Ultrasonic Pulse Velocity Test Information and Usage

Ultrasonic Pulse Velocity Test Information and Usage

This test method covers the determination of the propagation velocity of longitudinal stress wave pulses through concrete. This test method does not apply to the propagation of other types of stress waves through concrete. The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

Concrete is a complex composite material, which begins its life as a mixture of graded stone aggregate particles suspended in a fluid of cement and water and admixtures. Nominally, aggregate occupies 75% of the volume, cement about 15% and water content 10%. The priority when choosing a mix design is strength, which along with permeability of the concrete is governed by the water-cement ratio. For high strength and low permeability the water-cement ratio should be low.
It is usual for the coarse aggregate used in structural concrete to have a nominal maximum size of 20mm. Concrete reaches half its strength after about 3 days and 90% after 28 days. Concrete is a very versatile, potentially durable composite material, which is strong in compression but about 90% weaker in tension such that structural members subject to tensile stress are reinforced with steel bars. The setting of concrete is not a drying out process but a chemical reaction called hydration, where the calcium silicates in the cement react with the water to form hydrates and is accompanied by the evolution of heat.
In the early stages of hydration, water rises and aggregate settles, such that the surface concrete is not representative of the overall volume. The structure of the cement hydrate to a large extent determines the durability of the concrete. There are inherent pores of a few nm, and pores 50 to 100 times larger as a result of the presence of excess water above that required to complete hydration. Additionally there may be air pockets or volumes of lower density due to inadequate compaction. It is also likely that all concrete has an extensive crack system induced by shrinkage, thermal movements, loading and a number of other causes.
Concrete in service is exposed to a wide variety of environments and, because of its physical and chemical nature, may deteriorate as a result (Perkins, 1997). The pores and the crack system provide passage ways by which acidic moisture and gases that attack the alkaline concrete can penetrate. Once deterioration is apparent, its classification and extent need to be appraised so that appropriate remedial action can be specified. Routine testing of concrete is primarily concerned with assessing current adequacy and future performance (Bungey and Millard, 1996).
An initial visual inspection of the site can prove valuable in locating deterioration and aid the choice of an appropriate method of test. Although standard control cubes and cylinders are used to determine the strength of concrete produced on site, they cannot be truly representative of the insitu concrete strength of a structure. • However, extraction of cores is expensive and might weaken the structure. Various NDE methods which enable certain properties of concrete to be measured insitu, from which concrete strength is estimated, have been devised. Of the non-destructive techniques, the ultrasonic pulse velocity technique offers the lowest cost of use and is convenient as well as rapid to employ.

Ultrasonic pulse velocity
When the surface of a semi infinite solid is excited by a time varying mechanical force, energy is radiated from the source as three distinct types of elastic wave propagation. The fastest of these waves has particle displacements in the direction of travel of the disturbance and is called the longitudinal, compression or P-wave.

UPV(m/s) Vs Concrete quality
• Above 4500 Excellent
• 3500 to 4500 Good
• 3000 to 3500 Medium
• Below 3000 Doubtful

It is clear from the above equation that velocity is independent of geometry of material and depends only on elastic properties of the material through which it passes. Hence the principle of assessing quality of concrete is that comparatively higher velocities are obtained when the quality of concrete in terms of density, homogeneity and uniformity is good. In case of poorer quality, lower velocities are obtained.

Types of test
Direct Method
Semi Direct Method
Indirect Method

In the test described in BS (1881): Part 203 (BS, 1881, 1986), the time the pulses take to travel through concrete is recorded. Then, the velocity is calculated as:
= L / T

V = pulse velocity (m/s),
L = length (m), and
T = effective time (s), which is the measured time minus the zero time correction.
The zero time correction is equal to the travel time between the transmitting and receiving transducers when they are pressed firmly together.

The pulse velocity in concrete may be influenced by:
a) Path length
b) Lateral dimension of the specimen tested
c) Presence of reinforcement steel
d) Moisture content of the concrete

The influence of path length will be negligible provided it is not less than 100mm when 20mm size aggregate is used or less than 150mm for 40mm size aggregate.
Pulse velocity will not be influenced by the shape of the specimen, provided its least lateral dimension (i.e. its dimension measured at right angles to the pulse path) is not less than the wavelength of the pulse vibrations. For pulse of 50Hz frequency, this corresponds to a least lateral dimension of about 80mm

The velocity of pulses in steel bar is generally higher than they are in concrete. For this reason pulse velocity measurements made in the vicinity of reinforcing steel may be high and not representative of the concrete. The influence of the reinforcement is generally small if the bars runs in a direction at right angles to the pulse path and the quantity of steel is small in relation to the path length.
The moisture content of the concrete can have a small but significant influence on the pulse velocity. In general, the velocity is increased with increased moisture content, the influence being more marked for lower quality concrete.

The UPV results can be used:
To check the uniformity of concrete
To detect cracking and voids in concrete
To control the quality of concrete and concrete products by comparing results to a similar made concrete
To detect the condition and deterioration of concrete • To detect the depth of a surface crack • To determine the strength if previous data are available

Ultrasonic testing equipment includes a pulse generation circuit, consisting of electronic circuit for generating pulses and a transducer for transforming electronic pulse into mechanical pulse having an oscillation frequency in range of 40kHz to 50kHz, and a pulse reception circuit that receives the signal.
The transducer, clock, oscillation circuit, and power source are assembled for use. After calibration to a standard sample of material with known properties, the transducers are places on opposite sides of the material and readings are noted.