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Application of the Ultrasonic and Sclerometric Methods for the Assessment of the Structures Made of High-Strength Concrete (HSC) Prof. Leonard Runkiewicz, PhD, Civ.Eng. Building Research Institute (ITB), Warsaw Maciej Runkiewicz, MSc, Civ. Eng. Warsaw University of Technology Contact

This paper deals with the following issues:
• analysis of the concrete structures damages,
• analysis of the testing methods applied to estimate the in situ HSC,
• recommendations concerning testing and assessment of the design and factored strengths used to calculate the load capacity and serviceability limit states,
• new trends of application of the non-destructive testing methods for the testing of the in situ concrete.

1. INTRODUCTION

In Poland, in order to complete a reinforced structure high quality concrete class B30 to B100 is applied. Most often it is used to construct bridges, towers, core buildings, high structures, tanks, airports, highways, water industry buildings, tunnels, etc.
High quality concrete structures require new components, high technological standards and a strict quality control during the construction.
Strength and quality of concrete used in constructions in Poland are tested most frequently with use of non-destructive ultrasonic and sclerometric (rebound hammer) methods [1, 4, 5, 8, 9] and recently, given the recommendations of Eurocodes, methods of testing of drilled samples taken from the structures [7].
An introduction of HSC to norms and recommendations is mainly hindered by, inter alia, methods of testing and quality control.
In particular, they concern single and long-term tests of resistance to compression and tension as well as structure homogeneity, cracking, shearing, bonding, fatigue and durability.
In Poland, in concrete building , non-destructive testing methods are in particular applied to:
• estimate qualities of strength, homogeneity, density, humidity and radioactivity of materials in a construction;
• control quality of and discover defects in products during the manufacturing of elements and structures;
• control quality of joints between elements;
• change characteristic features of materials during their exploitation;
• conduct experimental testing of strength of materials under various exploitation conditions.

2. ANALYSIS OF THREATS IN CONCRETE STRUCTURES

Despite an extensive progress in designing modern concrete structures, they are still frequently threatened and damaged.
According to analyses of concrete structure threats in Poland carried out from 1963, 50% of damages and catastrophes have occurred to reinforced concrete structures.
Fig. 1 and 2 illustrate elements and structures threatened with damages according to a material characteristic and types of structures.

Fig 1: Percentage of the damaged elements and structures in the years1963-98 due to the applied materials

Fig 2: Types of the damaged elements and structures

Fig. 3 and 4 show elements damaged due to the place of production and types of errors.

Fig 3: Place of construction of the damaged elements

Fig 4: Construction failures influencing the damages, failures and catastrophes

3. RULES OF THE APPLICATION OF NON-DESTRUCTIVE METHODS

Rules and conditions of the application of non-destructive in situ testing methods in respect of reinforced concrete structures are set forth by Polish norms and instructions [4, 5, 8, 9] which are compliant with ISO and ENV norms.
Core elements of Polish norms and instructions include:
• a proper performance of in situ tests of elements and structures;
• a correct interpretation of test results.
During the testing, a choice of representative samples of elements or structures and proper measurements are of crucial importance.
While choosing representative samples, the following aspects should be particularly considered:
• sufficient tightness and stiffness of a tested element, e.g. a beam, a wall, a plate, a sub-floor, etc.;
• compact, homogenous structure of concrete at place of testing; and
• carbonation or surface destruction of concrete.
The correct interpretation relates to the following:
• sufficient number of testing places,
• selection of a correlative relation for a tested concrete;
• drilling for a more precise scaling of non-destructive methods;
• acceptance of all adjusting factors following their due justification;
• assessment of guaranteed, characteristic and analytical strengths to evaluate the structure.

4. APPLICATION OF NON-DESTRUCTIVE METHODS FOR HSC

It is extremely important for non-destructive methods to select appropriate correlative relations. The practice shows that empirical (correlative) relations are widely differentiated and their improper use impairs an exactness of the assessment even by app.100%.
Fig. 5 and 6 show the examples of relation arrangements for non-destructive ultrasonic and sclerometric methods set by centres in different countries.

Fig. 5. Examples of the characteristic f_c - V relations for the ultrasonic method

Fig.6 . Examples of the empirical f_c - L relations for the Schmidt's hammer of type N

HSC is produced with use of, inter alia, various additives. Such additives and admixtures strongly affect empirical relations.
Therefore, new additives used in production of HSC substantially alter empirical relations in respect of an assessment of, e.g. concrete strength.
As a result of the many-year research and implementation work with use of drilled samples, it was stated that for HSC with B40 - B120 class additives factors adjusting relations of the Building Research Institute were received in accordance with the following formulas:

• For the ultrasonic method (fig. 7)
f_c = (2,0 ÷ 2,7)(27,48V² - 8,12V + 4,8)MPa
• For the ultrasonic method (fig. 7)
f_c = (1,1 ÷ 1,4)(0,0409L² - 0,915l + 7,4)MPa

