What is plastic settlement? Plastic Settlement Cracks in Concrete

This article is about Plastic Settlement Cracks in Concrete and its Appearance and Prevention Methods. Plastic settlement cracks in concrete occur during the early stages of concrete setting. These cracks are caused by the settlement of concrete due to the weight of the aggregates and other constituents present in the mixture. The appearance of these cracks is typically in the form of a pattern of small cracks on the surface of the concrete.

To prevent plastic settlement cracks, several measures can be taken during the construction process. One way to prevent these cracks is by reducing the water content of the mixture. This reduces the amount of settlement that occurs during the initial setting of the concrete.

Another way to prevent plastic settlement cracks is by using a mix design that includes smaller aggregate sizes. This reduces the weight of the aggregates in the mixture, which in turn reduces the amount of settlement that occurs.

What is plastic settlement?

Plastic settlement is a type of concrete defect that occurs when the concrete is still in its plastic state. During the pouring and compaction of concrete, the heavier aggregates in the mix settle to the bottom due to gravity, leaving the lighter cement paste at the top. As the concrete begins to set, the trapped air bubbles in the paste rise to the surface and escape, causing the paste to shrink and pull away from the surface of the concrete below. This creates a depression or “plastic settlement” in the surface of the concrete.

Mechanism of Plastic Settlement Cracks in Concrete

Concrete is a commonly used construction material, and its strength and durability are crucial for the stability of structures. However, issues such as cracking can arise during the curing process. When solids in the concrete settle freely without hindrance, the volume and depth of the cast concrete will decrease, and cracking can be avoided. But if there is any restriction to this movement, such as reinforcement, plastic settlement cracks may appear.

The presence of cracks can cause the concrete to “break its back,” resulting in tears on the surface due to the tension exerted on it. Additionally, cracks may form at regular intervals that correspond to the layout of the reinforcement. Void formation is also a common occurrence during concrete curing, which can contribute to cracking.

Figure 1 shows the process of void formation, where crescent-shaped voids initially form under the reinforcement bars and may be filled with bleed water. The presence of voids reduces the region of bond between the bar and the concrete, potentially leading to more significant cracking over time.

Appearance of Plastic Settlement Cracks in Concrete

Plastic settlement cracks are a common occurrence in slabs, and are primarily caused by the restraint from the reinforcement. These cracks usually form on the top surface and follow the line of the uppermost bars, resulting in a series of parallel cracks. Shorter cracks may also appear at right angles over the bars running in the opposite direction. These cracks are typically around 1 mm in width and extend from the surface to the bars. In some cases, settlement may lead to visible undulations on the concrete surface, with the high points over the top reinforcing bars.

In certain instances, when the top layer of reinforcement bars is closely spaced, the entire surface layer of concrete may be supported by the reinforcement while the concrete beneath settles. This can create a horizontal break below the bars, causing a loss of bond and eventual delamination of the concrete cover that safeguards the reinforcing steel against corrosion.

These cracks are different from cracks that appear in hardened concrete due to overloading, as they occur at an early stage and run through the cement paste instead of the aggregate particles. The crack’s trajectory is more complex as a result. These types of cracks are particularly concerning since they run parallel to the reinforcement and can extend to the steel, negating the concrete’s ability to resist corrosion.

Fine cracks may emerge in relatively narrow form surfaces, such as columns. In such cases, the concrete may bend between the faces of the form, and as settlement occurs beneath the restrained concrete, a crack is formed, usually coinciding with the links. Plastic settlement cracks may also emerge on a vertical face when reinforcement restricts the flow of concrete within the formwork. These cracks might develop between the lines of the reinforcement.

The concrete may also be supported by the face of the formwork, creating a restraint between interconnected members. This is especially visible when changes in the section cause differential settlement, with the concrete settling more in the deeper section than in the shallower section. This can result in a crack between the flared column head (mushroom) and the plain column, as well as in trough and waffle slabs, where more settlement occurs in the web than in the relatively thin flange. These cracks may run through the flange and resemble shrinkage cracks.

Concrete structures are prone to cracking, which can occur in various locations. Although cracks in the mushroom heads of columns are usually horizontal and about 1 mm wide, they can extend across the entire section. Settlement of the concrete can also lead to cracking, particularly if the sub-base or the material adjacent to the concrete is highly absorbent. This can be exacerbated in cases where the material against which the concrete is placed is dry soil or permanent forms. In such situations, the cracks tend to follow the layout of the reinforcement.

