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◎ Sound velocity test 

The velocity of ultrasonic wave pulse passing in the solid material depends on the density and strain, so the properties of a material is related to its strain and the velocity of ultrasonic wave in these materials tell the properties of the material.

We can examine the inner defect or scratch by finding out the reflecting wave that is run and come against the defect when the ultrasonic wave test is applicable for metal.

However, the technique like this can not be used on the concrete. The reasons is following. Echoes occur in lots of aggregates boundaries contacted when the pulse goes through the inside of concrete and in case of going through the inside of concrete, the velocity of ultrasonic wave pulse isn't regular as the case of passing the inside of metal and become different depending on quality or composition.

Usually, like the case of going through the metal, the intensive beam of pulse can't go through the inside of concrete. The one of reasons is that if high audio frequency pulse necessary to make the intensive beam go through the inside of concrete, it lessons extremely. And also, because it reflects to overlap to overlap at the boundary of aggregate, vibration beam is scattered. Therefore, as the vibration energy of ultrasonic wave in concrete is scattered to all directions, the high audio frequency used on metal can't be used on the concrete, woods, stone.

And also because the higher frequency is, the narrower radiation and the more decreasing vibration of pulse, that is more understandable. So the velocity of pulse going through the inside of concrete is very close related to the elastic modulus, because the elastic modulus changes largely in proportion to the change of density and poison's ratio. The elastic modulus of concrete increases as the mechanical strength increases.

And the velocity of pulse may be related to the cube, strength or the seal strength. Since such regular cement content and the type of aggregate change as the combination ratio of water and cement, air void ratio or the age on test change, there is the rational correlation between the velocity and cube strength. And the definite value from the testing result about the single curve that clear the relationship between pulse velocity and concrete strength has not existed yet, but a test is said that we presume the strength inspection of concrete in extent of 20KHz frequency and 50KHz is proper to outdoor concrete strength inspection. Therefore the ultrasonic waves necessary for concrete test is the velocity of pulse going through the tested object and we can examine the void, strength, frost heaving and burn of the inside of concrete.

The distance of pulse passing the inside of object is calculated from the following formula.

How to calculate velocity of electric wave or the other things.

☞ ⓐ With measuring electric wave time and distance of vertical or cross wave, it is given by where,

V :velocity of electric wave

L : distance cm

t : electric wave time

10 : coefficient for adding unit

☞ ⓑ Calculating of concrete depth, it is given by where,

L : depth cm

Vt: velocity of electric wave

Tr: length of time needed for reflection

☞ ⓒ How to get the dynamic poison's ratio.

With velocity of vertical wave and cross wave, it is given by where,

uo: dynamic poison's ratio

Vi: velocity of vertical wave

Vs: velocity of cross wave

☞ ⓓ How to get the dynamic modulus of elasticity.

With speed of cross wave and density, it is given by where,

GD: dynamic modules of elasticity kg/sec

Vs: velocity of cross wave km/sec

p : density g/cm2

g : acceleration of gravity 980 cm/sec

W : the weight of specimen g

V : the volume of specimen ㎤