NISTIR 6975

Ultrasonic evaluation methods

applicable to polymer concrete

composites

5

10

15

20

25

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5 10 15 20 25 30 35

Andrzej Garbacz

Edward J. Garboczi
NISTIR 6975

Ultrasonic evaluation methods

applicable to polymer concrete

composites

Andrzej Garbacz

Warsaw University of Technology

Warsaw, Poland

Edward J. Garboczi

Materials and Construction Research Division

Building and Fire Research Laboratory

April 2003

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

TECHNOLOGY ADMINISTRATION

Phillip J. Bond, Under Secretary of Commerce for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Arden L. Bement, Jr., Director
III

EXECUTIVE SUMMARY

This publication is the final report on the three-year project entitled "Ultrasonic evaluation

methods applicable to polymer concrete composites." The project was sponsored by the

M.Skłodowska-Curie US-Polish Joint Fund II. The project was collaboratively carried out by the

National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA and the Institute

of Construction Engineering and Management (ICEM), Warsaw University of Technology,

Warsaw, Poland. Edward J. Garboczi (NIST) and Andrzej Garbacz (ICEM, from Prof. Lech

Czarnecki’s Building Polymer Composites Group), were the principal investigators.

The main objective of the project was to evaluate the possibility of implementing ultrasonic

methods for the nondestructive assessment of polymer composite properties. The two main fields

of polymer composite applications, anticorrosion protection of concrete structures (including

industrial floors) and polymer concrete pre-cast elements, were both taken into account. The

possibility of nondestructive evaluation of the quality of multi-layer repair systems, including

adhesion mapping, has arisen as the most interesting result of the project. The design of the

experimental program was developed by the principal investigators, and was carried out at the

ICEM laboratories. NIST also collaborated in the interpretation of the test results and preparation

of the report.
IV

ACKNOWLEDGEMENTS

The authors would like to thank Prof. Lech Czarnecki for inspiration, remarks and discussion

during preparation of this report. Marek Łuciuk and Mikołaj Krystosiak have prepared their

Master Theses during the realization of this M.Skłodowska-Curie US-Polish Joint Fund II

project.

Cover: industrial epoxy concrete floor and an example of adhesion mapping of a polymer

concrete on top of portland cement concrete using an ultrasonic pulse velocity

method
V

ABSTRACT

Polymer composites (PC) appear as useful materials for repair and protection of building

structures, as well as for manufacturing precast elements. In the case of pre-cast elements, as

well as repair materials, the usefulness and durability of polymer composites depend on the

selection of the material composition for obtaining the composite with controllable properties.

This task is a material design and optimization procedure. For repairs and protective coatings, the

main problem is good bonding between PC composites and concrete substrate for the length of

time desired. In both cases, there is a need for quality control (verification tests) and diagnostic

tests during structural service as well. This is available using destructive or semi-destructive tests.

Such a way of testing, however, is expensive and of limited usability due to its destructive

character. Therefore, the development of nondestructive assessment methods for polymer

composites is an important need. Ultrasonic methods are among the most common nondestructive

techniques used in material science and industry. Ultrasonic methods are well-known and

standardized towards traditional building materials: metals, cement concrete, and rocks. In the

case of polymer concrete composites, ultrasonic methods are at the introductory stage.

The results of the project confirmed the usefulness of ultrasonic methods for

nondestructive evaluation of polymer concrete composites in various applications. The pulse

velocity method can be used for evaluation of the properties and homogeneity of the pre-cast

elements made from polymer concrete as well as for evaluation of adhesion in the multi-layer

PC-CC systems (adhesion mapping). The ultrasonic echo method can be used as a

complementary method for nondestructive estimation of PC layer thickness.

Results of the project are presented in this report. The appendix contains a list of the eight

publications that were presented during international and domestic conferences and published in

proceedings and journals. The results are in general accord with worldwide scientific and

engineering activities in nondestructive quality control of repair, e.g. Guide for evaluation of

concrete structures prior to rehabilitation, ACI 364.1R-94, 2001; the family of European standards

1504: Products and systems for the protection and repair of concrete structure; and activities of

RILEM Technical Committees: TC-151 "Adhesion Technology in Concrete Engineering – Physical

and Chemical Aspects" and TC-184 IFE "Industrial floors for withstanding harsh environmental

attacks, including repair and maintenance".
VI
VII

TABLE OF CONTENTS

1. INTRODUCTION

2. NONDESTRUCTIVE TEST METHODS APPLICABLE TO CONCRETE-LIKE

COMPOSITES

3. CHARACTERISTIC OF THE ULTRASONIC TESTING

3.1. Characteristic of the ultrasonic waves

3.2. Attenuation of the ultrasonic wave

3.3. Methods used in the ultrasonic measurement technique

3.4. Application of ultrasonic methods for the concrete testing

4. GENERAL APPROACH FOR ULTRASONIC EVALUATION OF POLYMER CONCRETE

COMPOSITES

5. EVALUATION OF TECHNICAL PROPERTIES OF POLYMER CONCRETE ELEMENTS

5.1. Experimental procedure

5.2. Ultrasonic evaluation of selected technical properties of vinyl ester concrete

5.2.1. Materials

5.2.2. Results

5.2.3. Relationship between microstructure and ultrasonic waves propagation

5.3. Ultrasonic estimation of selected properties of epoxy concrete

5.3.1. Material and results

5.3.2. Discussion of results

5.3.3. Analysis of relationship between microstructure and ultrasonic wave propagation

5.4. Conclusions concerning ultrasonic evaluation of elements made from polymer concrete

6. ULTRASONIC EVALUATION OF MULTILAYER REPAIR SYSTEMS

6.1. Approach to evaluation of multi-layer repair systems

6.2. Evaluation of the multi-layer system with ultrasonic echo method

6.2.1. Procedure of the ultrasonic testing

6.2.2. Materials

6.2.3. Determination of adhesion

6.2.4. Results of testing using echo method

6.2.5. Results of testing using echo method

6.3. Evaluation of multi-layer system with pulse velocity method

6.3.1. Ultrasonic test procedure

6.3.2. Materials and testing floor systems

6.3.3. Ultrasonic evaluation of adhesion at boundary conditions

6.3.4. The effect of chemical composition and geometry of polymer coating

6.3.5. Relationship between pull-off strength and ultrasonic parameters

6.3.6. Ultrasonic detection of defects in PC-CC system

6.4. Conclusions concerning the ultrasonic evaluation of the multi-layer PC-CC system

7. SUMMARY AND CONCLUSIONS

8. REFERENCES

ENCLOSURE - List of publications involved with the activity on the project
VIII
1. INTRODUCTION

Polymer composites (PC) are useful materials for repair and protection of building structures, as well as

for the manufacturing of pre-cast elements. There are various fields of applications for these materials

[1-4]:

- repair, maintenance and anti-corrosion protection (protective and decorative coatings) of building

structures,

- flooring, mainly industrial floors, but also floors in hospital and school buildings, sport arenas and

other community structures,

- polymer concrete precast elements, like manholes, pipes and slabs as well as chemical resistant

vessels, e.g. electrolytic cells.

In the case of pre-cast elements and repair materials, the usefulness and durability of polymer

composites depend on the selection of the material composition for obtaining composites with

controllable properties. This task is a material design and optimization procedure [5,6]. For repairs

and protective coatings, the main issue is good bonding between PC composites and cement

concrete substrates, including the effect of time. In both cases, there is a need for quality control

(verification tests) and diagnostic tests during structural service as well. This is available using

destructive or semi-destructive tests, e.g. determination of compressive strength on samples cored

from structure or determination of bonding with the pull-off method. Such a way of testing,

however, is expensive and of limited usability due to its destructive character as well as because

information obtained is only local. Therefore, the development of nondestructive assessment

methods for polymer composites is an important need [7,8]. Ultrasonic methods are among the most

common nondestructive techniques used in material science and industry [9,10]. Ultrasonic methods

are well-known and standardized for traditional building materials, like metals [11], cement concrete

[12-14], and rocks [15,16]. In the case of polymer concrete composites, ultrasonic methods are at the

introductory stage [17,18].

 The aim of the project was analysis of the usability of ultrasonic methods for assessment of PC

properties in various applications.

2. NONDESTRUCTIVE TEST METHODS APPLICABLE TO CONCRETE-LIKE

COMPOSITES

Nondestructive test (NDT) methods are commonly used for quality control of various construction

elements [11]. The development of NDT methods is important from a technical and an economical

point of view. Contrary to destructive methods, NDT techniques give information about material

properties without deteriorating material microstructure and serviceability. The main advantages of

NDT methods are: the possibility of on-site evaluation, repeatability at the same place during structural

service, and quick test results.

By comparison to other construction materials like steel, the development of NDT methods for

concrete-like composites has progressed at a slower pace because these kind of composites are difficult

to test [10]. Concrete is heterogeneous, intrinsically conductive (because the pore solution is an ionic

electrolyte), and also usually contains steel reinforcement. For these reasons, NDT techniques used with

metals are not easy to implement for concrete and similar composites. Since it is difficult to apply to

concrete some of the techniques used with metals, alternative NDT methods have been considered.

Recently, the development of NDT techniques applicable to concrete has become of interest.

This has resulted from an increase in quality requirements for concrete in new construction. On the

other hand, the percentage of repair and rehabilitation in the total building market has increased. The

estimated cost of repair and rehabilitation in the US and Canada during the next 20 years is equal to

$1 · 1012 to $3 · 1012 (USD) or about 15 % to 50 % of the total North American building market
2

[19]. The situation in the Polish building market is similar but is actually closer to the upper

percentage limit. The evaluation of structures prior to repair, a proper selection of repair materials,

and quality control of the repairs should be done to assure the effectiveness of the repair process

[20]. Many institutions are involved with elaboration of repair guidelines [21,22]. This indicates the

importance of repair issues. The European Standardization Committee CEN/TC104 has been

working on the family of standards EN 1504 under the common title “Products and systems for the

protection and repair of concrete structure." The guidelines for structural evaluation prior to the

repair and quality control after repair are important part of standards [22]. In 1999, ACI Technical

Committee 364 - Rehabilitation has elaborated guidelines for evaluation of concrete structures prior

to rehabilitation [23]. In Poland, procedures of concrete structure evaluation are given in the Polish

Standard PN-88/B-01807 “Anticorrosion protection of building structures. Concrete and reinforced

concrete structures. Evaluation procedures” [24], and in the Guidelines of Building Research

Institute 361/99 "Guide for evaluation of safety of reinforced concrete structures" [25]. NDT

methods play an important role in evaluating concrete structure and in quality control of repair

work. Recently, the RILEM Technical Committee, "Non-destructive evaluation of concrete

structures," has been created. The main task of this Committee is an elaboration of the guidelines for

NDT evaluation of concrete structures and the promotion of a wider use of NDT techniques in

practical applications. The progress in this field has been made in the US. The review of different

NDT methods applicable to concrete structures (Table 1) was given in the Report of ACI Technical

Committee 228 – Non-Destructive Testing of Concrete [26], in which the capabilities, limitations

and potential application of various NDT methods were presented. The ACI Reports ACI 364.1R-94

and ACI 228.2R-98 have been published together in 1999 as “The Concrete Repair Manual.”

NDT methods are applied to concrete structures for four main reasons [26]:

- quality control of new structures,

- unexpected problems with new construction,

- evaluation of existing structures, including evaluation prior to repair,

- quality control of concrete repair.

In general, considering the measured parameters, NDT methods can be divided into the

following categories [9,10]:

- rebound hammer,

- acoustic methods – stress wave propagation (ultrasound, acoustic emission, impact-echo,

etc.),

- radiation methods (X-ray, gamma ray, neutron emission, etc.),

- electromagnetic methods,

- others – e.g. infrared thermography.

Another classification takes into account the aim of nondestructive evaluation. Two main categories

can be recognized [10]:

- evaluation of concrete strength and its homogeneity (e.g. rebound hammer, ultrasonic pulse

velocity),

- evaluation of structural integrity – detection of various types of defect in concrete, detection

and evaluation of steel reinforcement (e.g. visual inspection, stress wave propagation,

impact-echo, infrared thermography, radiation methods, electromagnetic methods).

Among various NDT techniques, ultrasonic methods, especially the ultrasonic pulse velocity

method, are still commonly used for the evaluation of concrete structures.
3

Table 1. Nondestructive methods for evaluation of concrete structures [9,10,12,26].

Rebound hammer Example of application:

− evaluation of homogeneity or consistency of

compressive strength of concrete structure,

− monitoring strength gain

Scheme of method:

Principle:

Measurement of rebound height after striking

concrete surface with spring loaded hammer;

correlation between rebound number and

compressive strength is determined

Limitations:

- evaluation of near-surface properties only,

- results depend on surface roughness,

- reference curve needed for strength

 estimation,

− rebound number affected by the orientation of

the apparatus,

Ultrasonic pulse velocity, UP-V Example of application:

- evaluation of degree of homogeneity of

 concrete structures,

- monitoring strength gain,

- determination of dynamic Young modulus if

Poisson’s ratio and mass density is known,

- estimation of concrete strength,

- complementary to other tests ,

Scheme of method:

Principle:

Measurement of a travel time of ultrasonic P

wave, over a known path length, calculation of

pulse velocity in concrete, regression analysis of

relationship between pulse velocity and concrete

properties (mainly compressive strength)

Limitations:

- needs coupling agent,

- reference curve needed for strength

 estimation,

- measurements with the transducers at the

same side of sample difficult to interpret

Ultrasonic pulse echo, UP-E Example of application:

- method developed to detect delamination,

discontinuities, and small cracks,

- measurements of slab thickness,

- monitoring of polymer adhesive curing,

Scheme of method:

Principle:

Propagation of a short pulse of ultrasonic wave;

measurement of travel time to boundaries

separating materials with different densities and

elastic properties; by knowing the wave speed the

distance to the reflecting interface is detected

Limitations:

- needs coupling agent,

- heterogeneous nature of concrete and of

reinforcement presence result in multitude of

echoes,

- difficult interpretation of results,

- relatively large “null-zone”
4

Table1. (cont’d)

Impact-Echo, IE

Example of application::

- defect detection in concrete slabs, like

delamination, flaws, large cracks,

honeycombing, debonding

- measurements of concrete slab thickness,

- evaluation of the quality of the bond between

overlay and base concrete,

- estimation of depth of surface-opening cracks,

including water-filled ones

Scheme of method:

Principle:

Development of echo method; mechanical, high

energy impact used to generate the stress wave;

high penetration of concrete, mainly by P wave;

frequency analysis of recorded waveform using

the fast Fourier transform

Limitations:

- detection of large defects, relatively deeply

located

- needs expert for interpretation of results,

Spectral Analysis of Surface Waves, SASW Example of application::

- determination of the stiffness profiles of

flexible pavements,

- measurement of changes in elastic properties

of concrete slabs during curing,

- estimate thickness of layers

Scheme of method:

Principle:

Analysis of the spectrum of the disperse

generalized Rayleigh surface wave in a layered

system; the received signal is analyzed to obtained

the dependence of phase velocity on the

frequency

Limitations:

- necessity of comparison of the theoretical and

experimental dispersion curves

- time consuming procedure,

- difficulties in interpretation of the results,

- requires Poisson’s ratio

Ground-Penetrating Radar, GPR Example of application:

- delamination detection,

- locating reinforcing bars in structures,

- measurement of pavement thickness,

- measurements of water content of fresh

 concrete

Scheme of method:

Principle:

Non-contact method; method analogues to UP-E

techniques, except that pulses of electromagnetic

waves are used instead of stress waves, results are

registered as a waterplot

Limitations:

- improper estimation of relative dielectric

constant resulted in large error,

- needs expert for interpretation of results,

- results depends on in-situ conditions: presence

of moisture and chlorides in concrete,

- expensive equipment

impact

R wave R wave

impact
5

Table1. (cont’d.)

