Se ve ral days after a fire,walls and ceilings are coldto both sight and touch.Ash and water or ice lie

over eve rything. Co n c rete is likelyto be spalled, exposing steel thatdrapes from beams in shallow arcs.Su rfaces may be blackened, cra c k-ing may be visible and aluminumrailings melted. By this time at leasta pre l i m i n a ry investigation of thecondition of the stru c t u re will per-haps have been made. If the firstg e n e ral assessment of the damagehas been ambiguous, a re l i a b l ee valuation of the condition of thes t ru c t u re by experts becomes nec-e s s a ry. There will be great intere s tin this evaluation and considerableu rgency for completing it becausei n s u rance companies will be con-c e rned about the extent of re p a i r sre q u i red and owners will want thestructure back in service with mini-mum delay.

Although the complete eva l u a-tion will begin with an examinationof the condition of va rious materi a l sthat went through the fire, such asc e ra m i c s, furn i t u re or metal hous-i n g s, the information pri m a ri l yneeded is the condition of the stru c-t u ral elements that sustain thebuilding loads. Many methods andva rieties of test equipment are ava i l-able for assessing the damage.Some tests can be made at the jobsite but others re q u i re studies ofspecimens in the labora t o ry. Al-though such studies may at timesre q u i re sophisticated equipmentmuch or all of the information can

often be obtained with quite simpleequipment or tools.

General surveySu rveys are useful for compiling

w h a t e ver information is alre a d yk n own about the nature of the fire.They begin by drawing on inform a-tion in fire department re c o rds andon re p o rts from witnesses. That i n f o rmation includes the extentand duration of the fire, and the pe-riod when failures of memberswe re observe d .

In f o rmation about the type andamount of combustible materi a l scan be used for estimating the pro b-

Part II of a three-part series on Repair of Fire Damage

Evaluating fire damageto concrete structures

When the structuralcondition is ambiguous,a reliable diagnosis canassure future safety andmay save money inrepairs

BY BERNARD ERLIN*,WILLIAM G. HIME*,WILLIAM H. KUENNING *

Spalling of concrete and melting of aluminum railing from gasoline truck firebelow a bridge.

* Erlin, Hime Associates,

Materials and Concrete Consultants

Northbrook, Illinois

** Consultant, Concrete Technology

Lombard, Illinois

able tempera t u res reached withinthe building. If the building hadcontained, for example, not muchother than about 10 pounds of ve g-etable oil per square foot of floora rea, all of which was consumed inthe fire, probable tempera t u re sreached within the building can becalculated by using the inform a t i o nin Table 1. The amount of ve g e t a b l eoil per square foot multiplied by thefactor 2.1 (obtained from the topsection of the table) gives a value of21 pounds per square foot, thee q u i valent wood load to pro d u c ethe same amount of heat. In thel ower part of the table it can be seenthat a wood load of 21 pounds pers q u a re foot, which falls in the ra n g eof 12.5 to 25 pounds, would supplythe amount of heat consumed in a2-hour fire test. If it is assumed thatconditions at the actual fire we renot significantly different fro mthose in a standard fire test it wouldmean that the probable tempera-t u re reached in the fire atmospherewas 1,850 degrees F (found in theFa h renheit column).

On-site surveyMo re reliable information about

t e m p e ra t u res can be obtained dur-ing on-site surve y s. One method isto note the condition of materi a l sexposed to the fire (Table 2). Ex a m i-nations of rubble might show thatw i n d ow glass had been melted andb rass cupboard knobs had becomerounded but that copper in electri cw i res had not been softened. Theseo b s e rvations would point to tem-p e ra t u res that had certainly exc e e d-ed 1,560 degrees F (indicated by theglass) and had probably been higherthan 1,850 degrees F (indicated bythe brass) but not as high as the2,000 degrees F re q u i red to meltc o p p e r.

Those tempera t u re s, howe ve r,might not truly reflect the heat in-tensity reached in the concre t e,which is likely to be less than in thea i r. Co n c rete color provides a bro a d ,g e n e ral guide of tempera t u re s,whether the color re p resents theo riginal surface or one re s u l t i n gf rom spalling. Crazing, cra c k i n g ,

TABLE I Estimating Temperatures Attained During Fires(Adapted from Reference 1)

Factors for Use in Calculating Wood Load Below

Multiply weight of material per unit area by the factorindicated to obtain the equivalent wood load.