Fig. 7. Empirical relations for the estimation of the strength of the HSC using ultrasonic method

Fig. 8. Empirical relations for the estimation of the strength of the HSC using sclerometric method

Application of the relationships taken from the Fig. 7 and 8 allow for the remarkable increase of the accuracy of the assessment of the properties in accordance with the norms [4,5]. These properties are:

• mean strengths of the concrete,
• standard deviations of the strengths,
• classes of the concrete (being equal to the minimal compressive strengths),
and compliant with the norms [2,3]:
• nominal strengths,
• factored strengths.
The mean compressive strength of the concrete is estimated based on the formulas:
• for the ultrasonic method:
  
• for the sclerometric method
  
  where:
  

The same way the sclerometric method is treated (by taking L->V).
The properties of the concrete quality used to assess the structures are calculated in accordance to PN-B-03264:1999 and EC2 [3,6]. The following properties are taken into account:

• class of the concrete
  
• nominal compressive strength of the concrete
  f_ck =0,78 ÷ 0,82 · f^G_ccube,MPa
• nominal tensile strength of the concrete
  f_ctk =0,048 ÷ 0,074 · f^G_ccube,MPa
• factored compressive strength of the concrete
  f_cd = f_ck/g_c
• factored tensile strength of the concrete
  f_ctd = f_ctk/g_c
  where:

  g_c - partial safety factors for the concrete=1,3÷1,8
  s_f - standard deviation of the concrete strength

5. NEW TRENDS IN THE APPLICATION OF NON-DESTRUCTIVE TESTING METHODS IN POLAND

New trends in the application of non-destructive methods for testing HSC in structures include laboratory tests and in situ tests on a construction site or in existing buildings. Therefore, two aspects of a tests development are important:
• technical - laboratory methods;
• constructive - methods applied in practice.
Most important testing concerns in respect of concrete structures with use of non-destructive methods include the assessment of the following:
• features of strength and homogeneity;
• cracks and other damages;
• thickness and rheological features;
• structure, porosity and non-continuity of concrete;
• humidity and its distribution in a given element;
• concrete corrosion;
• repairs and durability;
• density and its changes in time;
• inclusions and defects.
For the purpose of tests and controls of the features of concrete listed above in reinforced concrete elements and structures the following expert methods are improved and developed:
• ultrasonic and sclerometric methods to assess strength features;
• ultrasonic and acoustic emission methods to assess homogeneity and structure;
• electric and electrochemical methods to assess humidity and corrosion;
• interferometry to assess structure;
• holographic and magnetic methods to assess structure and inclusions;
• radiological methods to assess humidity and weigh;
• radar and thermographic methods to assess structure, homogeneity and durability;
• dynamic, laser, pull-out methods, radiographic methods with use of betatrons and microtrons, computer tomography, radiometric methods (gamma), methods of electromagnetic resistance, electro-acoustic methods, methods of spectroscopy, gas permeability, heat transmission, optical methods, etc. to assess other selected important features of concrete.

6. CONCLUSIONS

In Poland for the purpose of the assessment of HSC in structures non-destructive ultrasonic and sclerometric methods are used in connection with testing of drilled samples in accordance with norms [4, 5, 7] and also other methods scientifically based and adjusted to the practical use.
The strength assessment tests proved the existence of wide discrepancies between empirical relations for normal concrete (B10-B35) and relations for HSC (B40-B120).
Adjusting factors for HSC amount to:
• 2.0 - 2.7 for an ultrasonic method;
• 1.1 - 1.4 for a sclerometric method.
In order to raise the exactness of a HSC strength assessment respective empirical relations are defined precisely.
The process of improving quality and strength of reinforced concrete structures implemented under new circumstances in Poland, supported by a licence and certification compliant with requirements of the European Union, requires an extensive development and application of non-destructive methods. These methods are adjusted to requirements and conditions of building in modern reinforced concrete technology and in particular to in situ tests during the construction and monitoring during the exploitation.

7. BIBLIOGRAPHY

1. Runkiewicz L.: Effect of Factors on Results of Sclerometric Tests of Concrete Building Research Institute, Warsaw 1991
2. PN-80/B-06250 Normal Concrete
3. PN-B-03264:1999 Reinforced Concrete Structures and Pre-stressed Structures
4. PN-74/B-06261 Non-destructive Tests of Concrete Structures. Ultrasonic Methods
5. PN-74/B-06262 Non-destructive Tests of Concrete Structures. Sclerometric Method. Testing of Concrete Resistance to Stressing with use of Schmidt hammer N type
6. Eurocode 2. Building Research Institute 1995
7. ENV 206 Normal Concrete Building Research Institute 1996
8. Instruction of Building Research Institute No. 209. Ultrasonic Method for Testing Concrete Strength in Structures
9. Instruction of Building Research Institute No. 210. Sclerometric Method for Testing Concrete Strength in Structures

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