Infrared Thermography , IT

Example of application:

- locating near-surface defects, like

delamination and flaws,

- short-time period for scanning large surface

 areas

Scheme of method:

Principle:

Measurement of surface temperature differences –

thermographic image

Limitations:

- needs expert for interpretation of results,

- results depends on in-situ conditions (surface

quality, wind speed and ambient temperature)

Electro-magnetic (covermeters) Example of application:

− evaluation of concrete quality (moisture

content)

− detection of steel bars in concrete,

− evaluation of concrete cover around steel bars

Scheme of method:

Principle:

Interaction between the steel reinforcement and

low-frequency electromagnetic fields; two

principles are used: magnetic reluctance and eddy

currents

Limitations:

− method sensitive to concrete substrate quality

(dielectric constant depends on the concrete

moisture)

− results difficult to interpret

Radioactive (radiometric, radiographic) Example of application:

− detection of steel bars,

− detection of defects inside concrete,

− detection of low density regions

Scheme of method:

Principle:

High-energy electromagnetic radiation (X-ray,

gamma, neutron); concrete evaluation on the base

of changes in detected intensity of radiation; two

types of methods according to type of sensor:

radiometric (detector) and radiographic

(photographic film)

Limitations:

− limited thickness of tested elements (< 500

mm)

− very expensive,

− safety training and licensing personnel

required

infrared

scanner

film or

detector

source of

radiation

concrete
6

3. CHARACTERISTICS OF ULTRASONIC TESTING

3.1. Characteristics of ultrasonic waves

Ultrasonic waves are generally defined as a phenomenon consisting of the wave transmission of

a vibratory movement of a medium with above-audible frequency (above 20 kHz). Ultrasonic waves

are considered to be elastic waves. Ultrasonic waves are used in two main fields of materials

testing:

- ultrasonic flaw detection (detection and characterization of internal defects in a material),

- ultrasonic measurement of the thickness and mechanical properties of a solid (stresses,

toughness, elasticity constants), and analysis of liquid properties.

In all the above listed applications of ultrasound, the vibrations of the medium can be

described by a sinusoidal wave of small amplitude. This type of vibration can be described using the

wave equation:

2

2

2

2

2

x

a

c

t

a

∂

∂

∂

∂ = ⋅ 1)

where: a = instantaneous deflection in m; t = time in s; c = wave propagation velocity in m/s;

 x = position coordinate (path) in m.

The vibrations of the medium are characterized by the following parameters:

- acoustic velocity, ν = velocity of vibration of the material particles around the position of

equilibrium:

υ ω ωφ == − d a

d t

A t cos( ) 2)

where: a, t are as above; ω = 2πf, the angular frequency in rad/s; A = amplitude of deviation

from the position of equilibrium in m; φ = angular phase or deviation, at which the vibrating

particle reaches the momentary value of the deviation from position of equilibrium, in rad,

- wave period, T = time after which the instantaneous values are repeated,

- wave frequency, f = inverse of the wave period: f = 1/T in Hz,

- wave length, λ = the minimum length between two consecutive vibrating particles of the same

phase:

λ =⋅ = c T c

f

3)

In a medium without boundaries, ultrasonic waves are propagated spatially from their source.

Neighboring material, vibrating in the same phase, forms the wave surface. The following types of

waves are distinguished depending on the shape of the wave front (Fig. 1):

- plane wave – the wave surface is perpendicular to the direction of the wave propagation,

- cylindrical wave – the wave surfaces are coaxial cylinders and the source of the waves is a

straight line or a cylinder,

- spherical waves – the wave surfaces are concentric spherical surfaces; the waves are induced by

a small size (point) source; deflection of the particle is decreased proportionally to its distance

from the source. For large distances from the source, a spherical wave is transformed into a

plane wave.
7

Figure 1. Forms of the wave surface: a) plane wave, b) cylindrical wave, c) spherical wave

Ultrasonic waves differ in the direction of the medium vibration in relation to the wave

propagation direction. Three main types of ultrasonic waves are distinguished (see Table 2):

- longitudinal waves: the medium vibrates in rectilinear way, in the direction of the ultrasonic

wave propagation,

- transverse wave: the medium particles vibrate only in a direction perpendicular to the direction

of wave propagation,

- surface waves (Rayleigh waves): the waves propagate in an interface layer with two directions of

the vibrations of the particles: perpendicular and parallel to the direction of the wave

propagation,

- plate waves (Lamb waves): these waves propagate in media like plates and bars, one or two

dimensions of which are similar to the wave length; two forms of these waves are possible:

• anti-symmetrical wave (torsional) – the medium particles vibrate along the transversely

neutral axis and elliptical movement is done at the surface,

• symmetrical waves (dilatation) - the medium particles vibrate along the longitudinally

neutral axis and elliptical movement is done at the surface.

As it is seen from formulae above, the velocities of the longitudinal, transverse and surface

waves do not depend on the frequency f. Therefore, the formulae may be applied for both

continuous and impulse waves.

Impulse waves are most often used for concrete testing using the ultrasonic measurement

techniques. The velocity of the impulse propagation is characterized by the group velocity, which is

the velocity of propagation of the given wave amplitude, and by phase velocity, which is the

velocity of propagation of the given phase. In non-dispersive media, the group velocity of the wave

is equal to the phase velocity. In dispersive media, the spectrum of the impulse frequency is

broadband and therefore the group and phase velocities are different and difficult to measure. In

practice, wave impulse propagation is most often characterized by the velocity of the impulse front

or by the velocity of the signal (velocity of the given point of the main part of the signal).

λ

λ

λ

r

r
8

Table 2. Types of stress waves according to the direction of medium particle vibration

Wave type Symbol & scheme of

medium vibration

Wave velocity * Remarks

Longitudinal

(compressive) P or L

a) infinite medium

( )( ) c E L = ⋅ −

ρ + ⋅−

ν

ν ν

1

1 12

b) bounded medium

- plate: a >> λ, b << λ

 c E L = ⋅ ρ − ν

1

1 2

- bar: a >> λ, b >> λ

 c E L = ρ

Waves commonly used in

practice. Propagation in

solid, liquid and gas

medium. Propagation of

pure P-wave limited by

medium dimensions only –

should be large enough in

comparison to wave length.

Transverse

(shear) S or T

( ) c G E T = =⋅ ρρ ν ⋅ +

1

2 1

cT < cL

Polarized waves - rotation

of wave source causes

changes in plane of

vibration of medium

particles. Propagation in

solid media only, whose

dimensions are significantly

larger than the wave length.

Surface

(Rayleigh)

R

c c R T ≈ +

+ ⋅ 087 1121

. . ν

ν

cR ≈ 0,9 ÷ 0,95 cT (*?*)

Waves penetrate medium to

a depth close to the wave

length; vibration in the

perpendicular direction has

much higher amplitude than

in parallel direction.

Plate

(Lamb)

Dilatation, plate wave

Torsional, plate wave

Velocity of the Lamb

waves, cLamb, depends

on the wave frequency

and plate thickness

cR < cLamb < cT.

Influence of frequency on

the Lamb wave velocity

creates difficulties in

application of the Lamb

waves in NDT

*E - elastic modulus, Pa , G - shear modulus of elasticity, Pa, ν – Poisson’s ratio,

ρ – density, kg/m3, λ = wavelength
9

3.2. Attenuation of ultrasonic waves

The energy of an ultrasonic wave travelling through a medium is attenuated depending on

the properties of the medium. The reasons are:

• energy absorption, which occurs in every state of matter and is caused by the intrinsic friction of

the medium leading to conversion of the mechanical energy into thermal energy,

• reflection, refraction, diffraction and dispersion of the wave; this type of wave attenuation is

characteristic particularly for heterogeneous media like metal polycrystals and concrete.

The weakening of the ultrasonic wave is usually characterized by the wave attenuation coefficient α

[dB⋅m-1⋅Hz-1], which determines the change of the acoustic pressure after the wave has traveled a

unitary distance through the given medium.

In solids, the loss of energy is related mainly to absorption and dispersion. The attenuation

coefficient α is described by the relation:

α=α1+α2 4)

where α1 is the attenuation coefficient that describes how mechanical energy is converted into

thermal energy, and α2 is the attenuation coefficient that describes the decrease of wave energy due

to reflections and refractions in various directions.

For a majority of solids, the energy losses connected to the absorption are proportional to the

ultrasonic waves frequency, so that the attenuation of longitudinal waves is greater than the

attenuation of transverse waves.

In non-homogenous materials, the energy losses caused by dispersion of the ultrasonic wave

are more important. The most important parameter is the material grain size D, and the relation

between the wave length λ and D is given by the following formulae:

a) λ » D and 0.016 < D/λ < 0.16 – Rayleigh dispersion

α ≈ α1 f + α2 f

 4 5)

where the dispersion coefficient α2 ≈ D 3

b) λ ≈ D and 0.16 < D/λ < 1 – stochastic dispersion

α ≈ α1 f + α2 f

 2 6)

where the dispersion coefficient α2 ≈ D

c) λ < D – diffusion dispersion; attenuation coefficient α is defined by the previous equation and α2

≈ 1/D

d) λ « D and D/λ > 10 - absorption of the ultrasonic wave in every grain as well as reflection on the

interfacial surfaces; the average attenuation coefficient is defined by the following equation:

αα α ≈+ + 1 2

2 f f R

D

7)

where R is the average reflection coefficient for the interfacial surfaces, and the dispersion

coefficient α2 ≈ 1/D. If R is low, the attenuation is low, too.
10

3.3. Methods used in the ultrasonic measurement technique

The most often applied methods of ultrasonic testing are the pulse velocity method, the echo

method, and the resonance method. Visual and holographic methods, used for direct visualization of

the ultrasonic field in the given medium, are of lesser importance.

The ultrasonic pulse velocity method (called also transmission method) is one of the oldest

and simplest methods of materials testing. The method consists in the determination of the travel

time, over a known path length of the longitudinal ultrasonic wave after its transmission through the

tested medium (see Fig. 2). Both the emitting and receiving transducers are usually placed on the

opposite sides of the tested sample (coaxially if possible). Other transducer arrangements are also

used in concrete testing (Fig. 2b, c). They can be placed on the perpendicular surfaces (Fig.2b) or on

the same side of the tested member (Fig.2c).

a) b) c)

Figure 2. Ultrasonic pulse velocity method: a) direct method, b) semi-direct method, c) indirect

(surface) method

The ultrasonic echo method is often used for defect detection in metal members. The method

consists in generation of a short impulse of the ultrasonic wave by the transmitting transducer (Fig. 3).

After reflection by the material’s structural heterogeneity or by the limiting surface, the impulses are

recorded by the receiving transducer (dual transmitting-receiving transducers are also available). Part of

the ultrasonic wave is reflected by the material defect, returns to the receiving transducers and is

recorded as the defect’s echo. Another part of the wave passes by the defect and reaches the opposite

wall of the tested material, where it is reflected and returns to the receiver with some delay as the

bottom echo. The depth of the defect or the reflecting surface is determined on the basis of the travel

time of the impulse and the ultrasonic wave velocity. A small grain size of the tested material is

necessary for the echo method to be efficient. The grain size should be significantly smaller than the

searched-for defects; if not, then any defect echo will be overlapped by the echoes formed by the grain

boundaries.

a) b)

Figure 3. Testing the concrete by ultrasonic echo method: a) transmitting-receiving transducer,

b) double transducer
11

The resonance method consists in the introduction of an ultrasonic wave into the tested

medium, which is of the constant thickness g, in such a way that a resonant standing wave, of

wavelength λ, will be formed under the condition:

g n

nc

f =⋅ = λ

2 2

8)

where n = an integer that defines the harmonic number.

Contrary to the echo method, in the resonance method the interference of the incident and

reflected waves is observed. A continuous wave is usually emitted in this method. The

demonstration of the resonance of the continuous wave requires a large area of contact of the

transducer with the tested material. The transducers used usually have a diameter of about 30 mm

and should be well pressed against a smooth surface of the material. The main problem is the

finding of the resonance frequency for n = 1. The other limitation is that practical use of the method

is really only possible in the laboratory, not in the field.

3.4. Application of ultrasonic methods for concrete testing

At present, two ultrasonic methods are used for concrete testing: the pulse velocity method

and the echo method. These methods enable the evaluation of concrete strength and homogeneity. In

a limited range, the ultrasonic pulse velocity method is also applied for determination of the

elasticity constants [30,31], detection of crack geometry [30,32], evaluation of the degree of

concrete degradation, e.g. deterioration due to freeze-thaw attack. The structure of concrete, as

defined by the maximum aggregate size, requires low frequency ultrasonic waves, since the wave

length should be larger than the grain size for minimizing losses caused by dispersion. Ultrasonic

waves are diffracted by discontinuities smaller than the wave length. Assuming the pulse velocity in

concrete, cL , is 3 km/s to 5 km/s, the wave length, λ, ranges from 75 mm to 125 mm at a frequency

of 40 kHz [9]. The maximum diameter of aggregate grains, D, does not usually exceed 32 mm. In

this case, the ratio D/λ ranges from 0.25 to 0.41. From eqs. 5-8, it can be concluded that at least

Rayleigh and stochastic dispersions occur. As frequency increases, the wave length decreases and

becomes close to the grain diameter. This implies that other types of wave dispersion can occcur.

Therefore, the resolution in the concrete testing is worse than in the case of metals. In practice [27],

frequencies from 100 kHz to 1 MHz are used for testing concrete samples or members smaller than

0.5 m. Members larger than 0.5 m require low frequencies below about 100 kHz, and are usually

about 40 kHz.

The direct pulse velocity method is most often used to assess concrete structures. The

indirect pulse velocity method is used rarely in specific application. Only longitudinal ultrasonic

waves are used in practice, because transverse waves are difficult to generate in concrete and are

strongly attenuated in this material. The pulse velocity method is used mainly for evaluation of the

compressive strength vs. time as well as for evaluation of structural homogeneity.

