Fire resisting concrete

Transcription

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Fire resisting concrete B.P. Van den Bossche Concrete technology engineer ABSTRACT: Tunnel safety is gaining importance the last few years. Fire protection is one of the items that has to protect the users of the tunnel. Effects like explosive scaling occur on good quality concrete. Degeneration of the mechanical properties of concrete and rebar will also provoke dangerous instability of concrete constructions. During the tender on the Overkapping A2 in Utrecht the idea of fire resisting concrete was launched by the joint venture Besix-Dura Vermeer-GTI. After acceptance of tender, a nine-month investigation was performed, starting with a literature study, to trial mixes, testing of fresh concrete and tests on hardened concrete. Large post-tensioned specimens were made and tested on conformity with the RWS-fire curve. After investigation the concrete was applied on the building site. This paper will treat the whole investigation and testing of the fire resisting concrete, the dosing of the primary materials and the treatment of the concrete on the building site. 1 INTRODUCTION Disastrous fires in tunnels caused a lot of casualties in the past, and also the damage on the tunnels was very severe. The damage during these fires gave a progressive spalling, until 30 cm deep. With this deep spalling, the rebar came in contact with the fire, and lost its mechanical properties. Examples such as the fires in the Gottard tunnel in 1984, the Eurotunnel in 1996, the Mont Blanc tunnel in 1999 with 41 casualties, the Kitzsteinhorn tunnel in 2000 with 155 casualties pushed the public ministries to review the vision on structural design of tunnels regarding fires. The Dutch ministry of public works, RWS, translated these visions in ROBK rules. For the latest tenders of public tunnels, the fire protection is a hot issue. To meet with the specifications of the capping construction of the A2, RWS specified maximal temperatures on concrete and steel reinforcement in the construction. These temperatures need to be confirmed by tests. 2 SPECIFICATIONS BY RWS (CLIENT) The contract of the capping over the A2 highway in Utrecht is a Design and Build contract. The tunnel needs to be designed so that in case of fire the safety of the construction is guaranteed and Figure 1. Figure hour fire resistance. RWS fire curve. that there is enough time for the users of the tunnel to escape or help can be given. Two zones can be determined. The normal zone with 1 hour fire resistance and the safe zone (emergency corridors and technical buildings) with 2 hour fire resistance. The one and two hours fire resistance are according the RWS fire curve. For the one-hour-fire resisting, the area of the compressive zone of the concrete that reaches more than 380 C cannot be taken in account. 319

2 Limestone Gravel Thermal expansion 8 12 coefficient 10 6 / C E-modulus N/mm Figure 4. Table of differences between gravel and limestone. 4 PREPARATIONS AFTER SIGNATURE OF CONTRACT Figure 3. Calculated temperature distribution. For the zone with tensile stresses, temperature in the steel reinforcement cannot exceed the temperature necessary to maintain stability of the structure. Hereby the concrete at 25 mm of the reinforcement cannot exceed 380 C. For the two- hour-fire resisting the zone with tensile stresses temperature in the steel reinforcement cannot exceed 250 C. Hereby, the concrete at 25 mm of the reinforcement cannot exceed 380 C. For the zone with compressive stresses, the concrete that reaches more than 380 C cannot be taken in account. These temperatures needs to be verified conforming the document Fire protection for tunnels, number GT-98036a (98-CVB-R1161a). 3 INTERPRETATION DURING TENDER The alternative for using PP-fibers in the concrete to prevent it from progressive spalling was initiated by the engineering department of Besix. Therefore a literature study was made. Referential projects in Norway, England and Australia were used to make a feasibility study. The information of suppliers and fire test laboratories were not available to the public and were protected by intellectual ownership. The combination Besix-Dura Vermeer-GTI (contractors) took the risk to use the fire resisting concrete for the first time in a tunnel project in the Benelux although the exact dosages and type of PP-fibers, the workability of the fiber concrete were not known. In the design a temperature ingress in the concrete was calculated to specify the minimal concrete cover (with various option, fire protection, PP-fibers,...). With this information the fire resisting concrete was economically more interesting than the use of fire resisting insulation. Once the works were given to the winning contractor(s) the engineering started. Parallel with the engineering, the investigation of the concrete properties and constituents was started. First a guideline was made to fix all steps that had to be done. This Testing plan or Protocol beton was accepted by RWS in an early stage. In order to have the concrete mix fixed on time the planning was: Start preparation site : August 2006 Selection supplier(s) : September 2006 Trial mixes : September/October 2006 Specimens fire test : begin November 2006 Fire tests : February/March 2007 First pour : March 2007 The supplier of the PP-fibers was selected not only out of economical considerations, but also by technical competences and referential projects. PROPEX had the necessary international experience and references. They advised us to use 32 µm 12 mm fiber, instead of the usual 18 µm 6 mm fiber. Although in theory the 18 µm 6mm fiber should be the most effective, the 32 µm 12 mm gave sufficient results regarding spalling (see chapter 7), and had the advantage that less air was trapped in the fresh concrete and of better workability (and consequently less dosage of plasticizer). For the aggregates limestone is used. The choice of crushed limestone aggregate was made because the additional costs were low regarding the advantages. Limestone aggregates make a concrete that gives a lower Young modulus and a lower thermal expansion coefficient. Because the limestone aggregates has nearly the same properties as the cement paste during high temperatures, the effect of spalling will be less. 5 TESTING THE CONCRETE 5.1 Mix designs The structural design gave two concrete qualities: Ground Slabs, Walls : C28/35 XD3 Self Supporting slabs : C35/45 XD3 320

