COMPARISION OF NON DESTRUCTIVE TESTS RESULTS FOR FIRE AFFECTED AND UNAFFECTED CONCRETE STRUCTURE

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1 COMPARISION OF NON DESTRUCTIVE TESTS RESULTS FOR FIRE AFFECTED AND UNAFFECTED CONCRETE STRUCTURE M Yaqub*, University of Engineering & Technology, Taxila, Pakistan 30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 30 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore COMPARISION OF NON DESTRUCTIVE TESTS RESULTS FOR FIRE AFFECTED AND UNAFFECTED CONCRETE STRUCTURE M Yaqub*, University of Engineering & Technology, Taxila, Pakistan Abstract This paper presents the results of a case study carried out on fire affected and unaffected concrete structural components in sixteen storey reinforced concrete building damaged in Pakistan. Nondestructive testing technique was used to obtain information about the properties or the internal condition of the building without damaging. Semi destructive tests were also carried out on fire affected concrete and unaffected concrete. Thirty three core-samples were taken. Out of which eleven samples were taken from the unaffected portion of the building and remaining twenty two from beams and six samples from slab were taken. It was observed that fire affected concrete core wall, beams and slabs compressive strength were 34 Mpa, 13 Mpa and 16 Mpa while unaffected concrete core wall, beams and slabs compressive strength were 44 Mpa, 27 Mpa and 22 Mpa respectively. Fifteen ultrasonic pulse velocity tests were carried out from basement to top floor on fire affected concrete and unaffected concrete. It was observed that in slabs the average strength of unaffected floors was 39 Mpa and average strength of affected floors was 32 Mpa. In beams the average strength of unaffected floors was 37 Mpa and average strength of affected floors was 16 Mpa. Similarly the average column strength of unaffected floors was 28 Mpa and the average column strength of affected floors was 16 Mpa. Fifteen rebound hammer tests were conducted on different floors of fire affected concrete and unaffected concrete. It was observed that the average strength of slabs beams and columns of unaffected floors were 23 Mpa, 26 Mpa and 21 Mpa and on fire affected floors were 29 Mpa, 25 Mpa and 17 Mpa respectively. Keyword: Nondestructive, semi destructive, ultrasonic, rebound hammer, fire affected and unaffected concrete. 1. Introduction The behaviour of R.C. structure during fire and after fire is an important issue in Civil Engineering. Generally, concrete can be able to maintain its strength in substance at the range of 200C C. However, when temperature is above 300 C, its strength begins to decrease sharply, and damaged extent depends on the properties of aggregate and the temperature of concrete itself. The degeneration of strength is primarily caused by the dissolution of hydration products of cement, and the tensile stresses due to the variation of deformation under high temperature. The raising temperature would result in compatibility between hardened cement paste and aggregate as well as cracking at their interface [1]. The rate of heating would also affect the development of cracks. As the influence of time duration at peak temperature the greatest reduction of strength usually occurs within an hour by heating. The loss of concrete s strength primarily takes place within the duration of first two hours [2]. With regard to fire endurance, since the superior stability under high temperature as well as the lower thermal conductivity and expansion are inherent in lightweight aggregate, structural members made of light weight aggregate concrete (LWAC) such as beams, column. Slab and wall are generally considered performing a better performance than those made of normal concrete with the same thickness. [3].

