*Ultrasonic pulse velocity test*

Ultrasonic pulse velocity test is a nondestructive testing is performed by positioning the source and receiver on either side of the area in question. In almost applications, UPV source has resonant frequencies ranging from 20 to 150 kHz. By measuring transmit time of ultrasonic pulse between known distance of material, one can determine the velocity of test material. Velocity is a physical parameter determined material property. There are three basic ways in which the transducer may be arranged, these are opposite faces (direct transmision), adjacent faces (semi-direct transmision) and same face (indirect transmision).

In concrete testing, UPV method utilizes to determine homogeneity, identify and map voids and honeycomb, estimate the depth of cracks, monitor the change of concrete, calculate the dynamic elastic modulus and Poisson ratio.

Concrete strength can be determined by the relation of ultrasonic pulse velocity and concrete strength built in accordance with Vietnamese standard 3118: 1993 at cast samples or samples drilled from elements, or the combination between the ultrasonic pulse velocity and rebound index in accordance with Vietnamese standard 9335: 2012.

Concrete strength can be also determined more exactly if SONREB curve is built. In order to create a SONREB curve for estimating concrete strength of a particular concrete, it is necessary to carry out both UPV measurments and rebound measurements on samples of the concrete and then correlate this to the compressive strength determined in the same samples.

The SONREB curve is expressed in the format:

fck = a.VbSc

where fck is the compressive strength;

V is the ultrasonic pulse velocity;

S is the rebound value;

a, b and c are constants.

In addition, the ultrasonic pulse velocity is used to determine the homogeneity of concrete in the bored pile Vietnamese standard 9396: 2012.

*Rebound hammer test*

The underlying principle of the rebound hammer test is the rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the pring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer. Rebound hammer test is done to:

- Find out the compressive strength of concrete by the experiment relation between the compressive strength (R) and rebound index (n) received on the same samples;
- Estimate homogeneity of the concrete on elements.
- Evaluate concrete quality;
- Compare concrete quality of elements.

The rebound hammer test can be used to differentiate the acceptable part of element with the questionable part of element or compare two elements based on rebound index (strength).

The rebound hammer test is in accordance with Vietnamese standard 9334: 2012.

*Potetial method*

Potential method is used to determine corrosion activity of reinforcing steel in accordance with Vietnamese standard 9348:2012.

*Electromagnetic method*

Electromagnetic method is used to determine thickness of concrete-cover and location and diameter of steel bar in the concrete in accordance with Vietnamese standard 9356:2012.