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The assessment of fire damaged concrete
Buildingand Environment,Vol.28, No. 1,pp. 97-102,1993.
0360-1323/93$5.00+0.1)0 © 1992PergamonPressLtd.
Printedin GreatBritain.
The Assessment of Fire Damaged Concrete M I C H A E L Y. L. CHEW* The assessment of fire-damaged concrete and masonry to determine whether a structure is still structurally sound depends largely on accurately determining the maximum temperature distribution within the damaged structure. The merits and limitations of various techniques for the assessment of fire-damaged concrete were discussed. It has been shown that TL test is more sensitive than other techniques in the detection of fire damage.
estimation of the maximum fire temperatures reached. Materials used as fire temperature indicators are usually readily identifiable after a fire. Commonly used materials in this respect are timber (chars at about 250°C), aluminium alloys (melt at about 650°C), and sheet glass (melts at about 850°C) [2]. As an example, a general survey of the extent of damage caused by a fire at the lower ground floor of a multi-storey building is presented. In this case, the major fire load was the timber sheds built inside the basement. In the worst affected areas where the timber sheds were, copper pipes with a melting point of about 1080°C were found not to have melted. Glass sheets with melting point of about 850°C were found to have partially melted. The outer surface of some aluminium sheetings was found to have melted. The melting point of aluminium is known to be about 650°C. Most of the timber sheds have charred. The charring depth in the worst case has reached approximately 30 mm. Among the materials that have melted, glass has the highest melting point. As the temperature of the glass has reached 850°C, while copper pipes with a melting point of 1080°C were not melted, it is estimated that the corresponding air temperature should be between 850°C and 1080°C, setting the upper limit of the room temperature at approximately 1000°C. As the fire load (timber shed) was small, with the charring depth of timber being shallow, and as only the outer layers of glass and aluminium have melted, the time of exposure was estimated to be short. This estimate was later confirmed by site personnel as well as the results from the thermoluminescence (TL) tests.
INTRODUCTION DUE TO the high fire resistance of concrete, a fire in a concrete structure rarely results in such serious damage as to require substantial demolition. As the loss of use of a building could result in serious financial consequences to the owner, a call for immediate reinstatement is generally required. To work out a proper and efficient repair strategy, however, would require a thorough investigation of the effect of the fire on the structural properties of the concrete and steel; the significance which any permanent change in material characteristics may have on the future structural performance of the member ; the feasibility of repairs to compensate of any unacceptable reduction in structural performance, durability etc; and the influence which fire exposure of individual members may have on the performance of the entire structure. Tests available today to make such an assessment can basically be divided into two types. The first type gives and indication of the apparent reduction in strength (e.g. Schmidt hammer, ultrasonic pulse velocity and pull-off tests) and the second type the temperature history of the member assessed (e.g. thermogravimetric, colour and thermoluminescence tests). The first type of test is usually used to identify areas where detailed analysis is required. The maximum temperature distribution of these areas will then be estimated using the second type. Once the maximum temperature distribution of a member is known, its effect on the many physical and mechanical properties of a structure can be deduced. Plenty of such data relating elevated temperature to properties of concrete and steel are available [1]. Figures 1 and 2 show the effect of elevated temperature on the compressive strength of concrete and yield strength of steel respectively. This paper discusses the merits and limitations of various techniques being used today for the assessment of fire damaged concrete.
Visual survey o f concrete
Following the general survey of a structure, a more detailed visual examination should be undertaken to identify areas of spalling, exposed reinforcement, excessively deflected members and the like. Each structural element should be classified individually, and the location and severity of any defects recorded onto drawings showing the principle members of the structure. This allows areas of particular damage to be identified and any inherent failure trends to be isolated from the general damage.
ASSESSMENT General survey
A general visual survey can rapidly identify areas where additional support or more detailed examination is required. Fire debris when available can allow a quick
Sound test
Sound tests can be used as a screening test to locate areas where more detailed assessments are required. Ref-
* School of Building & Estate Management, National University of Singapore, Singapore. 97
98
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Fig. 1. The effect of elevated temperature on the compressive strength of concrete by various researchers.
