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    • Feb 26

The 3 Main Properties in Concrete That Fibers Address

Plastic Shrinkage Cracking

 

Plastic shrinkage cracking in concrete is a type of cracking that occurs during the early stages of the concrete's curing process, typically within the first few hours to days after placement. It is caused by the rapid evaporation of water from the surface of freshly poured and still-plastic (not fully hardened) concrete. This evaporation of water can lead to the weakening of concrete:

                  

-          Surface Drying: As the concrete is exposed to air and environmental conditions, especially in hot or windy weather, the surface layer of the concrete begins to lose moisture rapidly.

 

-          Differential Drying: The outer layer of the concrete dries more quickly than the inner portion, creating a moisture gradient within the concrete. This means that the surface layer contracts due to drying while the inner layer is still relatively moist and hasn't yet undergone significant shrinkage.

 

-          Tensile Stresses: The differential drying leads to the development of tensile stresses at the surface of the concrete. Since concrete is relatively weak in tension, these tensile stresses can result in cracks forming on the surface.

 

These cracks are referred to as "plastic shrinkage cracks" because they occur while the concrete is still in its plastic or unset state before it has fully hardened. These cracks are typically shallow and can be quite fine, resembling hairline cracks. They often radiate from a central point or appear in a random pattern on the surface.

Preventing plastic shrinkage cracking is important because these cracks can compromise the appearance and durability of the concrete surface. To mitigate the risk of plastic shrinkage cracking, concrete workers often employ various strategies, including:

                  

-          Use of Windbreaks and Sunshades: Protecting the concrete surface from direct sunlight and wind can help reduce the rate of surface moisture loss.

                  

-          Fogging or Wet Curing: Spraying water mist or covering the surface with wet burlap or curing blankets can slow down moisture evaporation and reduce cracking.

 

-          Use of Retarding Admixtures: These chemical additives can delay the setting time of the concrete, giving more time for finishing and reducing the risk of cracking.

 

-          Proper Timing for Finishing: Timing the finishing operations (troweling) to coincide with the initial setting of the concrete can help close surface pores and minimize the potential for cracking.

 

-          Concrete Fibers: Create a spider web-like matrix to keep concrete interconnected while simultaneously protecting against surface-level abrasion and erosion

 

It's important to know that plastic shrinkage cracks are typically surface-level and do not compromise the structural integrity of the concrete. However, they can detract from the appearance of the finished surface and may serve as potential pathways for the impediment of moisture and contaminants if not properly repaired or addressed.

 

Flexural Strength

 

Flexural strength is a material property that measures its ability to withstand bending or flexural stresses without breaking or fracturing. It is particularly important for materials like concrete, ceramics, and certain types of composites that are subjected to bending loads. Flexural strength is expressed in units of force per unit area, such as Pascals (Pa) or megapascals (MPa) in the International System of Units (SI).

 

In the application of concrete, flexural strength is determined by subjecting a beam or a specimen of concrete to a bending test, often referred to as a "three-point bending test" or "flexural test." Here's how the test works:

 

Step 1: A concrete beam or specimen, typically of standardized dimensions, is supported at two points (span) with a gap in between.

 

Step 2: A load is applied at the midpoint of the span, causing the beam to bend. The bending creates tensile stresses on the underside of the beam and compressive stresses on the upper side.

 

Step 3: The load is increased gradually until the concrete specimen fractures. The maximum load that the concrete can withstand before it breaks is recorded.

 

Step 4: The flexural strength is calculated by dividing the maximum load by the cross-sectional area of the specimen. It is typically reported in units of force per unit area.

Flexural strength is an important property for assessing the behavior of concrete, especially in structural applications where elements like beams, slabs, and columns are subjected to bending loads. It provides valuable information about the concrete's resistance to bending and its ability to support loads without experiencing failure.

 

It is important to discern key differences between flexural strength and compressive strength.

-          Flexural strength is related to, but distinct from, compressive strength, which measures a material's ability to withstand axial or compressive loads ( the load-bearing capacity of a concrete column). While compressive strength is crucial in assessing the capacity of concrete under compression, flexural strength is more relevant when evaluating how concrete applications perform under bending or tensile stresses.

 

Tensile Strength

 

Since we are speaking of tensile strength, this is the 3rd and equally as important property, that fibers in concrete address. Tensile strength is a material property that measures its resistance to a force attempting to stretch or pull it apart. In other words, it represents the maximum axial (along the length) stress a material can withstand while being stretched or pulled before it breaks or fractures. Tensile strength is typically expressed in units of force per unit area.

 

Tensile strength is an important mechanical property for various materials, including metals, polymers, ceramics, and construction materials like concrete and wood. It provides information about the material's ability to withstand tensional forces without undergoing permanent deformation or failure.

 

In concrete application testing, tensile strength is determined through a tensile test, where a sample of the material is subjected to an increasing axial load until it fractures. The test measures the stress (force per unit area) at which the material breaks and the corresponding strain (deformation) it undergoes during the process. The resulting stress-strain curve provides valuable information about the material's behavior under tensile loading. In this case how does the concrete perform when fibers are added into the concrete.

 

Tensile strength is crucial in engineering and concrete applications because it helps determine the suitability of a material for various applications. Materials with high tensile strength are often preferred for applications where the ability to withstand stretching or pulling forces is essential, such as in structural components, Bridge deck overlays, precast concrete, and high-impact concrete flooring applications that you see in a warehouse and large retail stores such as Lowes and Walmart’s.

 

In conclusion, tensile strength is a material property that measures its resistance to being pulled apart or stretched and is essential in determining the mechanical behavior and suitability of materials for various engineering and construction applications.


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