What actually is a cold joint?
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 more difficult, and sometimes not achieved. This could act as a potential plane of weakness.
Cold joints 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.
On this photo (right), three cold joints can be seen only after the injection procedure begins. Those were the consequence of delays in the pouring process, which they assured us were never longer than 15 minutes!
Generally, cold joints are not a problem structurally if the joint is under static conditions. Most civil engineers and on site technicians forget the ‘if’ part. Cold joints perform poorly when transmitting the vibration energy generated by the machine; thus the upper part of the foundation, above the cold joint, is essentially the only one dampening the vibrations.
To illustrate this point, a transmission test for a multifrequency signal was carried out:
As shown in the picture on the left, the transmission of the signal above the cold joint takes 78.4 μs with no signal lost. A second test through the cold joint was made (right), showing that the transmission time has increased by a factor of 10 and indicates (red light flashes) that a part or all the signal is lost. The explanation is quite simple, the signal emitted above the cold joint had had to travel through the concrete to the rebar, from the rebar across the cold joint to the lower part of the foundation and then from the rebar to the concrete below, reaching the sensor.
This indicates that although the foundation may seem sound, the vibration dampening is only carried out by the upper part of the foundation, not the whole of it. This increases the vibration levels in the machine, induces shear stresses where they shouldn’t be, and builds up the tension and compression cycles which produces abrasion, worsening its condition and allowing pollutants to enter it, worsening it even more. All this mechanisms eventually provide the cold joint with enough energy to grow and form a visible crack.
Next picture shows the result of a core drill test. The cold joints marked, present useful information about the actual building of the plinth.
Some of the other cracks are a consequence of the extraction procedure, and others are due to the high level of vibration suffered by the machine.
Not all the cold joints are due to delays between concrete-filled trucks. Most of them appear because the foundation is complicated and is built in stages. For example, the foundation of a big diesel engine is built in many different stages, depending on the intricate of the foundation. This approach, though, prevents the foundation to act as a whole for inertia dampering and vibration absorber if cold joints are not addressed. Using a small quantity of AT440 as a bonding agent between pours will guarantee the correct transmission of vibration energy and will allow the foundation to act as a whole.
Other cold joints are formed close by the ends of the anchor bolts. These are generated because when pouring concrete, there’s a time break when the concrete reaches the anchoring hight. The on-site supervisors normally stop the pouring to allow a re-check of the position, alignment, and the rest of parameters of the anchoring system. When the pouring of concrete is delayed to such an extent, and its not vibrated properly afterwards, the cold joint is very likely to appear. This cold joints are very prone to give problems, because the oil, grease and other pollutants will get inside the foundation through the interface between the anchor bolts and the concrete (due to the bad adhesion properties of cement grout to steel). When they reach the end of the bolts, they reach a weak spot because of two reasons: the cold joint, and the fact that some parts of the concrete there are in tension (because of the bolts). The pollutants will stay there, the crack will find there a sweet spot to begin existence, and will grow throughout the cold joint until it reaches all four sides of the plinth. Oil, grease and the other pollutants will help in its growth, and will slowly permeate everything that crosses its path.