Prevention of Plastic Settlement Cracks in Concrete

Preventing plastic settlement cracking in concrete is crucial, but it is challenging to avoid the restraints that cause it during construction. One approach to reducing the risk of such cracking is through mix design and suitable cohesion of the concrete to minimize bleeding. Abrupt changes in section depth can also be avoided at the design detailing stage to reduce the risk of cracking.

To prevent plastic settlement cracks, it is recommended to increase the sand content in the concrete mix. However, there is a limit to this approach beyond which bleeding may increase. The use of very clean sand, such as marine-dredged sand, can promote water movement, so it is advisable to blend it with dirtier sand that has a higher fines content (<150 micron).

The tendency to bleed largely depends on the properties of the cement used. Cement fineness plays a significant role, as finer particles hydrate more quickly, thereby reducing the rate of sedimentation. Rich mixes are less prone to cracking than lean mixes. Adding pozzolanic materials to the mix can reduce water content and add to the fines, which may also help in preventing plastic settlement cracks. However, high GGBS contents should be avoided, as they result in slower setting times, allowing for bleeding to continue longer.

In addition to the above measures, air entrainment admixtures can be used to stabilize the concrete matrix and reduce water movement, which may help in preventing plastic settlement cracks. Polypropylene fibers are another option, which can help to improve the stability of the concrete and reduce the risk of cracking. Overall, preventing plastic settlement cracking in concrete requires careful attention to mix design and appropriate construction practices.

Remedial Measures for Plastic Settlement Cracks

Plastic settlement cracks are not typically seen in the entire section of newly placed concrete, unless the structure is designed with trough and waffle slabs. These types of cracks are generally halted by the reinforcement, which causes the restraint. As a result, the structural integrity is usually not compromised.

However, it is important to seal the cracks, particularly on slabs, in order to decrease the risk of reinforcement corrosion. To prevent plastic settlement cracking, the concrete should be re-vibrated with a poker vibrator after the cracks have appeared but before the initial set. The appropriate time to perform this task varies depending on the mix characteristics and ambient conditions, and is the responsibility of the worker.

If carried out in a timely and appropriate manner, re-vibration can improve the situation or even fully restore the bond beneath the bar. Attempting to close the surface with tamping is unlikely to remove voids beneath the bar. If the cracks are noticed early on, and they remain clean and free of debris, cement can be brushed into the openings and allowed to set.

However, this method does not address any bond reduction caused by voids beneath the reinforcement. The treatment of cracks in older hardened concrete depends on the exposure class and the severity of the cracking. Resin injection can be used in some cases, but in others, full-depth breaking out and reinstatement may be necessary.

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Plastic Shrinkage Cracks in Concrete

Plastic shrinkage cracks are surface cracks that occur when the concrete is still in its plastic state, meaning it is still wet and has not yet hardened. These cracks typically form when there is a rapid loss of moisture from the surface of the concrete, which causes the surface to shrink more quickly than the interior. As a result, the concrete surface cracks, leaving a pattern of interconnected cracks that can resemble a dried-up mudflat.

Plastic shrinkage cracks are most common in hot and dry conditions, when there is high wind or low humidity. They can also occur when the concrete surface is exposed to direct sunlight or when the concrete is placed on a hot surface. The risk of plastic shrinkage cracks can be reduced by taking measures to slow down the evaporation of water from the surface of the concrete, such as covering the surface with wet burlap or using a fog spray. Proper curing of the concrete can also help prevent plastic shrinkage cracks.

What Causes Plastic Shrinkage Cracks in Concrete?

Plastic shrinkage cracks in concrete are caused by rapid moisture loss from the surface of fresh concrete during the early stages of hydration. When the surface of the concrete dries out faster than the interior, it creates tensile stresses in the concrete that can lead to cracking.

This can happen when concrete is placed in hot, dry, or windy conditions or when the surface is exposed to direct sunlight. Other factors that can contribute to plastic shrinkage cracking include improper mix design, inadequate curing, and improper finishing techniques. It is important to take steps to prevent plastic shrinkage cracks from occurring, such as using shading or windbreaks to protect the concrete surface and ensuring that proper curing methods are used.

FAQs About Plastic Settlement Cracks