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Diagnosis and Consolidation of the 'Business Centre UNIS' in Sarajevo, Damaged by the Bosnia War Dario Almesberger SER.CO.TEC. S.r.l - Piazza S. Giovanni 3 - Trieste Antonio Rizzo Dipartimento di Ingegneria Civile - Universitį degli Studi Di Trieste Milenko Toncic SER.CO.TEC. S.r.l. - Piazza S. Giovanni 3 - Trieste Raffaella Geometrante Dipartimento di Ingegneria dei Materiali e Chimica Applicata - Universitį degli Studi di Trieste Contact

ABSTRACT

The Business Centre UNIS (Fig. 1) had been built on the occasion of the winter Olympic games of Sarajevo (1984); it was constituted by two twin skyscrapers, very similar to the New York and Montreal ones, but smaller.

Fig 1: Business Centre UNIS


During wartime, both have been damaged but the building B resulted to be the most ruined (Fig. 4 and 5).

Fig 4: Damage in the Business Centre UNIS

Fig 5: Damage in the Business Centre UNIS


Non-destructive techniques have been applied so to perform a diagnosis of the state of conservation of skyscrapers B reinforced concrete. Concrete quality has been determined by the combined methods of sclerometry (ASTM C805 - Standard Test Method for Rebound Number of Hardened Concrete), ultrasound (ASTM C597 - Standard Test Method for Pulse Velocity Through Concrete) and pullout (ASTM C900 - Standard Test Method for Pullout Strength of Hardened Concrete). Reinforcing steel bars have been localised by magnetoscopic surveys while their state of corrosion has been determined by potential measurements (ASTM C876 - Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete). On the building, endoscopic and thermographic investigations have been also used and the microcores extracted have been characterised by specific laboratory tests.
The results achieved have been determinant for the elaboration of the project of conservative restoration.
Non destructive methods will be also used during the consolidation phase for the quality control of the refurbish treatments.

1.INTRODUCTION

The war of Bosnia and Herzegovina has seriously ruined industrial, residential and historical monuments. In particular, in Sarajevo most of the buildings have been damaged by artillery and some of them had been burning for several days (this is the case of the "UNIS" Business Centre, too).
The effect of high temperatures, and in particular of the fire, on the durability of reinforced concrete structures is rather complex. On one side it is necessary to compare structures which have not been exposed to high thermal gradients with parts directly involved in the fire. On the other side it is necessary to estimate the residual strength and the elastic modulus of the structure.
The analysis of fire effects can be conducted by means of visual surveys, non-destructive testing (sclerometry, ultrasounds, electric potential measurements) and also through the identification of mechanical and chemical properties of the microcores extracted "in situ". In particular, the alteration degree of the structures can be determined by the comparison of the characteristic parameters concerning elements which have been affected by fire, with the theoretical values or values relevant to similar elements which have not been damaged or were located far from the area affected by the fire.
An approximate evaluation of the duration and importance of the fire and of the temperature achieved, can derived from the analysis of "decolorization" of the concrete and from the conditions of steel reinforcement bars and wooden elements. Flaking and cracks can be detected by visual survey, too.
However, this kind of research cannot produce any information concerning recoverability of the structures so it has to be follow by non-destructive investigations.
In this case, in order to assess the state of conservation of the structures, a combined investigation utilising ultrasounds (ASTM C597) and Schmidt sclerometry was carried out. The results achieved by this method have been supported by the measurements of the potential of corrosion (ASTM C876) of the most significant areas. The elaboration of the results collected from these tests has allowed the evaluation of the material's residual resistance and deformability so to assess the type and the entity of the intervention necessary to recovered the damaged structures.