3 Explosive spalling may occur in moist, impermeable concrete structures exposed to fire. The deterioration process leads to reduction of the cross section, spalling of concrete cover and exposure of the reinforcement. If the fire has direct access to the reinforcement, the reinforcement temperature is increased dramatically leading to a substantial strength decrease. Present knowledge implies that conventional concrete usually does not spall whereas high performance concrete and self compacting concrete usually do since they normally have low permeability [4]. The influence of elevated temperatures on the mechanical properties of concrete is important for fire resistance studies. Many investigations on the effect of elevated temperatures on strength of concrete have been reported since 1956 and the thermal and mechanical properties of concrete at elevated temperatures must be considered. Most of the investigations were carried out to add to the knowledge and for understanding the changes occurring in concrete subjected to elevated temperatures. The structural property of concrete that has been studied most widely as a function of heat exposure is compressive strength. Less attention has been given to flexural strength and split tensile strength as influenced by heat exposure. To investigate the effect of elevated temperatures and to attain necessary information to evaluate the structural safety, the residual strength of concrete also has to be determined. Compared to normal strength concrete, high strength concrete is more brittle contains less water and the solid patches are more compacted. This conflicting data must be related to factors such as strength grade [5]. It seems that the most different observations have been reported in the range of 100 to 300 C, Above 300 C, there is a uniformly of comments concerning a decrease in mechanical properties. However, compressive strength remains the most important property of structural concrete from a engineering view point. Thus this research was carried out to evaluate the residual compressivestrength of normal strength concrete after exposure to high temperature up to 685 C in slabs, 800 C in columns and 510 C in beams. 2. Detail of Fire Affected Reinforced Building. The building was a reinforced concrete (R.C) frame structure with basement, ground plus 16 floors and a mezzanine floor. The fire brake out on the top floor and spread down to 4 th floor when it was extinguished after 10 hours. 3. Experimental Details. The following non-destructive and semi destructive tests were carried out to evaluate the residual compressive strength of different structural components. 4. Non-Destructive Tests. 4.1 Ultrasonic Pulse Velocity Test. Ultra sonic pulse velocity test was carried out to find the compressive strength of concrete from fire affected portion and unaffected portion of R.C. Structure. The results of ultrasonic pulse velocity tests of fire effected concrete and unaffected concrete are shown graphically in fig.1. Figure -1 Floor wise variation of ultrasonic pulse velocity

4 Figure -1 Floor wise variation of compressive strength from ultrasonic pulse velocity test 4.2 Schmidt Hammer R C Bound Test. Schmidt Hammer Rebound test was also carried out to find the compressive strength of concrete. This test is a measure of the hardness of concrete surface, operates by measuring the rebound of small spring-driven mass that strikes against a steel rod in contact with concrete. A rebound number above 40 indicates hard surface, while a number below 20 usually indicates cracking and delaminating near the surface. The results of Schmidt hammer rebound tests of fire affected concrete and unaffected concrete are shown in fig.2. Figure 2 Floor wise variation of values of rebound number

5 Figure 2 Floor wise variation of compressive strength of fire affected & unaffected concrete 5. Semi Destructive Tests Core testing is a very much reliable method. Cores can be used to detect segregation honey combing or to verify the thickness of the concrete layers. Cores of 73mm in diameter were extracted using electrical core cutter. The results of core testing are shown in fig.3. Figure - 3 Floor wise variation of core strength in slab

6 Figure 3 Floor wise variation of core strength in beams 6. Results and Discussion The comparison of test results of residual compressive strength achieved during non destructive and destructive testing indicates that the variation in rebound numbers and the corresponding compressive strength values calculated from graphs provided by the manufactures of the test equipment are less as compared to the core test results. This happened due to the fact that rebound hammer number primarily depends on the hardness of the test surface. The value are uniform in the fire affected portions of the structure and comparatively lower than those after unaffected areas. Since the pulse velocity in steel could be up to double of that in concrete, especially in poor concrete, pulse velocity measurements in the vicinity of the reinforcing steel mass be higher than in plain concrete of the same composition. The quantitative estimation of this effect is difficult because not only the direction of bars but also their quantity and diameters can influence significantly the measured velocity of pulse. The temperature in slab during fire varies between from 100 o C to 685 o C. The top concrete slab was exposed to lesser temperature due to the presence of 75mm of flooring. The top concrete therefore exposed to 90 o C to 240 o C. In beams the temperature was 90 o C to 510 o C and in columns the temperature was 100 o C to 800 o C. The loss of concrete strength was not much affected on the positive moment capacity of the slabs. Negative movement capacity was reduced due to spalling. In core tests results the average value of concrete strength of undamaged floors was about 22 Mpa. The compressive strength of fire damaged concrete was 12 Mpa. The compressive strength of concrete in the beams in undamaged floors was 28 Mpa and in several fire damaged areas this strength was reduced to 10 Mpa. The Schmidt hammer Rebould Test results shows that the average compressive strength of concrete in slabs of undamaged floors was 23 Mpa where as in fire affected areas this strength was reduced to 20 Mpa in beams the average compressive strength of concrete of unaffected floors was 26 Mpa while in affected regions it was 26 Mpa. In columns the compressive strength of concrete in unaffected floors was 21 Mpa where as in affected floors it was 17 Mpa. The results of ultrasonic pulse velocity tests showed that in slabs the average compressive strength of concrete was 39 Mpa and average compressive strength of concrete in affected floors was 32 Mpa. The average compressive strength of concrete beams in unaffected floors was 37 Mpa while the average compressive strength of affected floors was 16 Mpa.