3 The specifications lave the possibility to raise the water-cement ratio to 0.5 by using CEM III/B cement. For the cement CEM III/B 42.5 N LHHS of ENCI was used. The dosages of the PP-fibers were fixed per grade, on 1; 1.5, 2 kg/m 3 and 3 kg/m 3. That gave us eight mixes that needed to be designed and tested by the supplier MEBIN in Utrecht. 5.2 Trial mixes Following tests were performed during the test procedure: Fresh concrete: Slump 0 and 60 Shock table 0 and 60 Air content Mass Water content by burning Visual : workability and pomp ability Hardened concrete: Compressive strength 3, 7, 28 days Tensile strength 3, 7, 28 days by splitting Water penetration test Heat of hydration in insulated mould Maturity test graph Volume mass The first two trail mixes were done conforming the mix designs. Using the PP-fibers and hyperplasticizers the fresh concrete showed segregation. Dosing less plasticizers, the concrete was not workable. The solution to cope with this problem was to increase the quantity on fines (<250 µm) with a filler. Dosing 20 kg/m 3 of limestone filler extra to the concrete gave an acceptable result. The slump test conforming EN gave not a trustable result. The fibers connect the grains of the aggregates of the concrete and prevent the concrete to flow. The shock-table conforming EN gave a good alternative, and is within the specifications and the NEN EN Different hyper-plasticizers were used. Some of them gave a raise of air-content in the fresh concrete up to 3.6%. Using other types the air content was between 1.5 and 2%. The conclusions of the trail mixes were: Fixing the dosage of plasticizer Fixing the type of plasticizer Adding extra filler to stabilize the mix Slump test is not reliable Shock table test needs to be mm Workability was more than 60 Compressive strength is within the limits Figure 5. Photograph of fiber concrete. 5.3 Fire test specimens The fire tests needs to be performed to meet the specifications (see chapter 2). The goal of the test is to measure: Temperature at 25 mm of the steel reinforcements Temperature in the reinforcements Sensitivity of spalling of the concrete. The last item depends on quite some parameters in the concrete mix and the location of the structure. Therefore a study of how the specimens should be, was performed by the engineering department. During the test methods the differences in Wall or Slabs has to be taken into account.the maximum solicitation in the concrete of the tunnel has to be reflected in the specimens. The curing of the concrete surface is very important, and the degree of humidity in the concrete of the specimens has to be equal to the degree of humidity of the tunnel in service. The test will be accepted if two fire tests of one construction part is successful. Per concrete mix, two specimens are made. The concrete mixes with 1.5 kg/m 3 of PP-fibers was not tested. 12 specimens were made. 6 for the walls, and 6 for the roof-slabs. The specimens had to be as big as possible to prevent side-effects. For practical reasons the slabs were made 2.5 m 5m. The thickness was chosen equal to the smallest walls: 40 cm. Post-tensioning gains were inserted in order to applicate the designed normal force into the specimen. After pouring and unmoulding, the elements were cured in water retaining canvas, and stacked on the terrains of MEBIN in Utrecht for 3 months. In the first specimens to be tested, different types of spacers were inserted, in order to look at the influence of spacers regarding explosive spalling. 321