2. EXPERIMENTAL ANALYSIS

When the elements constituting the concrete are exposed to thermal shock, they can undergo to chemical, physical and chemical-physical modifications which imply discontinuities or gases volume increase inside the structure. In this case, the dynamic auscultation shows an evident decreasing in the propagation speed due to the very low transmissibility of the signal at the level of the separation surface between air and concrete; this is recorded as a decrease in the energetic content. In particular, this technique can detect cracking caused by constrain stress as a consequence of different thermal dilations related to uneven heat propagation in the structure. Moreover, increasing in micro and macro cracking pattern, due to physical and chemical transformations occurring at high temperature in concrete components, can be also determined by ultrasonic investigations (Fig. 3). In any case, before applying these techniques, a wide pacometric investigation is essential for the localisation of transversal and longitudinal steel reinforcement bars.

Fig 3: Ultrasonic investigation

Hydrochloric acid fumes released by burning PVC could have reacted with hardened concrete, developing calcium chloride. The possibility that important quantities of chloride may have penetrated deeply into the concrete must be taken into account since this would have seriously endangered reinforcement, in those cases where the combustion of large quantities of PVC has probably produced high concentrations of hydrochloric acid in combustion gases, or, alternatively, where acid fumes may have affected the concrete for several days.
War-damaged buildings rarely still have roofs. In this case, fume-polluted concrete has been saturated by water every time it rained. These conditions could have caused the corrosion of the steel bars, and the situation should be particularly serious in pre-stressed structures. In fact, almost all reinforced concrete buildings examined in Sarajevo and Mostar are affected by loss of compressive strength and increased corrosion of reinforcement bars.

3.NON DESTRUCTIVE TESTING INVESTIGATIONS

The two skyscrapers of the UNIS Business Centre, symbol of the winter Olympic games of 1984, have been damaged from the tenth floor on. Their structure is constituted by a central reinforced concrete core (where the stair-well and the elevator is located), by reinforced concrete slab and steel pillars. These last ones are filled with reinforced concrete and protected from the fire by fire-proof panels which have prevented the steel structure from deformations. This is not true for what the concrete structure is concerned: spalling of the concrete cover are widely present all other the reinforced concrete elements.
The chemical analysis of the microcores extracted "in situ" has determined the presence of chloride deriving from PVC combustion gases. The removal of the concrete polluted by chloride will be necessary in order to avoid further more damage to rebars.
The most damaged floor is the twentieth as, besides war damages, this has been also exposed to freeze-thaw cycles and the reinforcement bars have been corroded due to the environmental humidity.

Fig 2: Attenuation of ultrasonic wave through concrete on the 20th floor


The results of non destructive tests on concrete and steel reinforcement bars have been reported in Tab. 1. In Fig. 2 is reported the attenuation of an ultrasonic wave.

BUSINESS CENTER UNIS                                           Temp. 5.7°C - Rel. Hum. 85%
20TH FLOOR	PROBES DISTANCE[mm]	WAVE SPEED [m/s]	SCLERO METER	SONREB METHOD[N/mm2]	ELETTRIC POTENTIAL[MV]	NOTES
20 C1	250	4000	42	34		semi-indirect measure
20 U1	200	3090	38	14	420	indirect measure
	400	4400				superficial cracks reinforcement corroded
	600	2760
20 U2	200	2510	39	-	459	indirect measure superficial cracks
	400	2820				reinforcement corroded
	600	2770
Table 1: Results achieved by "in situ" non-destructive tests on the 20th floor

4.LABORATORY TESTINGS

Mechanical tests and chemical analysis have been conducted in the laboratories of the University of Trieste on concrete samples (Ę 50 mm microcores) and on steel reinforcement bars.
Compression strengths and chloride contents have been determined on concrete samples (Tab. 2). In order to obtain a correlation between non-destructive and destructive tests, before the compression test, ultrasound speeds have been measured. These results have been useful as a comparison with the strength values determined using the SONREB non-destructive method.
Steel bars have been submitted to tensile strength test which has pointed out that, in spite of the fire, steel has conserved a good yield and fracture strength (Tab. 3).

