Thermal cracking
One of the most significant challenges that arise from temperature variations is thermal cracking. For civil engineers, as soil expands and contracts in response to heat, it can develop cracks, affecting the lifespan of the structures they’re supporting. This singular point affects the very design of constructed structures, and planning processes.
What is thermal cracking: its definition, types and causes
Thermal cracking is the formation of cracks in concrete and asphalt structures due to temperature fluctuations that induce expansion and contraction of materials. This phenomenon compromises the structural integrity of infrastructure, leading to costly repairs and safety concerns. Thermal cracks are not limited to mass concrete structures but also others, such as pavements, concrete structures working in high-temperature environments, etc where mechanical expansion in response to temperature fluctuations occur.
Types of thermal cracking
Thermal cracking poses a great challenge that compromises the longevity and performance of structures. Understanding the different types of thermal cracking is essential for effective management and prevention strategies.
Transverse cracking
Transverse cracks are cracks that run perpendicular to the centerline of a roadway or a structure. As the pavement cools, it contracts, and if internal stresses exceed the material’s tensile strength, cracking occurs. This type is non-load related and appears irrespective of traffic loading conditions. The cause for it is a weak subgrade, or weak soil underlying the paved road.
Longitudinal cracking
Longitudinal cracks run parallel to the centerline of pavement or structure. These are caused by issues such as layers, fatigue from load stress, or poor joint construction. While planning, if the soil hasn’t been stabilized to handle repetitive loading, and compaction to lock the soil and improve its tensile strength, these types of cracks tend to form quite quickly.
Block cracking
Block cracking appears as interconnected rectangular patterns. It is predominantly caused by the shrinkage of asphalt pavements, which fail to expand and contract adequately with temperature changes. The condition is exacerbated when the asphalt binder is of low quality or if the mix was prepared too dry.
Plastic shrinkage cracking
Plastic shrinkage cracking occurs when the concrete is still plastic and is caused by surface shrinkage of the concrete. When the rate of moisture evaporation from the surface exceeds the rate of the bleed water (excess water that rises to the surface of freshly poured concrete), this type develops. Appearing in the first few hours after concrete placement, plastic shrinkage is often short and localized.
How can thermal cracking in concrete be predicted?
To figure out, if a concrete slab is prone to cracking, prediction methods are useful with each employing different methods.
PCA method
The Principal Component Analysis method is a multivariable statistical approach. The method analyzes and reduces the complexity of variables affecting concrete’s thermal behaviour. Relevant variables such as temperature gradient, heat of hydration(heat released as the cement reacts with water), cement content, and size of the concrete mass are collected. The method calculates 10°C temperature rise for every 100kg of cement.
Shmidt’s method
Shmidt’s method is a simplified procedure based on the size of the concrete structure and the heat of hydration. Using Shmidt’s equations, the thermal stresses are predicted based on the temperature differences between the center and surface of the concrete element. This method should be performed by an experienced engineer. However, this is less applicable to smaller concrete elements.
ACI 207.2R Graphical method
The ACI 207.2R method involves graphical charts and equations based on empirical data. The temperature gradients and stresses are analysed by evaluating the heat of hydration of the cement. Even though simple to use without extensive data collection, it can be less accurate in highly dynamic or complex projects.
Acoustic Emission Testing for Asphalt Pavements
The acoustic emission testing (AET) techniques assess low-temperature cracking performance in asphalt pavements. This approach allows the evaluation and characterization of various asphalt binders and mixtures, including recycled materials. Sensors are used to detect AE and then convert the waves into electrical signals so that it can be recorded.
Causes of thermal cracking
Thorough understanding of fissure formation is pivotal to protect our concrete infrastructures from the damaging effects of thermal stresses. To find out the reasons why heat stresses appear, the following reasons need to be distinguished.
Types of cement
Different types of cement generate varying amounts of heat. High-heat cements like refractory cements, cause major temperature differences within the concrete mass, leading to the risk of thermal cracking. Conversely, low-heat cements, such as specifically formulated for use in mass concrete, produce less heat being less prone to induce thermal stresses.
Temperature Differential
As concrete and asphalt are exposed to varying ambient temperatures, it expands when heated and contracts when cooled, thereby creating internal stresses. If these stresses exceed the material’s tensile strength, cracks develop. In cold climates, the freeze-thaw cycle exacerbates this effect, as moisture infiltrates small gaps, freezes, and expands, furthering the damage. Moreover, direct solar radiation causes external walls, particularly thin walls, to experience extensive thermal variations.
Creep movement
During the heating phase, compressive stresses may develop, which can be relieved through creep. Creep is the gradual, time dependent deformation of concrete under sustained loads. As concrete expands due to heat, creep movement causes localized tensile stresses in areas where the material is constrained, leading to fractures and deformation.
What are the effects of thermal cracking on concrete structures?
Thermal cracking can have detrimental effects on concrete structures. If left unaddressed, cracks will propagate and worsen, leading to more structural issues and costly repairs.
