Application of Non-Destructive Testing and Diagnostics in Investigating and Evaluating the Technical State of the Existing Reinforced Concrete Structures in Bulgaria
Assoc. Prof., Ph.D. (C. Eng.), University of Architecture, Civil Eng. and Geodesy,
1 Chr. Smirnenski Blvd, 1046 Sofia, Bulgaria
At present and for the past few years a large amount of existing buildings and bridges in Bulgaria have required reconstruction and renovation. This tendency poses a number of problems that have to be solved by our construction specialists. The main task is to determine the actual technical condition of the load-bearing elements and structures so as to provide a possibility for preserving and utilizing them in the future. This often includes specification of the types and description of apparent faults and damages, degree of wear of the elements, the quality of materials used and other important parameters needed even in the presence of their executive designs.
The present paper treats a complex method for assessing the condition of the Rotunda load-bearing structures in the open square in front of the Sofia Central Railway Station. This method is based on the practical application of various contemporary physical methods and apparatus for Non-Destructive Testing and Diagnostics of concrete and reinforcement in reinforced concrete elements and structures. As a result of their complex application, reliable input data have been obtained to help accelerate the design preparation and accomplish the reconstruction.
1. Aim and scope of investigation
The technical investigation presented was conducted in connection with the ongoing reconstruction and renovation of the open square in front of the Sofia Central Railway Station (Fig.1).
The aim is to specify the actual technical condition of the main elements and materials used in the load-bearing structures of the central part of the square (the Rotunda - Fig. 2) so that they can be used as input data for designing the reconstruction. This was done by carrying out complex non-destructive tests and diagnostic studies of:
- The quality and physico-mechanical properties of the utilized concretes by laboratory and in situ combined non-destructive tests;
- Diagnostics of the type, location and condition of the reinforcement of RC main load-bearing elements;
- Diagnostics of the thickness and degree of corrosion of the structural steel of the columns, and
- Investigation and assessment of the type, extent and quantity of manifested faults and damages.
2. Brief information about the tested structures
The load-bearing structure of the Rotunda at relevance elevation ±
000 of the fore-station square, designed in 1973 and executed in 1974 is divided into four sections by radial and circular dilatation joints (Fig. 2). In turn, they constitute four load-bearing structures almost uniform in appearance and configuration. Each one consists of:
- Monolithic reinforced top structures composed of continuous slabs, radial and circular beams;
- Erection steel columns with one or two spherical joints depending on their location in relation to the temperature center of the structures;
- Monolithic single reinforced concrete footings in which the steel columns are anchored.
3. Results of the investigations
3.1. Established faults and damages
During the technical in-situ inspections by well-known methods , the following faults and damages were found:
- Traces of atmospheric water leaks and carbonate extractions from the concrete of beams and slabs in the places of radial joints, most strongly expressed between rings 1 and 2;
- Initial to medium degree of corrosion of the slab and beam reinforcement with insufficient concrete cover in these places;
- Considerable 'flaky' corrosion of the lower unprotected parts of the steel columns at the radial joints and of the columns along the inner ring;
- Similar corrosion was also established on the spherical bearings (joints) of the same columns;
3.2. Establishing the quality of utilized concretes
3.2.1. By cutting off concrete test specimens
Thus two important problems were solved:
- The actual thickness of the beams and slabs was found and the physico-chemical properties of the utilized concrete were determined;
- The so-called 'correlation coefficient' of the standard dependence - the relationship between the established surface hardness by BSS (Bulgarian State Standard) 3816-84  and the actual concrete strength in the tested elements, obtained by BSS 505-84  and BS EN 12504-1  was directly determined by breaking the test specimens cut off the elements themselves.
For the purpose, a total of 7 cored specimens were cut off selected beams and slabs from the top structure in sections B and C of the Rotunda(Fig. 3), from places previously subjected to non-destructive testing (NDT) by a concrete test hammer.
15 cylindrical test specimens, Æ 74 mm in diameter and approx. 100 mm in height were additionally shaped from the cut off cored specimens. These specimens were tested under lab conditions for their volume density (whose average value was 2 340 kg/m3), their homogeneity (which turned out to be quite good) and the concrete compressive strength (which varied from 30 MPa to 40 MPa).
3.2.2. Concrete non-destructive testing (NDT)
NTD was performed by a concrete test hammer "Schmidt", type N34 (Fig. 4), in compliance with the standard requirements . A total of 110 spots (series), of which 95 series on radial and ring beams in sections A, B and D and 15 series on RC slabs from sections B, C and D. Single average probable concrete compressive strengths of 30 to 40-45 MPa were found with relatively low variation coefficients (0.05 - 0.10), according to which their characteristic strengths were obtained of the order of 30 to 35-36 MPa thus rating the utilized concretes in class B30 according to the existing BSS [2, 5].
3.2.3. Ultrasonic diagnostics of concrete and cracks
7 cross-sections of radial beams from sections A, B and D in a total of 33 measurement bases were tested by the two-sided ultrasonic method using a UNIPAN ultrasonic thickness gauge, model 543 (Fig.5), in compliance with BSS 15013-84 .
