CHAPTER 5 CONCRETE PAVEMENTS - TU Delft

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5.1 Introduction: Concrete pavements always require technical provisions to prevent uncontrolled cracking due to hardening shrinkage of the concrete and due to a decrease of the temperature. The possible measures are: - in plain (unreinforced) concrete pavements every 3 to 6 m a transverse joint is made, and in wide pavements also longitudinal joints are made; this means that the pavement is divided into concrete slabs - in reinforced concrete pavements such an amount of reinforcement (0.6 to 0.75%) is applied, that every 1.5 to 3 m a very narrow crack appears - in prestressed concrete pavements by prestressing such compressive stresses are introduced that the resulting flexural tensile stresses in the concrete due to shrinkage, prestressing, temperature and traffic loadings stay within acceptable values. The application of the ‘zero-maintenance’ but very expensive prestressed concrete pavements is limited to extremely heavily loaded pavements, especially airport platforms (for instance at Amsterdam Airport Schiphol). Reinforced concrete pavements, with a noise-reducing and permeable wearing course of porous asphalt (‘ZOAB’), are nowadays sometimes applied on Dutch motorways. In all other cases, however, plain concrete pavements are applied and for that reason in this lecture note only attention is paid to this type of concrete pavement. More extensive information about the design (and construction) of concrete pavements can be found in (1,2,3,4,5). 5.2 Structure of plain concrete pavements: 5.2.1 General: Figure 5.1 shows in general terms the plain concrete pavement structure. Plain concrete toplayer Base Sub-base Subgrade Substructure Figure 5.1: Plain concrete pavement structure. The toplayer consists of cement concrete, that exhibits an elastic behavior until the moment of failure. The Young’s modulus of elasticity of the concrete toplayer is much higher than that of the underlying layers, which results in a great load spreading in the toplayer and hence in low stresses in the underlying substructure (base plus subbase plus subgrade). Because of the great load spreading in the concrete toplayer, for reasons of strength a base is not (always) necessary. Nevertheless generally a base (with a high 161
esistance to erosion) is applied to prevent as much as possible the loss of support of the concrete toplayer, which could result in unevenness and/or early cracking of the concrete. A base is also necessary for carrying the modern, heavy construction equipment (slipformpaver) for the concrete toplayer. The sub-base is especially needed to protect the concrete pavement structure against frost/thaw damage. The minimum thickness of the sub-base is thus dependent on both the thickness of the overlaying layers and the frost penetration depth. 5.2.2 Subgrade: Because of the high Young’s modulus of elasticity of the concrete toplayer the bearing capacity of the subgrade has only a small effect on the stresses in the concrete layer due to a traffic loading. Due to this small effect it is common use in the design of concrete pavement structures to simply schematize the subgrade into a system of independent vertical linear-elastic springs with a stiffness (‘modulus of subgrade reaction’) k0 (see 5.3.2). The bearing capacity of the subgrade (modulus of subgrade reaction) does have a great effect on the vertical displacements (deflections) of a concrete pavement structure due to a traffic loading. Because of the characteristic behavior of a concrete pavement structure and the high repair costs in case of failure, it is important to use in the design of the concrete pavement structure a relative low modulus of subgrade reaction, for instance the value that has a 95% probability of exceeding. Besides the bearing capacity also the settlement behavior of the subgrade is important. Except the connection to bridges, founded on piles, equal settlements of the subgrade generally are not a problem. However, by unequal subgrade settlements extra flexural stresses are introduced in the concrete pavement structure. The magnitude of these stresses is dependent on the wavelength and amplitude of the settlement pattern (related to the dimensions of the concrete pavement) and on the velocity of the settlement process (because of stress relaxation in the cement concrete). In practice (plain) concrete pavements are only applied on subgrades that do not exhibit settlements (such as the sand subgrade in the southern and eastern parts of The Netherlands) or on subgrades with rather limited and uniform settlements (for instance at Amsterdam Airport Schiphol). 5.2.3 Sub-base: The sub-base may have the following functions: - raising the road surface above the level of the surrounding subsoil - preventing damage of the pavement structure due to frost/thaw action - temporary storage of rainwater that penetrated into the pavement structure - soil improvement, i.e. replacement of unsuitable subgrade material - platform for the construction of the overlaying pavement layers - spreading the traffic loadings. 162
Page 1: CHAPTER 5 CONCRETE PAVEMENTS 160
Page 5 and 6: 2. Not preventing the adhesion betw
Page 7 and 8: (‘wild’) cracking, due to the c
Page 9 and 10: 5.3 Stresses and displacements in p
Page 11 and 12: Example Sand subgrade: ko = 0.045 N
Page 13 and 14: 1. positive temperature gradients m
Page 15 and 16: where: σ = flexural tensile stress
Page 17 and 18: The equations 5.5 and 5.6 and figur
Page 19 and 20: Figure 5.12: Design model for plain
Page 21 and 22: Axle load group (kN) Wheel load gro
Page 23 and 24: where: wl = deflection (mm) of the

CHAPTER 5 CONCRETE PAVEMENTS - TU Delft

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