erence 2 states that 'It may be sufficient to take soundings on the d a m a ~ ! concrete to determine the degree of deterioration. The "ring" of sound concrete and the "dull thud" ofweak material are readily distinguished, and this test may be ~ y done with hammer and chisel'. Schmidt hammer test The Schmidt hammer test gives an indication of strength based on the impact hardness of the concrete. The rebound of a standardiznd sping-loaded impact hammer is m c a s u r ~ both on damaged and undamaged areas. The aim is to have a quick indication of the effect of a fire on the impact hardness and thus indirectly on strength of a concrete. The author has carried out numerous Schmidt hammer tests for the ~ t of concrete quality [3]. It is important to u n l k ~ t a n d when using a Schmidt hammer in the ~ t of fire-damaged concrete that it can only indicate areas where the surface strengths are relatively lower compared with the undamaged areas. It is also
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Colour test Concrete cast with certain aggregates shows distinct and reproducible colour changes when heated. These changes normally occur between temperatures of 300° and 600°C, and are most pronounced in sificeous fiver gravels, sandstones, and the like. The most common alteration is the development of a pink colour, due to a change in the hydration states of iron oxides and other salts within the coarse or fine aggregates (see Fig. 3). From Fig. 1, it is known that a reduction of up to 60% in compressive strength can occur when concrete is heated to 300°C. The test is thus useful in identifying the depth of a damaged concrete member that needs to be discarded. Observation by the author from laboratory specimens as well as from actual sites that have been subjected to a temperature of above 300°C all showed some kind of colour change. However, difficulties were found in many cases in the determination of the boundary where colour change diminishes.
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important to be aware of a number of irregularities in the tested concrete such as : • small air pockets ; • aggregate consistency and hardness; • lamination within the concrete; and • the inclusion of impurities in the concrete.
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Te~pem~m~ (c) Fig. 2. Typical strength properties of reinforcingand prestressing steels tested at room temperature after heating to an elevated temperature.
Fig. 3. Distinct change in colour shown on brick mortar heated to 300°C from one side.
Assessment o f Fire D a m a g e d Concrete Ultrasonic pulse velocity test ( UP V) In this test, a pulse of longitudinal vibrations is produced by an electro-acoustical transducer which is held in contact with one surface of the concrete under test. After traversing a known path length L in the concrete the pulse of vibrations is converted into an electrical signal by a second transducer and electronic timing circuits enable the transit time T o f the pulse to be measured. The pulse velocity V is given by : V = LIT.
(1)
Browne, Geoghegan and Baker [4] suggested values that can be used to assess the condition of concrete as follows : Velocity (km/s) >4 3-4 <3
Quafity Good Fair Poor
In general, the strength of concrete can be related to the UPV as :
F' c = aV b
99
Table 1. PUNDIT on concrete [grad with and without reinforcement
Number I 2 3 4
5 6 7 8 9
10 11 12 13 14 15 16 17 18
Path
Steel
Concrete
length (ram)
diameter (ram)
cover (nun)
150 300 150 300 150 300 150 300
--25 25 20 20 25 25
--46 46 29 29 29 29
150
--
--
300 150 300 150 300 150 300 150 300
-----12 12 ---
-----19 19 ---
Time Velocity (s) (m]s) 40 90 46
3.75 3.33 3.26
100
3.00
33 72 33 70 39 92 40 95 39 95 32 73 38 92
4.54 4.17 4.54 4.28 3.85 3.26 3.75 3.16 3.85 3.16 4.69 4.28 3.95 3.26
(2)
where a and b are constants. The values of constants a and b are not unique and depend upon a number of factors such as mix design, type of aggregates, age of concrete, moisture content and size of the specimen. A number of other factors such as the surface condition, whether smooth or rough, temperature of concrete, presence of reinforcement etc. also affect the UPV to a considerable extent. The strength of concrete can vary over a wide range for a given UPV. This is because UPV varies as the fourth root of concrete strength, which causes a much smaller change in UPV for a given variation in the strength of concrete. The presence of reinforcement in the structure further adds to the difficulties. It is essential to understand the influence of these additional parameters like orientation, quantity and distribution of steel [5]. A study using a portable ultrasonic nondestructive digital indicating tester (PUNDIT) to investigate the effect of the diameter of reinforcement and concrete cover on the pulse transmission was carded out. Tests were undertaken on a reinforced concrete panel with a series of reinforcement of various diameters and concrete covers. Only indirect transmission was employed. The results are shown in Table 1 [6]. The inclusion of reinforcement was found to increase the pulse velocity. The lower the cover, the higher is the pulse velocity. This phenomenon is not difficult to explain as steel is a better conductor than concrete. However, as the concrete cover increases to a certain value (more than 40 mm in this case), the effect of reinforcement on the pulse velocity diminishes. The diameter of reinforcement does not appear to have an effect on the pulse velocity.
Pull-off test The test involves sticking a steel plate on the area to be tested using a fast, high strength adhesive. After the adhesive is set, the plates are connected to a jack and the strength required to pull off the concrete from the surface is measured. This test provides useful information of the in-situ surface tensile strength of the concrete.
