Concrete does indeed shrink as it dries, experiencing volume reduction that can lead to cracking, warping, and structural issues if not properly managed. This shrinkage occurs primarily due to moisture loss and is influenced by numerous factors including mix design, environmental conditions, and curing practices. The phenomenon involves complex physical mechanisms that operate from the initial placement of concrete through its hardening and long-term aging.
Types of Concrete Shrinkage
Plastic Shrinkage
Plastic shrinkage occurs in freshly placed concrete when water evaporates from the surface faster than it can be replaced by bleed water rising from below. This type of shrinkage happens before the concrete has hardened, typically within the first few hours after placement. On hot, windy days, the surface of concrete may lose moisture faster than the concrete can tolerate, causing surface shrinkage known as plastic shrinkage cracking. These cracks are typically random in pattern but may be influenced by finishing operations. They tend to be 1-2 mm wide, 300-500 mm long, and 20-50 mm deep, though they can sometimes extend through the full depth of a member.
Drying Shrinkage
Drying shrinkage is defined as the contracting of hardened concrete due to the loss of capillary water over time [1]. Unlike plastic shrinkage, drying shrinkage occurs in hardened concrete and can continue for months or even years after placement. This type of shrinkage causes an increase in tensile stress, potentially leading to cracking, internal warping, and external deflection. The volume reduction happens primarily because concrete typically contains more water than is required for cement hydration, and as this excess water evaporates, the concrete contracts.
Other Forms of Shrinkage
In addition to plastic and drying shrinkage, concrete can experience other volume changes:
- Autogenous shrinkage: Occurs as a result of chemical reactions during hydration, independent of moisture loss to the environment [2].
- Thermal contraction: As fresh concrete cools from the heat generated during hydration to ambient temperature, it contracts.
- Carbonation shrinkage: Results from the reaction between carbon dioxide in the air and calcium hydroxide in the cement paste.
Mechanisms of Concrete Shrinkage
The Role of Water in Concrete Shrinkage
For concrete to hydrate and gain strength, a minimum amount of water is needed. However, additional water is typically added to improve workability and pumpability of the mixture. This excess water is not used in the hydration process and eventually bleeds out of the concrete, creating a volume change known as drying shrinkage.
Capillary Tension and Microstructural Changes
When concrete dries, moisture loss from its fine pores results in shrinkage. In the finer water-filled capillary pores (2.5 to 50 nm size), the loss of moisture forms curved menisci, and the surface tension of water pulls the walls of the pores. This creates internal negative pressure that develops when the meniscus forms in the capillary pores, resulting in a compressive force that leads to shrinkage.
The shrinkage of the hydrated cement paste itself is rather large, but in concrete, this effect is moderated because aggregates occupy 65-75% of the volume and resist the shrinkage forces [3]. This resistance creates internal tensile stresses in the concrete matrix.
Factors Affecting Concrete Shrinkage
Water Content and Water-Cement Ratio
The water content in concrete is perhaps the most significant factor affecting shrinkage. Higher water content leads to greater drying shrinkage. Research clearly shows that at a given water-cement ratio, drying shrinkage is reduced as the aggregate-cement ratio is increased.
Aggregate Properties and Proportions
Aggregates play a critical role in limiting concrete shrinkage. The mineral composition, stiffness, shape, surface texture, and grading of the aggregate all influence the shrinkage potential. Since aggregates generally do not shrink, they provide internal restraint to the shrinking cement paste. The stiffer the aggregate, the more it will resist the shrinkage of the paste.
Environmental Conditions
Particularly in Australia, environmental factors significantly influence the rate and extent of shrinkage:
- High wind speed accelerates surface water evaporation
- Low relative humidity increases moisture loss
- High ambient and concrete temperatures speed up the drying process
When the rate of surface evaporation exceeds about 0.1 lb/ft²/h (0.5 kg/m²/h), the loss of moisture may exceed the rate at which bleed water reaches the surface, initiating the mechanisms that cause plastic shrinkage cracking.
Mix Design and Construction Factors
The concrete mix design directly impacts shrinkage potential. This includes:
- Cement type and content
- Admixtures used
- Mixing procedures
- Time elapsed after water addition
- Placement and finishing techniques
Consequences of Concrete Shrinkage
Cracking Patterns and Structural Integrity
When shrinkage is restrained (which is virtually unavoidable in real structures), internal tensile stresses develop. Since concrete is weak in tension, cracking occurs when these stresses exceed its tensile strength. The pattern of cracks depends on the type of shrinkage and restraint conditions. While plastic shrinkage cracks are typically random and surface-level, drying shrinkage cracks are often more uniform, evenly distributed, and can extend through the full depth of the concrete.
An extensive survey revealed that concrete cracking can be attributed to:
- Construction and supervision problems (approximately 36%)
- Design defects (approximately 27%)
- Ambient conditions like temperature and humidity (approximately 21%)
- Quality of materials (approximately 17%)
Curling and Warping
Differential shrinkage between the top and bottom surfaces of a concrete slab can cause curling or warping. This occurs because the top surface is exposed to air and dries faster than the bottom surface, which remains in contact with the moist subgrade. The resulting deformation can affect the serviceability of floors and pavements.
Prevention and Control of Concrete Shrinkage
Proper Curing Techniques
Curing is the protection of fresh concrete from evaporation and is crucial in controlling shrinkage. Proper curing ensures hydration by controlling the rate of moisture loss from the concrete. If concrete is allowed to dry too quickly, it may cause shrinkage, cracking, or a weak powdery surface on the slab.
Depending on the climate, concrete may be kept wet using hessian bags, plastic membranes, or similar materials. Chemical compounds can also be sprayed or brushed on the surface to ensure adequate curing. The compressive strength of properly cured concrete can be double that of of concrete that has not been cured.
Mix Design Optimisation
Several strategies can reduce shrinkage through mix design:
- Reduce water content: The most obvious way to reduce shrinkage is to mix less water into the concrete. However, this creates a stiffer mix that is more difficult to work with.
- Increase aggregate content: Higher aggregate-cement ratios reduce shrinkage substantially.
- Use admixtures: Various additives can reduce water requirements without affecting workability, though their effectiveness in reducing shrinkage is debated.
Contraction Joints and Reinforcement
In slabs, contraction joints are provided so that concrete can crack due to shrinkage at predetermined locations rather than randomly. Reducing the spacing of contraction joints can minimise curling effects. For other structural members, appropriate detailing of reinforcement can help minimise cracking due to shrinkage.
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