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There are many different types of joints in concrete construction. While most are deliberate and strengthen the structure, one, in particular, does not: the cold joint.
This article explores the causes of cold joints, how they can be prevented, and their distinctions from other types of joints in concrete construction.
Table of Contents
1. What Are Cold Joints?
2. Why Do Cold Joints Occur?
3. How Do You Prevent Cold Joints?
4. Cold Joints vs. Construction Joints
5. Cold Joints vs. Control Joints
6. Cold Joints vs. Pour Lines
When fresh concrete is placed against concrete that has already begun to set, it creates a visible discontinuity where the two layers do not bond properly. This is known as a concrete cold joint.
The visible change between the two concrete surfaces could be a slight difference in color or texture, or as bad as unsightly exposed aggregate.
Cold joints have a negative effect on the structure; they can reduce the load-bearing capacity by creating weak points in areas subject to shear or tensile forces.
They are also possible entry points for water or other contaminants, which could cause a concrete crack or deterioration.
Obviously, no contractor would intentionally build a structure with weak points. So, why would they allow a cold joint to happen?
Factors beyond human control sometimes create a cold joint during a concrete project. However, the majority of cold joints are caused by a lack of foresight and knowledge of their causes.
Understanding these and taking preventative measures can help reduce the chances of cold joints occurring in a concrete slab.
In the field of concrete, timing is everything.
If not managed correctly, delays between pours can jeopardize concrete structural integrity. Equipment malfunctions like pump failures can halt a project until repairs or replacements are made, resulting in a critical delay.
Shortages in materials or manpower also create significant obstacles, with each component being essential to the pour’s success.
Delivery issues, like traffic or logistical delays, can further prolong the arrival of concrete trucks. This complicates coordination between pours and increases the risk of cold joints.
Extreme temperatures pose unique challenges during concrete pouring and curing.
Hot weather reduces the setting time of the first batch, often leaving a narrow window to pour the next batch before it hardens. This can result in a weak connection between layers, leading to structural issues down the line.
Conversely, low temperatures slow the setting process and cause surface drying, which makes it harder for the new concrete to bond with the old concrete.
Proper consolidation is crucial to eliminate voids and ensure that the new layer of concrete adheres well to the previous one.
Without sufficient vibration or tamping, the fresh pour might not fill the gaps, creating weak points that could compromise the structure’s strength.
Consolidation failures often occur due to a lack of attention to the curing process or equipment use, resulting in poor bonding and reduced structural longevity.
Every concrete pour requires meticulous planning for all variables, from environmental conditions to the project’s complexity.
Inadequate planning can lead to unexpected delays, miscalculations in timing, or neglect of weather conditions that influence curing times.
For example, pouring all the footings first may leave enough time to create hardened concrete before the concrete floor is poured over them to bond.
Related: 5 Types of Concrete Joints and Their Uses
Now that we’ve identified the causes of cold joints, let’s discuss how they can be prevented.
Most of these measures involve:
However, as mentioned, some cold joints can’t be avoided. For those, some measures can be taken between pours to help form a deeper bond between the two layers of concrete.
Large concrete pours, or vertical elements, require continuous pouring to ensure each layer bonds effectively with the next. For example, there is no time for delays when pouring concrete walls or a concrete foundation.
Continuous pouring helps the concrete layers blend seamlessly, creating a robust, unified structure without weak points.
Also, when pouring the footings of a concrete foundation, starting from one end of the structure and pouring the footing and slab monolithically to the opposite end will ensure no cold joints form between elements.
Several admixtures are invaluable in managing concrete’s curing rate and adapting it to specific environmental needs:
Effective concrete vibration is crucial to integrating each new layer into the previous one. This compaction of the concrete reduces air bubbles and fills any voids, ensuring that fresh concrete fully adheres to the existing surface.
This process:
Regular maintenance and careful equipment checks are essential to prevent unexpected breakdowns that could delay pours. Before starting a job, any required equipment should be thoroughly inspected to confirm it’s working properly.
Concrete pumps, mixers, and other tools have to be reliable to support a continuous pour without interruptions, ultimately helping avoid delays that might lead to cold joints.
A skilled, well-coordinated team is key to achieving a smooth, uninterrupted concrete pour. Good teamwork and communication ensure that every worker understands their role and the importance of staying on schedule.