The common procedure for evaluation of cement concrete properties with the pulse velocity

method (Fig. 4) consists in regression analysis of the experimental relationship between the pulse

velocity and selected technical properties (mainly compressive strength), leading to development of

suitable reference curves (called also calibration curves, correlation curves or ISO-strength curves)

[9,10,13]. There are many recommendations and national standards for the ultrasonic evaluation of

concrete compressive strength. They define the type of reference concrete as well as the materials

parameters that can be varied to develop the reference curve. Komlos et al. [14] have analyzed the

standards concerning the rules for ultrasonic testing by the pulse velocity method. In general, three

methods of reference curve development can be recognized:
12

0

10

20

30

40

50

60

70

80

90

3.8 4.0 4.2 4.4 4.6

pulse velocity, km/s

compressive strength, MPa

a) calibration curve developed for cube concrete specimens with the same composition and cured

in the same way as the concrete in the investigated structure (Fig. 4a). The number of specimens

needed to develop the curve depends on its universality = range of strength variability; as the

universality of the reference curve increases, the number of samples necessary to develop it

increases. To develop reference curves for a wide range of strength variability it is recommended

to change the quantity of mixing water, the degree of compaction, age of the concrete, the curing

or storage conditions, and if necessary, the proportion of fine material and cement content,

b) calibration curve experimentally established from samples taken from the structures from zones

of different pulse velocity (Fig. 4a); at least three individual transit time measurements should be

carried out in each location and cores should be taken from the same location to obtain the

compressive strength, the number of cores depends on the concrete volume,

c) calibration curve established with inversion procedure (Fig. 4b) using the reference curve for the

concrete with similar composition and specimens taken from structures (number of specimens

lower than in case (b) and depends on the concrete volume - at least three). This procedure is

often used in practice for structures with unknown concrete composition or high age concrete

and for structures where possibility of coring is limited. To obtain a recalculated reference curve

the inverse coefficient should be determined. The compressive strength is calculated from the

following equation:

fc

ef = Ciexp fcref (9)

where: fc

ef = the effective compressive strength of tested concrete, fcref = the compressive

strength determined from a reference curve on the base of the ultrasonic measurements, Ci

exp =

the total coefficient of influence obtained from the tests on the cores.

Figure 4. Scheme of reference curve development for a) ultrasonic evaluation of concrete

compressive strength on the base of investigation of concrete cube samples with the same

composition as that in the structure or on the basis of the investigation of cores taken from the

structure; b) ultrasonic evaluation of concrete compressive strength with inverse procedure using

reference curve for similar concrete, where Ci

exp = the coefficient of influence, fc,i

ref = the strength

determined from the reference curve from the ultrasonic measurements on specimen i, fc,i exp = the

measured strength of specimen i, and n = the number of specimens tested.

0

10

20

30

40

50

60

70

80

3.8 4.0 4.2 4.4 4.6

pulse velocity, km/s

compressive strength, MPa

a) b)

∑=

=

n

i

ref

ci

ci

i f

f

n

C

1

exp

exp 1

fci

exp

fci

ref
13

Most standards and guidelines recommend two regression equations for description of the

relationship between pulse velocity and concrete strength:

- linear : fc = a0 + a1 cp (10)

- exponential: fc = a0 exp (a1 cp) (11)

where: fc = compressive strength, cp = longitudinal pulse velocity, and a0 a1 are regression

coefficients.

However, others types are allowed:

- fc = a0 + a1 cp + a2 cp

2 (12)

- fc = a0 cp

a1 (13)

where the symbols are the same as in eq. (10)

The measure of the accuracy of strength estimation from a reference curve is the coefficient

of standard deviation, Cd:

[ ] ( )/ 100 %

1

1 1/ 2

1

2 exp

,

exp

, , f f f x

n

n

i

c i c i

ref Cd c i 



 



 − − = ∑=

(14)

where: fc,i

ref = the strength determined from the reference curve from the ultrasonic measurements

on specimen i, fc,i exp = the measured strength of specimen i, and n = the number of specimens tested.

If Cd is less than 12 %, the estimation of compressive strength with the ultrasonic method is

considered satisfactory.

The ultrasonic pulse velocity method is commonly used for compressive strength estimation.

However, many authors have stressed that various factors can affect pulse velocity and overshadow

changes resulted from strength changes [10,23]. Komlos et al. [14] have also concluded that

applications of longitudinal waves for concrete evaluation may be classified in the following way,

according to decreasing precision of measurement:

- monitoring of how concrete properties change with time,

- control of homogeneity of the structure of concrete (possible disturbances of the signal from the

reinforcement),

- estimation of the compressive strength (necessity of calibration),

- determination of the elasticity constants (doubtful as concrete is a heterogeneous composite),

- detection of defects – least attractive of all (possibility of obtaining faulty results with dangerous

and expensive consequences).

-

They have also stressed the necessity of enhancing ultrasonic measurement techniques in the

following proposed directions:

- using waves others than longitudinal, e.g. surface wave, plate waves,

- using wave parameters other than wave propagation velocity,

- using advanced methods for analysis of the ultrasonic signal.

Recently, many institutions have become focused on the improvement of ultrasonic test methods,

e.g. using surface waves and advanced signal processing [35, 36]

The ultrasonic echo method is rarely used for concrete, and mainly only for flaw detection.

This method gives lower resolution for concrete compared to metal testing. Impulse duration is long

at a frequency of 100 kHz; this leads – in the case of the single transducer in the echo method – to a
14

long “dead zone”. Double transducers cannot be applied here due to the small directivity of the

ultrasonic beams emitted with low frequencies and the possibility of “cross-talk” between them, as

well as the reflection of the wave by the surfaces parallel to the beam axis. In the case of the testing

of the concrete using the echo method, the obtained results are difficult to interpret due to the

multiple echoes caused by material heterogeneity – the presence of coarse aggregate and steel

reinforcement. Recently, some work on improving the echo method has been carried out, focused on

using ultrasonic waves with higher frequency and a new data processing procedure, the so-called

split spectrum processing [39].

The resonance method is mainly used in the laboratory for the determination of dynamic

elastic moduli of concrete-like composites [18,40].

4. GENERAL APPROACH TO ULTRASONIC EVALUATION OF POLYMER CONCRETE

COMPOSITES

From the engineering point of view, the nondestructive assessment of the properties of polymer

composites should be developed for three main fields of PC/PCC application (Fig. 5):

- repair,

- protective coatings (including industrial floors),

- pre-cast elements.

Usually, different procedures for nondestructive evaluation should be used because of different

purposes for evaluating PC materials or the systems in which they are used.

NDT evaluation of PC pre-cast elements

Due to the similarity of the geometrical features of the microstructure (Fig. 6) of both cement

concrete and polymer concrete, it would seem possible that experience using ultrasonic techniques on

cement concrete can be implemented into polymer composites technology for PC pre-cast elements.

The ultrasonic pulse velocity can be used for this purpose. This implies that reference curves should be

determined for a given type of PC (see Section 3.4). However, the differences in properties should be

taken into account, especially differences in elastic properties between cement paste and resin

binder, which can affect ultrasonic wave propagation in PC. Ultrasonic wave propagation depends

generally on material composition and composite microstructure. In the specific case of polymer

concrete composites, the ultrasonic wave propagation is influenced by: type of the binder and filler,

content and grain-size distribution of the aggregate, and microfiller content and porosity. The

adhesion between resin binder and aggregate is also important. For example, using a wet aggregate

can result in a lack of adhesion, and by using coupling agents the adhesion can be increased [41,42].

NDT evaluation of multi-layer systems

As a result of repair and applying anti-corrosion protection to a building structure, a multilayer system consisting of portland cement concrete (CC) in contact with polymer composite (PC) is

produced. In this case, quality control of the repair application is one of the most important purposes

for applying NDT methods. These are mainly focused on nondestructive estimation of the system

geometry (layer thickness), and detection of flaws, voids and places with lower adhesion [43-46] at

the interface zone of the repair material (PC/PCC) and the concrete substrate. Lack of adhesion can

be a result of technological error, actual material incompatibility in the PC-CC system, as well as

from the change of properties during the service time of the repair, which are termed durability

problems.

The nondestructive evaluation of layer thickness, flaws, disbonds, and areas of poor adhesion

under the top PC layer (Fig. 7) is a difficult issue and requires a careful selection of the most
15

suitable NDT methods. The NDT techniques selected should give a possibility for testing the PC-CC

system from one side of the structure due to the specific repair system. Two ultrasonic methods can

be considered for this purpose: the echo method and the indirect pulse velocity method. Figure 7

illustrates details of this method, which will be discussed in more detail later in this report.

The relation between the specific acoustic impedance of the components of the PC-CC multilayer system is one of the most important factors influencing ultrasonic wave propagation in

particular elements of the system, as well as through the internal interfaces. The heterogeneous

nature of both PC and CC, including “cohesion” defects of their microstructure (like voids, porosity,

cracks), and the presence of steel reinforcement can additionally complicate the nondestructive

assessment of adhesion between PC and CC.
16

Figure 5. The general concept of the research project for the evaluation of polymer concrete

composite properties in various applications using non-destructive ultrasonic methods.
17

CC PC

E aggregate ∼ E cement paste E aggregate >> E resin binder

Figure 6. Example of similarity of microstructure geometry of portland cement concrete (CC) and

polymer concrete (PC)

Table 3. Approximate value of specific acoustic impedance, Z, for various building materials:

 Z = density x pulse velocity

Material Density Pulse velocity Specific acoustic

impedance, Z

(kg/m3) (m/s) (kg/m2s)

Concrete 2300 3500 to 4500 (8 to 10) ·106

Mortar 2100 3500 7 ·106

Air 1.2 343 0.411

Water 1000 1480 1.5 ·106

Soil 1500 500 to1500 (1 to 3) ·106

Clay 1800 1300 2 ·106

Sand 1700 1500 2 ·106

Timber 900 800 to1200 (0.7 to 1) ·106

Bitumen 1200 500 1 ·106

Asphalt 1900 2500 5 ·106

Steel 7800 5900 4.6 ·107

Granite 2700 5500 to 6100 (15 to 17) ·106
18

Figure 7. (a) Scheme of determination of reflection coefficient R; (b) R value for common "building

interfaces"; Z1, Z2 - acoustic impedance (computed from data in Table 3); (c) sketch of defects in PCCC system

pores cracks

concrete substrate

polymer composite

non-hardned

resin

non-zero

volume disbond

zero-volume

disbond

Ai

Atr

Aod

Z1

Z2

Aod = Ai Z2 Z1

1 Z2 - Z

+

Atr = Ai Z2 Z1

2 2 Z

+

R =

Z2 Z1

Z2 - Z1

+ = Ai

Atr

Interface Reflection

coefficient R

Concrete / air - 0.99

Concrete / water - 0.71

Concrete / soil - 0.63

Granite/concrete - 0.28

Asphalt / concrete + 0.29

Concrete / steel + 0.67

Bitumen/concrete + 0.80

b)

a)

c)

(16)
19

5. EVALUATION OF TECHNICAL PROPERTIES OF POLYMER CONCRETE

ELEMENTS

5.1. Experimental procedure

The assessment of PC precast elements was performed according to the normal procedure for

cement concretes (see Section 3.4). In this case, suitable variability of ultrasonic wave velocity

and PC properties is the main issue. There are many recommendations and national standards for

assessment of cement concrete structure with the pulse velocity method; however, a similar

guideline does not exist for PC. On the basis of literature data, as well as the authors’ experience,

the resin binder content, the content of sand, the micro-filler fraction of aggregate, the porosity,

and if necessary, the use of a wet aggregate, can all be varied to develop a reference curve for PC

(Fig. 8). In this work, the usefulness of a material optimization approach [6] for suitable

variability of PC properties was analyzed.

The surface-transmission pulse velocities in polymer concretes were measured by a commercial

concrete tester, using sets of associated transducer pairs. The source pulse frequency was 100 kHz,

and the distance between transducers was 80 mm. A petroleum jelly was used as a coupling agent to

improve the acoustic contact between the samples and the transducers. The propagation times were

measured for all PC samples before mechanical testing, with later calculation of the ultrasonic wave

velocities.

The compressive strength, fc, and flexural strength, fb, were tested for each sample. The static

modulus of elasticity (Young’s modulus) was calculated from the load-deflection curve at 50 % of

ultimate load. Volume density (total mass divided by total volume) and porosity, which can be

treated as bulk material parameters, were determined for characterization of the PC

microstructure. The porosity, p, was calculated from the formula: p=1-(Dv/Ds), where the volume

density, Dv, and specific density (total mass divided by pore-free volume), Ds, were measured for

each sample. Additionally, the dynamic elasticity modulus (see Table 1) was estimated from the

following formula: Ed = Dv vp

 2 , where vp = longitudinal pulse velocity and Dv = the volume

density.

Beam-shaped samples (40 mm x 40 mm x 160 mm) were prepared and used for compressive

and bending tests. The ultrasonic measurements and determination of mechanical properties were

carried out after 14 d of PC curing.

5.2. Ultrasonic evaluation of selected technical properties of vinylester concrete

5.2.1. Materials

Introductory investigations of the ultrasonic evaluation of vinyl-ester polymer concrete properties

were carried out during the preparation of a proposal for the M.Skłodowska-Curie US-PL Fund II

[51-53]. An analysis of PC composition effect on ultrasonic wave propagation was one purpose of

that investigation. The pulse velocity of ultrasonic waves mainly depends on the elastic properties of

the constituent material, the volume density, and the geometry of the structures tested. The geometry

effect can be minimized by using standard sample and test procedures. The elastic properties and

density are direct results of the material microstructure. The basic elements of polymer concrete

structure can be classified, in descending order of the elastic properties, as follows: filler (course

aggregate), micro-filler (fine sand), resin binder, and pores (to be thought of as a filler with zero

elastic properties). Some factors like the porosity, or low adhesion on an aggregate/binder interface,
20

can also affect the ultrasonic wave attenuation. On the other hand, increasing the filler and microfiller content can increase the ultrasonic wave velocity.