Material Factor Material Factor Material Factor

Wood 1.0 Hay or straw 0.8 Cotton 0.9Cellulose 1.0 Silk 1.2Paper 0.9 Asphalt 2.1 Wool 1.2

Coal tar 2.1Coal 1.8 Petroleum products 2.5 Nitrocellulose 0.5Coke or charcoal 1.5Lignite 0.8 Animal or vegetable oil 2.1 Rubber 2.1Peat 1.3 Fats and waxes 2.1

Wood load Equivalent Probable temperaturefire test period reached in fire atmosphere

Pounds Kilograms (hours) per square feet per square meter Degrees F Degrees C

12.5 61 1 1,700 92712.5 - 25 61 - 122 2 1,850 1,01025 - 50 122 - 244 4 2,000 1,093

TABLE II Conditions of Materials Useful for Estimating TemperatureAttained Within a Structure During a Fire (Adapted from Reference 1)

Material Typical Examples Condition Degrees F Degrees C

Pluming lead; Sharp edgesLead flashing; storage rounded or 550 - 650 300 - 350

batteries; toys drops formed

Plumbing fixtures;Zinc flashing; galvanized Drops formed 750 400

surfaces

Small machineAluminum parts; brackets;and its alloys toilet fixtures; Drops formed 1,200 650

cooking utensils

Softened or adherent 1,300 - 1,400 700 - 750

Glass block; jarsMolded and bottles; glass tumblers; solid Rounded 1,400 750

ornaments Thoroughly 1,450 800flowed

Window glass;Sheet glass plate glass; Rounded 1,450 800

reinforced glass Thoroughly 1,560 850flowed

Sharp edgesSilver Jewelry; rounded or 1,750 950

tableware; coins drops formed

Door knobs;Brass furniture knobs; Sharp edges

locks; lamp rounded or 1,650 - 1,850 900 - 1,000fixtures; buckles drops formed

Sharp edgesBronze Window frames; rounded or 1,850 1,000

art objects drops formed

Sharp edgesCopper Electric wiring; rounded or 2,000 1,100

coins drops formed

Pipes; radiators;Cast iron machine pedestals Drops formed 2,000 - 2,200 1,100 - 1,200

and housings

popouts caused by quartz or cherta g g regate part i c l e s, spalling, andd e h yd ration (crumbling and pow-d e ring of paste) are general indica-tions of tempera t u res to which con-c rete has been exposed, as shown inFi g u re 1. Co n c rete tempera t u res willva ry from one enclosed area to an-other and certainly from room toro o m .

The relationship of tempera t u reand strength loss was discussed inthe preceding paper of this seri e s. Asindicated there, surface tempera-t u res usually are quite differe n tf rom tempera t u res at differe n tdepths in the concre t e, and thus es-timates of strength of the intern a lc o n c re t e, when based on surf a c ec o l o r, are speculative. If precise in-f o rmation on strength is neededother methods that will be dis-cussed are re q u i re d .

The amount and condition of ex-posed re i n f o rcing bars or pre s t re s s-ing wires and strands are helpful in-dications for assessing damage. Ifthe steel is draping or shows embri t-tled surf a c e s, serious concrete dam-age can be anticipated. It does notf o l l ow, howe ve r, that unexposedsteel has been as seriously affected,if affected at all. The locations andlengths of exposed steel may be use-ful for reviews and recalculations ofdesign re q u i re m e n t s.

Di s t o rtions and cracking of stru c-t u ral members, as well as their loca-t i o n s, are useful for evaluating theefficacy of building units for sus-taining loads. If cracks are located inc ritical places such as support are a s,if deflections are observed any-w h e re, or if there are misalignments,the support capabilities may havebeen seriously impaired, whether

s u p p o rt is carried by individualmembers or integrated membersthat constitute a support system.

Equipment commonly used at the site

Two ve ry useful tools for examin-ing field concrete are a hammer andchisel. Their resistance to impactsand their resonance when stru c kcan re veal information about hard-n e s s, integri t y, depth of damage, se-riousness of cracking, and the con-dition of the steel.

Other useful tools and test instru-ments applicable in field studies orin removal of specimens for labora-t o ry studies include: impact ham-m e r, measuring tape, transit ort h e o d o l i t e, level, dial gages, fieldm i c ro s c o p e, gri n d e r s, coring ma-chine, wear test devices, steel hard-ness tester, soniscope, and load testdevices.