Disruption of Hydration
Cracks formed during the curing process disrupt the uniformity of hydration. Through creating weak points where cement particles cannot fully hydrate and bond, temperature gradients compromises the overall strength development of the concrete.
Reduced load bearing capacity
Ruptures disrupt the material’s continuity, compromising the ability to resist applied loads. These act as stress concentrators, which propagate under loading conditions, leading to a reduction in the cross-sectional area of the concrete. It also affects the bond between steel reinforcement and concrete, reducing the structural strength.
Increased permeability
Cracks act as a channel for the ingress of water, salts and chlorides. Increased permeability facilitates the penetration of moisture, which promotes the corrosion of reinforcing steel. This leads to spalling, loss of cross sectional area of reinforcement, and further degradation of concrete.
Impact on structural integrity
Thermal cracking slows down the strength gain of concrete and accelerates corrosion of reinforcements. Over time, these worsen with repeated temperature cycles, leading to structural fatigue, reduced stiffness, and increased maintenance costs.
Design considerations for minimizing thermal cracking
Effective strategies begin in the design phase, where restraints in various concrete elements should be accounted for to accommodate thermal expansion and autogenous shrinkage.
Material selection
The use of air-entraining mixtures like Vinsol resin improve concrete’s workability, resistance to freeze-thaw cycles, and overall durability. Moreover, durable materials like modified asphalt or polymer additives being resilient to temperature fluctuations and organic acid exposure mitigate adverse effects on concrete integrity. Supplementary cementitious materials like fly ash, slag or portland cement generates lower heat of hydration.
Construction Practices
Maintaining strict quality control during batching, mixing, and placing the concrete cease inconsistencies that lead to weak spots in the concrete. Properly
proportioning mix constituents and avoiding excessive water in the mix are essential to achieve optimal strength and durability. Appropriate curing maintains uniform temperature and moisture levels throughout the concrete, minimizing the risk of thermal cracking.
Reinforcement detailing
Reinforcement must be placed strategically throughout the concrete mass to resist internal resistance and crack propagation. Close spacing prevents larger gap widths. Also, mesh reinforcement or woven wire fabric offers more uniform stress distribution. Furthermore, ensuring the proper expansion joints of the structure hold thermal movement of concrete.
Adequate formwork techniques
Formwork technique is the process of creating temporary or permanent molds called ‘forms’ into which fresh concrete is poured. One effective technique is the use of insulating formwork. It aids in providing uniform temperatures within the concrete mass by reducing the rate of heat loss from the surface. This technique is highly beneficial during cold weather conditions when the risk of temperature differentials is high.
Repair methods for thermal cracking
Despite our best efforts, cracks might appear under certain circumstances. It is vital to address these promptly to limit the further damage. Here are some of the common repair methods:
Crack injection
In the crack injection method, a liquid epoxy or polyurethane resin is injected into the fractures to fill and seal it. The resin prevents water penetration and maintains the structural integrity of the concrete. This method is effective for narrow cracks.
Routing and sealing
In the routing and sealing process, the crack is widened and cleaned using specialised tools. Then, a suitable sealant is applied to fill the void which provides a flexible barrier. This is recommended for wider cracks.
Overlay or resurfacing
In cases where the thermal cracking is extensive, overlay or resurfacing is applied. The method involves applying a new layer of concrete or a specialised overlay material to the existing surface. The overlay not only covers the gaps, but also reinforces the structure, enhancing its durability.
How Geosynthetics Help Mitigate Thermal Cracking?
Geosynthetics mitigate the heat- induced cracks in concrete and asphalt structures, especially in pavements, roads, and large concrete slabs.
Geotextiles provide flexibility to the surface , allowing it to move more freely in response to temperature changes. By preventing the intermixing of different soil types, geotextiles reduce the possibility of uneven settlement, which could otherwise cause cracks. The porous nature of geotextiles allows for moisture drainage, maintaining optimal curing conditions for concrete.
Geocells limit the movement of confined soils. Through helping to distribute applied loads across the soil, geocells reduce the concentrated stress points. The cellular structure of geocells provide thermal insulation, reducing temperature differentials between the surface and subgrade.
Geogrids hold the aggregate together, hindering the excessive movement under stress. By reinforcing the soil or base layer, geogrids also reinforce the stiffness of the overall pavement structure. By anchoring the surrounding material, these reinforced polymer grids limit the deformation that leads to crack formation.
When used as an underlying layer beneath concrete slabs or pavements, geomembranes inhibit moisture ingress from below, reducing the risk of freeze thaw cycles. This is critical in maintaining consistent moisture levels during curing, thereby reducing shrinkage cracks caused by rapid drying.
Thermal cracking in concrete is a significant concern in civil engineering, especially as the thickness of concrete structures increases. To avoid this, it is crucial to understand and control the forces within a concrete element starting from the planning stage.