Page 128 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

The design process begins with determining the inputs, which fall into these categories:
• Pavement structure
• Concrete mix properties and strength
• Loading: number of axles and the axle load spectra
• Climate: temperature, rainfall and humidity

In the cncPAVE implementation, many of the input parameters are input as distributions, to account for variability
and to enable Monte Carlo type statistical analyses.

Suitable design models were developed from finite element and multilayer evaluations. These models are used to
determine:
• Shrinkage, which affects crack width as well as curling of the slab
• Effective subbase support by combining all support layers into one layer
• Erosion characteristics of the subbase
• Load transfer across joints from crack width, aggregate size and dowels defined in terms of relative vertical
movement at joints/cracks under moving loads
• Development of voids under the slabs
• Structural capacity for the following three mechanisms:
− Shattered slabs
− Faulting
− Pumping

The models were calibrated by performance data from different concrete paved sections, including roads, streets and
hardstandings, some of which were accurately instrumented.

The damage is determined as the ratio of the number of axles n, and the structural capacity, N, for each distress
mechanism. The number of instances in the statistical simulation, where n>N determines:
• Percentage shattered concrete surface
• Percentage pumping concrete surface
• Faulting in the concrete pavement (in the case of plain and dowel concrete)

The performance of the concrete pavement is determined by predicting the change in IRI with time and loading.

The final step is to calculate the life cycle costs to enable selection of an appropriate and cost-effective pavement
design. The decision criteria for plain, dowel and continuously reinforced concrete are shown in Table 52.

Table 52. Typical Decision Criteria for Plain and Dowel Jointed and Continuously Reinforced
Concrete
Plain and Dowel Jointed Concrete
Decision Variable Good Acceptable Excessive
% shattered concrete < 2% 2% to 5% > 5%
% pumping < 2% 2% to 5% > 5%
% faulting < 2% 2% to 5% > 5%
Continuously Reinforced Concrete
Decision Variable Good Acceptable Excessive
% shattered concrete < 0.2% 0.2% to 0.8% > 0.8%
From % pumping < 2% 2% to 5% > 5%
1.0 m to 1.5 m Below 1.0 m or more than
Crack spacing From 1.5 to 2.0 m
2.0 m to 2.5 m 2.5 m

Section 7: Structural Capacity Estimation: Concrete Pavements
Page 117