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The residual deformations of the reinforcing bars and of the prestressing tendons should be checked and mapped, and the smoke and soot particles (that may be corrosive) should be analysed. It is advisable to investigate the residual properties of the steel, by clearing a few bars from the surrounding concrete and by cutting out a few samples to be tested in tension. In this way the full σ-ε-relationships can be determined.

In most cases the in-situ measurement of the hardness may be sufficient to have the required information on steel residual properties. In other cases a more complete screening is required, and coupling cut-out samples and hardness tests is necessary. Metallographic analyses on ground slices are required only in specific cases.

7.4 Diagnosis

Generally concrete structures (either made of plain concrete, or reinforced/prestressed) can be repaired even after a severe fire (see Section 7.9). However, for each element one should estimate and compare the repair costs with the costs for a complete or partial reconstruction . The larger the structure and the more severe the damage, the more likely the reconstruction of the whole structure or of some members.

Only a specific investigation allows to understand whether the residual deformations can be tolerated or more comprehensive measures are necessary for the rehabilitation (such as treating the cracks for static and durability purposes).

For concrete it is possible to define “damage factors” (see Figs. 7.2 and 7.3). For instance, in all the regions, where the temperature did not exceed 100 °C, the damage factor is 1, while it is 0.85 in all the regions, where the maximum temperature was between 100 °C and 300 °C, 0.4 everywhere the maximum temperature reached 300 °C to 500°C, and 0.0 in all the regions were the temperature exceeded 500 °C.

The same applies to the reinforcement, that is practically unaffected by the temperature up to 400°C. For temperatures above 400°C, the residual properties of hot-worked carbon steel are better than those of tempcore steel, of cold-worked carbon steel and of cold-drawn prestressing steel, but are worse than those of hot-rolled stainless steel. However, to take care of the mechanical decay of the reinforcing bars, additional reinforcement may be added.

As for prestressed concrete, on the whole it is more fire-sensitive than ordinary concrete, but it depends on the prestressing system (pre-tensioned members are more heat-sensitive than post-tensioned members) and on the type of the section. However, a comprehensive investigation on the residual load-bearing capacity and on the deformations is mandatory, because of the reduction of concrete Young’s modulus and of the great sensitivity of cold- drawn tendons and strands to high temperature.

In general, had the original design barely respected the safety margins, the occurrence of a fire could damage the structure beyond the level that is considered acceptable for its rehabilitation.

Sometimes, repairing a fire-damaged prestressed structure can be achieved by changing the load-bearing system from prestressed concrete to reinforced concrete. In other cases, structural repairing can be achieved by adding prestressing tendons to heat-damaged RC structures.

7.5 Damage classification

The effects of high temperature and fire on buildings and structures can be characterized by introducing a few “classes” (Table 7-2). This classification makes it possible to define different strategies for the future use of the damaged building or structure:

• complete repair

• combination of partial repair and partial reconstruction • change of the destination or use

• demolition and rebuilding

7.6 Repair criteria

The main objective of repairing fire-damaged concrete structures is to bring back the structure to its original state and destination, through the following steps:

- the reinforcement should be refurbished and protected, and the concrete sections should be brought back to their original size;

- the repaired structure should have the same residual life as before the fire;

- the repaired structure should have the same load-bearing capacity as before the fire; - the repaired structure should meet the same fire-safety requirements as before the fire.

Table 7-2: Classes of damage

Class Characterization Description

1 Cosmetic damage, surface Characterized by soot deposits and discoloration. In most cases soot and colour can be washed off. Uneven distribution of soot deposits may occur. Permanent discoloration of high-quality surfaces may cause their replacement. Odors are included in the class (they can hardly be removed, but chemicals are available for their elimination).

2 Technical damage, surface Characterized by damage on surface treatments and coatings. Limited extent of concrete spalling or corrosion of unprotected metals. Painted surfaces can be repaired. Plastic-coated surfaces need replacement or protection. Minor damages due to spalling may be left in place or may be replastered.

3 Structural damage, surface Characterized by some concrete cracking and spalling, lightly-charred timber surfaces, some deformation of metal surfaces or moderate corrosion. This ype of damage includes also class 2 damages, and can be repaired in similar ways. 4 Structural damage, cross-section Characterized by major concrete cracking and spalling in the web of I-beams,

deformed flanges and partly charred cross-sections in timber members, degraded plastics.

Damages can be often repaired in the existing structure. Within the class are also (a) the large structural deformations that reduce the load-bearing capacity, and (b) the large dimensional alterations, that prevent the proper fitting of the different substructures and systems into the building. This applies in particular to metallic constructions.

5 Structural damage to members and

components

Characterized by severe damages to structural members and components, with local failures in the materials and large deformations. Concrete constructions are characterized by extensive spalling, exposed reinforcement and damaged compression zones. In steel structures extensive permanent deformations due to diminished load-bearing capacity caused by high temperature. Timber structures may have almost fully charred cross-sections. Mechanical decay in materials may occur as a consequence of the fire. Class 5 damages usually will cause the dismissal of the structure.

7.7 Repair methods

Depending on the damage class, repairing should be performed with one - or more - of the following techniques:

- Cleaning and aesthetical renovation.

- Repair of concrete surfaces by using approved materials (polymer-modified mortars and coatings).

