Praised for its strength and durability, concrete is one of the most widely used building materials today. However, it can also be prone to expansion, which could disastrously affect any building’s longevity and structural integrity. So what causes expansion in concrete?
In this guide, we’ll take a closer look at concrete expansion. From the factors causing it to how to prevent or minimise it, everything you need to know is down below!
Common Causes of Concrete Expansion
Let’s begin with what causes expansion in concrete. Understanding these is crucial if you want to prevent damage to your buildings, not to mention ensure their long-term stability. Anyway, the most common causes include:
Chemical reactions can occur within any building material, concrete included. One of the most notable examples is the Alkali-Silica Reaction (ASR). This reaction happens when there are reactive silica minerals in the aggregates. These elements react with the alkali in the cement’s moisture, forming a gel that will eventually cause the entire concrete structure to expand under pressure.
Soil and water naturally contain sulphates. When this element comes into contact with cement, it triggers a chemical reaction that forms expansive minerals. This phenomenon is known as a sulphate attack, and it can cause concrete to expand, compromising structural integrity in the process.
Temperature fluctuations can also induce concrete expansion. Concrete expands when hot and contracts when cool (this has something to do with its coefficient of thermal expansion, which we won’t talk about for now). Constant exposure to these temperature changes will leave the concrete continuously expanding and contracting, eventually leading to microcracks and other issues.
Carbon dioxide from the atmosphere can penetrate the concrete surface and react with the calcium hydroxide in cement paste. This will form calcium carbonate, reducing the concrete’s volume and causing internal stress and potential cracking.
Delayed Ettringite Formation (DEF)
DEF is a chemical reaction that occurs when concrete is exposed to high temperatures during curing. It can form expansive mineral compounds, again leading to internal stress and cracks within the concrete structure.
Factors Influencing Concrete Expansion
As a ubiquitous construction material, several factors can cause concrete to expand or contract. These include:
The type, size, and characteristics of aggregates significantly impact concrete expansion. For example, a study by Weihua Jin and his team found that using glass as an aggregate can lead to ASRs. However, non-reactive aggregates pose a lower risk of inducing expansion.
The composition of cement, particularly its alkali content, is a crucial factor in concrete expansion. Higher alkali levels increase the potential for ASR. Using low-alkali adhesives or supplementary cementitious materials can help mitigate this risk.
The environment in which a concrete slab is placed plays a pivotal role. Moisture and humidity levels affect expansion, with higher moisture content accelerating chemical reactions like ASR or sulphate attack. The presence of sulphates in soil or water can intensify expansion issues.
Concrete’s response to temperature fluctuations is a significant consideration. Daily and seasonal temperature changes cause concrete to expand and contract. The cumulative effects of these cycles can lead to microcracking and potential deterioration.
Proper curing is crucial to control expansion. Inadequate curing can increase the risk of chemical reactions and expansion issues. Ensuring adequate moisture levels and temperature during curing helps reduce the potential for expansion-related damage.
The methods used during concrete placement and finishing can influence expansion. Improper consolidation or finishing can increase porosity, allowing moisture to penetrate the concrete and accelerate chemical reactions.
The design of a structure can affect its vulnerability to expansion. Building with intricate shapes or enclosed spaces might experience higher temperature differentials and expansion. Architects and engineers must consider expansion-related risks during the design phase.
Rigorous quality control measures are essential to prevent expansion-related problems. Regular testing of aggregates, cement, and the overall mix ensures that the materials used meet the required specifications and minimise the potential for expansion.
Types of Concrete Expansion
As an engineer or construction professional, you need to be aware that several common types of concrete expansion can negatively affect the integrity and longevity of your buildings. These include:
Chemical reactions within concrete can lead to expansion. One notable example is the Alkali-Silica Reactions (ASRs) mentioned earlier. This occurs when reactive minerals within the aggregates react with alkalis from the moisture present in the cement. This reaction will form a gel-like substance that will swell upon moisture absorption, thus causing cracks and expansion.
Exposure to sulphates in the environment can trigger sulphur attacks, leading to concrete expansion. Sulphates react with compounds in the cement, forming expansive minerals that disrupt the concrete’s structure. This type of expansion is particularly problematic in areas with high sulphate concentrations in the soil or water.
As touched on above, concrete’s volume changes whenever the temperature does. It expands whenever the temperature rises and contracts as it goes down. Daily and seasonal temperature changes will leave concrete structures experiencing cycles of expansion and contraction, eventually resulting in cracks and other problems.
Hydration Heat-Induced Expansion
Heat is generated whenever concrete undergoes hydration processes. This heat can lead to localised expansion in large concrete structures like dams or skyscrapers. The temperature difference between the core and the structure’s surface can also induce stress and cracking.
