COLD JOINT
What is cold joint?
When we do concrete installation of multiple times in 1 block of structure, freshly mixed concrete before and after that did installation does not unify it, so that time lag occurs between each installation. We name adhesive failure of this placing joint generally and say cold joint. Cold joint does not breed if we do installation of concrete in succession. However, it is very difficulty practically that installation of concrete continually by what we transfer it from a freshly mixed concrete factory to the spot and the installation place. We set up a freshly mixed concrete plant in the spot, and there are methods to do installation of 1 block in succession, but for a quantity of use of concrete, such a facility is difficulty so that it costs expense very much.
The point that occurs of cold joint
A cause of cold joint
It is the phenomenon that is not avoided by construction of concrete structure in the present that a placing joint occurs in installation of concrete. It is supposed that the following thing is generated when time lag occurs in installation in 1 block.
1. Between the freshly mixed concrete which we turned installation into first and the next concrete, cold joint occurs when we did not enough hit it and harden it. However, we can not hit it and harden it by shape.
2. Because levee crown of the concrete which did installation first touches it with air, hardening progress earlier than the inside, and cold joint occurs by what we do not unify with concrete doing installation next.
3. Surplus water rises in levee crown of the concrete which did installation first (bleeding), and cold joint occurs by what we do not unify with concrete doing installation next.
4. Cold joint occurs so that there is a big difference in materials in the freshly mixed concrete which did installation first and the concrete which did installation next.
In addition to the cause that we listed in the above, there are a lot of the causes of temperature difference, a humidity difference, the rain by having done installation.
A percussion test / a visual inspection
The external wall investigation situation
A figure of external wall percussion test / visual inspection result
A figure of tunnel percussion test / visual inspection result
The tunnel investigation situation
More than one cold joint (elastic wave)
Even if some cavity exists, elastic wave is propagated and reflects it in each boundary, and it is received by vibration. In addition, there is not broken-out section, but characteristic reflected wave is propagated from the boundary when materials are heterogeneous.
Cold joint elastic wave investigation situation
An elastic wave output sample
The elastic wave investigation situation
Cold joint (a ultrasonic)
When there is cold joint, and concrete does break, the ultrasonic which turned oscillation into from a transmitter does not propagate in a receiver. In addition, when the inside does break even if there is not a crack forward, data of anything are indicated by a measure. When a cavity exists, a ultrasonic is not propagated.
Cold joint ultrasonic investigation situation
Ultrasonic investigation situation
Adhesion test
We extract a core in a section of a point supposed that it is cold joint and do tensile test.
