Cold Joint (Part-01)

Hello, Good-Day to Everybody.

Today I will tell you about a topic which is frequently seen in practical life of a Civil Engineer but rarely read at your books, but in practical life usually it can be seen always.

Now, I am going to explain you about “Cold Joint”.

------ Engr.Snehashish Bhattacharjee (Tushar).

Cold Joint:

A Cold Joint is a plane of weakness in concrete caused by an interruption or delay in the concreting operations. It occurs when the first batch of concrete has begun to set before the next batch is added, so that the two batches do not intermix properly. Sometimes Cold Joints occur because of emergency interruptions & delays and sometimes because of the work stoppage at the end of the day, but they can also occur from poor consolidation.

Figure-01 indicates a cross section of a road with cold joint,

Figure-01: Cold Joint

To prevent or avoid Cold Joint in walls, beams and other structural components it is necessary to place concrete in layers about 18 inches deep and intermix each layer with the previous one by using a vibrator. Placement of concrete should begin in the corners and work toward the center. When slabs are placed the concrete should be placed against the preceding batch and not dumped in an individual pile. On sloping grades the work should proceed uphill. In hot weather a retarding admixture may be needed to slow the setting time.

Cold Joint is simply a joint of concrete pour which is usually seen in construction field, which will happen naturally for long length structures with a slope such as long length span of a bridge, long length slab of a building structures etc.

For Example,

Suppose we have a long span of 38 meter of a Flyover and we are unable to pour concrete the total span at a time. Now see the following figure,

Figure-02 which indicates how can Cold Joint forms in concrete in practical life,

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

 Figure-02: Cold Joint forms in concrete (at practically)

Now let’s analyze this Figure-02. From this Figure-02, there are two Piers and a 38 m span is in between them. The distance between Pier-A & Pier-B is 38 m. This is a Cast-In-Situ Span. So, we need to cast this span at site. It is needed to be remembered that there is a slope between two piers, the slope is downward from Pier-A to Pier-B.

Now, what will be the concreting process of this Cast-In-Situ span?

At first let’s start our concreting process from Pier-A. Our destination is to concrete pour up to 13 m from Pier-A. This 13 m is not any fixed distance, actually there is a slope between two Pier (downward slope from Pier-A to Pier B), that’s why at the time of concreting when concrete was in liquid state, it behaves like water & wanted to go to downward. Because of more amount of liquid material will create a free flow & will want to go downward. It causes the damage/not proper quality of casting process.

So that, we need to stop at a distance (here is 13 m, just say). Then after casting at Pier-A, now we will go to the other side at Pier-B, here we will do concreting process at the distance 13 m also from Pier-B.

So, we have completed concreting process 13 m from Pier-A & 13 m from Pier-B (total 13 m+13 m=26 m), other 12 m (38 m-26 m=12 m) in between is not be filled up yet by concrete. Now when we re-back to start our concreting process at the last 12 m space then the new concrete pour will go to create a bond with the concrete which we have already concreted (26 m). The concreting process which is already completed (13 m from Pier-A & 13 m from Pier-B) going to be hardened and already become hard than new concrete pour (12 m space). Here bonding between the new concrete pour and old concrete pour not be happened properly and create a weak point as a joint which we have called “Cold Joint”.

Actually Cold Joint is not acceptable in case of any concreting process. Any Engineer does not like/appreciate Cold Joint, but in some situation Cold Joint is performed where we can’t do anything.

For Example, in this situation (Figure-02), we have nothing to avoid Cold Joint because,

        ---This is a span with long length so that we could not continue concreting process simultaneously,

       ---There is a slope so that we could not pour the concrete continuously, because concrete is in liquid state which behaves like water and want to go downward automatically during concreting time, so quality can’t be maintained properly, so that we have a distance limitation.

        ---Here, It will be helpful for us to do concreting process at last 12 m space because concreting at the other two sides (Pier-A & Pier-B) have already done & it become hardened during the last casting process at last 12 m space, so that concrete could not go anywhere from its own place.

Different types of Cold Joint:

Cold Joint can be forms in concrete in Vertical & Horizontal both direction also,

Figure-03 indicates Vertical Cold Joint forms in concrete,

Figure-03: Vertical Cold Joint

Figure-04 indicates Horizontal Cold Joint forms in concrete,

Figure-04: Horizontal Cold Joint

How can we avoid Vertical and Horizontal Cold Joint:

Assume that any foundation contractor is not able to pour all of the concrete for a foundation at once, leaving vertical and horizontal cold joints. They can prepare the surface of the concrete after the first pour by,

By using epoxy bonding agents named Anti-Hydro mixtures:

---They can use a concrete additive called Anti-Hydro. It’s available in one-gallon jugs and five-gallon cans at most builder supply stores for about $13 per gallon. Mix one part Anti-Hydro to three parts clean water, and add enough fresh (not old, lumpy, or partially hydrated) Type I Portland cement to make a thick, rich slurry, or "Slush Coat." Apply this mixture liberally with a stiff brush to the clean cold joint immediately before you pour. If they are on a big pour and the slurry has started to set up by the time they get to it, just brush on more. This will result in a strong, continuous bond.

---Anti-Hydro increases the percentage of hydration in concrete without affecting its chemical composition, resulting in a denser, harder, stronger, and more waterproof material. It also accelerates the rate of hydration, so it has many uses with all Portland cement-based products.

---There are also many different Epoxy Bonding Agents available from a variety of manufacturers (including Anti-Hydro). The better quality epoxies are expensive, but are supposed to provide a joint at least as strong as the concrete.

By using Extra Reinforcement method:

If anyone have extra reinforcement that extends out of the cold joint 30 to 40 bar diameters (at least 15 inches for #4 bar, 19 inches for #5 bar), he can tie in the rest of this re-bar and continue the pour.

What an Civil Engineer will do to avoid Cold Joint at present instantly in the construction field:

Any Civil Engineer do not like cold joint, but Cold Joint happens. If engineer know they have got another one coming in the construction field, here are several things they can do,

---Try to avoid cold joints in the middle of the wall, where the loads are high.

---Let the re-bar run 2 to 3 feet out of the concrete at the joint so you can tie into it when you continue the pour.

---If there is not already re-bar in place where an engineer stop the pour, put some in before the concrete begins to set.

To Be Continued...... ...... ...... ...... ...... ......