GUIDELINES FOR ULTRASONIC ASSESSMENT

OF CONCRETE-LIKE COMPOSITE PROPERTIES

Cement concrete:

RILEM draft recommendation and many

national standards and recommendations

Polymer concrete:

None exist

TYPE OF REFERENCE CONCRETE

CC PC (proposal)

∇ type of cement

∇ cement content

∇ type (nature) of aggregate

∇ granularity of aggregate

∇ admixture type and content

∆ type of resin binder

∆ type (nature) of aggregate

∆ binder content ⇒ aggregate

to binder ratio by weight

∆ granularity of aggregate

∆ micro-filler type and content

MATERIAL PARAMETERS WHICH CAN BE VARIED

TO DEVELOP ISO-STRENGTH CURVE

CC PC (proposal)

∇ quantity of mixing water

∇ degree of compaction

∇ age of concrete (3 - 90 days)

∇ curing or storage conditions

+ if necessary:

∇ proportion of fine

material (± 8 %)

∇ cement content (± 10 %)

∆ aggregate (A) and resin

binder (B) - aggregate

to binder ratio by weight ⇒ A/B

∆ content of sand fraction

of aggregate ⇒ S/A

∆ micro-filler content ⇒ M/A

∆ porosity

+ if necessary:

∆ wet (coarse) aggregate

Figure 8. Research approach to evaluation of PC precast element properties

MATERIAL OPTIMIZATION APPROACH

Material model of PC

PC properties = f (A/B, S/A, M/A)
21

In the present work, eight types of model polymer concretes, differing in structure, were

tested. A commercial vinyl-ester resin with low viscosity was used as the binder. A natural multisize fraction quartz aggregate with grain sizes ranging from 0 mm to 8 mm, was used as the filler.

A silica flour (>90 % SiO2) with a high specific surface area (1800 m2/kg to 2000 m2/kg) was

used as the micro-filler. On the basis of previous investigation of various vinyl-ester concretes,

the aggregate (A) to resin binder (B) ratio, A/B (by mass), was equal to 8 for the polymer

concretes tested. The following eight types of polymer concrete, differing in aggregate composition,

were investigated. Each composite used a different size range of aggregates. The table below shows

the size fraction of the quartz aggregates used in each material, which were labeled A-H. For

composites D-H, the amounts of different aggregates shown are by mass.

• single-fraction composites:

A: size fraction 0 mm to 2 mm,

B: size fraction 2 mm to 4 mm,

C: size fraction 4 mm to 8 mm,

• multi-fraction composites (Fig. 9):

D: 1/3 (0 to 2) mm + 1/3 (2 to 4) mm + 1/3 (4 to 8) mm,

E: 50 % (0 to 2) mm + 50 % (2 to 4) mm,

F: 48 % (0 to 2) mm + 48 % (2 to 4) mm + 4 % silica flour,

G: 46 % (0 to 2) mm + 46 % (2 to 4) mm + 8 % silica flour,

H: 44 % (0 to 2) mm + 44 % (2 to 4) mm + 12 % silica flour.

The composition of the PC of types F-H was selected on the basis of the non-continuous sieve

curve method, with silica flour included as the micro-filler. Composite E served as the reference

composition for composites F-H. The particular types of composite differed in structure. Due to the

low workability of the polymer concrete mix when high contents of micro-filler were present, the

samples of composite H were characterized by a high irregularity in the micro-filler distribution. The

relationship between ultrasonic pulse velocity and mechanical properties for a vinyl-ester concrete,

which was representative of industrial precast elements (see Fig. 9f), was analyzed. In this case, the

aggregate to resin binder ratio was A/B = 10 (by mass). Quartz aggregate with a grain size ranging

from 0 mm to 5 mm (using a continuous sieve curve) was used, with silica flour used as the microfiller. The basic differences between the model and industrial vinyl-ester concrete samples were the

grain size distribution of the aggregate and the type of vinyl-ester binder. Six rectangular-shaped

samples, 40 mm x 40 mm x 160 mm, were prepared for each type of composite.

5.2.2. Results

Ultrasonic testing with the indirect pulse velocity method on the rectangular samples was

compared to tests of ultrasonic wave propagation with the direct method was carried out for

representative sample of each vinyl-ester concrete type. The results of these tests showed that the

pulse velocity measured with both methods was practically the same, with a ratio of 1.00 ± 0.02 for

both the model and the industrial vinyl-ester concretes. This confirmed the usefulness of the indirect

pulse velocity method for evaluation of PC properties.

The results of measurements of the parameters of ultrasonic wave propagation (see Table 4)

indicated that the propagation time was characterized by a low coefficient of variation CT, ranging

from a low of 0.9 % for composite F to a high of 6.2 % for composite C. The high value of the

coefficient of variation for the amplitude, as high as 50 %, made this parameter practically useless

for estimation of the technical properties of polymer concrete. On the basis of a statistical analysis

(significance level α=0.05), it can be concluded that the values of propagation time obtained for the

types of composites tested were significantly different and therefore can be used for characterization

of the technical properties of concrete. The pulse velocity, vp, for each sample was calculated, based

on the values of the propagation time. The pulse velocity was lower – up to 30 % - for the single-
22

size-fraction composites (A,B,C) compared to the multi-size-fraction composites (D,E,F,G,H). This

result indicates a significant effect of aggregate grain composition, with wider size range resulting in

better filling of the volume and with lower porosity. For example, pulse velocities for the singlesize-fraction composites, A, B, and C, were lower than for composite D, which consists of all the

aggregate size fractions used in A, B, and C. In general, the addition of micro-filler increased the

pulse velocity. Only in the case of composite H, which had the highest micro-filler content, the pulse

velocity was lower than in the reference composite E (Fig. 10a). In this case, the lower workability

of the polymer concrete mix when large amounts of micro-filler was present caused an irregular

distribution of the micro-filler and formation of micro-filler agglomeration (Fig. 10b), often with

microcracks in the interior of the agglomerates (see Fig.10c). This agglomeration was the probable

cause of the significant decrease of pulse velocity for composite H.
23

Figure 9: Examples of microstructure of tested vinyl ester concretes (SEM - BSE mode):

(a - f) model multi-fractional vinyl ester concretes (types D – H, respectively (magnification. 25x);

(g) microstructure of industrial vinyl ester concrete (magnification 13x)

a)

b)

c)

d)

f)

g)
24

Figure 10. (a) Influence of the micro-filler content on the pulse velocity for composites E-H, (b)

example of composite H microstructure with agglomeration of micro-filler, (c) microcracks inside

agglomeration.

Table 4. Parameters of ultrasonic wave propagation for vinyl ester concretes tested

Statistical parameter

Composite type

ABCDE FGH

propagation time, μs

Mean value, μm 22.67 23.67 24.77 19.63 19.18 18.63 17.96 19.78

Std.dev., μm 0.50 0.86 1.52 0.37 0.26 0.15 0.19 0.54

Coeff. of variation, % 2.22 3.66 6.15 1.89 1.36 0.82 1.10 2.73

Amplitude

Mean value 166.83 212.06 125.28 128.17 123.89 138.83 130.67 130.33

Std..dev. 43.94 45.72 24.30 17.47 10.77 13.37 7.27 8.27

Coeff. of variation, % 26.34 21.56 19.39 13.63 8.69 9.63 5.56 6.34

Mean value of pulse velocity, m/s

---- 3530 3388 3230 4076 4172 4293 4454 4047

Mean value of dynamic elasticity modulus, GPa

---- 25.2 22.2 21.1 37.0 38.0 40.4 43.6 35.6

The technical properties of the model PC composites types used are shown in Table 5. The

composites differed in porosity. The highest porosity was observed for the single-fraction

composites, greater than 10 %. An increasing width of aggregate size distribution and the addition of

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5

0 4 8 12

microfiller fraction in aggregate, %

pulse velocity, km/s

E F G H

a) b) c)
25

silica flour tended to decrease the porosity down to about 6 % (a porosity of around 5 % is a typical

value for polymer concretes).

The different PC structures resulted in different properties. The compressive strength of multisize-fraction (D-H) composites was about 50 % higher than for single-size-fraction composites (AC). The addition of micro-filler (composites E - H), in relatively small amounts, did not affected the

compressive strength by very much compared to the standard deviations. The lowest values of fc in

the micro-filler group, however, were obtained for composite H. The average compressive strength

for composite H was about 30 % lower than that for reference composite E, and were probably due

to the high irregularity of the micro-filler distribution. The flexural strength for the microfiller group

compared to the other PC types was changed in a relatively smaller degree. However, the nonhomogeneous distribution of micro-filler in composite H caused a 45 % lowering of fb compared to

Table 5. Technical properties of tested vinyl ester concretes

Statistical parameter Composite type

ABCDE FGH

compressive strength, MPa

Mean value, MPa 24.8 13.2 13.2 44.2 47.4 48.2 47.4 35.1

Std.dev.,MPa 3.4 1.6 3.7 6.2 2.4 4.2 4.8 2.5

Coeff. of Var., % 13.8 12.0 28.0 14.1 5.1 8.6 9.8 7.1

flexural strength, MPa

Mean value, MPa 15.7 14.1 11.9 18.5 20.8 22.1 21.5 12.3

Std.dev.,MPa 2.0 2.0 0.5 1.0 1.1 1.1 2.1 1.2

Coeff. of Var., % 12.7 13.9 4.2 5.2 5.5 5.1 9.8 9.7

static elasticity modulus, GPa

Mean value, GPa 9.6 4.0 2.0 12.8 15.9 18.8 20.4 19.5

Std.dev.,GPa 3.5 1.2 0.1 1.3 1.7 1.5 5.8 2.8

Coeff. of Var., % 36.6 31.4 6.2 9.8 10.8 7.9 1.2 19.5

porosity, %

Mean value, % 10.9 17.8 14.9 7.0 7.7 5.8 5.8 6.1

Std.dev., % 1.1 1.2 1.7 0.9 0.9 0.6 1.2 1.2

Coeff. of Var., % 9.8 6.5 11.3 12.8 11.9 9.9 20.2 19.2

volume density, kg/m3

Mean value, kg/m3 2018 1932 2025 2224 2184 2191 2196 2176

Std.dev., kg/m3 30 17 41 12 8 9 8.9 5.4

Coeff. of Var., % 1.5 0.9 2.0 0.5 0.36 0.4 0.4 0.3
26

the reference composite E, just like in the case of compressive strength. The micro-filler composites

also had higher values of elasticity modulus, Eb, than did the non-micro-filler composites. This can

be explained by a modification of the resin binder by the micro-filler resulting in higher values of Eb

of the binder phase, e.g., the binder phase becomes a polymer micro-mortar. In the case of

composite H, a small decrease in the value of Eb was found, although within the standard deviation.

Clearly, a non-homogenous micro-filler distribution affects strength more than modulus of elasticity,

which is a reasonable result, since strength is much more sensitive to flaw size than is the modulus

of elasticity.

5.2.3. Relationship between technical properties and ultrasonic wave propagation

The possibility of application of an ultrasonic method for the non-destructive evaluation of the

mechanical properties of PC composites depends upon determining an adequate regression function

a with a high value of correlation coefficient (close to 1). If good empirical relations are found

between ultrasonic pulse velocity, for example, and other properties, then one measurement of pulse

velocity can be used to accurately determine many other properties as well. In this work, the

relationships (Fig. 11) between pulse velocity, vp, and the structural parameters p and Dv and the

mechanical properties fc, fb and Eb were analyzed. The accuracy of fitting by various regression

functions was characterized by the value of the coefficient of standard deviation, Cd, calculated from

Eq. 14 (see Table 6 for a summary of results). Analysis of the results indicated that in all cases the

best fits were obtained using a quadratic polynomial, e.g., the highest regression coefficients and the

lowest values of Cd. For both the relationships of porosity and volume density vs. pulse velocity,

statistical significance was obtained, denoted by a value of the correlation coefficient r > 0.90. As

the porosity, p, decreased and volume density, Dv increased, the pulse velocity increased. This

results confirms the effect of the PC structure on the propagation of ultrasonic waves. Micro-filler,

which is added to PC in order to modify the elastic properties of the resin binder, also increases the

pulse velocity. The effect of PC microstructure on PC properties was also confirmed also in the

relationships between mechanical

Table 6. Regression functions for pulse velocity and selected technical properties of tested vinylester concretes.

Relationship Regression function Correlation

coefficient, r

Coeff. of standard

deviation Cd,

eq. (14)

p – vp p = 6.80 vp

2 – 61.14 vp + 143.32 0.91 17.3

Dp –vp Dp = -105.39 vp

2 + 1021.31 vp – 242.42 0.90 2.2

fc – vp fc = -16.13 vp

2 + 161.43 vp – 340.22 0.94 21.4

fb – vp fb = -2.32 vp

2 + 25.60 vp – 46.05 0.90 9.0

Eb –vp Eb = 14.20 vp – 43.20 0.94 25.5

properties and pulse velocity. The regression functions obtained for fc, fb, and Eb were characterized

by a high value of the correlation coefficient (r > 0.90). The results of the t-Student test confirmed

the statistical significance of the analyzed functions. It should be noted that non-homogeneous
27

structure (e.g. irregular distribution of micro-filler due to the low workability of PC mix) affects the

propagation of ultrasonic waves to a smaller extent than do the mechanical properties of PC.

The regression analysis of the relation between the static, Eb, and the dynamic, Ed, modulus of

elasticity showed (Fig. 8f) that this relationship (α=0.05) is statistically significant (correlation

coefficient r=0.94). Similar relations exist for the other composites.

Statistically significant relationships between pulse velocity and compressive and flexural

strength were also obtained for representative samples of industrial vinyl-ester concrete (Fig.12).

The regression functions obtained for fc and fb were characterized by lower correlation coefficients

(0.84 and 0.71) in comparison to the corresponding properties of the model concretes. However, the

shape of the regression function plot is similar in both cases and the low values of the coefficient of

standard deviation, Cd, indicated a reasonable fit to experimental results.

The results obtained confirmed that the pulse velocity method is useful for nondestructive

evaluation of polymer concrete properties. However, further investigation should be continued to

improve the accuracy of the reference curve.
28

p = 6.80vp

2

 - 61.14vp + 143.32

r = 0.91

0

5

10

15

20

25

3.0 3.4 3.8 4.2 4.6

vp, km/s

p, %

a) Dp= -105.39vp

2 + 1021.31vp - 242.42

r = 0.90

1800

1900

2000

2100

2200

2300

2400

3.0 3.4 3.8 4.2 4.6

vp, km/s

Dp, kg/m3 b)

fc= -16.13vp

2

 + 161.43vp - 340.22

r = 0.94

0

20

40

60

80

3.0 3.4 3.8 4.2 4.6

vp, km/s

fc, MPa

c) fb = -2.32vp

2

 + 25.60vp - 46.05

r = 0.90

5

10

15

20

25

30

3.0 3.4 3.8 4.2 4.6

vp, km/s

fb, MPa

d)

Eb = 14.20vp - 43,20

r = 0.94

0

5

10

15

20

25

3.0 3.4 3.8 4.2 4.6

vp, km/s

Eb, GPa

e)

Edyn = 1.047Estat + 16.113

r = 0.94

0

10

20

30

40

50

0 10 20 30

Estat, GPa

Edyn, GPa

f)

Figure 11. Regression functions describing relationships between the pulse velocity and: (a)

porosity, p, (b) volume density, Dv, (c) compressive strength, fc, d) flexural strength, fb, and (e) static

elasticity modulus, Eb , for tested model vinyl ester concretes tested. Graph f) shows the dynamic

elasticity modulus, Edyn, plotted against the static modulus of elasticity, Estat .
29

Figure 12. Relationships between pulse velocity, vp, and: (a) compressive strength, fc, and

(b) flexural strength, fb, for representative samples of industrial precast elements made from vinyl

ester concrete. The dashed line shows the corresponding regression functions for the model vinyl

ester concretes, obtained previously (see Table 6).