Timeliness and shared responsibility help eliminate delays and streamline the pour, especially when working under a tight deadline or harsh environmental conditions.
Workers also need to be meticulous. All surfaces must be kept clean before pouring the concrete. Any dirt or debris between the layers can break the bond.
When dealing with large structures, one way to prevent them from developing weak spots from cold joints is to pour in sections that connect with rebar and/or keyways.
Before pouring the concrete, rebar protruding about two feet can be installed into the form. Then, the rebar can be tied from the other side of the form before the next section is poured.
Keyways require a form to be placed before the concrete is formed that will leave a groove for the proceeding pour to fill.
Once the first pour is set, these forms must be removed, and a bonding agent is applied to the indent before pouring the next layer.
It’s vital to adapt to environmental conditions when pouring concrete.
In hot weather, keeping the surface moist helps prevent premature drying, while in colder conditions, insulating blankets or space heaters can be used to maintain a stable temperature.
Erecting wind-breaking tents can shield the pour from harsh weather conditions, further promoting a steady, controlled curing process and aiding in the formation of a durable, cohesive concrete structure.
Learn More: Can You Build Concrete Projects in Cold Weather?
When cold joints are unavoidable, applying a bonding agent to the initial layer can help create a strong link with the next pour.
Bonding agents act as a glue between layers, ensuring a more cohesive bond and reducing the risk of separation. This technique is especially useful for ensuring the long-term stability of the structure when continuous pouring isn’t feasible.
Non-structural cold joints can be fixed using a grout pump by injecting polyurethane or epoxy as waterproofing or sealants.
Not all joints in concrete construction are unintended. Joints are intentionally placed in concrete structures to allow controlled pauses during construction or to connect structural elements.
Unlike cold joints — which are unplanned — construction joints are designed with careful consideration to support load transfer and accommodate movement.
Construction joints are often placed at specific points, such as where a wall meets a beam or where long stretches of concrete need segmenting. These joints are known as isolation joints.
Construction joints require prepared, clean surfaces reinforced with elements like rebar to secure a strong bond between pours.
This strategic placement ensures these joints help — rather than hinder — the concrete’s load-bearing capabilities.
Cold joints, in contrast, lack this level of preparation and reinforcement. This makes them unintended weak points in the structure that could impact long-term durability.
Control joints, also known as contraction joints, are another type of planned joint in concrete.
They’re installed deliberately to manage and guide cracking occurring naturally due to things like:
A structural engineer typically specifies the placement of these joints, which are either saw-cut or pre-formed at precise intervals. These neat, straight lines guide inevitable cracks along predefined paths, preserving the slab’s strength as a whole.
While control joints are neat and deliberate, cold joints are unintended, often uneven lines or planes in the concrete that don’t benefit from pre-planning. Control joints play a positive role in concrete work by reducing the impact of cracking, while cold joints present potential structural risks.
Expanding joints are very closely related to control joints, which is well explained in our full article here.
Pour lines are often confused with cold joints, but they differ in one fundamental way: pour lines are primarily cosmetic and do not impact the structural strength of the concrete.
Pour lines occur when a new batch of concrete is poured over a layer that has started to cure but hasn’t fully set.
This distinction between batches often results in subtle surface-level differences, such as slight discoloration or minor texture changes, visible along the pour lines.
While both pour lines and cold joints may not be entirely intentional, unlike cold joints, pour lines don’t compromise the concrete’s strength or durability.
Distinguishing between pour lines and cold joints can be challenging but must be done. Even the trained eye can have difficulty simply looking at the concrete’s surface.
However, understanding the difference between these visual cues helps builders ensure that any weaknesses are identified and addressed early in construction, preserving the quality and stability of the concrete.
A deeper investigation must be done, which involves extracting core samples from the location in question, with the surface line at the center of the sample.
These samples are then examined under a microscope and tested to determine the extent of damage and how far down the line goes beneath the surface.
As this article has highlighted, cold joints are unintentional and often problematic occurrences in concrete structures that can be avoided.
By recognizing and planning for these potential issues, construction teams can avoid common pitfalls and build a strong, structurally sound concrete structure without cold joints.
Contact FMP Construction today for your concrete structure. Our professional team can ensure a seamless, resilient final product that will stand the test of time.