Core test It is common to take core specimens from concrete suspected of being damaged by fire. Compression tests of such cores must be used with caution. If such a test indicates good strength, then one can be confident about the condition of the member from which the cores were taken. If the measured compressive strength is low, then such a result may be due to local failure close to the exposed surface and the main body of the specimen may be in good condition. Careful inspection of the failure mode is needed in such cases. Thermogravimetric and Dilatometry test When cement paste is heated, the dehydration of Ca(OH)2 (taking place generally between 400°C and 600°C) is recognizable from a rapid weight loss or shrinkage indicated by thermogravimetric and dilatometric tests. Since all dehydration reactions are more or less irreversible, the heating of the cement paste can be construed as a process of chemical stabilization. This is always accompanied by loss of weight and shrinkage, thus either thermogravimetry or dilatometry can be used to determine the maximum temperature distribution within a concrete member by testing samples extracted from various depths of a member [7]. Tests carried out by the author showed that in many cases, the process of rehydration can cause the technique to produce inaccurate results. Previous tests have also shown that samples have to be collected within one or two days after a fire to achieve reasonably accurate results [6]. Thermoluminescence test ( TL ) Thermoluminescence (TL) is the light produced when certain materials are heated. Its application for the assessment of fire-damaged concrete was first proposed by Placido [8]. Plotting the light output of a heated concrete or masonry specimen against its temperature in a TL test produces a characteristic glow curve of a specimen. By comparing the glow curves of field samples extracted
M . Y. L. Chert'
100
Perhaps it is easier to visualize the phenomenon by using a glass of water as an example. Imagine that if water is flowing into a glass at a very slow rate, the a m o u n t of water in the glass will increase with time, symbolizing the accumulation of TL in grains over the years. If the glass is subjected to a high temperature, water will evaporate, symbolizing the loss of TL when subjected to a high temperature. It follows that using the relationship between temperature and the loss of TL by measuring the residual TL left in a sample of concrete with appropriate normalization and calibration--the temperature history of the sample can be estimated [1 I, 12]. Figure 4 shows the simplified sequence of sample preparation of concrete specimen for a TL test. Grains from various depths are extracted using a small masonry drill and water lubrication to prevent a significant temperature rise. The author has, through his experience in extracting samples from various depths in concrete members for durability tests such as CO2 and CIcontent, developed a simple method which can reduce
from various locations and depths of a structure after a fire against standard specimens processed in the laboratory, an estimate can be made of the temperature history within the structure. Natural unheated grains extracted from concrete give a very large TL signal as they have been exposed to ionizing radiation (e.g. uranium, thorium, potassium, cosmic ray etc.) for millions of years. Highly thermoluminescent minerals present in concrete include felspar, quartz, calcite, dolomite, nepheline, zicron, apatite etc. This signal, however, will be substantially reduced if the concrete has been exposed to a high temperature such as a fire according to the Randall-Wilkins [9, 10] exponential expression : ,~ = sexp ( - E / k T )
(3)
where 2 = electron release rate; s = 'attempt to escape' frequency factor; E = trap depth or activation energy; k = Boltzmann const~tnt ; T = absolute temperature.
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Assessment of Fire Damaged Concrete Table 2. Comparison of results between different tests
Element
Schmidt hammer test (number)
Pull-off test (MPa)
Compressive test (residue F'c of normal in %)
C1
28.0
1.66
W1
28.1
W7
TL test Depth (mm)
Max. temp (°C)
82
3-7 13-17 23-27 28 32
< 200 < 150 < 120 < 100
1.43
87
3-7 13-17 23-27
< 200 < 150 < I00
28.8
1.56
I00
3-7 13-17
< 150 <100
B2
40.6
1.40
3-7 13-17 23-27
< 150 < 120 < 100
$2
33.0
1.66
3-7 13-17 23-27
< 200 < 150 < 100
the possibility of collecting samples other than from the target depth. In the method, different drill sizes (say 20 mm to 5 mm) are used. The largest drill bit is used to reach the first depth, say 5 mm from the concrete surface. The depth of the hole is measured using a calibrated steel rod. Before reaching the 5 mm range, air is blown through the hole using an empty squeezer to remove grains from earlier depths. Water is squeezed through the hole to wash away any remaining grains which are still loosely intact. The empty squeezer is used again to blow away the remaining water and accelerate the drying process. A smaller drill bit is then placed at the center of the previous hole and is used to drill to the 5 mm range, with a specially-made steel-dish container corresponding to the size of the previous drill placed before the range to collect
I.A
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the sample. The schematic diagram of the process is shown in Fig. 5. It should be noted that only a very small amount of sample (a few milligrams) is needed from each depth. After the sample preparation, the grains were mounted on 10 mm diameter aluminium discs using a fixed volume aliquot of suspension of grains in acetone. The discs at this stage are ready for TL tests to produce glow curves as shown in Fig. 4.
