Plastic shrinkage cracking remains one of the most common and frustrating challenges faced by concrete contractors, particularly during hot weather or adverse wind conditions. These cracks appear within the first few hours after concrete placement, often before finishing operations are complete, and can compromise both the appearance and long-term durability of concrete flatwork. Understanding the mechanisms behind plastic shrinkage cracking and implementing effective control measures is essential for delivering quality concrete installations. For a broader perspective on cracking phenomena, see our article on Thermal Cracking in Concrete Causes Prevention and Control, which covers another major category of early-age concrete cracking.
Understanding Plastic Shrinkage Cracking in Concrete
Plastic shrinkage cracking occurs when the surface moisture of freshly placed concrete evaporates faster than it can be replaced by rising bleed water from within the concrete mass. This differential moisture loss causes the surface concrete to shrink more than the interior concrete, which remains relatively unaffected. Because the interior concrete restrains this surface shrinkage, tensile stresses develop across the surface. When these stresses exceed the tensile capacity of the still-plastic concrete, cracking occurs.
Characteristics of Plastic Shrinkage Cracks
Plastic shrinkage cracks exhibit several distinct characteristics that help differentiate them from other types of concrete cracking:
- They appear within the first few hours after placement, while the concrete is still in its plastic state.
- Cracks are typically discontinuous and relatively short, ranging from a few inches to several feet in length.
- They are usually shallow, though occasionally they can propagate to become full-depth cracks.
- Cracks may appear in random patterns or in roughly parallel arrangements spaced from a few inches to several feet apart.
- The timing of crack formation typically coincides with the period when bleeding ceases and the concrete transitions from a liquid to a solid state.
At this critical stage of hardening, the concrete has essentially zero tensile capacity. If concrete hardening is delayed for any reason, the window of vulnerability widens, significantly increasing the risk of plastic shrinkage cracking. This is why conditions that slow the setting time, such as cool temperatures or certain chemical admixtures, can paradoxically increase cracking risk even when evaporation rates appear moderate.
When and Where Plastic Shrinkage Cracking Occurs
Plastic shrinkage cracking occurs primarily in concrete slabs and other flatwork with large surface areas exposed to drying conditions. While commonly associated with hot weather concreting, these cracks can develop whenever surface moisture evaporates faster than bleed water can replace it. This includes conditions such as:
- Hot, dry weather with low relative humidity
- Windy conditions that accelerate surface moisture loss
- High concrete temperatures that increase the rate of evaporation
- Indoor environments with high air movement or low humidity
For more detailed information on how shrinkage cracks develop across different concrete applications, refer to our detailed article on What Is Shrinkage Cracks in Concrete Types and Causes.
Key Factors That Influence Plastic Shrinkage Cracking Risk
The risk of plastic shrinkage cracking depends on a complex interaction of environmental conditions, concrete mixture properties, and construction practices. Understanding each factor allows contractors to assess risk accurately and implement targeted control measures.
Environmental Conditions
Ambient jobsite conditions during concrete placement and finishing have the most immediate impact on cracking risk. The four primary environmental parameters that influence surface evaporation are:
| Environmental Factor | Effect on Evaporation Rate | Impact on Cracking Risk |
|---|---|---|
| Air temperature | Higher temperature increases evaporation rate | Higher air temperature increases cracking risk |
| Relative humidity | Lower humidity accelerates moisture loss | Low humidity conditions raise cracking risk |
| Wind velocity | Wind removes moist air from the surface | Wind speeds above 5 mph significantly increase risk |
| Concrete temperature | Hotter concrete increases evaporation rate | High concrete temperature magnifies cracking potential |
In addition to pattern cracks that develop across large surface areas, plastic shrinkage cracks can also occur as isolated single cracks across flatwork. These individual cracks form when drying and subsequent surface shrinkage create stresses that exceed the tensile capacity of the plastic concrete at a specific weak point.
Concrete Mixture Properties
The concrete mixture itself plays a significant role in determining cracking susceptibility. Key mixture considerations include:
- Bleeding characteristics: Mixtures with lower bleeding rates provide less internal moisture to replace surface evaporation, increasing cracking risk.
- Pozzolanic materials: Conventional and ultra-fine pozzolans, fly ash, and slag can reduce the rate of bleeding and delay concrete hardening, extending the vulnerable period.
- Water-cement ratio: Lower water-cement ratios reduce available bleed water, while higher ratios increase bleeding but may compromise strength.
- Microfiber reinforcement: Polypropylene or other microfibers can increase the tensile capacity of plastic concrete, helping resist cracking.
- Setting time: Mixtures with delayed setting times extend the window during which cracking can occur.
Construction Operations
Even routine construction operations, including screeding and finishing practices, can influence cracking risk. Delays between finishing passes expose the concrete surface to evaporation for longer periods. Overfinishing can work water to the surface, temporarily increasing bleeding but potentially weakening the surface. The timing of finishing operations relative to the onset of bleeding cessation is critical, as finishing after bleeding has stopped can disrupt the surface and accelerate moisture loss.
