Joints in concrete: why do you need to saw finished structure?

Concrete is a durable construction mix. However, it also has a number of disadvantages that appear after it has gained strength. Main concrete disadvantages are susceptibility to cracking and crack formation.

The cracking process does not stop once the poured mixture is fully cured. Cracks in the structure appear during use and only get worse over time. For this reason, all concrete structures require periodic maintenance and repairs.

Cracks are initially formed at a level that is invisible to the human eye. The smallest particles of the concrete mix change size even after concrete has hardened.

Water molecules freeze and expand as air temperature drops, and evaporate when it gets warmer, leaving voids in concrete structure. Therefore, repetitive freeze-thaw cycles lead to cracks in the structure.

Deformation occurs due to several factors affecting concrete:

• Shrinkage during drying and operation
• Temperature and humidity changes
• Material mobility
• Chemical reactions occurring within a building structure or building element

At the production stage of concrete this problem is solved by adding special chemical additives. However, it is not possible to stop the process of cracking completely. In order to minimize cracking and negative consequences of this process, special concrete or expansion joints are made in concrete structures by sawing it.

Such joints allow you to determine where cracks are forming, where it will be possible to control and seal them. Sometimes different widths and lengths of concrete joints are required to improve material performance and allow materials to expand/shrink or move without damaging other structures.

Joints types

Shrinkage joints

Based on the title, it is already clear that such joints eliminate the consequences of uneven shrinkage of concrete mixture. Concrete structure surface dries much faster than lower layers. As a result, stresses will occur in concrete as soon as it has reached full strength. Unless special joints are used, cracks will appear immediately after setting and will only increase in size over time. Construction workers use a jointing machine that can cut the structure in depth by about a third of the full depth of the structure to make such gaps.

Insulating

Insulation joints are made to reduce stress between parts of the entire structure, for example, to separate slabs from other parts of the building, such as walls and foundations. Joints allow free vertical and horizontal movement between adjacent parts of structure and help reduce cracking when these movements are limited. Such joints are made around the perimeter of a room or around a column.

Structural

It is necessary if concreting does not occur all at once, but with technological interruptions. Structural joints allow you to avoid stress at the joints of the concrete already hardened and just laid. Notch is cut in the place of the joint with the help of tools. Such joints are similar to a “dovetail and sinking” joint in terms of connection.

Compensation or expansion joints

These joints are designed to minimize stress from rising temperatures or freeze-thaw cycles.

The coefficient of concrete expansion in volume under the temperature influence is 0.00001 per degree Fahrenheit, although this expansion can’t be noticed. Concrete deformation will be significant in large structures such as bridges and sidewalks.

If there is not enough space for material to expand, stresses due to temperature fluctuations will lead to cracking.

Joints are made in several places, providing a gap to allow the concrete to move freely. Expansion is a function of length, so concrete structures that are typically longer than 49 yards require one or more expansion joints.

Expansion joints create a weakened area in concrete and regulate the locations of cracks, usually in a straight line. Expansion joints should be placed so that panels are as square as possible and never exceed a length to width ratio of 1.5 to 1. Joints are usually spaced at distances that are 24 to 30 times the thickness of the structure itself. When using a joint groove for expansion joints, joint depth should be at least 1/4 of the thickness of slab.

Joint spacing greater than 15 feet requires the use of additional load transfer measures beyond the gaps themselves. Joints may be caulked into the concrete surface during placement. They may also be cut into the cured concrete surface. It is worth noting that the longer sawing is delayed, the greater likelihood of cracking before the sawing is completed.

Tools for joints

The following tools are commonly used to create concrete joints, although these tools may vary depending on the size and scope of the project. Here are the most commonly used tools:

• Handheld or step gutter: depending on the slab size , you can choose one of these two
• Self-powered or battery-powered tools: ideal for use on small to medium-sized projects
• Concrete saw: ideal for cutting concrete, but you need to calculate how deep the joint should be

Sometimes a straight grinder with a masonry blade is used instead of a concrete saw for cutting joints in dry concrete.

The cuts should be made at a predetermined distance and only after the concrete has gained sufficient strength, but before internal cracking begins. The timing of the cuts is therefore critical. Determining when to make the cuts depends on many factors, including the type of additives and aggregates used in concrete, air temperature and concrete mix design.

Joints technology

Determination of concrete sawing time depends on concrete hardness, type of cutting equipment used to saw, concrete mixture and weather conditions. Cutting too early causes cracking. This creates the effect of pulling the aggregate out of position by the saw blade, leaving a messy, loose edge along the cut.

This also leads to wear problems on diamond concrete saws. Sawing too late can lead to uncontrolled cracking as the concrete shrinks during curing.

Sawing can be started three to six hours after the concrete is poured, depending on weather conditions. The best way to determine if the slab is ready is to make test cuts to check for cracking.

Some contractors postpone sawing to protect their equipment and saw blade to reduce abrasion. There are several types of saw blades that can be used, depending on the type of concrete and how soon you can start cutting. Other factors that can cause excessive blade wear and joint cracking include:

• Cutting with a saw at high speed
• Using a saw with a bent spindle
• Using the wrong saw blade

Such factors as curing methods, board thickness and substrate type should be analyzed before choosing where the seams should be cut. After the analysis is done, mark the seams with a chalk line. If operating equipment with a water-cooled saw blade, make sure the water drains down to the blade. Allow blade to reach the proper depth, then start walking or moving the tool, following the chalk mark. Here are some guidelines when sawing concrete:

• Do not twist the saw blade.
• Do not allow the blade to rotate in the cut, as this increases wear on the bond.
• Soft metal blades should be used when cutting concrete with thick rebar.

It is necessary to cut joints one-quarter to one-third of the thickness of the slab. For example, for a slab that is 6 inches thick, this means cutting to a depth of 1.5 to 2 inches. The cut depth should be consistent with the specifications of the structure. If the cut is too shallow, accidental cracks may occur.

Filling joints

Open gaps in the structure can lead to other damage. Dirt or water entering the joints causes corrosion of both concrete itself and rebar in structure. It is necessary to seal concrete joints to avoid such negative consequences. The following materials are commonly used to fill the gaps:

• thermoplastics based on butyl rubber or bitumen, which provide for two technologies of use – by hot or cold curing
• silicone sealants
• mastics based on polybutylene
• thermosetting plastics made of polyurethanes, polysulfides and vinyl acetates