Here are the key takeaways about daywork joints:
- A daywork joint is where new concrete meets already hardened concrete
- Daywork joints are needed when a pour can’t be completed all at once
- Daywork joints should transfer force and not leak
- There are different types of daywork joints like butt joints and keyed joints
- Daywork joints must be built carefully for strength and sealing
When buildings, roads, and other structures are made out of concrete, the concrete has to be poured into forms. But sometimes the whole project can’t be poured all at one time. Reasons might be that the structure is too big, there aren’t enough workers, or not enough concrete or other materials are available.
In cases like these, the concrete will be poured in sections. The first section will be poured and allowed to harden. Later, fresh concrete will be poured next to the hardened section. The place where the new concrete meets the old hardened concrete is called a “daywork joint” or “construction joint.”
This article will fully look at what daywork joints are, why they are needed, and how they are made. It will cover the different types of joints and what is important in building them correctly. Properly understanding daywork joints helps ensure strong, lasting concrete structures.
Making daywork joints right is an important skill. This article will provide a deep understanding of daywork joints. It will help anyone involved with concrete construction, from workers to engineers. Keep reading to learn all about the what, why, and how of daywork joints.
What Exactly Are Daywork Joints??
Daywork joints, also called construction joints, are where fresh poured concrete comes together with a previous section that has already hardened. The joint is the interface between the wet concrete and the hardened concrete.
Daywork joints happen when the entire concrete pour can’t be completed in one continuous process. The concrete sets and hardens too fast to allow this. Instead, the project has to be done in stages.
The first section is poured and allowed to cure. When work resumes, new concrete is poured right next to the existing section. The place where the two meet is the daywork joint.
Daywork joints are needed for large projects like buildings, bridges, and dams. There may not be enough workers, concrete, or other resources to complete the pour all at once. Daywork joints allow the work to be staged over days, weeks, or even months.
Daywork joints are also used if concrete needs to be poured in a certain sequence. Some parts may need to harden first to support later sections. Daywork joints make this possible.
Why Are Daywork Joints Important??
Daywork joints serve important functions. Here are the main reasons daywork joints are needed:
- Allow large or complex pours to be done in stages
- Allow construction sequence where some parts must harden first
- Make concrete structures possible that can’t be poured all at once
- Provide a strong connection between concrete sections
- Prevent leakage between sections in some cases
The most basic reason for daywork joints is that massive concrete structures can’t be poured all in one step. There are limits on how much concrete can be mixed and poured at one time. Daywork joints divide the project into manageable sections.
Daywork joints also allow the pour sequence to match construction needs. Some parts of a structure have to harden first to support other sections layered on top. Daywork joints permit this proper strength sequencing.
For concrete structures to perform right, daywork joints must correctly transmit forces between sections. They also need to prevent leakage in structures like dams and aqueducts. Daywork joints are carefully designed and built to serve these purposes.
Overall, daywork joints are an essential technique that allow large and complex concrete structures to be built in stages. They make possible many structures that otherwise could not be built from concrete.
How Are Effective Daywork Joints Constructed??
There are important considerations in forming daywork joints correctly:
- The fresh concrete must integrate well with the hardened section
- The joint must transmit forces across the two sections
- In some cases, the joint must prevent fluid leakage
- The joint may require added hardware like dowels or keys
- Joint formation depends on the type of joint being constructed
The interface between the fresh poured concrete and the existing hardened section is critical. The joint might be an uneven surface, or use dowels or keys. This creates a strong mechanical connection to transmit forces.
For joints that must be watertight, care must be taken to avoid gaps or voids. Sealing compounds may be used. The opposing surfaces should mate tightly.
Some joints use hardware like dowel bars placed into the existing concrete. The fresh concrete will flow around these. Other joints have protruding keys that the fresh concrete surrounds and locks into.
Butt joints just connect directly without special hardware. Keyed joints use protruding shapes to interlock the sections. Doweled joints use embedded rods. Different daywork joints serve different purposes.
Careful forming, preparation, material selection, and workmanship ensures daywork joints are strong, water-tight if needed, and durable. Well-built joints prevent future structure failures.
What Are the Different Types of Daywork Joints??
There are several standard types of daywork joints used in concrete construction:
- Simplest type of daywork joint
- Fresh concrete poured directly against existing hardened section
- No special keys, dowels, etc.
- Rely only on bonding of two sections
- Use keys or tabs protruding from existing concrete
- Fresh concrete flows around and locks into keys
- Keys provide shear transfer across joint
- Grooves also improve bond
- Use dowel rods placed in existing concrete
- Fresh concrete flows around dowels
- Dowels transfer load across sections
- Often used in slabs and pavement
Doweled and Keyed Joints
- Combination of the two methods
- Uses both dowel rods and shear keys
- Provides reinforced load transfer
- Very strong and secure joint
The type of daywork joint used depends on the structure, stresses, strength needs, and purpose. Many factors are considered in engineering effective daywork joints.
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How Should Daywork Joints Be Formed in Walls??
Vertical daywork joints in walls have some special considerations:
- Often use formwork stop ends to shape joint
- Stop end removed after concrete hardens
- Alternative is expanded metal mesh stop end
- Mesh left in place provides good bond
In walls, daywork joints are usually vertical since concrete is placed horizontally in lifts. The joint will be vertical between adjacent lifts.
For clean flat joints, a formwork stop end is placed at the end of the formwork. After the concrete hardens, the end can be removed, leaving the imprint of the joint.
An alternative is to use expanded metal mesh as the stop end. The roughened surface mesh leaves behind provides excellent mechanical grip without needing to remove the mesh.
For very heavy reinforcement, a standard stop end may be hard to place. The expanded metal mesh alternative is easier since it can simply be wrapped around the rebar cage. The mesh avoids congestion at the joint location while creating a rough joint.
How Can Daywork Joints Fail??
Daywork joints are vulnerable to certain types of failures:
- Cracking from force transfer issues
- Spalling from water intrusion
- Corrosion from water and contaminants
- Misalignment preventing load transfer
- Shear failure from inadequate keys or dowels
- Bond failure allowing separation
Cracks can form next to poorly designed joints as forces are not correctly transmitted across the joint. Related crack failures are spalling and corrosion.
Contaminants and water can be introduced at poorly sealed joints. This can corrode reinforcing steel or compromise the concrete itself through freeze/thaw cycling.
Inadequate shear keys, dowels, or bond prep can prevent the joint from properly transferring forces. This can lead to cracking or even full separation of the sections.
Proper daywork joint design, construction, materials, and preparation is crucial to avoiding future failures and compromising overall structural integrity.
Daywork joints, or construction joints, play a vital role in staged concrete construction. They allow large and complex concrete structures to be built in a sequence by separating the work into smaller sections. Fresh concrete is poured against existing hardened concrete at the joint.
For the structure to perform properly, daywork joints must correctly transmit forces across sections. Joints also need to seal out moisture in applications like water reservoirs. Several types of joints using different methods are used.
Careful joint design, preparation, materials, and construction practice results in durable, crack-free daywork joints. This allows sequentially poured concrete structures to act monolithically. Understanding daywork joints is key for civil and structural engineering