SAMPLE	Cl- % in concrete	Cl- % in cement
9	0.025	0.15
10C	0.038	0.22
11C	0.018	0.11
11P	0.028	0.16
11Z	0.032	0.19
12C	0.043	0.25
13C	0.043	0.25
14C	0.055	0.33
15C	0.099	0.59
17	0.074	0.44
18	0.082	0.49
19	0.067	0.40
20	0.028	0.16
Table 2: Chloride ion contents for different floors

SAMPLE	DIAMETER [mm]	UNIT MASS [kg/m]	DIAMETER EQUIHEAVY BAR [mm]	TRACTION
				UNIT LOAD		% strain after fracture
				yield [N/mm2]	break [N/mm2]
10Z	12	0.954	12.4	581	723	30.0
11Z	12	0.948	12.4	556	698	28.3
13P	12	0.943	12.4	571	710	23.3
14P	12	0.970	12.5	564	677	26.7
15P	12	0.911	12.2	558	706	26.7
16Z	12	0.921	12.2	575	633	26.7
17P	12	0.923	12.2	596	659	23.3
Table 3: Tensile strength values of steel reinforcement bars of different floors

5. CONCLUSIONS

The elaboration of the data collected with non-destructive techniques (ultrasounds, sclerometer, pull-out, micro-drilling and corrosion potential) has allowed an evaluation of the state of conservation of the structure. In particular, most of the reinforced concrete structures are suffering damage only on the superficial part of the concrete cover while the core of the structure is in a quite good condition.
The rehabilitation intervention will consist in the reconstruction of the concrete cover using anti shrinkage cementitious mortars. The hydrodemolition will be used to remove the damaged concrete in order to take away the chloride transported by the gases produced during the combustion of PVC.
In the building an "healthy" area, which did not enter in contact with the fire, has been identify; from this area a microcore has been extracted to be used as reference sample. The ultrasonic speed determined on this sample (Vr) was varying between 4300 and 4500m/s.
In terms of the ultrasonic speeds measured (Vm) in the areas which have been affected by the fire, four categories, related to four different concrete conditions, can be identified:

a) Vm ³ 0.87 Vr	Ž        sound situation
b) 0.87 Vr > Vm ³ 0.70 Vr	Ž        medium intensity damage
c) 0.70 Vr > Vm ³ 0.48 Vr	Ž        high intensity damage
d) Vm < 0.48 Vr	Ž        very high intensity damage

Most of the structure of "Business Centre UNIS" skyscrapers is constituted by concrete with medium intensity damage so it will be possible to restore these buildings by a partial or complete substitution of the concrete cover, by injection of epoxy resin in the deeper cracks and by the consolidation of the most damaged areas by means of metallic plates or new concrete layers.
Once the rehabilitation works will be concluded, a safety assurance will be done through a dynamic identification. The seismic resistance and the quality control of the restoration works will be checked studying the propagation of the waves produced by a vibratory equipment

6. REFERENCES

1. D. Almesberger, "Riparazione e durabilitą delle strutture in cemento armato dall'azione del fuoco", lezioni tenute al convegno di studio su manutenzione e durabilitą delle strutture in cemento armato, Udine 9 - 13 giugno 1986.
2. M. Mele, P. Rocchi, D. Almesberger, M. Ciampoli, C. Filipuzzi, F. Sabato, "Consolidamneto, risanamento e protezione antincendio del patrimonio edilizio", edizioni ASS.I.R.C.C.O. 1986.
3. Relazione tecnica: "Dijagnoza ratom oštecenog poslovnog centra UNIS, Gradske vjecnice i Gazi Husrevbegove dzamije u Sarajevu pomocu kontrola bez razaranja", SER.CO.TEC. 1995.
4. Relazione tecnica: "Dijagnoza ratom oštecene Titove vile, zgrade robne kuce Razvitak i zgrade Beko-Energopetrol u Mostaru", SER.CO.TEC. 1995.
5. Relazione tecnica: "Dijagnoza požarom oštecene zgrade u Ulici od Puca 16 u Dubrovniku", SER.CO.TEC. 1995.
6. Relazione tecnica: "Dijagnoza i sanacija konstrukcije kompleksa hotela Lipik oštecene u domovinskom ratu - Hrvatska", SER.CO.TEC. 1994.

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