Concrete Joints — Purpose, Types, and Tips for Placing and Saw Cuts
Concrete isn’t a ductile material. It does not bend or stretch without breaking and both are considered its greatest strength and weakness. Concrete’s high compressive strength and hardness is the main reason why it is always used in construction. However, concrete does move as well — it expands, shrinks and makes different parts of buildings move in many ways. This is where joints come into play.
While many building elements are built and typically designed with joints, including foundations and walls, we’ll limit the discussion to joints in concrete slabs. For this blog, here is an overview of joints, including its purpose, types, how to saw cuts, and some helpful tips in locating and installing joints.
The Purpose of Joints in the Concrete Slabs
So as the concrete moves, especially when it is tied to itself or to another structure, it’s called restraint. It causes tensile forces, which lead to cracking. In simple terms, restraint means that the concrete elements, whether a wall/foundation or the slab, isn’t being allowed to shrink freely as it contracts or expand with temperature changes.
Joints then allow a concrete element to move freely of other parts of the structure or building. These joints also let concrete shrink whenever it dries, thus, preventing it from the internal restraint. For the record, internal restraint is typically created whenever one part of the slab shrinks in a different direction. Think about when you feel a part of you wants to do something, and another part prefers to do something else. Concrete feels the same way!
In slabs, there are three major types of joints:
· Isolation joints (sometimes functioning as expansion joints)
· Contraction joints (sometimes called control joints)
Concrete joints are definitely used to compensate whenever concrete shrinks or expands while temperature changes. Sometimes because the material and span or width of the concrete joints are often required to improve the material’s performance, it allows the materials to contract/expand or move without damaging any structures.
Differentiating Contraction Joints and Isolation Joints
1. Concrete contraction joints
This type of joint is intended to create weakened areas in the concrete. It must also regulate where cracks usually will occur, typically in a straight line. The contraction joints must be placed to produce panels that should be square as possible. The length to width should never exceed a ratio of one and one half to one. Apparently, these joints are typically spaced at distances equal to twenty-four to thirty times the thickness of the slab.
Joint spacing greater than fifteen feet may require the use of load transfer devices. At the time of placement, contraction joints might be tooled into the surface. The joints might also be sawed into the hardened concrete surface. It is very crucial to know that the longer sawing is delayed, the potential for cracks to establish will get higher.
2. Concrete isolation joints
Concrete isolation joints are typically used to separate concrete forms and slabs from other parts of a structure. These joints allow independent movement between adjacent structural members, reducing cracks whenever such actions are being restrained. It allows for contraction and thermal expansion without inducing extreme stress to the system.
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How to Place Concrete Joints
The following tips are highly recommended and should be observed when placing concrete joints:
· Joints must be spaced for about ten feet and a maximum of fifteen feet.
· When utilizing a joint groove, especially for contraction joints, the joint must be a minimum depth of the slab’s one-fourth thickness.
· Keyed joints aren’t recommended for industrial floors.
· Maximum joint spacing must be twenty-four to thirty-six times the slab’s thickness.
· The pre-molded joint filler must be used to separate slabs from footings or building walls.
· Place two inches of sand over the top of the footing to avoid bonds.
· Dry-cut joints must be made between one to four hours right after completion of finishing.
· Joint spacing must also be chosen. As a result, the concrete areas are approximately square.
· If the slab contains some wire mesh, it is highly suggested to discontinue the mesh across contraction joints.
· Metal dowels must be used in slabs since they can carry heavy loads.
· It is suggested to have concrete joints alongside column lines, either keyed or sawed.
· Use isolation joints between columns, walls, and footings or where sidewalks or curbs meet other concrete structures.
· Plan exact locations on all joints, including saw cutting and its timing.
· Always look for materials that are penetrable and are easily bonded to concrete.
· Materials used on concrete should be flexible enough to deform or absorb as needed. It should be able to restore back to its original state.
Where, When and How to Saw Cuts for Concrete Joints
Saw cuts are often used to create controlled concrete joints, which helps control areas where cracking occurs because of shrinkage. The cuts must be made at a prearranged spacing and only right after the concrete has started obtaining enough strength. Everything should be set before internal cracking begins. Hence, the timing of saw cuts is considered crucial. Determining when to make cuts heavily relies on many factors, including the concrete’s aggregates, type of admixtures, the concrete’s mix design, and air temperature.
When to Saw-Cut Concrete
In addition to the weather conditions and concrete mix, determining when to saw concrete is usually based on the concrete’s hardness and the cutting equipment used for saw cuts. Cutting too early may cause raveling, a certain effect created when the saw blade pulls the aggregates out of its position. As a result, it leaves a weakened, messy edge along with the cut. Sawing too late may result in uncontrollable cracking since the concrete contracts during curing.
Some contractors tend to delay sawing in order to protect all of their equipment. Delaying can be necessary when sawing blades to minimize blade abrasion. There are various types of blades you can use, depending on the concrete type and how soon you started creating cuts. Here are other factors that can lead to extreme blade wear that you should be aware of:
· Saw cutting at increasing speed
· Pushing the blades too hard
· Using the wrong saw blade
· Using a saw that has a ben spindle
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Where to Saw-Cut Concrete
Before you begin saw cutting concrete, it is crucial to select where the cuts are done. It is suggested to start saw cuts at or on the center of column lines. Concrete joints must be spaced at twenty-four and thirty-six times the slab thickness. Joint spacing typically ranges ten to eighteen, depending on the amount of strength/reinforcement the slab has.
If you are using higher shrinkage concrete, you may want to reduce the cut spacing instead. Below are some factors that will help you in determining where to saw cut:
· Make saw cuts nonstop
· Try to form some square patterns
· It is crucial to choose a specific area that does not have consistent steel reinforcement from one square or slab to another.
· It is much better to get smaller cracks than to increase expenses by maintaining several joints.
How to Saw-Cut Concrete
Factors like slab thickness, curing techniques, base type, and slab length must be analyzed before choosing where joints must be cut. Once you laid out the concrete joints that are subject to cutting, mark them with a chalk line. If you are using water cutting equipment, ensure that the water is flowing down to the blade. Let all the blades to reach the required depth, then move or operate the equipment following the chalk line mark.
Below are some recommendations and tips when sawing concrete:
· Do not allow the blade to spin in the cut since it increases wear on the bond.
· Do not twist the saw blades.
· When cutting concrete using heavy rebar, use blades that have softer metal segment bonds.
· Use a construction planner to ensure that everything is completed safely and on time.
· Always make use of the required PPE (personal protection equipment).
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Key Takeaways
It is important to note and understand the various types of concrete joints and be active in deciding where, when, and how to saw cuts for these joints. Oftentimes, jointing isn’t taken seriously enough these days. However, specifications for control joints such as timing, depth, and exact place have always played an essential role in controlling the efficiency of concrete joints.