Additionally, the effect of aggregate moisture content on polymer concrete properties and the

pulse velocity was tested. The investigations were carried out for vinyl-ester mortar (A/B = 4 by

mass) and vinyl-ester concrete (A/B = 8 by mass) with aggregates of different moisture contents,

wagg (amount of water by mass). The results obtained showed that the decrease of interior

adhesion (Fig.13) affected both the PC mechanical properties and the pulse velocity (Fig.14). As

the moisture level of the aggregates increased, the mechanical properties and the pulse velocity

significantly decreased. This indicates that aggregates with different moisture levels can be used

for development of the lower region of the reference curve, for low values of the strength and the

pulse velocity. In other words, the variation of these properties with aggregate moisture content is

statistically significant.

Figure 13. SEM micrographs showing examples of PC microstructure with dry (a) and wet (b)

aggregate, which had caused different levels of adhesion.

fb = -2.32vp

2

 + 25.60vp - 46.05

fb = -3,90vp

2

 + 35,22vp - 50,53

r = 0.71 Cd = 3.6%

5

10

15

20

25

30

35

3.0 3.5 4.0 4.5 5.0

vp, km/s

fb, MPa

b)

fc = -16.13vp

2

 + 161.43vp - 340.22

fc = -27.48vp

2

 + 252.03vp - 489.47

r = 0.84 Cd =6.9%

0

20

40

60

80

100

120

3.0 3.5 4.0 4.5 5.0

vp, km/s

fc, MPa

a)
30

Figure 14. Effect of aggregate moisture content on a) mechanical properties: compressive

strength, fc, and flexural strength, fb, and b) pulse velocity, vp, for vinyl-ester mortar and

concrete; w agg – water to aggregate ratio by mass. Error bars represent variation over several

nominally identical samples.

5.3. Ultrasonic estimation of selected properties of epoxy concrete

5.3.1. Materials and results

The second stage of the experiment was concerned with the ultrasonic evaluation of epoxy

concrete with different fractions of resin binder (different A/B ratio). The relation between their

properties and the ultrasonic pulse velocity was tested. A commercial epoxy resin was used as a

binder. The same natural quartz, with diameters of (0 to 1.5) mm, (1 to 2) mm, and (2 to 5) mm, was

used as an aggregate, and the same microsilica was used as a micro-filler. Seven types of epoxy

concrete, differing in microstructure and properties, were obtained (Table 7). The material

optimization approach was used to formulate the composition of the tested epoxy concretes [54]

and to obtain a wide range of variability of both concrete properties and pulse velocity. Beamshaped samples (40 mm x 40 mm x 160 mm) were prepared and used for compressive and bending

tests.

5.3.2. Discussion of results

The relationship between the ultrasonic pulse velocity, vp, and the mechanical properties fc and fb

and the structural parameters p, and Dv were analyzed (see Fig.15). Analysis of the results indicated

that, just as in the vinyl-ester concretes, the best fits to experimental data were obtained using

quadratic polynomial functions (Table 8). For both porosity and volume density vs. pulse velocity, a

satisfactory level of statistical significance was obtained. With decreasing p and increasing Dv , pulse

velocity increased. The relationships between the mechanical properties and the propagation of the

ultrasonic wave were also statistically significant. The regression functions obtained for fc and fb

were characterized by a high value of the correlation coefficient, r > 0.93. In the case of epoxy

concretes with a lower A/B ratio, a relatively high effect of micro-filler addition on pulse velocity

was observed – for concrete with A/B=9 and M/A=0.18 the pulse velocity was higher and

0

20

40

60

80

0 0.5 1 1.5 2

wagg, %

fc, fb, MPa

a)

 fc - mortar K/S = 4

 fc - concrete K/S = 8

 fb - mortar K/S = 4

 fb - concrete K/S =8

2

3

4

5

0 0.5 1 1.5 2

wagg, %

vp, km/s

 vp - mortar K/S = 4

 vp - concrete K/S = 8

b)
31

mechanical properties lower in comparison with concrete with A/B=7 and M/A=0.12. This

confirmed the conclusion from the previous investigation of vinyl-ester concrete.

Table 7. Properties of tested epoxy concretes (standard deviation)

Property Composite:

KI KII KIII KIV KV KVI KVII

Composition*,

by mass ratios

A/B=14

S/A=0.60

M/A=0.04

A/B=14

S/A=0.60

M/A=0.12

A/B=11

S/A=0.42

M/A=0.04

A/B=11

S/A=0.42

M/A=0.12

A/B=9

S/A=0.41

M/A=0.18

A/B=7

S/A=0.36

M/A=0.04

A/B=7

S/A=0.36

M/A=0.12

Pulse velocity,

vp [m/s]

3883

(23)

3972

(22)

3989

(17)

4189

(19)

4509

(20)

4278

(19)

4405

(26)

Flexural strength,

fb [MPa]

12.2

(0.5)

11.7

(0.8)

13.1

(1.4)

17.5

(1.9)

20.9

(1.1)

19.5

(1.9)

23.8

(0.9)

Compressive

strength, fc [MPa]

27.6

(2.3)

25.1

(0.3)

38.4

(5.3)

52.6

(4.1)

81.5

(4.9)

81.0

(3.2)

86.4

(1.5)

Porosity, p [%] 15.4

(0.5)

13.9

(0.8)

12.2

(0.9)

11.5

(2.9)

5.5

(0.6)

6.4

(0.4)

5.1

(0.8)

Volume density,

Dv [kg/m3

]

2115

(8)

2155

(11)

2151

(20)

2207

(49)

2273

(9)

2198

(4)

2234

(5)

* B- binder, A – total aggregate, S – sand, M – micro-filler

Table 8. Regression functions for pulse velocity and selected technical properties of epoxy

concretes tested.

Relationship Regression function Correlation

coefficient, r

Coeff. of std. dev.

Cd, eq.(15)

p – vp p = 16.1 vp

2 – 152.3 vp + 143.3 0.95 15.4

Dp –vp Dp = -129.6 vp

2 + 1311.1 vp – 1014.9 0.90 1.2

fc – vp fc = -47.9 vp

2 + 513.1 vp – 1248.2 0.95 19.5

fb – vp fb = -11.1 vp

2 + 112.6 vp – 258.7 0.93 9.1

In the case of the epoxy concretes, the correlation coefficients obtained were higher and the

coefficients of standards deviation lower in comparison to the corresponding coefficients determined

for the vinyl-ester concrete (see Fig. 15 and Table 6). This indicates that the material optimization

approach (material model of PC) gives the possibility of developing a reference curve for a given PC

type with high accuracy and uniform distribution of data points in the range of mechanical properties

tested. The basic issue is how big the variation in the PC composition can be to develop the

reference curve for the given PC type. It seems that the variation in the micro-filler content is the

most important. Figure 16 shows the relationship between PC properties and the pulse velocity for

epoxy concretes that differ in the micro-filler content, M/A. This figure indicates that two main

types of concrete can be recognized: those with low values of M/A (0.04) and those with high values

of M/A (0.12). The results for concrete with the highest M/A=0.18 are close to the PC type with the

lowest M/A=0.04. This indicates that the relative variation of micro-filler content (M/A) in
32

Epoxy concrete: Dmax = 4 mm Vinyl-ester

concrete

Composition A/B=14

S/A=0.60

M/A=0.04

A/B=14

S/A=0.60

M/A=0.12

A/B=11

S/A=0.42

M/A=0.04

A/B=11

S/A=0.42

M/A=0.12

A/B=9

S/A=0.41

M/A=0.18

A/B=7

S/A=0.36

M/A=0.04

A/B=7

S/A=0.36

M/A=0.12

A/B=8

Dmax= 2-8 mm

Symbol

Figure 15. Regression functions describing relationships between pulse velocity and a) porosity, b)

volume density, c) compressive strength, and d) flexural strength, for epoxy and vinyl-ester concretes

differing in microstructure.

y = -2. 32x2 + 25.60x - 46.05

r = 0.8959

y = -11.11x2 + 112.55x - 258.70

r = 0.9281

0

5

10

15

20

25

30

3.0 3.5 4.0 4.5 5.0

vp, km/s

fb, MPa

y = -16.13x2 + 161.43x - 340.22

r = 0.9409

y = -47.9x2

 + 513.08x - 1248.2

r = 0.9528

0

10

20

30

40

50

60

70

80

90

100

3.0 3.5 4.0 4.5 5.0

vp, km/s

fc, MPa

y = 0.3917x2 + 2.0667x - 2.244

r = 0.9263

y = 9.2867x2

 - 34.971x + 36.205

r = 0.9021

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

1.80 1.90 2.00 2.10 2.20 2.30 2.40

Dv , g/m3

vp, km/s

y = 0.0059x2 - 0.2218x + 5.3593

r = 0.9160

y = 0.0015x2

 - 0.0813x + 4.8098

r = 0.9469

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

0 5 10 15 20

p, %

vp, km/s

b)

c) d)

a)
33

Figure 16. The pulse velocity vs. a) porosity, b) volume density, c) compressive strength, and d)

flexural strength, for epoxy concretes with different micro-filler contents, M/A.

composition of the reference PC type should be not too high. On the basis of the results obtained it

can be proposed that the variation in M/A should be not higher than ± 0.02.

5.3.3. Analysis of relationship between microstructure and ultrasonic wave propagation

Fractography, which was used to determine the fracture surface roughness ratio, RS, was

then used for characterization of the microstructure of the epoxy concretes tested (Fig.17). It has

been shown [55,56] that the fracture surface roughness ratio, RS, is an important parameter that can

be used to characterize the toughness of materials. This parameter is defined as the true fracture

 M/A = 0.04 - M/A = 0.12 M/A = 0.18

0

20

40

60

80

100

3.8 4.0 4.2 4.4 4.6

vp, km/s

fc, MPa

0

5

10

15

20

25

30

3.8 4.0 4.2 4.4 4.6

vp, km/s

fb, MPa

0

2

4

6

8

10

12

14

16

18

3.8 4.0 4.2 4.4 4.6

vp, km/s

p, %

2000

2100

2200

2300

3.8 4.0 4.2 4.4 4.6

vp, km/s

Dv, kg/m3

a) b)

c) d)
34

surface area, S, divided by the apparent projected area, So: Rs = S/So. In the case of polymer

concrete, the topography of the fracture surface depends not only on the composition (resin binder to

aggregate ratio, A/B; porosity and aggregate grain size distribution) but also depends on the

adhesion between binder and aggregate [57]. In this case, the Rs parameter can be treated as a factor

describing the PC microstructure in a satisfactory way [58]. The relationship between Rs and pulse

velocity can be used for the nondestructive estimation of polymer concrete toughness.

The value of Rs was determined (Fig.18) using the method of vertical sections [59,60], for

representative samples of each type of epoxy concrete at magnifications of 10x, 25x, 63x, 160x,

and 400x (see Table 9).

Table 9. The surface roughness ratio, RS, for the epoxy concretes tested

Property Composite:

KI KII KIII KIV KV KVI KVII

Composition*,

by mass ratio

A/B=14

S/A=0.60

M/A=0.04

A/B=14

S/A=0.60

M/A=0.12

A/B=11

S/A=0.42

M/A=0.04

A/B=11

S/A=0.42

M/A=0.12

A/B=9

S/A=0.41

M/A=0.18

A/B=7

S/A=0.36

M/A=0.04

A/B=7

S/A=0.36

M/A=0.12

Surface fracture

ratio, Rs at mag.:

10x

25x

63x

160x

400x

2.213

2.101

1.985

1.949

1.834

2.308

2.357

1.970

1.889

1.743

2.324

2.144

1.927

2.093

1.924

2.011

1.972

1.869

2.072

1.942

1.660

1.598

1.654

1.712

1.819

1.607

1.571

1.559

1.851

1.815

1.708

1.665

1.685

1.819

1.760

* B- binder, A – aggregate (in total), S – sand, M – micro-filler

The results obtained indicated that the relationship between the pulse velocity and the fracture

surface geometry (Fig.19) depends on the observation level or magnification. The relationship

between pulse velocity and Rs is statistically significant up to a magnification of 160x. As the

magnification increases, the values of the correlation coefficients decrease. At a magnification of

400x, which starts seeing the length scales at the micro-filler level, the relationship is no longer

statistically significant. In this case, binder modification by the micro-filler affects pulse velocity to

larger extent than does the purely geometrical features of the microstructure resulting from the

composition. This confirmed previous conclusions that to develop the reference curve for a given PC

type the variation in micro-filler content should be limited.
35

Figure 17. Examples of microstructures and fracture surface profiles of the epoxy concretes tested.

Concrete compositions were designed based on the material model of epoxy concrete [54].

KVII: A/B = 7; S/A = 0.36; M/A = 0.12

KI: A/B = 14; S/A = 0.60; M/A = 0.04

KIII: A/B = 11; S/A = 0.42; M/A = 0.04

KV: A/B = 9; S/A = 0.41; M/A = 0.18 KVI: A/B = 7; S/A = 0.36; M/A = 0.04

KII: A/B = 14; S/A = 0.6; M/A = 0.12

KIV: A/B = 11; S/A = 0.42; M/A = 0.12

A – aggregate in total (mass);

B – binder (mass);

S – sand (mass);

M- micro-filler (mass)
36

Figure 18. Scheme of determination of fracture surface roughness ratio using vertical section

method and image analysis.