Concrete Color Temperature Other Possible Physical Effects

Buff

950 C, 1,740 F 1,650 F, 900 C Powdered, light colored, dehydrated paste

1,450 F, 800 C Spalling, exposing not more than 25 percent ofBlack reinforcing bar surface

ThroughGray

to Buff

600 C 1,100 F1,070 F, 575 C Popouts over chert or quartz aggregate particles1,000 F, 550 C Deep cracking

Pinkto Red

300 C, 550 F 550 F, 300 C Surface crazing

Normal

100 F, 40 C None

Fig. 1 Visual evidence of temperature to which concrete has been heated(Adapted from References 1 and 2)

Methods for evaluatingmaterials

By this time a good idea of thecondition of the stru c t u re will havebeen achieved, and in a few casesenough information will be ava i l-able to close the investigation. Bu toften there will be questions thatcan be re s o l ved only by detaileds t u d i e s. Methods that can be used toobtain answers to questions re g a rd-ing any of a number of pro p e rties ofc o n c rete or the condition of stru c-t u ral members are given in Table 3.

Co res can be used for eva l u a t i n gs t rength and modulus of elasticity.Co res should be taken judiciouslyand from locations where their ef-fect on strength will be minimalthough they provide necessary data.Co m p a risons of these data shouldbe made with data obtained fro mc o res taken from areas that we re notexposed to elevated tempera t u re s.These comparisons provide themost reliable information onchanges in concrete caused by thet e m p e ra t u res reached. Co res are al-

so useful for giving incidental infor-mation about cracking in the interi-or of a member, the bond to re i n-f o rcing steel, and interi o rt e m p e ra t u res as re vealed by colorc h a n g e s. Some of the inform a t i o nthat can be obtained is shown in theschematic drawing of Fi g u re 2.

The impact hammer is useful forestimating compre s s i ve stre n g t h ,but only when a considerable num-ber of measurements are made andc o m p a red to undamaged concre t eof the same quality from within thesame stru c t u re.

De t e rminations of pulse ve l o c i-t i e s, using a soniscope, provide indi-cations of strength and modulus,and attenuated signals are indica-tions of cracking. The soniscope issingularly the most effective nonde-s t ru c t i ve instrument for detectingc ra c k s.

Cracking can also be studied vi-sually in cores or fra g m e n t s, but de-tailed information about cra c k i n gusually must be obtained by usingp e t ro g raphic methods. Su i t a b l yp re p a red specimens can re veal mi-c ro c ra c k s, their relationship to ag-gregate and steel, and their internalorientation.

The extent of concrete dehyd ra-tion can be determined by usingp e t ro g raphic methods or by differ-ential thermal analysis (DTA). Sp e c-imens for DTA studies should bec a refully selected, pre f e rably afterthe petro g raphic studies. The petro-g raphic and DTA studies can be ex-tended to assessment of the condi-tion of the aggre g a t e.

The hardness and abrasion re s i s-tance of surf a c e s, if they are of inter-est, can be measured by standardmethods and compared to similardata from undamaged concrete inthe stru c t u re.

Condition of steelOften a major question in deter-

mining what stru c t u ral repairs willbe needed can be answe red by de-t e rmining the condition of steel. Infloors and walls, exposed pre s t re s s-ing steel can be measured in placeusing metallurgical methods. If ac-cess to the steel is limited because of

TABLE III Methods for Details Appraisals of Condition of Fire-Damaged Concrete

Condition of Property Methods Notes

Actual temperature Examination of building contents See Tablesreached in building 1 and 2

Actual temperature Visual examination of concrete, Petrographic,reached in concrete DTA, and metallurgical studies of steel. See Fig. 1

Compressive strength Tests on cores. Impact hammer studies.Penetration resistance. Soniscope studies.

Soundness at highlystressed areas (upperside at center of Hammer and chisel. Visual examination.beam; beam supports; Soniscope studies.anchorages for reinforcement nearsupport; frame corners)

Modulus of elasticity Tests on cores. Soniscope studies.

Dehydration of DTA. Petrographic analysis.concrete Chemical analysis.

Aggregate performance Visual examination. Petrographic analysis.

Spalling Visual examination.Petrographic analysis.

Cracking Visual examination. Soniscope studies. Petrographic analysis.

