Concrete cracks are the most serious concern. It can be witnessed in both old and new structures, leading to significant issues in the future. They are unavoidable and can occur due to several reasons, particularly plastic shrinkage structural cracks. It is a common cause witnessed only in the freshly placed concrete within a few hours of pouring, when the concrete is still in a paste-like consistency.
As a builder, engineer, or homeowner, you should be aware of this cause before starting construction. This article will help you learn its significant information and apply solutions early.
Overview of Plastic Shrinkage Cracks in Concrete
Plastic shrinkage cracks occur on the fresh concrete surface, typically due to the rapid loss of moisture content before the concrete sets. They appear in an irregular pattern approximately 3 to 10 mm deep and 0.1 to 0.3 mm wide. The formation of such cracks happens very soon, often before or shortly after finishing. They are caused when the rate of moisture evaporation from the surface exceeds the rate at which bleed water rises, creating tensile stresses that the concrete cannot withstand in its plastic state. Factors contributing to this include high air temperature, wind velocity, low humidity, and elevated concrete temperature..
Causes of Plastic Shrinkage Cracks
As stated earlier, plastic shrinkage crack is a result of rapid loss of moisture content from the concrete surface. When it directly reacts with environmental factors, the concrete experience differential volume change. Beyond that, here are some significant reasons mentioned:
Concrete Temperature: High temperature accelerates the water evaporation on the concrete’s exterior surface. Therefore, the outer layer shrinks more rapidly than the interior concrete, resulting in plastic shrinkage cracks that gradually grow downward.
Faster Evaporation Rate: The cracks develop when the evaporation rate exceeds 1.0 kg/m²/hour. It is quite greater than the rate at which bleed water rises to the concrete surface.
Low Relative Humidity: Relative humidity means the amount of water vapour that exists in the air. At the stage of hardening, if the dry air absorbs water content on the concrete surface, it can result in plastic shrinkage cracks.
Low Bleeding: The reduced or low quantity of bleed water mixes with concrete and is highly susceptible to cracks. The common factors that actively reduce the bleeding rate are
- Entrained air
- High cementitious materials content
- Low water content
- High fines content
- Thinner sections
- High concrete temperature
Delayed Hardening: The delayed setting time of concrete increases the chances of plastic shrinkage cracks occurring.
Identifying Plastic Shrinkage Cracks
These are the key characteristics that help identify plastic shrinkage cracks on the surface of concrete.
Early Appearance: This crack appears very soon after the concrete placement, typically before it achieves the initial set and strength. They are often created before or during the finishing operations. This timing is crucial to differentiating plastic shrinkage cracks from others that may probably appear once the concrete has hardened.
Pattern: You can identify parallel cracks, which are perpendicular to the direction of prevailing wind. It can also be a series of random, discontinued patterns spaced a few inches to several feet apart.
Length and Width: Plastic shrinkage cracks are typically individual and relatively short, ranging from 25 mm to 2 meters in length and up to 3 mm in width, although most are narrower. Their size and pattern can vary depending on surface conditions, ambient weather, and the rate of moisture loss during the early setting period.
Lack of Connection with Joints: Unlike other cracks, this will rarely extend to the edges or joints. This is because the concrete edges have less restraint, so they will experience less tensile stress from surface shrinkage.
No Broken Aggregates: During the occurrence of plastic shrinkage cracks, the concrete is still in a plastic state. Therefore, it has not attained its complete strength and bond between aggregate particles. Rather than breaking inside the particles, the cracks will appear around them.
Tools Required to Repair Plastic Shrinkage Crack
These are the tools required to repair the cracked concrete:
Vibratory Screed: This tool helps consolidate the concrete and close shallow cracks, specifically while repairing the larger slabs.
Power Float/Trowel: It has rotating blades to run over the surface, which helps rework the concrete and close cracks.
Garden or Pump Sprayer: They help to apply water-based chemical solutions over the surface to reduce rapid water evaporation.
Plastic sheeting or curing blankets: They help to cover the concrete surface and avoid exposure to harsh environmental conditions.
How to Repair Plastic Shrinkage Cracks in Concrete
Repair methods of concrete differ based on whether it is in a plastic or hardened state.
Methods to Repair Shear Cracks in Fresh (Plastic) Concrete
Identify the Cracks Early: Earlier identification is a key to preventing the cracks from spreading over the surface. It usually happens within a few hours of concrete placement. Keeping your eye on freshly placed concrete will help you encounter the cracks early.
Refinishing/Refloating: Minor or hairline cracks are easily repairable. You can use either a low-viscosity polymer or a cement grout to refloat the area. Applying them with a brush or injection can ensure the cracks are properly filled. Care must be taken to avoid over-troweling, which may cause other surface defects.
Surface Vibrators: The usage of surface vibrators helps you to re-consolidate the deeper plastic cracks across a larger area. This is a superficial method that encourages the concrete to settle and fill the gaps efficiently.
Methods to Repair Shear Cracks in Hardened Concrete
Cement Grout or Slurry: If the hardened concrete contains very fine or hairline cracks, apply a thin mixture of Portland cement and water (cement grout or slurry). This method involves filling the cracks effectively and improving the aesthetic appearance. It is likely to be prepared and applied directly into the gaps.
Concrete Patching Compounds: The ready-to-use patching compound is suitable for applying to cracks typically with 2mm or more. It can be epoxy-based, cementitious or acrylic latex-modified. Once the compound efficiently fills the cracks, it can be smoothed to blend with the surrounding concrete surface.
Low-Viscosity Polymer Injection: The concrete with slightly wider cracks can be repaired using low-viscosity polymer injections. It is highly durable and designed to cure narrow crack widths and patch up the concrete together. This method can effectively stand against moisture content and restore some degree of structural integrity.
Preventive Measures During Concrete Placement
Make use of these preventive measures to avoid plastic shrinkage cracks:
Pre-Planning: Pre-check the weather conditions before applying concrete. If the conditions seem critical, you can reschedule it. Generally, applying concrete at cooler times, especially in the early morning or early evening, can prevent this plastic shrinkage crack in hot weather.
Site Preparation: Moisten the forms and subgrades to stop them from absorbing water from the freshly placed concrete. Using sunshades actively prevents the direct exposure of sunlight over the concrete surface.
Concrete Mix Design: Optimize the water-cement ratio to improve the strength and workability of concrete without excessive water content. Avoid using shrinkage-reducing admixtures, which notably decrease the cracking potential.
Last Few Words
Plastic shrinkage cracks, though superficial, can compromise the surface durability and appearance of concrete if not addressed promptly. Early identification and timely repair—whether through refinishing in the plastic stage or using suitable injection and patching techniques in the hardened stage—are key to maintaining concrete integrity. Alongside proper curing and mix design, these repair strategies ensure long-term performance and reduce the risk of future cracking..
Apoorva H P Achar