4 Figure 6. Picture of prefabricated rebars for specimens. Figure 8. Photograph of the roof fire-test. Figure 7. Pouring of the specimens, and cured specimens. The specimens were equipped with thermocouples at 75 mm, 100 mm and 125 mm of the heated surface. At minimal ten locations, these three thermocouples were inserted. Some spare locations were foreseen. With these thermocouples in function of the depth, it is able to make a temperature distribution in the 125 mm concrete cover. 5.4 Fire test testing at efectis (Rijswijk- Nl) In order to avoid discussion between RWS and Besix- Dura Vermeer-GTI after testing, a test protocol was made by the contractors and accepted by the client. This document was based on the contractual document Fire protection for tunnels, number GT-98036a (98- CVB-R1161a), and had the following requirements: Oven specifications RWS temperature curve Temperature tolerance in the oven Temperature registration Analyze of the test Test reports Video recording Figure 9. Photograph of the wall fire-test. In total five fire test were done. Three roof specimens and two wall specimens. During the tests temperature (for all the thermocouples) was continuously monitored, and displayed on real time in the laboratory. The video recording was also in real time visible in the lab, and recorded together with a chronometer. Before the specimens were installed in the oven, they were stacked in the laboratory (not heated) for minimum 3 days. This is done to have a comparative relative humidity in the concrete, as it should be in the tunnel. During this time, the post-tensioning was applied. For the roof-specimens it was 14 N/mm 2, done with 11 Ø 32 FeP1230 post-tension bars, spanned up to 880 kn per bar. These bars were put in eccentrically, to simulate the eccentric forces in the roof slab. The wall specimens needed 15,3 N/mm 2, done with 6 Ø 32 FeP1230 post-tension bars, spanned up to 910 kn per bar. These bars were put in centrically. 322

5 Figure 12. Depth of spalling, roof specimen after cooling down. Figure 10. Temperature evolution in time, in the middle of the roof specimen, on three depths (75,100 and 125 mm). Figure 13. Temperature evolution in time, in the middle of the wall specimen, on three depths (75,100 and 125 mm). Figure 11. Picture of heated surface, roof specimen, after cooling down. 5.5 Results of the fire tests On of the five tests had to be stopped because of excessive explosive spalling. This mix was therefore not accepted. The other four gave satisfactory results regarding spalling and also enough results to interrelate the temperature distribution, which gave the concrete cover for 1-hour fire resisting and for 2-hour fire resisting. On the next page, the results of the fire-tests, for the roof and the wall are displayed. In order to fix the necessary concrete cover, a statistical analysis was made with all the results of the four accepted fire tests. For a 60 fire, 50 mm of cover should be sufficient when taking in account only the temperature distribution. Nevertheless, the raining off has to be added to the concrete cover. Therefore the minimal cover regarding fire resisting is 75 mm for 60 fire. Extra execution tolerance was added by the contractor. The same is done for a 120 fire. Here a 75 mm cover is needed regarding temperature distribution. Taking in Figure 14. Picture of heated surface, wall specimen, after cooling down. account the raining off, the minimal cover is 100 mm. Extra execution tolerance was added by the contractor. 5.6 Not accepted test As earlier stated, one of the five test gave excessive explosive spalling. This resulted into a high raise of 323

6 Figure 15. down. Depth of spalling, wall specimen after cooling The lime stone aggregates, together with PP-fibers gave a positive result on E-modulus and the linear expansion coefficient. Together with the use of Low Heat cement, the measures to be taken regarding thermal cracks are minimal. Following test were made: Shrinkage on young age concrete Creep on young age concrete E-modulus on young age concrete and 28 days Adiabatic evolution Linear expansion coefficient 28 days 7 CONCLUSIONS Figure 16. Picture of heated surface, roof specimen with explosive spalling, after cooling down. temperature in the specimen. The risk of collapsing was severe, so the test was stopped after 35 minutes. The picture shows the result of the spalling, with a maximum depth up to 125 mm. 6 EXTRA TESTING In order to calculate the effect of shrinkage of the mass concrete of the capping, extra tests were performed on the concrete mix that was accepted during the fire tests. It is possible to engineer a tunnel construction with fire resisting concrete and a sacrificial concrete layer. There are more advantages in using fire resisting concrete than in using fire protective layers. The integration of the passive fire protection in the construction is cheaper, less sensitive to leakages, more resistant to mechanical damages during accidents. During construction time the fabrication cycle can be shortened with one day, resulting in a productive week cycle. PP-fibers have beneficial properties to frost-thaw attack. Fore every concrete mix proportions, and every structural case, intensive investigation needs to be performed. Spalling and temperature distribution depends on various parameters. The results of this project cannot be generalized or used in other projects. REFERENCES Ernst & Shon Bautechnik 78 (2001) :Vershuche zum Brandverhalten van Tunnelinnenschalenbeton met Faserzusatz. Neville A.M. : Properties of concrete (1991). Efectis : report nr R0347 (2007). 324