Case study To compare the relative sensitivity of these various techniques, the same case as mentioned earlier under the section 'General Survey' is used. The average results of all the tests carried out on each element are summarized in Table 2. Column 1 of the table represents the element on which tests were taken, Columns 2, 3, 4 and 5 are the average results of Schmidt hammer test, pull-off test, compressive strength test and TL test. As the compressive strength test on cores is still the most direct and realiable test to date, it was used as the standard for comparison. From the compressive strength test results in Column 4, the residual strength of CI, Wl and W7, when compared with the control specimens are approximately 82%, 87% and 100% of the normal strength respectively. The sensitivity of the Schmidt hammer test was found to be low. The rebound number shows a difference of less than 3% corresponding to a reduction of 18% reduction in F %. Pull-off test results were unable to detect the relative damage. The highest pull-off strength was found on element C1 where the highest reduction in F'c was observed using the compressive strength test. The TL test, on the other hand, appeared to be more sensitive in detecting relative damage. For C1 and W l where the residual F'c was 82% and 87% respectively, TL detected a maximum temperature of 200°C in the 3-7 mm region and 150°C in the 13-17 mm region. CONCLUSION
Fig. 5. Schematicdiagram showing the drilling method to reduce the possibility of collecting samples from the previous depths.
To use the many data available on the effect of elevated temperature on the properties of concrete and steel, it
102
M.Y.L.
is essential to be able to estimate the confidence, the maximum temperature distribution within the member. This is particularly true for prestressed concrete. Although the most direct and reliable method is by carrying out tests on cores drilled from suspected areas, the drilling of cores may cause further damage to an already weakened structure. Technique such as the colour tests, thermogravimetry and dilatometry test sometimes give
Chew good indications of the maximum temperature reached, but both have their shortcomings. The case study presented has also shown that Schmidt hammer test and pull-off tests, are not sensitive enough to detect small damage in concrete caused by a fire. TL tests, on the other hand, were found to be more sensitive than other tests employed in the study.
REFERENCES 1. M . Y . L . Chew, The assessment of fire damaged concrete and masonry using thermoluminescence, Thesis presented for PhD (Building), The University of NSW (1990). 2. Assessment of fire damaged concrete structures and repair by Gunite. Report of a Concrete Society Working Party, Concrete Society Technical Report No. 15. The Concrete Society (1978). 3. M. Marosszeky and M. Y. L. Chew, Concrete durability--Final report, R1.87, Building Research Centre, The University of NSW (August 1987). 4. R.D. Brown, M. P. Geoghegan and A. F. Baker, Analysis of Structural Condition .for Durability Results, Corrosion of Reinforcement in Concrete Constructions, pp. 193-222. A. P. Cron. Ellis Horwood Ltd. (1983). 5. D.S. Prakash Rao, Some pitfalls in the ultrasonic velocity method of testing hardened concrete, Indian Cone. J., October, 254-257 (1984). 6. M. Marosszeky and M. Y. L. Chew, Concrete repair--Interim report, Building Research Centre, The University of NSW (July 1988). 7. T. Z. Harmathy, Determining the temperature history of concrete constructions following fire exposure, A CI J. November, 959-964 (1968). 8. F. Placido, Thermoluminescence test for fire-damaged concrete, Mag. Cone. Res., 32, 112-116 (June 1980). 9. J. T. Randall and M. H. F. Wilkin, Phosphorescence and electron traps 1. The study of trap distributions, Proceedings of the Royal Society of London, A184, 366-389 (1945). 10. J.T. Randall and M. H. F. Wilkin, Phosphorescence and electron traps 2. The interpretation of long period phosphorescence, Proceedings of the Royal Society of London, A184, 390-433 (1945). 11. M.Y.L. Chew, P. T. McMillan and J. C. Kelly, The temperature history of heated concrete measured by thermoluminescence, Non-Destructive Testing, 24, 5, Sep/Oct. (1987). 12. M.Y.L. Chew, Assessing heated concrete and masonry using thermoluminescence, A CI Material J., Nov/Dec. (1988).
0360-1323/93$5.00+0.1)0 © 1992PergamonPressLtd.
Printedin GreatBritain.