Concrete can absorb water, leading to swelling and subsequent expansion. Freeze-thaw cycles in colder climates can further exacerbate this problem. As trapped water freezes and expands, it can impose additional stress on the concrete.
Speaking of which, according to D.W. Hobbs, freeze-thaw attacks on their own can also cause concrete expansion. In regions with frequent freeze-thaw cycles, water can penetrate the concrete pores and freeze during cold temperatures, causing it to expand. When the ice thaws, it contracts, leading to internal stresses and damage to the concrete.
While concrete is relatively corrosion-resistant, the steel bars that reinforce it are not. Over time, rust will form within these support materials due to moisture and other environmental factors. Eventually, this will take up space within the structure, causing the concrete and expansion joint to expand and crack.
Autogenous Shrinkage Expansion
During hydration, a gel is produced from the reaction between the cement and water. This gel will eventually absorb moisture within the cement mix and reduce the concrete’s mass, a process called autogenous shrinkage. Left unchecked, it can cause microcracks within the structure, potentially leading to expansion since water will be able to penetrate the building through these vulnerable spots.
Best Techniques to Monitor Concrete Expansion
Construction and building experts can easily detect and manage potential expansion-related issues. Some of the most reliable techniques for monitoring concrete include:
This involves creating concrete samples and subjecting them to conditions that mimic real-life exposure. By monitoring these samples for cracks, deformations, or dimensional changes, experts can provide valuable insights into your building’s potential expansion problems.
Non-Destructive Testing (NDT)
These include techniques like ultrasonic testing, ground-penetrating radar, and electrical resistivity. Through such methods, professionals can assess the internal condition of your concrete structure without causing any damage. NDTs can also help identify areas of potential expansion, including spots with a high presence of cracks or moisture.
This involves attaching strain gauges to concrete surfaces. Experts can detect any problems that could lead to concrete expansion by monitoring these strains. It also allows them to evaluate whether or not the structure is within acceptable deformation limits.
This involves regularly checking and recording any changes in temperature within the structure. Since temperature fluctuations are one of the most common causes of concrete expansion, it helps professionals get valuable data on potential risks.
Digital Image Correlation (DIC)
DIC uses images of the concrete’s surface to track minute deformations over time. This technique is beneficial for monitoring localised expansion and identifying potential problem areas.
Tips to Prevent or Reduce Concrete Expansion
Preventing and mitigating expansion in concrete is crucial to ensure concrete structures’ integrity and longevity. Here are a few tips to follow:
Optimise Mix Design
Pay careful attention to the concrete mix design, including the type and amount of cement, aggregates, and supplementary cementitious materials. Use low-alkali adhesive to reduce the risk of alkali-aggregate reactions (AAR) and carefully select aggregates to avoid those with potential reactivity.
Control Water-Cement Ratio
Maintaining an appropriate water-cement ratio ensures sufficient hydration while minimising excess water that can lead to chemical expansion. A lower water-cement ratio results in denser and more durable concrete.
Use Admixtures Wisely
When using chemical admixtures, ensure they do not promote expansion or adversely affect the concrete’s long-term performance. Choose admixtures that improve workability and strength without compromising stability.
Consider Environmental Factors
Assess the environmental conditions where the concrete will be placed. Implement temperature control measures to reduce the impact of thermal expansion and contraction. Additionally, protect the concrete from exposure to sulphate-rich environments to prevent sulphate attack.
Perform Proper Curing
Adequate curing is critical to prevent early-age shrinkage and cracking. Apply appropriate curing methods, such as wet or membrane curing, to maintain the necessary moisture content and temperature during the initial stages of concrete hardening.
Control Aggregate Reactions
Identify aggregates with the potential for reactivity and consider using alternative materials or surface treatments to minimise their impact on concrete expansion.
Apply Protective Coatings
Use protective coatings and sealants to create a barrier that prevents water penetration into the concrete. This helps reduce the risk of chemical reactions and water-induced expansion.
Monitor and Maintain
Regularly inspect and monitor concrete structures to identify early signs of expansion or distress. Timely maintenance and repairs can prevent minor issues from escalating into major problems.
Find Professional Expertise
Engage experienced engineers and construction professionals knowledgeable about concrete expansion and its prevention. Their expertise can ensure proper design, construction, and maintenance practices.
So wrapping up, we learned what causes expansion in concrete, as well as a truckload of other important stuff. Here’s a list of what we discussed earlier:
- Many things can cause concrete expansion, including temperature fluctuations, sulphate attacks, and delayed ettringite formations (DEFs).
- Certain factors can also influence concrete’s tendency to expand, including cement composition, curing practices, and aggregate properties.
- There are plenty of ways to prevent or reduce concrete expansion, from optimising cement mix design to applying protective coatings.
At any rate, concrete expansion is a serious problem that should always be addressed. By following the guide above, you’ll familiarise yourself with what’s causing it and get tips on how to avoid or reduce its effects.