400x

10x

25x

160x 63x



RS (magnification) 

fracture

suface

cutting

i sample

thinning
37

Figure 19. Pulse velocity plotted against surface roughness ratio at different magnifications for

epoxy concrete. The correlation coefficient vs. the logarithm base ten of the magnification, N, is also

shown.

y = -0.682x + 5.517

r = 0.903

3.6

3.8

4.0

4.2

4.4

4.6

4.8

1.4 1.6 1.8 2.0 2.2 2.4 2.6

fracture surface roughness ratio

pulse velocity, km/s

magn. 10x

y = -0.674x + 5.460

r = 0.870

3.6

3.8

4.0

4.2

4.4

4.6

4.8

1.4 1.6 1.8 2.0 2.2 2.4 2.6

fracture surface roughness ratio

pulse velocity, km/s

magn.25x

y = -1.205x + 6.347

r = 0.868

3.6

3.8

4.0

4.2

4.4

4.6

4.8

1.4 1.6 1.8 2.0 2.2 2.4 2.6

fracture surface roughness ratio

pulse velocity, km/s

magn.63x

y = -1.219x + 6.500

r = 0.702

3.6

3.8

4.0

4.2

4.4

4.6

4.8

1.4 1.6 1.8 2.0 2.2 2.4 2.6

fracture surface roughness ratio

pulse velocity, km/s

magn.160x

3.6

3.8

4.0

4.2

4.4

4.6

4.8

1.4 1.6 1.8 2.0 2.2 2.4 2.6

fracture surface roughness ratio

pulse velocity, km/s

magn.400x

0.0

0.2

0.4

0.6

0.8

1.0

0.5 1.5 2.5 3.5

logN

correlation coefficiant r

statisticaly significant value

of correlation coefficiant
38

5.4. Conclusions concerning ultrasonic evaluation of elements made from polymer concrete

On the basis of the results obtained, the following main conclusions about ultrasonic

assessment of polymer concrete precast elements can be formulated:

• the regression functions obtained for vinyl-ester and epoxy concretes justify the possibility of

 applying ultrasonic methods for the nondestructive evaluation of properties of precast elements

 made from various polymer concretes;

• in engineering practice, a reference curve (ISO-strength curve) should be made for the given

type of polymer concrete, taking into account the type of resin binder and the type of aggregate;

• to develop a reference curve for calibration procedures the following parameters can be varied:

 the aggregate to resin binder ratio, A/B, the sand fraction, S/A, and in a limited range,

 the micro-filler content, M/A (the variation in M/A should be about 50 %, and the maximum

 value of M/A should be less than 0.12), and, if necessary, the aggregate moisture content;

• the material optimization approach (material model of polymer concrete) gives the possibility of

developing a reference curve for the given PC type with high accuracy and uniform distribution

of data points in the tested range of mechanical properties and pulse velocity;

• the pulse velocity, as calculated from the transit time and as the parameter describing the

ultrasonic wave propagation in a material, is practically useful for the estimation of PC

properties. The amplitude was found to be useless for estimating polymer concrete properties

because of the high level of scatter;

• the obtained results indicate the necessity for further investigation in order to increase the

accuracy of ultrasonic evaluation of polymer concrete properties using advanced methods of

data processing, e.g, fuzzy logic or Bayes's theory [61], or an advanced method of signal

analysis [62].
39

6. ULTRASONIC EVALUATION OF MULTI-LAYER REPAIR SYSTEMS

6.1. Propagation of ultrasonic waves through the medium boundary

Quality control of the efficiency of repair applications is the basic purpose behind the

nondestructive evaluation of the multi-layer systems of polymer concrete on top of portland

cement concrete (PC-CC) that are created as a result of repair techniques. This quality control is

focused mainly on the estimation of the thickness of system elements and the detection of various

defects at the PC-CC interface. Echo methods are preferred in this field [10]. Recently, the impactecho method, which was developed at the National Institute of Standards and Technology and

Cornell University, has appeared as the most promising NDT method for the quality assessment of

concrete structures [62,63]. However, detection of relatively large and deeply located defects only

plus expensive equipment are disadvantages of this method.

The adhesion between PC and CC is a crucial property of a repair system. Quality control of

the adhesion level is available using a semi-destructive method, e.g. a pull-off test (Fig. 20). In a

new European Standard, ENV 1504, a pull-off strength higher than 1.5 MPa is recommended. The

measurement of pull-off strength is usually restricted by contractors to a localized area due to the

destructive character of pull-off strength measurement. Therefore, the development of

nondestructive assessment methods for evaluation of the adhesion between polymer composites and

concrete substrates, and especially a possibility of mapping the adhesion level over a wide region, is

one of the important needs in the PC repair field.

The complexity of ultrasonic wave propagation through the PC-CC multi-layer system (see

Section 4) means that for proper nondestructive evaluation of a multi-layer system the proper

selection of investigation method is very important. In general, the usefulness of two ultrasonic

methods, the echo method and the indirect pulse velocity method, can be considered.

Figure 20. Example of deteriorated industrial floor and evaluation of adhesion with pull-off test
40

6.2. Evaluation of the PC - CC system with echo method

6.2.1. Procedure of the ultrasonic testing

The purpose of this stage of research was the evaluation of the possibility of using the

ultrasonic echo method for estimating the thickness of industrial resin floors and adhesion levels

in the PC-CC system. The investigations were carried out using four commercial transducers* of

various frequency characteristics: 0.5LN50, with a frequency of 0.5 MHz, S12HB0.8-3, with a

frequency range of 0.8 MHz to 3 MHz, 1V102, with a frequency 1 MHz, and 10V202, operating

at a frequency of 10 MHz and having a delay line.

The floor thickness was determined based on the location of the echo from the PC-CC

interface and the previously determined velocity of the wave in the floor material. The influence

of adhesion differences on the value of the echo amplitude was assumed during analysis of the

possibility of using the echo method for evaluation of the adhesion at the PC-CC interface. The

amplitude was amplified to a level equal to 0.8 of the maximum amplitude. The value of the

amplification was defined to be W0.8H. It was assumed that value of W0.8H was a measure of the

wave attenuation at the PC-CC interface and corresponded to a measure of the adhesion in the

PC-CC system. A commercial ultrasonic gel was applied as the coupling agent. The ultrasonic

tests of coatings were carried out after 7 d and 28 days of the curing of the portland cement

concrete substrate.

6.2.2. Materials

Tests were performed on samples corresponding to commercial epoxy industrial floors.

The polymer coating was placed on the concrete substrate of the B35 class, with sample sizes of

500 mm x 1000 mm x 50 mm. For obtaining various adhesion levels, coatings were placed on

concrete substrate having 8 different kinds of surface states: dry, wet, oiled, and with primer

(Fig. 21a). The variability of the coating thickness over a sample surface was obtained by a slight

inclination of the substrate. Additionally, beam samples from the floor material, 40 mm x 40 mm

x 160 mm were prepared for determination of the ultrasonic wave velocity in the tested polymer

coating and for calibration of the measurement equipment. The measurements were carried out

also for a floor that was disbonded to the concrete substrate. In this case, the polymer coating

was applied to a layer of metallic foil and after hardening placed onto a concrete substrate

(Fig.21b). This corresponded to a "zero adhesion" state of the PC-CC system.

 

*

 Certain commercial equipment is identified in this report in order to adequately specify the experimental

procedure. In no case does such identification imply recommendation or endorsement by the National Institute of

Standards and Technology, nor does it imply that the equipment used is necessarily the best available for the

purpose.
41

Figure 21. Scheme of ultrasonic evaluation of (a) different adhesion levels and (b) the “zeroadhesion” state.

6.2.3. Determination of adhesion

 Adhesion between the polymer coating and the concrete substrate has been characterized

by the bond strength in the pull-off test. Steel cylindrical “dollies” of 50 mm diameter were

glued to the coating with epoxy glue. The floor layer and concrete substrate were drilled to a

depth of 10 mm. The pull-off bond strength was determined using a digital tester after ultrasonic

testing was completed.

6.2.4. Results of testing using echo method

The reflection coefficient, R, was determined for the floor and concrete substrate system.

For that reason, the density and the ultrasonic pulse velocity for the floor material and concrete

substrate were first determined separately. The ultrasonic wave velocity obtained for the epoxy floor

was equal to 2650 m/s ± 10 m/s. This value agrees well with literature data [64]. The apparent

density of the floor was 1589 kg/m3

 ± 12 kg/m3. The ultrasonic velocity and density for the concrete

substrate were 4100 m/s ± 24 m/s and 2220 kg/m3

 ± 22 kg/m3, respectively. Using these values, the

acoustic impedances were calculated:

- epoxy floor: Z1 = cp · ρp = 4.21 · 106 kg/m2

s

- concrete substrate: Z2 = cb · ρb = 9.102 · 106 kg/m2

s

The value of the reflection coefficient for the epoxy floor – concrete substrate system, calculated

from eq.16, was then:

R =

2 1

2 1

Z Z

Z Z

+

− = 0.37 kg/m2

s

The value obtained is relatively high (Fig. 7), showing that obtaining information on the floor –

concrete interface separation surface using ultrasonic techniques is possible even without complete

delamination.

Measurements of the floor thickness using the echo method after 7 days of curing were

carried out for checking the usefulness of the transducers for evaluating the state of the PC-CC

a) variable adhesion b) zero adhesion

PC coating

Concrete substrate

with different

 surface quality

Concrete substrate
42

interface. Transducers S12HB0.8-3 and 10V202 appeared to be the most useful compared to the

other transducers. The echo from the floor – substrate separation interface was visible in the

recorded signal (Fig. 22 a, b), but the echo from the bottom of the substrate was not visible. This

is favorable for the testing of coatings due to the reduction of disturbing echoes, e.g. from the

reinforcement. The remaining transducers were not useful for evaluating the state of the PC-CC

system. Transducers 0.5LN50 and 1V102 had too large of a dead zone and the floor signal

interfered with the input impulse (Fig. 22 c, d). However, transducer 0.5LN50 recorded the echo

from the substrate bottom. Transducers S12HB0.8-3 and 10V202 were selected for further

testing.

The floor thickness and amplification coefficient W0,8H were measured in the first stage

using the ultrasonic echo method. The tests were performed in 9 uniformly distributed measuring

points of every field. Then the pull-off bond strength was determined in 4 places of each field. After

detachment of the dollies, the real thickness of the floor was measured in 4 places of each dolly

using a Brinell magnifier. Results of these measurements are given in Table 10. The distributions of

floor thickness over a given slab, measured directly with the magnifier and estimated by ultrasonic

methods, are presented in Fig. 23. In all cases, the thickness distribution was determined using the

least squares method of estimation with the same parameters.
43

Figure 22. Registered signal (A-scan) after testing the floor system of 2 mm thick layered on

concrete substrate with echo method using: a) S12HB0.8-3, b) 10V202, and c) 0.5LN50, 1V102.

The signals recorded in (a) and (b) contain echoes from the interface floor - concrete substrate.

a)

c)

depth

depth depth

Relative amplitude

Relative amplitude

Relative amplitude

Relative amplitude

2.2 mm – echo from

interface PC-CC

56 mm – echo

from bottom

of concrete

substrate

 depth

2.0 mm – echo from

interface PC-CC

Overlapping

with echo from

transmitter

d) depth

echo from

delay line

 depth

b)
44

Figure 23. The floor thickness distributions measured with magnifier (a) and estimated with

ultrasonic echo method (b,c) for the S12HB0.8-3 and 10V202 transducers, respectively.

 1

 1,5

 2

 2,5

 3

 3,5

 4

 4,5

 5

 ponad

length, cm

width, cm

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

 1

 1,5

 2

 2,5

 3

 3,5

 4

 4,5

 5

 ponad

length, cm

width, cm

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

 1

 1,5

 2

 2,5

 3

 3,5

 4

 4,5

 5

 ponad

length, cm

width, cm

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

a)

a)

b)

c)

above

above

above
45

Table 10. Results of floor thickness estimation and pull-off strength measurements

Ultrasonic testing

Transducer S12HB0.8-3 Transducer 10V202

Thickness tU-E, mm Amplification, dB Thickness tU-E, mm Amplification, dB

"true" thickness

t, mm

Pole

Mean

value

Standard

deviation

Mean value Standard

deviation

Mean value Standard

deviation

Mean value Standard

deviation

Mean value Standar

deviatio

After 7 d of hardening

1 4.14 0.56 48.32 1.81 4.180 0.540 74.100 1.208 --2 4.57 0.55 51.11 3.37 4.460 0.918 74.080 4.479 --3 3.04 0.38 49.86 3.08 2.840 0.358 67.620 3.776 --4 2.69 0.56 48.78 3.43 2.500 0.430 69.040 2.752 --5 3.37 0.43 51.00 1.99 3.280 0.363 71.100 0.806 --6 2.92 0.34 47.41 1.70 2.960 0.477 72.013 5.276 --7 2.73 0.17 48.97 2.64 2.640 0.329 71.060 1.960 --8 2.17 0.23 47.67 2.57 2.200 0.453 65.720 2.729 --

After 28 d of hardening

1 4.44 0.60 63.73 2.32 4.080 0.638 86.560 2.482 3.88 02 4.72 0.39 66.11 3.11 4.600 0.696 86.760 3.692 4.21 03 3.10 0.36 63.67 3.21 2.860 0.428 83.340 3.303 2.29 04 2.81 0.61 64.47 3.45 2.440 0.503 81.000 3.156 1.98 05 3.52 0.52 65.21 2.18 3.200 0.424 82.440 2.981 2.96 06 3.24 0.37 60.93 2.46 3.000 0.361 82.960 1.071 2.67 07 2.84 0.22 62.62 3.17 2.540 0.365 79.720 1.221 2.36 08 2.32 0.29 59.67 3.10 2.220 0.377 78.260 2.269 2.07 0
46

6.2.5. Discussion of results

The floor thickness distributions presented in Fig. 23 indicated that the ultrasonic

measurements with the echo method reproduced the trends in floor thickness very well. However,

in comparison to the thickness distribution obtained directly with the Brinell magnifier, the

ultrasonic method overestimated the floor thickness by about 0.3 mm to 0.5 mm, depending on

the transducer type. The thickness measurement results were statistically analyzed to find a

relationship between the thickness as determined with different methods. The floor thickness as

estimated by the ultrasonic method after 7 d and 28 d of curing is in very good agreement

(correlation coefficients r > 0.95) with the "true" thickness (Fig. 24) for both transducers. The

thicknesses as estimated by the two transducers are close to each other. This displays the good

repeatability of the ultrasonic method. In general, the relationship between thickness measured

with magnifier, t, and thickness as estimated with ultrasonic method, tU-E, can be described by a

simple linear function:

 t = A tU-E (17)

The correction coefficient A equals 0.88 for the S12HB0.8-3 transducer and 0.90 for the 10V202

transducer. The value of the A coefficient mainly depends on the transducer type, but hardening

time and presence of the coupling agent can also affected the estimated value of thickness. It was

observed that as the hardening time of the floor increased the A coefficient increased at an

approximately constant dosage of coupling agent. This implies that during nondestructive

evaluation of floor thickness a suitable correction coefficient should be introduced taking into

account the device type, coupling agent used, and hardening time. The results obtained indicated

that a value of the "A" coefficient equal to 0.89 is acceptable for both transducers. Surprisingly

good results were also obtained for the 10 MHz transducer, which was theoretically more

susceptible to ultrasonic wave attenuation than were the lower frequency transducers.

Analysis of the amplification coefficient (W0.8H ) results showed that a relation between

the level of amplification and the pull-off bond strength did not exist (Fig. 25) for both hardening

times. The value of attenuation W0.8H for a "zero-adhesion" state of polymer coating (see Fig. 21)

was located between the points corresponding to different levels of adhesion. This indicated that

evaluation of the adhesion in PC-CC systems with an ultrasonic echo method is difficult because

of the high difference between the acoustic impedances of the PC coating and concrete substrate.

In the case of the 10V202 transducer (frequency 10 MHz), a statistically significant relation

between thickness and amplification was observed. This implies that the differences found in the

values of the amplification coefficient W0.8H measured on fields with various adhesion levels may

be at least 70 % the result of the differences in the floor thickness (Fig. 26 a). The effect of the

floor thickness was less significant in the case of the S12HB0.8-3 transducer (Fig. 26b). A

statistical significance with high determination coefficient was obtained for the relationship

between attenuation W0.8H and the "true" thickness as measured with the magnifier (Fig. 26c).

