Cold Joint (Part-02)

How Cold Joints forms in concrete:

A Cold Joint is the intersection between the end of one concrete pour and the beginning of a new pour. The basic rule is to try to avoid Cold Joints by pouring straight through until the job is finished. The Cold Joint is a weak area and could allow the entry of water. If it must be done, inserting reinforcing bars in the fresh concrete of the fresh concrete of the old pour will tie in the new pour more effectively.
Figure-01 indicates how can a Cold Joint will form in a Concrete Slab,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-01: Cold Joint in a Concrete Slab

Cold Joints are formed primarily between two batches of concrete where the delivery and placement of the second batch has been delayed and the initial placed and compacted concrete has started to set. The full knitting together of the two batches of concrete under vibration to form a homogeneous mass is therefore not possible, unlike the compaction of two fresh workable batches of concrete. This could be a potential plane of weakness. Cold joints, unlike cracks that form in hardened concrete through tensile restraint, are not gaps in the concrete but merely seams containing no appreciable void structure. They are usually linear, closely joined and bonded. However, there is a danger of small voids in areas where the concrete is not fully compacted, as with any concrete pour.
Figure-02 indicates the formation of a Cold Joint in a Concrete Slab,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-02: Cold Joint forms in a Concrete Slab

Problems associates with Cold Joint:

A cold joint is an undesirable discontinuity between layers of concrete that occurs when one layer of concrete is allowed to harden before the rest of the concrete is poured in what is meant to be a single, solid mass. The discontinuity occurs between the layers due to the inability of the freshly poured, wet concrete to intermix with and bind properly to the hardened concrete. Such a discontinuity is often the result of logistical issues such as a contractor’s work schedule or an unexpected material shortage.

Figure-03 indicates undesirable continuous layer of Cold Joint which caused discontinuity between two concrete layer,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

 Figure-03: Undesirable continuous layer of Cold Joint & discontinuity between two concrete layer

Problems associated with cold joints range from the relatively minor to the very serious. At the less serious end of the spectrum, a cold joint may result in a visually unappealing discontinuity, called a cold joint line, that is visible on the surface once the concrete has hardened. This kind of aesthetic defect may simply be concealed rather than repaired.

------A more serious problem associated with a cold joint is the possibility of moisture intrusion into the concrete section. If water settles in a cold joint, it may lead to degradation of the concrete under certain environmental conditions. For example, as water expands when it freezes and then contracts when it melts, water trapped in a cold joint may cause cracking or erosion of the material. Moisture may also damage other things beyond the concrete mass if it is able to seep all the way through it.

Figure-04 indicates water absorption into old concrete section during new concrete portion is poured,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-04: Effect of water absorption on the new poured section

Additionally, a cold joint is an area of compromised strength. Concrete is notable for its high strength under compression, but it is much weaker under tension. A cold joint is even weaker under tension, and it is susceptible to shearing at the discontinuity.

Whenever possible, cold joints should be avoided in concrete construction by completing the entire pour for a given section in one session. This allows the entire section to harden in a continuous, solid mass. If this is not possible, several steps can be taken to mitigate the more serious problems.

Specialized waterproofing joint sealant may be applied to the joint to make it watertight, thereby protecting against potentially damaging moisture intrusion. A special surface preparation may be applied to the hardened layer before applying a fresh layer of concrete. This will strengthen the bond between the two layers. Another way to increase the strength of the cold joint is to insert reinforcing bar, or re-bar, into the first layer before pouring the next layer. This will better help tie them together and increase the tensile strength of the joint. It is also sometimes possible to locate the weakened joint in an area that is not critical to supporting a large load.

Repairing of Cold Joints:

Repairing of Cold Joints While Pouring Cement:

------ Mix the concrete from the earlier portion of the pour with the fresh concrete, if possible. Use a concrete vibrator to work the two sections into a single mix of concrete as much as possible. How hard, or setup, the first pour of concrete has become will determine if this is possible. If successful, this blending of the two concrete pours will provide the strongest slab that is the least likely to crack.

Figure-05 indicates concreting process to avoid/repair Cold Joint,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-05: Concreting process to avoid/repair Cold Joint 

------Place reinforcing rods between the first portion of the slab and the new pour. If the second portion of the pour will be more than two or three hours after the first portion the re-bar should be set in the first portion while it is still wet. After the second portion of the concrete is poured, the re-bar serves as a connection between the two slabs. The week joint of the cold slab may still crack, but with the re-bar connecting the two portions of the slab, the cement will not shift and form a displaced fracture of the cement.

Trowel the cement as smoothly as possible over the cold joint. Cold joints often remain visible after all the concrete has cured due to differences in the color of the concrete.

Figure-06 indicates the re-bar process to repair/avoid Cold Joint,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-06: Re-bar process to repair/avoid Cold Joint

Repairing of Cold Joints in Cured Slabs:

Repair small cracks at cold joints with thin mix or a concrete crack sealant. Closing and sealing the cracks prevents water from entering the crack and causing damage through freeze and thaw cycles.

Cut out bigger cracks using a concrete saw. This becomes necessary if the small crack has deteriorated over time and is causing breakage of the concrete, resulting in an open crevice with unstable concrete on the edges. Cut the concrete back far enough so both edges of the opening are high quality cement.

Figure-07 indicates how cracks form because of Cold Joint & how it can be removed,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-07: How cracks form because of Cold Joint & how it can be removed