7 The average compressive strength of columns in unaffected floors was 28 Mpa while the average compressive strength in affected floors was16 Mpa. 7. Conclusion Based on the destructive and non-destructive test results obtained in this study, the following conclusions can be drawn. 1. Concrete compressive strength is higher for unaffected floor than fire affected floors. 2. Concrete compressive strength has not significantly affected up to rise in temperature 300 o C. 3. When temperature rises above 300 o C the concrete compressive strength decreases significantly. 4. Above 300 o C the cement paste tends to contract due to loss of water while the aggregate expands at high temperatures, the bond between the aggregate and the paste is weakened, this reducing the concrete strength. 5. Ultrasonic pulse velocity method shows greater compressive strength of concrete both in affected and unaffected concrete as compared to core testing and Schmidt Hammer Rebound Test methods. 8. References [1] Mineless, S. and Young, J.f. (1981), Concrete, Prentice Hall, Inc. Englewood Cliffs, New Jersey, pp [2] Bilodeau, V. Kodur, K.R. and Hoff, G.C (2004), Optimization of the type and amount of Polypropylene fibres for preenting the spalling of light weight concrete subjected to hydrocarbon fire Cement and cconcrete composites, Vol. 26, pp , [3] Holm, T.A (1994), lightweight concrete and aggregate, Standard Technical Publication 196 C ( 1994). [4] Pewrsson, B. (2003), Self Compacting Concrete at Fire Temperatures, Report No. TVBM 3110, Lund University of Technology, LKund, Sweden. [5] Felicetti, R. and Gambarova, P.G (1998) Effect of the High Temperature on the Residual Compressive Strength of the High Strength Siliceous Concrete ACI Materials Journal V. 95, No. 4, American Concrete Institute, Farmington Hills, Mich., 1998, pp [6] Chan, S. Peng G. and Anson, M. (1999) Fire Behaviors of High performance Concrete Made with Silica Fume at various Moisture Contents, ACI Materials Journal V. 96, No. 3, American Concrete Institute, Farmington Hills, Mich., 1999, pp [7] Eneng, F., Kodur, V.K., and Wang, T.C (2004). Stress Strain Curves for High Strength Concrete at Elevated Temperatures, Journal of Materials in Civil Engineering, V.16 No.1, 2004, pp [8] Phan, L.T. and Carino. N. (2002) Effects of Test Conditions and Mixture Proportions on Behaviour of High Strength Concrete Exposed to High Temperatures ACI Materials Journal V. 99, No. 1, American Concrete Institute, Farmington Hills, Mich., 2002, pp [9] Savva, A. Manita, P., and Sideris. K.K (2005), Influence of Elevated Temperatures on the Mechanical Proporties of Blended Cement Concretes Prepared with Limestone and Siliceous Aggregates, Cement and Concrete Composites. V.27, No.2, 2005 pp [10] Ichikawa, M. and Kanaya, M. (1997). Effects of Minor Components and Heating Rates on Fire Textures of Alite in Portland Cement Clinker, Cement and Concrete Research V. 27. No. 7. Pergamon Press, pp