The results obtained confirmed that the ultrasonic echo method can be useful for the nondestructive evaluation of the polymer coating floor thickness. However, there is need for further

investigation to establish optimal parameters for the ultrasonic measurements, which are mainly

the type and the frequency characteristics of the transducers used.
47

Figure 24. Relationship between "true" value of floor thickness and estimated with ultrasonic echo

method using transducer a) S12HB0.8-3, b) 10V202, and c) thickness estimated with both

transducers.

 7 d 28 d of hardening

a)

b)

c)

y = 0.882x

R2

 = 0.913

y = 0.838x

R2 = 0.930

1

2

3

4

5

123456

t U-E, mm

t, mm

y = 0.904x

R2

 = 0.961

y = 0.886x

R2

 = 0.968

1

2

3

4

5

123456

t U-E, mm

t, mm

y = 1.002x

R2

 = 0.967

y = 1.078x

R2

 = 0.9764

1

2

3

4

5

12345

t(S12HB0,8-3), mm

t(10V202), m

m
48

 a) b)

Figure 25. Pull-off strength against amplification W0.8H for tested epoxy floor system with

transducers: a) S12HB0.8-3 and b) 10V202

 7 d 28 d of hardening

44

46

48

50

52

54

01234

pull-off strength, MPa

W0,8H, dB

W0,8H for "zero 

adhesion" state

60

62

64

66

68

70

72

74

76

01234

pull-off strength, MPa

W0,8H, dB

W0,8H for "zero 

adhesion" state

c)

Figure 26. Amplification coefficient W0.8H

plotted against the estimated thickness tU-E for

epoxy floor systems tested with transducers: a)

S12HB0.8-3 and b) 10V202; c) amplification

coefficient W0.8H vs. "true" thickness for both

transducers after 28 d of floor hardening.

a) b)

 7 d 28 d of hardening

R2 = 0.79

R2

 = 0.31

40

45

50

55

60

2345

tU-E, mm

W0,8H, dB

R2 = 0.90

R2

 = 0.70

60

65

70

75

80

2345

tU-E, mm

W0,8H, dB

S12HB 0.8-3: R2

 = 0.72

10V202: R2

 = 0.90

40

50

60

70

80

2345

t, mm

W0,8H, dB
49

6.3. Evaluation of multi-layer system with pulse velocity method

6.3.1. Ultrasonic test procedure

The ultrasonic evaluation of adhesion in the PC-CC system was carried out using the indirect

pulse velocity method. It was assumed that areas with poor adhesion influence the recorded

waveforms (Fig. 27) by attenuating the reflections more rapidly. In consequence, the changes in

parameters describing the waveform were used to describe the adhesion in the PC-CC system.

Figure 27. Received ultrasonic pulse for (a) good adhesion and (b) poor adhesion in the PC-CC

system.

The investigations were carried out with a commercial concrete tester with sets of associated

transducer pairs (Fig. 28). The source pulse frequency of the compressive wave was 100 kHz. The

distance between transducers was fixed and equal to 80 mm. A gel was used as a coupling medium

to improve acoustic contact between the samples and the transducers. The transmitted wave pulse

was transformed into digital signals (sampling period 0.2 μs) by an A/D converter system and then

fed into a microcomputer for a waveform analysis. The time versus voltage record was averaged

with six previously recorded pulse signals to reduce the effects of random noise and the

heterogeneity of the microstructures of both the polymer composites and the concrete substrate.

Each ultrasonic pulse was recorded after tester stabilization was indicated. These nondestructive

measurements were carried out after 7 d of hardening of each type of coating.

The propagation of ultrasonic waves through the PC-CC system was characterized by the

pulse velocity, calculated from the transit time, and by the changes of a mean square value

parameter, MS(t), defined in the time domain. The MS value at a given time was calculated from the

formula:

( ) i o

i

i i

n n

n

n

A A

MS t

o

o

− =

∑

−

( ) (18)

where Ai is the amplitude of the ith-recorded point [V] and no is the number of the first point with

amplitude different than zero. The plot of the MS value describes the amplitude variance in the time

domain and is a representation of the attenuation of the wave pulse. In this work, we assumed that

for a sample area of poor adhesion, the MS value would statistically decrease faster than for an area

of high adhesion. This assumption is justified by results like those shown in Fig. 27.

a) b)

-12

- 9

- 6

- 3

0

3

6

9

1 2

tim e [ μ s ]

amplitude [V]

-12

- 9

- 6

- 3

0

3

6

9

1 2

tim e [ μ s ]

amplitude [V]
50

Figure 28. Scheme of ultrasonic procedure for evaluation of adhesion between polymer composites

and concrete substrate.

6.3.2. Materials and tested floor systems

The investigations were carried out for the polymer industrial floor systems described in

Table 11: epoxy (EP), polyurethane (PUR) and vinyl ester (VE) coatings. All coating systems were

layered on a portland cement concrete substrate (B35 class, which has a characteristic compressive

strength of 35 MPa), prepared from the same concrete mix. This concrete class was selected to have

a relatively strong substrate in order to obtain failure in the PC-CC bond line (adhesive failure mode)

and to be close to the real concrete substrate used in the floor industry (concrete with compressive

strengths greater than 25 MPa). For the evaluation of the usability of the pulse velocity method for

assessment of the adhesion, three experiments were carried out:

travel time

pulse velocity

Mean square value, MS

Time [μs]

Transmitting

transducer

Receiving

transducer

i

i

n

A

MS(t)=

2 ∑

80
51

- ultrasonic evaluation of adhesion at maximum and minimum adhesion levels,

- analysis of relationship between pull-off strength and ultrasonic parameters,

- ultrasonic detection of defects in the PC-CC system.

Table 11. Chemical composition and pull-off strength of tested polymer coatings

 (from producer technical data sheets)

Property Symbol of polymer coating

EP-1 EP-2 PUR VE

Chemical composition Water

dispersion of

epoxy resin

Epoxy

resin

Polyurethane

resin

Vinyl-ester

resin

Number of components 2 2 2 1+

3 (hardening system)

Max. grain size of fine filler, mm < 0.1 < 0.1 < 0.1 < 0.125

Nominal thickness, mm 0.7 - 3 1 - 5 0.5 - 4 1 - 3

Pull-off strength (concrete substrate:

compressive strength > 25MPa)

> 1.5 MPa > 2.5 > 2 > 1.7

6.3.3. Ultrasonic evaluation of adhesion at boundary conditions

The pulse waveforms were analyzed for two boundary conditions (Fig. 29):

- maximum adhesion (for a given type of polymer coating),

- zero-adhesion (corresponding to delamination).

To simulate delamination the polymer coating was prepared on a metallic foil with an antiadhesion agent, and next put onto a cement concrete substrate. The investigations were carried out

for three commercial floor systems: EP, PUR and VE, of nominal thickness 3 mm and containing a

quartz filler (maximum grain diameter Dmax < 0.1 mm).

a) b)

Figure 29. Scheme of ultrasonic evaluation of adhesion for two conditions: (a) maximum adhesion

and (b) zero-adhesion.

The average pull-off strength obtained for the EP, VE and PUR coating systems was

equal to 3.23 MPa, 3.45 MPa, and 2.10 MPa, respectively. Cohesive failure in the concrete

substrate was obtained during the pull-off test for the EP and VE coatings. In the case of the PUR

coating, a mixed failure mode (70 % cohesive failure in concrete and 30 % adhesive failure in the

3mm

maximum adhesion

zero adhesion
52

PC-CC bond line) was observed. This confirmed that for the “maximum adhesion” state high

adhesion in the PC-CC system was actually developed. On the basis of waveform analysis, the

transit time and the MS value (Fig. 30) were selected as useful parameters for further

investigations. The amplitude values registered by the tester were not significantly correlated

with the adhesion level for all tested systems.

The MS values for coatings with maximum adhesion and zero adhesion were significantly

different. The MS plot determined for the concrete substrate without any polymer coating was

located between the MS plots for the coatings with maximum adhesion and with zero-adhesion.

In the case of the epoxy coating, the pulse velocity for the zero-adhesion state, vp = 2623 m/s,

was close to the pulse velocity determined with the direct method, vp = 2650 m/s (see. p.6.2).

This result confirmed that the ultrasonic wave penetrated both the polymer coating and the

concrete substrate. In the case where delamination was present, the ultrasonic wave traveled only

through the polymer coating. The highest difference of the MS value between the maximum

adhesion state and the zero adhesion state was observed to fall in the range of 400 μs to 700 μs in

the time domain. The MS value at 500 μs in the time domain was used in most of the analysis in

this report.

Figure 30. The pull-off strength, pulse velocity and MS distribution in the time domain for the

tested polymer coating systems.

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200 1400 1600

time of pulse propagation [μs]

Mean squere value MS(t) [V*V]

VE(1)

EP(2)

PUR(3)

CC

EP(5)

PUR(6)

VE(4)

Floor

system

Pull-off

strength

[MPa]

Pulse velocity

[km/s]

(stand. dev.)

Floor

system

Pull-off

strength

[MPa]

Pulse velocity

[km/s]

(stand. dev.)

VE(1) 3.45 4.460 (0.032) VE(4) - 2.703 (0.012)

EP(2) 3.23 4.460 (0.032) EP(5) - 2.623 (0.007)

PUR(3) 2.10 4.420 (0.020) PUR(6) - 2.339 (0.010)

CC: pulse velocity 4.908 km/s (stand. dev. 0.030)
53

6.3.4. The effect of chemical composition and geometry of polymer coating

Ultrasonic wave propagation through the PC-CC system [64] was analyzed for each type of

floor system and for three thickness values of the polymer coating: 1 mm, 2 mm, and 4 mm.

Additionally, the effect of using coarse filler, 0.3 mm to 0.7 mm in size, on wave propagation was

tested. This kind of filler is commonly added to improve the abrasion resistance of a polymer

coating.

The results obtained indicate that ultrasonic wave propagation through the PC-CC system is

affected by the type of resin binder, the presence of coarse filler, and the thickness of the polymer

coating. The addition of the coarse filler (Fig. 31) significantly decreased the pulse velocity and the

MS value for the EP-1 coating, while for the PUR system this effect was not significant. As the

coating thickness increased, the pulse velocity and MS value decreased (Fig. 32). The largest

changes were observed for the EP-1 and PUR floor types. In the case of the EP-2 coating, the effect

of coating thickness was less significant. The results indicate that for proper ultrasonic evaluation of

adhesion in a PC-CC system, a suitable reference curve should be determined, with coating

composition and thickness considered as factors affecting the propagation and attenuation of

ultrasonic waves. The value of the coating thickness can be verified using the echo method (see

p.6.2).

Figure 31. MS value distribution for (a) EP-1 and (b) PUR coatings with and without coarse filler

0

10

20

30

40

50

60

0 500 1000 1500

time (μs)

mean square value MS (V*V)

with filler

without filler

1 mm

4 mm

1 mm

a)

0

10

20

30

40

50

60

0 500 1000 1500

time (μs)

mean square value MS (V*V)

with filler

without filler

1 mm

4 mm

b)
54

Figure 32. Pulse velocity (a) and MS value distribution (b) for three values of the thickness of

the polymer coatings.

4.79

4.59

4.28 4.30

4.10

3.89

4.12

3.94

3.71

3.0

4.0

5.0

6.0

1 mm 2 mm 4 mm 1 mm 2 mm 4 mm 1 mm 2 mm 4 mm

pulse velocity (km/s)

EP-1

EP-2

PUR a)

10

20

30

40

50

0 500 1000 1500

time (μs)

mean square value MS (V*V)

EP-1 (1 mm) EP-1 (2 mm) EP-1 (4 mm)

EP-2 (1 mm) EP-2 (2 mm) EP-2 (4 mm)

PUR (1 mm) PUR (2 mm) PUR (4 mm)

b)
55

6.3.5. Relationship between pull-off strength and ultrasonic parameters

The relationship between the “pull-off” strength and the ultrasonic parameters was tested for

four floor system types (Fig. 33): VE (3 mm thickness), EP-1, EP-2 (2 mm thickness), and PUR (0.7

mm thickness). The concrete substrates differed in moisture content, and were prepared with and

without primer in order to obtain a continuous range of adhesion in the PC-CC system. Each variant

of polymer floor was prepared on a concrete plate of dimension 250 mm x 250 mm x 50 mm. The

pull-off test was measured at five different locations and the ultrasonic measurements were tested at

fifteen different positions for each floor specimen in order to determine average values. Statistical

analysis of the results was next performed to establish the regression function between the “pull-off”

strength and the ultrasonic parameters.

Figure 33. Scheme of ultrasonic evaluation of the relationship between pull-off strength and wave

parameters for “thick” VE and “thin” PUR coatings. The concrete substrates differed in moisture

content, surface quality, and presence of primer.

For the thick VE coating and the thin PUR coating the pulse velocity (Fig. 34), and the MS

value were measured for points corresponding to 500 μs and 1500 μs in the time domain (Fig. 35)

and were selected as the parameters describing ultrasonic wave propagation and attenuation,

respectively. For both coating systems, the relationship between pull-off strength and pulse velocity

was characterized by a relatively low correlation coefficient, r = 0.52 (VE) and r = 0.62 (PUR).

Higher correlation coefficients were obtained for the regression functions determined from the MS

values. For the VE system, the highest correlation coefficient (r > 0.86) was obtained for the

relationship MS (500 μs) vs. time. In the case of the PUR system, the highest correlation coefficient

(r > 0.95) was obtained for the relationship MS (1500 μs) vs. time. These results indicated that for

evaluation of the adhesion in PC-CC system a suitable reference curve can be determined, with

coating thickness considered as a factor that affects the attenuation of the ultrasonic wave.

2 - 3 mm

EP-1, EP-2,VE PUR

0.7mm

25

25 cm

25

25 cm
56

Figure 34. Pulse velocity against pull-off strength for (a) "thick" VE and (b) “thin” PUR coatings

Figure 35. Relationship between mean square value MS and pull-off strength calculated for the VE

coating: (a) MS (500 μs) and (c) MS (1500 μs), and for the PUR coating: (b) MS (500 μs) and (d)

MS (1500 μs).

