Structural Design of Pavements PART VI Structural ... - TU Delft

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25 In section 1 of table 3, location 0.85 has the SCI value (453) that comes closest to the mean SCI value of that section being 434, while location 0.75 has the SCI value (525) that comes closest to the 85% profile of that section being 521 (mean plus one standard deviation). These locations are then selected as being the representative locations for this section. Cores are taken at those locations to obtain accurate information on the thickness of the layers. This information is needed to allow accurate back calculations of the layer moduli to be made. Figure 13: Frequency distribution of the measured SCI values.
26 6. Back calculation of layer moduli: Back calculation of layer moduli is quite often considered as an important step in pavement evaluation. The reason for this is quite simple; the magnitude of the back calculated stiffness modulus quite often reveals whether or not the pavement layer is damaged or not. If e.g. a stiffness modulus of 600 MPa is back calculated for a cement treated layer, this layer should be in a rather deteriorated state because the modulus of a sound cement treated layer is substantially higher. One of the drawbacks of back calculating layer moduli is the fact that accurate information on the thickness of the various layers should be available. We know that the deflections are heavily influenced by the product E.h 3 , which means that a small error in the layer thickness can have a large effect on the magnitude of the back calculated modulus. Although computer programs are available that back calculate the layer moduli automatically when the deflections, the load configuration and the thickness of the different layers is known, back calculation of layer moduli is certainly not as straightforward as it may look like because in many cases the solution is not unique. This implies that some pre-treatment of the data is necessary before the actual back calculation process is started. In the sections hereafter the surface modulus diagram will be discussed first of all. This diagram provides insight in how the overall stiffness of the pavement develops from bottom to top and whether or not weak interlayers are present. After that the actual back calculation process will be discussed. It should be noticed that the procedures described are especially valid for the analysis of FWD measurements. They can however also be used for the analysis of BB and LD measurements provided that the appropriate corrections are applied. These correction procedures will be described in a later section. 6.1 Surface modulus: According to Boussinesq’s theory, the elastic modulus of a homogeneous half space can be calculated from the deflection measured at a given distance following: E = σ . a 2 . (1 - µ 2 ) / d r . r E = 2 . σ . a . (1 - µ 2 ) / d 0 Where: E = elastic modulus, a = radius of loading plate, µ = Poisson’s ratio, σ = contact pressure under loading plate. The question now is whether this formula can be of use in analysing the stiffness development in a pavement. Let us consider therefore figure 14. geophones a b Figure 14: Distribution of the vertical stress in a pavement.
Page 1 and 2: CT 4860 Structural Design of Paveme
Page 3 and 4: 2 Table of contents: Preface 3 1. I
Page 5 and 6: 4 1. Introduction: These lecture no
Page 7 and 8: 6 2. Usage and condition dependent
Page 9 and 10: 8 When a swimmer makes a nice dive
Page 11 and 12: 10 Figure 5: Principle of the autom
Page 13 and 14: 12 From the text given above it is
Page 15 and 16: 14 As shown in figure 7, 3 zones ca
Page 17 and 18: 16 Figure 9: Placement of loading p
Page 19 and 20: 18 5. Statistical treatment of raw
Page 21 and 22: 20 Figure 10: Results of a deflecti
Page 23 and 24: 22 Figure 12: Cumulative sum of the
Page 25: 24 Section Locations Mean SCI SD SC
Page 29 and 30: 28 The drawn line indicates a pavem
Page 31 and 32: 30 The clayey subgrade was covered
Page 33 and 34: 32 Section I Temp Force Layer E-mod
Page 35 and 36: 34 [N] [mm] [mm] [MPa] [N] [mm] [mm
Page 37 and 38: 36 Based on these calculations, eva
Page 39 and 40: 38 8. Estimation of the remaining p
Page 41 and 42: 40 Figure 25: Load configuration us
Page 43 and 44: 42 If we assume e.g. that the defle
Page 45 and 46: 44 Figure 27: Relationship between
Page 47 and 48: 46 resistance, some parts of the pa
Page 49 and 50: 48 Log ε = 0.481 + 0.881 log SCI 3
Page 51 and 52: 50 b 3 = 1.0153 b 4 = -7.73 x 10 -6
Page 53 and 54: 52 10. Extension of the simplified
Page 55 and 56: 54 Compressive vertical strain at t
Page 57 and 58: 56 This relationship opens possibil
Page 59 and 60: 58 The remaining pavement life can
Page 61 and 62: 60 estimation of the bitumen and mi
Page 63 and 64: 62 the composition and the degree o
Page 65 and 66: 64 The life of the overlay N can si
Page 67 and 68: 66 Table 7b: Relationship between K
Page 69 and 70: 68 Figure 41: Relationship between
Page 71 and 72: 70 From the nature of the equation
Page 73 and 74: 72 σ σ t Γ ε Figure 43: Strengt
Page 75 and 76: 74 14. Effects of pavement roughnes
Page 77:
76 16. Bekker, P.C.F. Pavement perf
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