Using the Menzel Formula and Nomograph for Evaporation Rate Estimation
Most project specifications rely on limiting the rate of evaporation from the concrete surface as the primary means of controlling plastic shrinkage cracking. The default maximum allowable rate of evaporation commonly used across the industry is 0.20 pounds per square foot per hour. However, the American Concrete Institute (ACI) 305.1 Specification for Hot Weather Concreting recommends reducing this maximum allowable rate for concrete mixtures that contain pozzolans or other cementitious materials that may reduce bleeding rates or delay hardening.
The Menzel Formula
For compliance with ACI 305.1, the estimated rate of evaporation must be calculated using the Menzel Formula rather than estimated from the nomograph alone. The nomograph is intended only as a graphical guide to determine an approximate solution of the Menzel Formula. The formula incorporates four key measurements:
- Average horizontal wind speed measured at 20 inches above the concrete surface
- Air temperature at the jobsite
- Relative humidity of the air above the concrete surface
- Concrete surface temperature
To properly use either the nomograph or the Menzel Formula, all input measurements must be taken in a consistent manner that matches the conditions under which the formula was originally developed. Portable weather meters are available that integrate these measurements and calculate the estimated rate of evaporation directly using the Menzel Formula, providing contractors with real-time data for decision-making.
Interpreting Evaporation Rate Data
When the estimated rate of evaporation exceeds the maximum allowable specified rate, specifications typically require the contractor to take preventative measures to control moisture loss. The relationship between evaporation rate and cracking risk is not linear, and other factors such as concrete temperature, bleeding rate, and setting time must also be considered. ACI 305R-20 Guide to Hot Weather Concreting provides additional guidance on evaluating these combined factors.
For information on how moisture-related issues in concrete can lead to broader deterioration problems, see our article on Concrete Deterioration and Repair Causes Assessment Methods Repair, which covers assessment methods and repair strategies for concrete structures.
Prevention and Control Strategies for Plastic Shrinkage Cracking
The most effective approach to preventing plastic shrinkage cracking involves a combination of preplacement planning, environmental monitoring, and proactive measures during finishing. No single strategy is sufficient on its own, and contractors should implement multiple complementary measures based on the specific conditions of each pour.
Preplacement Measures
Before concrete placement begins, several preparations can reduce cracking risk:
- Dampen the subgrade and forms: Dry base materials and forms can absorb moisture from the fresh concrete, reducing available bleed water and increasing the rate of surface drying.
- Lower concrete temperature: Using chilled water or chipped ice in the concrete mixture reduces the concrete temperature, which lowers the evaporation rate. Each 10 degrees Fahrenheit reduction in concrete temperature can significantly reduce evaporation.
- Schedule placement strategically: When possible, schedule concrete placement during cooler times of the day, such as early morning or late evening, or during periods of lower wind speed.
- Erect windbreaks: Temporary wind barriers can significantly reduce wind velocity at the concrete surface, one of the most influential factors in evaporation rate.
During Placement and Finishing
Active measures during concrete placement and finishing provide the most direct control over surface evaporation:
- Apply evaporation retardants: Sprayable evaporation retardants form a moisture-retaining film on the concrete surface that significantly reduces the rate of evaporation between finishing operations. These products typically consist of one part retardant and nine parts water after mixing and should not be used as finishing aids.
- Use fog spraying: Fog spraying increases the relative humidity in the air immediately above the concrete surface, which reduces the evaporation rate. Fogging can also help replace bleed water that has prematurely evaporated from the surface. Care must be taken not to work accumulated surface water back into the concrete.
- Cover with plastic sheeting: Covering flatwork with plastic sheeting between finishing passes creates a physical barrier to evaporation. This is particularly effective during construction delays.
- Include microfibers: Adding polypropylene or other microfibers to the concrete mixture increases the tensile capacity of the plastic concrete, providing additional resistance to cracking when evaporation conditions are less than ideal.
Post-Finishing Protection
After finishing is complete, proper curing practices remain essential for preventing surface drying and ensuring long-term concrete performance. While plastic shrinkage cracks are usually superficial and seldom require structural repair, they may create aesthetic concerns, particularly for architectural concrete applications. Depending on crack severity, width and depth, and the concrete exposure conditions, plastic shrinkage cracks can create pathways for moisture and aggressive chemicals to penetrate the concrete, potentially leading to durability issues over time.
For additional guidance on managing moisture in concrete structures, see our article on Solving Moisture Problems in Concrete Block Crawlspaces Causes Prevention and Remediation, which covers moisture management strategies relevant to concrete applications.
Monitoring and Documentation Best Practices
Contractors should implement a systematic monitoring program for all concrete placements where cracking risk is elevated. The following steps represent industry best practices:
- Measure and record air temperature, relative humidity, wind speed at 20 inches above the surface, and concrete temperature before and during placement.
- Calculate the estimated rate of evaporation using the Menzel Formula or a calibrated electronic instrument.
- Compare the calculated rate against the maximum allowable rate specified in the project specifications.
- Implement preventative measures whenever the estimated rate approaches or exceeds the allowable threshold.
- Document all measurements, calculations, and actions taken for quality control records and future reference.
The best way to avoid both aesthetic and durability concerns related to plastic shrinkage cracks is to understand the susceptibility of each concrete mixture, monitor jobsite conditions during placement, and take actions to minimize rapid moisture loss from the concrete surface. By combining preplacement planning with active monitoring and timely intervention, contractors can successfully manage plastic shrinkage cracking risk across a wide range of conditions.