In the case of the EP-1 and EP-2 coatings [64], the pulse velocity was approximately

constant (Fig. 36a) for a wide range of the pull-off strength, 1 MPa to 3.5 MPa, and its value was

y = 0.1872x + 4.1077

R2

 = 0.2758

4.3

4.4

4.5

4.6

4.7

4.8

0246

pull-off strength, MPa

pulse velocity, km/s

a)

y = 0.0776x + 4.3326

R2

 = 0.3813 4.3

4.4

4.5

4.6

4.7

4.8

0246

pull-off strength, MPa

pulse velocity, km/s

b)

y = 14.547Ln(x) + 33.406

R2

 = 0.7327

0

10

20

30

40

50

60

01234

pull-off strength, MPa

MS(500), V*V

a)

y = 9.8131Ln(x) + 39.042

R2

 = 0.7508

10

20

30

40

50

60

01234

pull-off strength, MPa

MS(500), V*

V

b)

y = 8.4589Ln(x) + 16.769

R2

 = 0.646

0

10

20

30

40

50

01234

pull-off strength, MPa

MS(1500), V*V

c)

y = 17.055Ln(x) + 18.826

R2

 = 0.9028

0

10

20

30

40

50

01234

pull-off strength, MPa

MS(1500), V*V

d)
57

significantly higher in comparison with the velocity measured for the "zero-adhesion" state. For both

epoxy coatings, the relationship: MS (500 μs) vs. pull-off strength (Fig. 36b) was characterized by a

high correlation coefficient value: r > 0.97. The relationship of MS (500 μs) vs. the pull-off strength

can be described by the following formula:

EP-1: MS (500 μs) = -3.43 fa

2

 +23.17 fa + 3.58 (19)

EP-2: MS (500 μs) = -2.68 fa

2

 +18.44 fa + 8.64 (20)

where the MS (500 μs) value is in V*V and the pull-off strength, fa , is in MPa.

For all coating systems tested, adhesive or mainly adhesive failure modes were observed up

to approximately 2.0 MPa. As the pull-off strength increased above this point, the failure mode

approached pure cohesive failure in the concrete substrate. The results obtained indicate that

relationships between the pull-off strength and the parameters of ultrasonic wave propagation are

valid for only a limited range of pull-off strength. This is a result of the nature of the pull-off test –

maximum bond strength corresponds to the tensile strength of the concrete substrate. On the other

hand, if the adhesion of the polymer coating to the concrete substrate is high the acoustic coupling is

enough to propagate an ultrasonic wave through the PC-CC system. Figures 35 and 36 show that the

MS (500 μs) value attained a maximum when the pull-off strength achieved a maximum value close

to the tensile strength of the concrete substrate.

Figure 36. Pulse velocity (a) and MS (500 μs) value (b) vs. the pull-off strength for the epoxy

coatings tested.

6.3.6. Ultrasonic detection of defects in PC-CC system

The possible ultrasonic detection of defects in a PC-CC system was tested for the EP-1 and

EP-2 epoxy coatings that had artificial defects introduced at the PC-CC interface In the first part of

experiment, ultrasonic defect detection was performed for floor systems having an unknown

relationship between the pull-off strength and various ultrasonic parameters. In second part of the

 EP-1 - EP-2; results for PUR coating at maximum and zero adhesion state

2

3

4

5

01234

pull-off strength (MPa)

pulse velocity (km/s)

a) y = -2.68x2 + 18.44x + 8.64

r = 0.999

y = -3.43x2 + 23.17x + 3.58

r = 0.997

0

10

20

30

40

50

01234

pull-off strength (MPa)

MS(500 

μs), V*V

b)
58

experiment, an approximate relationship between the pull-off strength and the MS value was used to

compute the adhesion mapping.

In the first part of the experiment, the EP-2 epoxy coating (3 mm thick) was layered on a

concrete plate having dimensions 600 mm x 30 mm x 50 mm. Artificial defects were prepared in the

form of polyethylene thin film sheets of different shape and dimensions, which were put on the

concrete substrate under the top coating (Fig. 37). The EP-2 floor sample was divided into a regular

grid. At the nodes of this grid, six ultrasonic measurements were carried out in two perpendicular

directions. After the ultrasonic measurements were performed, the “pull-off” strength was also

measured at the nodes of the grid.

Figure 37. Sample for ultrasonic detection of defects in PC-CC system

The results obtained for the ultrasonic measurements and the pull-off strength at particular

nodes of the grid were analyzed next. The results were created in two separate groups of

measurement points – one for a low and one for a high value of the pull-off strength. This implies

that from a statistical point of view, the plots obtained for pull-off strength vs. pulse velocity (Fig.

38a) and pull-off strength vs. the MS (1500 μs) value (Fig. 38b) could not be used as reference

curves for nondestructive estimation of the pull-off strength. The negative exponential approach to

estimation was used to determine the distributions of the measured pull-off strength (Fig. 38c) and

the MS (1500 μs) value (Fig. 38d). In general, the pull-off strength and the MS (1500 μs) value were

decreased in the defect areas. However, the MS value distribution, in contrary to the pull-off strength

distribution, indicated the defect presence (coordinates: 200 mm, 200 mm). It was confirmed in the

pull-off test. The results obtained indicate that the ultrasonic pulse method can be used for defect

detection in PC-CC systems, at least as an introductory test.

The same experiment was carried out for the EP-1 and EP-2 floors [64], for which the

relationship between the pull-off strength and MS (500 μs) value was already established. The EP-1

and EP-2 epoxy coatings (2 mm thick) were layered on concrete substrates of dimensions 400 mm x

400 mm x 50 mm. Artificial defects were prepared in the same way as previously, and the

specimens were divided into a regular grid. At the nodes of this grid, eight ultrasonic measurements

were carried out in directions inclined at 45o

 to each other. In selected nodes, the pull-off strength

was also determined. The results of the ultrasonic measurements were statistically analyzed. The

least squares method of estimation was used to determine the distribution of the MS (500 μs) value

for the EP-1 and EP-2 coatings (Fig. 39). Equations 2 and 3 were used to obtain the corresponding

pull-off strength distribution (see the right scale in Fig. 39). A low pull-off strength was obtained

3 mm

EP

60

30
59

only when a steel dolly was placed completely in the defect area. In general, the experimental pull

off strength was higher than 2 MPa, even at the points where steel dollies were partially placed in the

defect area. This can lead to overestimation of the pull-off strength. Contrary to the experimental

results of the pull-off strength, the MS value distribution properly indicated the presence of defects.

The MS (500 μs) value decreased at defect sites and the corresponding variational coefficient of the

MS value increased.
60

Figure 38. Relationships: pull-off strength vs. (a) pulse velocity, (b) the MS (500 μs) value and the

distributions of (c) pull-off strength and (d) the MS (500 μs) value on the PC coating sample with

artificial defects.

3.5

3.7

3.9

4.1

4.3

4.5

01234

pull-off strength, MPa

pulse velocity, km/s

0

5

10

15

20

25

30

01234

pull-off strength, MPa

MS(1500 

μs), V*V

a) b)

 0,5

 1

 1,5

 2

 2,5

 3

 3,5

 ponad

length, cm

width, cm

3,38 3,21 3,28 3,21 1,02

0,84 3,06 3,21 1,24 0,91

8

10

12

14

16

18

20

22

5 10 15 20 25 30 35 40 45 50 55

 10

 12,5

 15

 17,5

 20

 22,5

 25

 ponad

length, cm

width, cm

21,1765 25,863 20,3915 18,0395 10,196

14,902 22,745 21,961 18,0395 10,1965

8

10

12

14

16

18

20

22

5 10 15 20 25 30 35 40 45 50 55

c)

d)

above

above
61

Figure 39. Results of the ultrasonic mapping of the adhesion for (a) EP-1 and (b) EP-2 coatings (2

mm thick) with artificial defects (dotted line contour); on the left the scale for the MS value and on

the right the scale for the pull-off strength recalculated using the regression functions (see Fig. 35).

The experimental values of the pull-off strength (measured in the solid line circle) are included in the

white rectangles.

 15

 20

 25

 30

 35

 40

 45

 ponad

length, cm

width, cm

5

10

15

20

25

30

35

5 10 15 20 25 30 35

0,4MPa

2,4 MPa

3,1MPa

3,0MPa

2,3MPa

MS(500)

[V*V]

fa = f(MS(500))

[MPa]

above above

a)

 15

 20

 25

 30

 35

 40

 45

 ponad

length, cm

width, cm

5

10

15

20

25

30

35

5 10 15 20 25 30 35

0,5MPa 0,6MPa

3,2MPa

2,8MPa 2,2MPa

MS(500)

[V*V]

fa = f(MS(500))

[MPa]

above

above

b)
62

6.4. Conclusions concerning ultrasonic evaluation of the multi-layer PC-CC system

On the basis of the results obtained and described in this report, the following conclusions

about ultrasonic evaluation of the multi-layer PC-CC system, which is created as a result of repair or

anticorrosion protection application, are as follows.

1. The ultrasonic echo (U-E) method can be used for nondestructive estimation of the polymer

coating thickness. The possibility of defect detection in the PC-CC system with this method

needs further investigation and the evaluation of adhesion with this method seems to be less

promising.

2. The regression functions obtained for various coating systems justify potential application of the

indirect pulse velocity method for nondestructive evaluation of the adhesion between the

polymer coating and the concrete substrate, as well as for mapping of the adhesion distribution.

3. A reference adhesion curve should be developed for a given type of polymer coating (including

its composition and thickness), for a wide range of adhesion, from the "zero adhesion" state to

the maximum adhesion state. On this basis, the following steps for the nondestructive

assessment of adhesion between polymer coating and concrete substrate (Fig. 40) can be

formulated:

(a) selection of measurement points,

(b) determination of the MS(500 μs)REF value distribution with indirect pulse velocity

methods; measurement of MS(500 μs)EXP value around the selected points (at least four

measurements at each point),

(c) evaluation of the coating thickness with ultrasonic-echo method,

(d) recalculation of MS(500 μs)EXP into MS(500 μs)REF for measured coating thickness (eg.

on the basis of a suitable regression function or graph MS(500 μs) value vs. coating

thickness),

 (e) estimation of adhesion strength from the reference curve: pull-off strength vs.

 MS(500 μs)REF.

Figure 40. General scheme of ultrasonic evaluation of the adhesion between polymer coating and

concrete substrate (description above in text).

 4. The relationship between the pull-off strength and the parameters of ultrasonic wave

 propagation are valid for only limited range of pull-off strength. If the adhesion of the

 polymer coating to the concrete substrate increases the MS (500 μs) value approaches to the

 maximum for the pull-off strength value close to the tensile strength of the concrete

 substrate.

MS(500),V*V

pull-off strength, MPa

MS (500 μs)EXP

MS (500 μs)REF
63

5. The results obtained indicate also the need for further investigation to improve the

accuracy of the ultrasonic estimation of the adhesion in the PC-CC system using an

advanced method of signal analysis (e.g. spectral analysis) or data analysis (e.g. multiple

regression).
64

7. SUMMARY AND CONCLUSIONS

The main goal of the project entitled "Ultrasonic evaluation methods applicable to polymer

concrete composites" was the evaluation of the potential of ultrasonic methods for assessment of

polymer concrete composite properties. The project was carried out the framework of the

M.Skłodowska-Curie US-Polish Joint Fund II. The research task was analyzed taking into

account the two main fields of polymer composite application: anticorrosion protection of

concrete structures, including industrial floors, and pre-cast elements made from PC. For all these

applications, one of the most important issues is quality control (verification tests) and diagnosis

tests during structural service. For these purposes, the application of two ultrasonic methods, a

pulse velocity method and an echo method, were considered. In the case of pre-cast elements, this

work focused on the elaboration of principles of how to evaluate polymer concrete properties

with the pulse velocity method. The main goal of the ultrasonic evaluation of a multi-layer PCCC system created during repair and/or anticorrosion protection was quality control of the repair

efficiency, particularly an estimation of layer thickness, detection of defects in the interface

region, and mapping of the adhesion between polymer composites and concrete substrate. In this

field, the usefulness of the echo method and the indirect pulse velocity method were considered.

The results obtained, in the form of regression functions, confirmed the usefulness of the

ultrasonic methods for nondestructive evaluation of polymer composites in various applications.

The pulse velocity method can be used for evaluation of the properties and homogeneity of the

precast elements made from polymer concrete as well as for evaluation of adhesion in the multilayer PC-CC systems (the adhesion mapping). The ultrasonic echo method is useful for

nondestructive estimation of PC layer thickness.

The project results were presented in scientific journals and at international conferences.

Nondestructive evaluation of multi-layer repair systems, including especially adhesion mapping, has

appeared as the most interesting possibility. A more detailed report reviewing nondestructive

methods applicable to the evaluation of adhesion will be published as a subchapter of the final report

of RILEM Technical Committee TC-151 "Adhesion Technology in Concrete Engineering –

Physical and Chemical Aspects." The results will be also used during preparation of a book chapter

on the nondestructive evaluation of industrial floors. This chapter will be published in the report of

the new RILEM Technical Committee, RILEM TC-184 IFE "Industrial floors for withstanding

harsh environmental attacks, including repair and maintenance."
65

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68

List of publications involved with the activity on the project

1. Czarnecki L., Garbacz A., Łuciuk M.: Ocena wybranych właściwości betonów żywicznych

metodą ultradźwiękową (Evaluation of selected technical properties with ultrasonic methods),

Building Industry & Engineering (in Polish), 10 (1997), pp.514-516.

2. Garbacz A., Łuciuk M., Czarnecki L.: Przydatność metody ultradźwiękowej do oceny cech

technicznych betonów żywicznych (Usability of ultrasonic pulce velocity for assessment of

polymer concrete properties), Proc.26th Polish Conference on NDT Techniques, Szczyrk

(1997), pp.193-198.

3. Garbacz A., Czarnecki L., Clifton J.R.: “Ultrasonic Methods To Assess Polymer

Concrete Properties”, Proc. of the IX Inter.Congress “Polymers in Concrete” ICPIC’98,

Bologna (Italy), 1998, pp. 761-769.

4. Garbacz A., Czarnecki L., Clifton J.R., ”Non-destructive methods to assess adhesion

between polymer composite and portland cement concrete,” Proc. 2nd Int. RILEM Symposium

“Adhesion between polymers and concrete”- ISAP’99, Dresden (1999), pp. 467- 474.

5. Czarnecki L., Garbacz A., Clifton J., “Evaluation of polymer concrete properties with

ultrasounds,” Concrete & Reinforced Concrete (in Russian), 4 (1999), pp. 29-30.

6. Garbacz A. “Nondestructive assessment of adhesion between polymer composite and

concrete substrate,” chapter 8.4 in State of the Art. Report of RILEM TC-151 APC, 1999.

7. Garbacz A., Czarnecki L., Garboczi E., “Ultradźwiękowa ocena przyczepności między

kompozytem polimerowym a podkładem betonowym, (Ultrasonic evaluation of adhesion between

polymer composites and concrete substrate),” Proc. Polish Conf. "Materials in Building Industry"

Matbud '2000, Kraków (2000), pp. 99-106.

8. Garbacz A., Czarnecki L., Garboczi E.: "Ultrasonic Assessment of Adhesion between

Polymer Composites and Concrete Substrate", in the Proc. of the 10th International Congress

Polymers in Concrete - ICPIC 2001, USA.