The Assessment of Fire Damaged Concrete M I C H A E L Y. L. CHEW* The assessment of fire-damaged concrete and masonry to determine whether a structure is still structurally sound depends largely on accurately determining the maximum temperature distribution within the damaged structure. The merits and limitations of various techniques for the assessment of fire-damaged concrete were discussed. It has been shown that TL test is more sensitive than other techniques in the detection of fire damage.
estimation of the maximum fire temperatures reached. Materials used as fire temperature indicators are usually readily identifiable after a fire. Commonly used materials in this respect are timber (chars at about 250°C), aluminium alloys (melt at about 650°C), and sheet glass (melts at about 850°C) [2]. As an example, a general survey of the extent of damage caused by a fire at the lower ground floor of a multi-storey building is presented. In this case, the major fire load was the timber sheds built inside the basement. In the worst affected areas where the timber sheds were, copper pipes with a melting point of about 1080°C were found not to have melted. Glass sheets with melting point of about 850°C were found to have partially melted. The outer surface of some aluminium sheetings was found to have melted. The melting point of aluminium is known to be about 650°C. Most of the timber sheds have charred. The charring depth in the worst case has reached approximately 30 mm. Among the materials that have melted, glass has the highest melting point. As the temperature of the glass has reached 850°C, while copper pipes with a melting point of 1080°C were not melted, it is estimated that the corresponding air temperature should be between 850°C and 1080°C, setting the upper limit of the room temperature at approximately 1000°C. As the fire load (timber shed) was small, with the charring depth of timber being shallow, and as only the outer layers of glass and aluminium have melted, the time of exposure was estimated to be short. This estimate was later confirmed by site personnel as well as the results from the thermoluminescence (TL) tests.
INTRODUCTION DUE TO the high fire resistance of concrete, a fire in a concrete structure rarely results in such serious damage as to require substantial demolition. As the loss of use of a building could result in serious financial consequences to the owner, a call for immediate reinstatement is generally required. To work out a proper and efficient repair strategy, however, would require a thorough investigation of the effect of the fire on the structural properties of the concrete and steel; the significance which any permanent change in material characteristics may have on the future structural performance of the member ; the feasibility of repairs to compensate of any unacceptable reduction in structural performance, durability etc; and the influence which fire exposure of individual members may have on the performance of the entire structure. Tests available today to make such an assessment can basically be divided into two types. The first type gives and indication of the apparent reduction in strength (e.g. Schmidt hammer, ultrasonic pulse velocity and pull-off tests) and the second type the temperature history of the member assessed (e.g. thermogravimetric, colour and thermoluminescence tests). The first type of test is usually used to identify areas where detailed analysis is required. The maximum temperature distribution of these areas will then be estimated using the second type. Once the maximum temperature distribution of a member is known, its effect on the many physical and mechanical properties of a structure can be deduced. Plenty of such data relating elevated temperature to properties of concrete and steel are available [1]. Figures 1 and 2 show the effect of elevated temperature on the compressive strength of concrete and yield strength of steel respectively. This paper discusses the merits and limitations of various techniques being used today for the assessment of fire damaged concrete.
Visual survey o f concrete
Following the general survey of a structure, a more detailed visual examination should be undertaken to identify areas of spalling, exposed reinforcement, excessively deflected members and the like. Each structural element should be classified individually, and the location and severity of any defects recorded onto drawings showing the principle members of the structure. This allows areas of particular damage to be identified and any inherent failure trends to be isolated from the general damage.
ASSESSMENT General survey
A general visual survey can rapidly identify areas where additional support or more detailed examination is required. Fire debris when available can allow a quick
Sound test
Sound tests can be used as a screening test to locate areas where more detailed assessments are required. Ref-
* School of Building & Estate Management, National University of Singapore, Singapore. 97
98
M. Y. L. Chew % OF NORMAL STRENGTH 11111
......................
!
il
•
0 I
I
0
I
200
I
4oo
600
I
800
1000
TEMPERATURE (C)
- -
__/z~Lgli~___
'--"-- MORLEY(l:6)
--+--
1~1:8)
~
MALIHIO'IP.A(I:3) -'e'- GUSTN:B:IRO(1.'6)
+
UE(1:4,5)
+
HE(l:6)
-a--
MAU,K)IRA (1,'6)
Fig. 1. The effect of elevated temperature on the compressive strength of concrete by various researchers.
erence 2 states that 'It may be sufficient to take soundings on the d a m a ~ ! concrete to determine the degree of deterioration. The "ring" of sound concrete and the "dull thud" ofweak material are readily distinguished, and this test may be ~ y done with hammer and chisel'. Schmidt hammer test The Schmidt hammer test gives an indication of strength based on the impact hardness of the concrete. The rebound of a standardiznd sping-loaded impact hammer is m c a s u r ~ both on damaged and undamaged areas. The aim is to have a quick indication of the effect of a fire on the impact hardness and thus indirectly on strength of a concrete. The author has carried out numerous Schmidt hammer tests for the ~ t of concrete quality [3]. It is important to u n l k ~ t a n d when using a Schmidt hammer in the ~ t of fire-damaged concrete that it can only indicate areas where the surface strengths are relatively lower compared with the undamaged areas. It is also
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.