Surface hardness Dorry hardness or other tests

Abrasion resistance Los Angeles abrasion test on concrete chips*

Visual examination for spalling, cracking.Depth of damage Color variation in cores. Chipping. See Fig. 1

Petrographic analysis.

Deformation of beams, Visual examination. Straightedge and scale.Dial gages or theodolite if needed.

Gross expansion Visual examination. Checking of dimensions and levels.

Differential thermal Visual check of cores for loss of bond to steel.movements Color change in concrete next to steel.

STEEL CONDITIONReinforcing steel, Physical tests. Metallurgical studies.structural steel or Dimensional changes, displacementprestressing steel and distortion.

Load carrying capacity Load tests on structure

* Of uncertain value for this purpose.

height or stru c t u ral conformation ofthe member, short lengths of steelcan be re m oved for labora t o ry stud-i e s. A micro h a rdness test can beused for determining the tensiles t rength of pre s t ressing steel. This isp a rticularly useful because it israpid and accurate and re q u i res on-ly a small sample. It correlates we l lwith tensile strength determ i n a-t i o n s. Tensile and other tests on pre-s t ressed or re i n f o rcing steel samplesa re valuable because they give a di-rect measurement of the pro p e rty ofg reatest interest. Studies of steel mi-c ro s t ru c t u re are useful for eva l u a t-ing exposure tempera t u res andtheir effect on micro s t ru c t u re, andhence on physical pro p e rt i e s.

Load testsAlthough a compendium of indi-

rect information about the stru c t u r-al capability of individual elementscan be obtained, it may still becomen e c e s s a ry, and is indeed fre q u e n t l yd e s i ra b l e, to determine the actualloading capacity of an integra t e dsystem of elements. For this an ac-tual load test must be perf o rm e d .That test can be made before re i n-statement has been completed, af-t e rw a rd, or both. Tests completedp rior to reinstatement will establishwhat re p a i r s, if any, may be re-q u i red; tests completed after re i n-statement will indicate whether cor-rections have been adequate.

Us u a l l y, organizations experi-enced in job-site testing are bestqualified for completing the loadt e s t s. They are well informed on theva rious methods that can be used,safety precautions that must be em-p l oyed, and the interpretation oftest data.

ServiceabilityAside from stru c t u ral considera-

t i o n s, maintenance and safety ofs t ru c t u rally noncritical conditionsshould be evaluated. For example, ac o n c rete member may be stru c-t u rally adequate, yet careful study ofthe surface region of the membermay re veal incipient spalling orc racking that could result in eve n t u-

Sonic signal being sent through fire-damaged concrete. Sender and receiver arebeing held tightly against the concrete; signal is recorded on a test instrumentat floor or ground level.

Fig. 2 Much information can be obtained by petrographic methods from corestaken at appropriate intervals. Data can be used to evaluate the extent, typeand severity of damage and to estimate the temperatures to which the concretewas subjected.

al dislodgment of concrete sections,small or larg e. Detailed studies, us-ing petro g raphic methods, may thusbe desira b l e.

Final evaluationWhen all of the data are in and

b e f o re repairs are undertaken thereremain two determinations to bemade: (1) what parts of the stru c-t u re cannot be left in their pre s e n tcondition? and (2) should corre c-tions be made by re p a i r, re p l a c e-ment, or a combination of the two?These subjects will be discussed inthe final paper of this series enti-tled “Reinstating Fi re - Da m a g e dSt ru c t u re s.”

REFERENCES

1. Green, J. K., “Reinstatement ofConcrete Structures After Fire, (Part 1),”The Architects’ Journal (England),July 14, 1971, pp. 93-99

2. Smith, Peter, “Investigation of Repair ofDamage to Concrete Caused by Formworkand Falsework Fire.” ACI Journal, Proceed-ings V. 60, No. 11, November 1963, pp.1535-1566.

Part I of this three-part article on firedamage to concrete structures ap-peared in CONCRETE CONSTRUC-TION in March 1972. Part III will ap-pear in May 1972

Photomicrograph shows a major crack, approximately parallel to a formedsurface, that outlines an incipient spall caused by thermal shock. Theintricate network of fine cracks in the mortar was caused by drying. Morecracks were seen by direct microscopic viewing. Petrographic evidenceshowed temperatures had been high enough to dry the concrete but not todecompose components of the paste or aggregates.

Core removed for examination.

P U B L I C AT I O N# C 7 2 0 1 5 4Copyright © 1972, The Aberdeen Gro u p

All rights re s e r v e d