulsified in water or dissolved in a solvent. This membrane helps retain moisture in the concrete. Applying a curing compound is much easier and more effective to prevent early hydration of fresh concrete pavements.

Buildrite Cure Sil EB forms a premium-grade, emulsion-based membrane that retains an optimum amount of water in freshly placed concrete for complete hydration of the cement. Obtaining the normal concrete curing provides the optimum compressive strength and prevents crazing, shrinkage, dusting, and scaling.

  • Diagonal Corner Cracks 
As the concrete dries, differences in temperature or moisture content between the top and bottom layers can lead to cracking.
    • Curling

Concrete curling happens due to the differences in top and bottom temperature, which causes portions of either the top or bottom later to curl upwards/downwards. 

    • Warping 

The differences in the rate of moisture evaporation can cause concrete warping, or the upward deformation of the slab at cracks or joints. When the moisture in drying concrete evaporates only from the top, it causes the slab to shrink faster (concrete shrinks as it dries) on the top than the bottom, and curl upward in the process. The reverse happens when the bottom layer cures faster. 

Empty spaces created by concrete curling and warping created diagonal corner cracks. These are the cracks that run from one joint to its perpendicular joint at the corner of a slab. 

3. Hardened State

The hardened state takes place when concrete has already developed its strength and can be used. However, cracks can still form in this state for the following reasons: 

  • Chemical Reactions 
One common chemical reaction that can cause concrete to crack is the alkali or silica reaction. These chemicals can be found in cement, other parts of the concrete mix, admixtures, or outside sources like curing water, ground water, and alkaline solutions used in the finished concrete.  When they react chemically, they produce a swelling gel that eventually causes concrete expansion and cracking.
  • Corrosion of Reinforcement 

    Corroded reinforcements are commonly referred to as "concrete cancer." When steel corrodes, the rust has a greater volume than the original metal. As a result, it will create tensile stresses in the concrete and cause cracks. In worse cases, concrete cancer will lead to concrete spalling, where large chunks of concrete chip off. 

    • Errors in Design and Detailing 

      There are many mistakes that can be made after the concrete has hardened. These include improper installation of precast concrete, insufficient contraction joints, the use of poorly detailed re-entrant corners in walls, etc. When you commit these errors, cracking can eventually occur.

      • Foundation Settling 

      After a few years, some structures sink and their foundation shifts. Foundation settling or settlement can be caused by seismic activity, growing tree roots, the soil and the changes happening in it, etc. Cracks formed when your house begins to settle are called settling cracks.

      • Overloading 

        There are two reasons why overloading causes concrete cracks:

          • Overloading of the actual concrete slab happens when you place excessive weight on the concrete slab. You will determine it by knowing the compressive strength of the concrete.
          • Overloading of the ground below the slab occurs when the ground or soil becomes wet and soft due to continuous rain and other circumstances.
        • Lack of Control Joints 

          Basically, control joints, or contraction joints, are planned cracks. They are placed on slabs to minimize cracking of the concrete slab during thermal expansion or contraction. To make sure that control joints will help solve your cracking problem, you have to take note of the proper spacing and proper timing of cutting control joints.

          • External Forces

          Sometimes, concrete’s rigidity cannot handle the movement or stress caused by external forces. That’s why cracking takes place. These forces or stress inducers can be:

            • Externally applied loads: Concrete structures such as dams and retaining walls are most prone to cracking due to externally applied loads. 
            • Vehicle and foot traffic: The stress of vehicle and foot traffic might also cause cracking, especially if concrete was not properly prepared.
          • Exposure to Environmental Factors
          Thermal variation, solar radiation, and rain can affect the tensile strength of concrete even if it has already hardened, which can lead to cracking. 

            Structural Cracks vs. Non-Structural Cracks

            Cracks are classified as either structural or non-structural

            Structural cracks are often caused by the movement of the foundation. Having these at home threatens the structural integrity of your house.

            On the other hand, non-structural cracks, or commonly called hairline cracks, are caused by shrinkage or external impacts. They often take place during concrete’s plastic or curing state. 

            To further differentiate structural from non-structural cracks, you have to consider these:

          • Crack Width
              1. Structural cracks: wider than 1/10 inch
              2. Non-structural cracks: less than 1/10 inch in width 

          • Crack Location
              • Structural cracks appear on poured concrete structures, such as columns, beams, pedestals, retaining walls, etc. 
                • Non-structural cracks: cracks isolated in one block of concrete element, especially on walls made of concrete hollow blocks (CHB) 

                How to Repair Structural Cracks

                The presence of cracks, whether structural or non-structural, can speed up the deterioration of any concrete structure. If not repaired immediately, it could easily lead to expensive repair-and-replace scenarios. 

                There are two ways to address cracks: 

                1. For structural cracks, we recommend using Confix IJ. It is a two-component solvent-free, non-pigmented epoxy resin compound with low viscosity. It is chemical-resistant and virtually impermeable, protecting the concrete against carbonation. It also has high compressive, tensile, and slant shear bond strengths.

                Watch this video to learn more about Confix IJ:

                2. For non-structural hairline cracks, we recommend using Sapal 2 in 1. It is a ready-to-use, paintable, elastic, solvent-free, waterproof, weather-resistant, and water-based acrylic sealant. It forms a tough elastic film that can withstand the substrate's thermal expansion, contraction, and dynamic movements.

                Watch this to learn more about Sapal 2in1:

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