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Colour test Concrete cast with certain aggregates shows distinct and reproducible colour changes when heated. These changes normally occur between temperatures of 300° and 600°C, and are most pronounced in sificeous fiver gravels, sandstones, and the like. The most common alteration is the development of a pink colour, due to a change in the hydration states of iron oxides and other salts within the coarse or fine aggregates (see Fig. 3). From Fig. 1, it is known that a reduction of up to 60% in compressive strength can occur when concrete is heated to 300°C. The test is thus useful in identifying the depth of a damaged concrete member that needs to be discarded. Observation by the author from laboratory specimens as well as from actual sites that have been subjected to a temperature of above 300°C all showed some kind of colour change. However, difficulties were found in many cases in the determination of the boundary where colour change diminishes.
".... N,
\
-
important to be aware of a number of irregularities in the tested concrete such as : • small air pockets ; • aggregate consistency and hardness; • lamination within the concrete; and • the inclusion of impurities in the concrete.
I I
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~ u ~ ~aata
\
0.4 o
I
I
I
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200
300
400
500
60o
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Te~pem~m~ (c) Fig. 2. Typical strength properties of reinforcingand prestressing steels tested at room temperature after heating to an elevated temperature.
Fig. 3. Distinct change in colour shown on brick mortar heated to 300°C from one side.
Assessment o f Fire D a m a g e d Concrete Ultrasonic pulse velocity test ( UP V) In this test, a pulse of longitudinal vibrations is produced by an electro-acoustical transducer which is held in contact with one surface of the concrete under test. After traversing a known path length L in the concrete the pulse of vibrations is converted into an electrical signal by a second transducer and electronic timing circuits enable the transit time T o f the pulse to be measured. The pulse velocity V is given by : V = LIT.
(1)
Browne, Geoghegan and Baker [4] suggested values that can be used to assess the condition of concrete as follows : Velocity (km/s) >4 3-4 <3
Quafity Good Fair Poor
In general, the strength of concrete can be related to the UPV as :
F' c = aV b
99
Table 1. PUNDIT on concrete [grad with and without reinforcement
Number I 2 3 4
5 6 7 8 9
10 11 12 13 14 15 16 17 18
Path
Steel
Concrete
length (ram)
diameter (ram)
cover (nun)
150 300 150 300 150 300 150 300
--25 25 20 20 25 25
--46 46 29 29 29 29
150
--
--
300 150 300 150 300 150 300 150 300
-----12 12 ---
-----19 19 ---
Time Velocity (s) (m]s) 40 90 46
3.75 3.33 3.26
100
3.00
33 72 33 70 39 92 40 95 39 95 32 73 38 92
4.54 4.17 4.54 4.28 3.85 3.26 3.75 3.16 3.85 3.16 4.69 4.28 3.95 3.26
(2)
where a and b are constants. The values of constants a and b are not unique and depend upon a number of factors such as mix design, type of aggregates, age of concrete, moisture content and size of the specimen. A number of other factors such as the surface condition, whether smooth or rough, temperature of concrete, presence of reinforcement etc. also affect the UPV to a considerable extent. The strength of concrete can vary over a wide range for a given UPV. This is because UPV varies as the fourth root of concrete strength, which causes a much smaller change in UPV for a given variation in the strength of concrete. The presence of reinforcement in the structure further adds to the difficulties. It is essential to understand the influence of these additional parameters like orientation, quantity and distribution of steel [5]. A study using a portable ultrasonic nondestructive digital indicating tester (PUNDIT) to investigate the effect of the diameter of reinforcement and concrete cover on the pulse transmission was carded out. Tests were undertaken on a reinforced concrete panel with a series of reinforcement of various diameters and concrete covers. Only indirect transmission was employed. The results are shown in Table 1 [6]. The inclusion of reinforcement was found to increase the pulse velocity. The lower the cover, the higher is the pulse velocity. This phenomenon is not difficult to explain as steel is a better conductor than concrete. However, as the concrete cover increases to a certain value (more than 40 mm in this case), the effect of reinforcement on the pulse velocity diminishes. The diameter of reinforcement does not appear to have an effect on the pulse velocity.
