Summary 42012 127summaryThemeShear strength of existing structuresThe national motorway network is being usedmore and more, and subjected to increasinglylarge loads. In 2007 Rijkswaterstaat(Directorate-General for Public Works andWater Management) started an investigationinto the shear strength of existing concretebridges and viaducts built before 1975.Approach to shear force problems (pp. 4 7)The increase in the number of lanes andvehicles and in the permitted weight of heavygoods vehicles gives rise to the question ofwhether existing viaducts and bridges stillhave enough load-bearing capacity becausethey were designed on the basis of otherregulations.Safety philosophy for existing structures(pp. 8 16)The new Buildings Decree came into force on1 April 2012. The Eurocodes apply to newstructures and NEN 8700 applies to existingones. How should the acceptance of certainreductions in safety margins be interpretedand how should the limited lifetime of thestructure be taken into account?Assessment framework for existing structures(pp. 18 25)In 2007 Rijkswaterstaat started a largeinvestigation into the assessment of existingconcrete structures. An assessmentframework for this investigation describes thedifferent parts of the investigation that werecarried out.From RBBK to RBK (pp. 26 29)Rijkswaterstaat has been using the Guidelinefor Assessment of Existing Structures (RBBK)to assess existing structures since 2004. Thisguideline is in line with the old Dutch TGBstandards. The RBBK will be replaced by theGuidelines for Assessment of Structures (RBK)for assessment in accordance with theEurocode. This has been done with a view tothe uniform assessment of structural safety.Modelling traffic loads (pp. 30 35)In the Netherlands the NEN-EN 1991-2standard is used for determining the trafficloads on road bridges. Factors are prescribedin the accompanying national appendix forthe purposes of calculating country-specifictraffic loads. TNO carried out a study incollaboration with Rijkswaterstaat in order tomake it plausible that a bridge designed inaccordance with this load standard, togetherwith the associated material-relatedstandards, is safe enough according toNEN-EN 1990. The national appendix ofNEN-EN 1991-2 was specified on the basis ofthis study.Strength of concrete in existing structures(pp. 36 39)Concrete strength is one of the determiningparameters for ascertaining the shear forceload-bearing capacity of structures. Over timeit increases significantly as a result of ongoinghydration. The strength that is actuallypresent can be used in the assessment ofstructures. As a result of this, approval is givento structures that in the first instance do notcomply on the basis of calculations with thespecified safety requirements with regard toshear force.Shear loadbearing capacity of slab viaducts(pp. 40 45)In recent years, at the initiative ofRijkswaterstaat research has been done byTNO (Netherlands Organisation for AppliedScientific Research) and Delft University ofTechnology into the current reserve capacityin bridges and viaducts made from reinforcedconcrete. There were two key issues relatingto the many existing slab viaducts withoutshear reinforcement: a) which concretestrength may be taken into account in theassessment of the shear load-bearing capacityand b) can direct load transmission also beassumed at intermediate support points?Distribution of point loads in slab viaducts(pp. 46 50)Based on experimental and literatureresearch, Delft University of Technology hasdeveloped a new load distribution model forapplication in Rijkswaterstaat's quick scanmethod for the assessment of the shearload-bearing capacity of slab viaductswithout shear reinforcement.Assessment of prestressed structures(pp. 52 55)Rijkswaterstaat has conducted an extensiveinvestigation into concrete structures in itshighway network in which their shearstrength was assessed. These structures wereclassified by type because of the largenumbers involved.Longterm load (pp. 56 60)The long-term effect concerns 28-dayconcrete. The increase in strength as a resultof ongoing hydration of the cementthereafter, until the structure starts to beused, compensates for any negativelong-term effect. The question is, however,whether this also applies to structureswithout shear reinforcement that aresubjected to shear forces. This categorycontains many existing slab bridges, the shearload-bearing capacity of which is underdiscussion.Assessment of reinforced structures(pp. 62 67)Both the Guideline for Assessment of ExistingStructures (RBBK) and the later Guidelines forAssessment of Structures (RBK) ? introducedin 2012 ? are based on the simplest possiblerecalculation in order to assess the safety ofRijkswaterstaat's existing structures. Both setsof guidelines moreover take into account thefact that, unlike new build structures, it is noteasy to add additional reinforcement toexisting ones.Compressive membrane action (pp. 68 74)Compressive membrane action in thintopping layers can create a higher ultimatebearing capacity. Research has been carriedout at Delft University of Technology into howcompressive membrane action can be takeninto account in the reinforced topping layersof viaducts with inverted T-beams.Guideline for nonlinear finite elementcalculations (pp. 76 81)The first version of the guideline fornon-linear finite element concrete structuredesign calculations was recently issued as aRijkswaterstaat Technical Document. Thisguideline, which is embedded in Eurocode 2and the new fib Model Code 2010, providesadded reliability for experienced users.Test loading bridges (pp. 82 86)The test loading of bridges is a method fordetermining whether the structure canwithstand the calculated load. This isimportant for older structures with anunknown load-bearing capacity, or structuresthat are currently being subjected to a loadthat is greater than their original design load.Assessment with CRIAM (pp. 88 94)CRIAM (Structural Risk Indexing AssessmentModel) is a model that can be used toinventory and assess concrete structures atreduced costs. Experience from theRijkswaterstaat investigation into the shearload-bearing capacity of existing structureshas been incorporated into the model and ispartially determining for the results of theassessments.Outside the themeSymbiosis of in situ and prefab (pp. 106 109)The LaM ? Lille Museum ? is situated in abeautiful park in Villeneuve d'Asq. The existingbuilding, which was designed by RolandSimounet in 1983, has been extended byManuelle Gautrand Architecture with long,flowing organic volumes, constructed using acombination of in situ and prefab concrete.Crackfree concrete ? a dream? (pp. 110 115)Cracking in hardening concrete is anundesirable but not uncommonphenomenon, even in new structures with anintended lifespan of 50 to 100 years. Crackspermeable to water have a direct impact onthe durability of the structure because thechance of damage (carbonatation, chloridepenetration, etc) is increased. How should thechance of cracking be specified?`Flexible' formwork with vacuumatics(pp. 116 121)The creation of imaginative shapes is possibleby using so-called vacuum structures(or vacuumatics). These can be used as aformwork system with a changeable shape. Adoctoral research project is currently beingconducted at Eindhoven University ofTechnology into the structural properties andthe geometric preconditions of thesestructures. The design, load-bearing capacityand feasibility were addressed in a focusedstudy.Lifelong durability, but not as a matter of course(pp. 122 126)In the second part of his inaugural lecture,Rob Polder discusses experiences with thedurability of concrete in practice and theapplication of modern methods for extendingthe lifetime of existing structures. There is alsoa look into the future based on currentresearch.If you would like a whole article from Cement translated, why not let us arrange it for you?Prices start from 100 per page (actual price on request).We can supply either bare text or a professionally laid out article. For more informationplease contact ?neas: 0411 65 00 85, lezersservice@aeneas.nl.