The results obtained showed very good homogeneity and density (rates from 3 900 to 4 400 m/s), linear deformation moduli from Ebm=34 300 MPa to 36 500 MPa and compressive strengths
R bm = 32.4 MPa to 37 MPa which are common for high quality concretes. The linear deformation moduli were calculated by the known dependences :
|Eb,din = k r Vbm 2 where,
k - coefficient accepted to be equal to 1 for linear elements and cross-sectional sonic testing of plane elements;
r b =
gb / g - acoustic density of concrete in kN s2 / m4, and
whereas the concrete strength was calculated by BSS 15013-84  by the dependence:
3.2.4. Establishing the depth of carbonation of the concrete protective coating of the reinforcement by a chemical method
The depth of carbonation of the concrete of the structural elements was determined by a chemical method by using 1% solution of phenolphthalein in alcohol as a reagent . When freshly damaged surfaces of concrete coating are moistened, the coating being carbonated and not protecting the reinforcement from corrosion efficiently, then the concrete is not colored. When treating the surface of non-carbonated concrete with the above-mentioned reagent, its surfaces are colored in crimson (Fig.6).
From the testing of 14 zones of radial beams and slabs it was found that the depth of carbonation of the tested RC beams at the radial joints where continuous atmospheric water leaks occur, is of the order of 15-20 mm. It comprises the stirrups and partly the longitudinal bearing reinforcement along which surface corrosion is established at a depth of up to 50-80mm whereas in the RC slabs and beams without leaks,failures and cracks it is from 0 to 5-6 mm and the reinforcement there is still in non-carbonated concrete.
3.3. Establishing the type and condition of the utilized reinforcements
3.3.1. Establishing the type and method of reinforcing the main bearing elements
The specification of the method of reinforcing the main bearing elements was carried out by a Ferroscan FS10 scanner manufactured by the HILTI Corporation(Fig.7).
Thus, more than 30 area zones of slabs and beams of RC structures, mainly from section B and partly sections A and C were tested (Fig.8). It was found that the bearing reinforcement of the RC slabs (one-way reinforced with N16 and N14 of class A-III - Figs. 13 and 14) and of the radial and ring beams (N32, class A-III) is correctly positioned and corresponds to the schemes accepted in the calculations, whereas the stirrups in them (Fig. 12 of class A-I) also correspond in number and type to the ones accepted in the calculations, but in some places have been accomplished with lateral deviation (inclined) and spacings of 10-12 cm to 25-26 cm for a design spacing of 20 cm.
3.3.2. Establishing the reinforcement type and condition
From the local additional investigations made it was found that the stirrups of the radial beams next to the joints along the inner I ring and of the beams and slabs in the leak zones occurring in fully carbonated concrete have an initial to medium degree of corrosion at a depth of 50-80mm and the longitudinal bearing reinforcement of the beams in these zones and in the established local spots with surface cracks and failures has corroded only superficially up to 5-10mm.
3.4. Condition of the steel columns and the footings underneath
3.4.1. Diagnostics of the column walls
It was performed by an ultrasonic device DM1 manufactured by KRAUTKRAMER Co., with an accuracy of 0.1 mm and a range of 100 mm. The sections under the pavement of the uncovered 6 columns were diagnosed as well as the visible part of 12 steel columns (Fig.9).
From the analysis of the results it can be seen that only the thickness of the walls under the column pavement level along the inner ring 1 and of those along the radial joints along rings 1 and 2 has considerably decreased. In these wall the minimum thickness with 95% security is 17.6 mm (12% corrosion wear), whereas the overground visible parts of the walls of the same columns have corroded much less - approx. 3%.
3.4.2. Condition of footings under the columns
From the investigations made it was found that the monolithic RC footings under the steel columns are in a comparatively good technical condition without traces of faults or damages on the concrete surface and the NDT of the concrete in them showed that at present it has good homogeneity and can be referred to concrete class B20.
The technical investigations and complex non-destructive and diagnostic tests performed enables us to:
- Assess objectively the general technical condition of the top structure of the Rotunda, which besides the cited damaged sections and elements, turned out to be relatively good;
- Establish the actual strength and deformation properties as well as the homogeneity, dimensions, quantity and quality of concrete, reinforcement and structural steel utilized in the bearing elements and structures, which can be used as input data for designing the present reconstruction;
- Give adequate recommendations to the designs for rehabilitation and strengthening of the individual elements and parts with established degree of wear in order to guarantee the necessary security and durability of the structures as a whole by the efficient utilization of existing bearing elements in accomplishing the reconstruction.
- Concrete. Non-destructive testing for determining the probable compressive strength by the surface hardness. BSS 3816-84.
- Concrete. Strength control and assessment. BSS 9673-84.
- Concrete. Pulse ultrasonic method for non-destructive testing. BSS 15013-84.
- Plain concrete. Test methods. BSS 505-84.
- Concrete. Classification and basic specifications. BSS 7268-83
- Concrete. Method of analyzing corroded concrete. BSS 12705-75.
- Protection of building structures from corrosion. Design norms and rules. BCA No.8, 1980.
- Hot rolled steel for reinforcing RC structures. BSS 4758-84.
- Luhzin O. V., E. Pol, et al. Non-destructive concrete testing. Stroyizdat, 1985.
- Tetior, A. N. & V. N. Pomeranets. Investigation and Testing of Structures. Kiev, Vysha Shkola, 1988.
- BS EN 12504-1. Testing concrete in structures-Part 1: Cored specimens - Taking, examining and testing in compression.