Pull-off test The test involves sticking a steel plate on the area to be tested using a fast, high strength adhesive. After the adhesive is set, the plates are connected to a jack and the strength required to pull off the concrete from the surface is measured. This test provides useful information of the in-situ surface tensile strength of the concrete.
Core test It is common to take core specimens from concrete suspected of being damaged by fire. Compression tests of such cores must be used with caution. If such a test indicates good strength, then one can be confident about the condition of the member from which the cores were taken. If the measured compressive strength is low, then such a result may be due to local failure close to the exposed surface and the main body of the specimen may be in good condition. Careful inspection of the failure mode is needed in such cases. Thermogravimetric and Dilatometry test When cement paste is heated, the dehydration of Ca(OH)2 (taking place generally between 400°C and 600°C) is recognizable from a rapid weight loss or shrinkage indicated by thermogravimetric and dilatometric tests. Since all dehydration reactions are more or less irreversible, the heating of the cement paste can be construed as a process of chemical stabilization. This is always accompanied by loss of weight and shrinkage, thus either thermogravimetry or dilatometry can be used to determine the maximum temperature distribution within a concrete member by testing samples extracted from various depths of a member [7]. Tests carried out by the author showed that in many cases, the process of rehydration can cause the technique to produce inaccurate results. Previous tests have also shown that samples have to be collected within one or two days after a fire to achieve reasonably accurate results [6]. Thermoluminescence test ( TL ) Thermoluminescence (TL) is the light produced when certain materials are heated. Its application for the assessment of fire-damaged concrete was first proposed by Placido [8]. Plotting the light output of a heated concrete or masonry specimen against its temperature in a TL test produces a characteristic glow curve of a specimen. By comparing the glow curves of field samples extracted
M . Y. L. Chert'
100
Perhaps it is easier to visualize the phenomenon by using a glass of water as an example. Imagine that if water is flowing into a glass at a very slow rate, the a m o u n t of water in the glass will increase with time, symbolizing the accumulation of TL in grains over the years. If the glass is subjected to a high temperature, water will evaporate, symbolizing the loss of TL when subjected to a high temperature. It follows that using the relationship between temperature and the loss of TL by measuring the residual TL left in a sample of concrete with appropriate normalization and calibration--the temperature history of the sample can be estimated [1 I, 12]. Figure 4 shows the simplified sequence of sample preparation of concrete specimen for a TL test. Grains from various depths are extracted using a small masonry drill and water lubrication to prevent a significant temperature rise. The author has, through his experience in extracting samples from various depths in concrete members for durability tests such as CO2 and CIcontent, developed a simple method which can reduce
from various locations and depths of a structure after a fire against standard specimens processed in the laboratory, an estimate can be made of the temperature history within the structure. Natural unheated grains extracted from concrete give a very large TL signal as they have been exposed to ionizing radiation (e.g. uranium, thorium, potassium, cosmic ray etc.) for millions of years. Highly thermoluminescent minerals present in concrete include felspar, quartz, calcite, dolomite, nepheline, zicron, apatite etc. This signal, however, will be substantially reduced if the concrete has been exposed to a high temperature such as a fire according to the Randall-Wilkins [9, 10] exponential expression : ,~ = sexp ( - E / k T )
(3)
where 2 = electron release rate; s = 'attempt to escape' frequency factor; E = trap depth or activation energy; k = Boltzmann const~tnt ; T = absolute temperature.
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101
Assessment of Fire Damaged Concrete Table 2. Comparison of results between different tests
Element
Schmidt hammer test (number)
Pull-off test (MPa)
Compressive test (residue F'c of normal in %)
C1
28.0
1.66
W1
28.1
W7
TL test Depth (mm)
Max. temp (°C)
82
3-7 13-17 23-27 28 32
< 200 < 150 < 120 < 100
1.43
87
3-7 13-17 23-27
< 200 < 150 < I00
28.8
1.56
I00
3-7 13-17
< 150 <100
B2
40.6
1.40
3-7 13-17 23-27
< 150 < 120 < 100
$2
33.0
1.66
3-7 13-17 23-27
< 200 < 150 < 100
the possibility of collecting samples other than from the target depth. In the method, different drill sizes (say 20 mm to 5 mm) are used. The largest drill bit is used to reach the first depth, say 5 mm from the concrete surface. The depth of the hole is measured using a calibrated steel rod. Before reaching the 5 mm range, air is blown through the hole using an empty squeezer to remove grains from earlier depths. Water is squeezed through the hole to wash away any remaining grains which are still loosely intact. The empty squeezer is used again to blow away the remaining water and accelerate the drying process. A smaller drill bit is then placed at the center of the previous hole and is used to drill to the 5 mm range, with a specially-made steel-dish container corresponding to the size of the previous drill placed before the range to collect
I.A
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the sample. The schematic diagram of the process is shown in Fig. 5. It should be noted that only a very small amount of sample (a few milligrams) is needed from each depth. After the sample preparation, the grains were mounted on 10 mm diameter aluminium discs using a fixed volume aliquot of suspension of grains in acetone. The discs at this stage are ready for TL tests to produce glow curves as shown in Fig. 4.