- Repair of concrete members and recreation of the original shape (for instance in damaged sections) by using shotcrete (according to DIN 18551 or to equivalent EN or ISO standards).

- Replacement of single elements (in the case of steel or prefabricated-concrete members).

- Addition of extra reinforcement, by using glued carbon- or glass-fibre laminates (FRP).

- Addition of extra fire-safety equipments.

- Repair of concrete cracks by injecting resins or cement slurries. - Pull-down of the structure and build it anew.

In the case of concrete members, first all the external layers that have been subjected to temperatures in excess of 300°C should be removed according to the following, well-proved techniques:

• Chiselling and sand blasting.

• Cleaning with high-pressure water jets.

Then the tensile strength of the new concrete layers should be assessed, for instance by means of pull-off tests. The correlation between the pull-off load and concrete tensile strength makes it possible to have sufficient information on concrete strength for the application of mortar and protective coatings, as well as for the instalment (if required) of anchors of different types. The following pull-off strengths are generally considered adequate (Table 7-3).

Table 7-3: Minimum values for the pull-off strength of the concrete substrate

System Mean value [N/mm²] Min. single value [N/mm²]

Concrete replacement 1.5 1.0

Coating

(paint without fine mortar)

1.0 0.6

Coating systems with fine mortar 1.3 0.8

Coating systems under motorcar lanes 1.5 1.3

Should the sections be brought back to the initial size and shape by using shotcrete, the following steps would be appropriate:

• Instalment of auxiliary moulds (if necessary).

• Placement of additional reinforcement as required by the load-bearing capacity. • Washing and moistering of the substrate.

• Shotcreting in subsequent layers not thicker than 30 mm each. • Special treatments (if necessary) of the surfaces after shotcreting. • Application of a mortar layer (if requested for architectural reasons).

The application of special adhesives between the subgrade (= original undamaged concrete) and the shotcrete is generally superfluous, since the rebound of the coarse aggregates contained in the shotcrete leaves a thin transition layer between the subgrade and the shotcrete. Since this transition layer is very rich in fine aggregates, its “bridging” properties (between the original concrete and the new concrete) are very good.

When using shotcrete, there is no need to apply specific anti-corrosion products to the reinforcement, because shotcrete is very similar to ordinary cast-in-situ concrete. It is true that shotcrete is an extremely-dense material, that may spall under fire, but several additives have been developed - and are available on the market – aimed at preventing shotcrete spalling at high temperature.

The repair with the aid of repair mortars requires the following steps:

•

Painting of the cleaned bars (a) with an anti-corrosion epoxy-based coating (containing corrosion inhibitors and pigments), or (b) with a slurry (containing Portland cement, sand and styrol-butadien acrylate), that later solidifies and becomes elastic.

• Application of an adhesive layer on the original cleaned concrete, to favour the cohesion between the subgrade and the mortar. (This layer may be based either on epoxy resin or on a polymer-modified cementitious mortar).

• Recreation of the original size and shape of the member in question, by using polymer- modified repair mortars, that should fulfill the following requirements:

- Pull-off strength larger than that of the subgrade.

- High capability of remaining stuck to the subgrade (high sticking capability). - Good capability of retaining the water added to the mix (as in ordinary

cementitious materials).

- Same strength as in the subgrade (= original undamaged and cleaned concrete). - Same thermal expansion as in the subgrade.

- Modulus of elasticity from 1/3 to 1/4 of that of the subgrade. - Frost resistance.

- Good handling in ordinary building-site conditions.

•

For applications on small thin spots also polymer-bound concretes are now available. The previously-mentioned mortars have been available for the last twenty years: they are obtained by mixing together water, fine/medium aggregates and a soluble polymeric powder. These mortars are generally prepared by using different max.-size aggregates (da between

1-2 mm and 8 mm). The mortar should be applied in layers when the damaged areas are relatively deep (max. thickness of each layer close to 30 mm; da up to 8 mm). When the

damaged areas are shallow, smaller aggregates should be used (da = 1-2 mm).

Whenever necessary, a fine mortar should be applied as a finish, to close the surface pores and to adjust each member to the contiguous members.

Last but not least, a protective layer can or must be applied, depending on specific requirements and specific types of exposure.

The many coatings available today can be classified as shown in Table 7-4.

In some cases, crack filling with special resins or mortars to re-establish structural continuity is required by statics.

Table 7-4: Description of the various protective surface coatings

Description Main type of binder

Hydrophobic Impregnation Silan, Siloxane, Silicon resins

Coatings for non accessible surfaces with very low crack-bridging

capability (a) polymer (b) polymer/cement mix

Coatings for non accessible surfaces with low crack-bridging

capability (a) polymer/cement mix (b) polymer dispersion

Coating with enhanced crack-bridging capability for non

accessible surfaces (a) polyurethane (b)two-component polymethylmethacrylate-

modified epoxy resins Coatings below bituminous or other protective layers on

accessible surfaces Polyurethane

Coatings with low or high bridging capability for accessible

surfaces (a) multilayer epoxy resins (b) epoxy resins and polyurethane resins

7.8 Real fires

In document PONTIFICIA UNIVERSIDAD CATÓLICA DEL PERÚ (página 60-64)