Case study To compare the relative sensitivity of these various techniques, the same case as mentioned earlier under the section 'General Survey' is used. The average results of all the tests carried out on each element are summarized in Table 2. Column 1 of the table represents the element on which tests were taken, Columns 2, 3, 4 and 5 are the average results of Schmidt hammer test, pull-off test, compressive strength test and TL test. As the compressive strength test on cores is still the most direct and realiable test to date, it was used as the standard for comparison. From the compressive strength test results in Column 4, the residual strength of CI, Wl and W7, when compared with the control specimens are approximately 82%, 87% and 100% of the normal strength respectively. The sensitivity of the Schmidt hammer test was found to be low. The rebound number shows a difference of less than 3% corresponding to a reduction of 18% reduction in F %. Pull-off test results were unable to detect the relative damage. The highest pull-off strength was found on element C1 where the highest reduction in F'c was observed using the compressive strength test. The TL test, on the other hand, appeared to be more sensitive in detecting relative damage. For C1 and W l where the residual F'c was 82% and 87% respectively, TL detected a maximum temperature of 200°C in the 3-7 mm region and 150°C in the 13-17 mm region. CONCLUSION
Fig. 5. Schematicdiagram showing the drilling method to reduce the possibility of collecting samples from the previous depths.
To use the many data available on the effect of elevated temperature on the properties of concrete and steel, it
102
M.Y.L.
is essential to be able to estimate the confidence, the maximum temperature distribution within the member. This is particularly true for prestressed concrete. Although the most direct and reliable method is by carrying out tests on cores drilled from suspected areas, the drilling of cores may cause further damage to an already weakened structure. Technique such as the colour tests, thermogravimetry and dilatometry test sometimes give
Chew good indications of the maximum temperature reached, but both have their shortcomings. The case study presented has also shown that Schmidt hammer test and pull-off tests, are not sensitive enough to detect small damage in concrete caused by a fire. TL tests, on the other hand, were found to be more sensitive than other tests employed in the study.
REFERENCES 1. M . Y . L . Chew, The assessment of fire damaged concrete and masonry using thermoluminescence, Thesis presented for PhD (Building), The University of NSW (1990). 2. Assessment of fire damaged concrete structures and repair by Gunite. Report of a Concrete Society Working Party, Concrete Society Technical Report No. 15. The Concrete Society (1978). 3. M. Marosszeky and M. Y. L. Chew, Concrete durability--Final report, R1.87, Building Research Centre, The University of NSW (August 1987). 4. R.D. Brown, M. P. Geoghegan and A. F. Baker, Analysis of Structural Condition .for Durability Results, Corrosion of Reinforcement in Concrete Constructions, pp. 193-222. A. P. Cron. Ellis Horwood Ltd. (1983). 5. D.S. Prakash Rao, Some pitfalls in the ultrasonic velocity method of testing hardened concrete, Indian Cone. J., October, 254-257 (1984). 6. M. Marosszeky and M. Y. L. Chew, Concrete repair--Interim report, Building Research Centre, The University of NSW (July 1988). 7. T. Z. Harmathy, Determining the temperature history of concrete constructions following fire exposure, A CI J. November, 959-964 (1968). 8. F. Placido, Thermoluminescence test for fire-damaged concrete, Mag. Cone. Res., 32, 112-116 (June 1980). 9. J. T. Randall and M. H. F. Wilkin, Phosphorescence and electron traps 1. The study of trap distributions, Proceedings of the Royal Society of London, A184, 366-389 (1945). 10. J.T. Randall and M. H. F. Wilkin, Phosphorescence and electron traps 2. The interpretation of long period phosphorescence, Proceedings of the Royal Society of London, A184, 390-433 (1945). 11. M.Y.L. Chew, P. T. McMillan and J. C. Kelly, The temperature history of heated concrete measured by thermoluminescence, Non-Destructive Testing, 24, 5, Sep/Oct. (1987). 12. M.Y.L. Chew, Assessing heated concrete and masonry using thermoluminescence, A CI Material J., Nov/Dec. (1988).