MTO OVERLAY DESIGN The following presentation contains references to Figures 6.02 and 6.03, Tables 6.01 and 6.05, “Overlay Design Model” and “Overlay Example Problem”, all of which are posted under subsection 2.6 of the course notes on the instructor’s website. Viewer discretion is advised as some scenes contain material of a graphic nature.

TTTThe basic problem is to test the pavement’s strength and compare it to the strength required to carry an estimated number of standard axle load repetitions IIIInstead of coring surveys and guesstimates of strength, its more economical to measure deflections under standard loads as with…  T T T THE BENKELMAN BEAM or  T T T THE DYNAFLECT

Deflection Tests TTTThe Dynaflect is more efficient for mass inventory (many tests) but the original strengthening technology is based on Benkelman Beam Rebounds (BBR’s)  T T T The Dynflect Sensor 1, (DYN) readings must first be converted into BBR’s  i i i in Ontario this conversion is made using the following equation:

Statistical Criteria PPPPlotting deflections along the road alignment gives a deflection profile: Deflection Chainage Standard Deviation =

Design Deflection, DD  A A A Assuming the variation in deflections is random and therefore normally distributed, using the mean plus two standard deviations will ensure that the maximum deflection is taken into consideration 95% of the time  E E E Engineers always like to consider the worst case scenario in design  T T T The pavement is at its weakest in the early spring, when the frost has just come out of the ground...

Design Deflection, DD (Continued)  T T T The tests are usually taken s s s some time later, quite often in the autumn when the pavement structure has stabilized  A A A An adjustment is therefore made to reflect the pavement’s strength in the early spring To Summarize: Design Deflection is calculated as the mean plus two standard deviations of the Benkelman Beam Rebounds on a pavement adjusted to spring conditions by factoring by a spring/fall ratio

Maximum Tolerable Deflection, MTD Studies in Ontario (and elsewhere) have correlated the design deflection of a pavement with its ability to carry traffic loads as characterized by the DTN. Say, for example, that it is desired to know whether or not a pavement with a DD of 0.067” will be able to carry another 10 years of traffic as reflected by a DTN of 125. The MTD for a DTN of 125 is ”. Since DD > MTD, this pavement can’t handle this much traffic. If the MTD was 0.067” then how much traffic could it carry? It could carry a DTN of DD = 0.067” DTN = 125DTN = 48.9

For the portion of this relationship for DTN > 10, the following equation can be used instead of the graph:

Required Strengthening  T T T Through extensive research, the Asphalt Institute and MTO have developed design curves relating Design Deflection to Strengthening Requirements  E E E Each curve is for a different MTD value which range from 0.02” up to 0.10”  T T T The Strengthening is in terms of inches of GBE (new Granular A)

Interpolation  W W W What happens when the MTD is between the curves? EEEExample: DD = 0.045” & MTD = 0.033” T 0.03” = 6.913” T 0.04” = 2.026” M T D = ” DD = 0.045” T 0.033” = 5.447”

Overlay Thickness Equations MTDab ab Each curve on these graphs has an equation with two coefficients, a and b in the form… WARNING Only the MTD values tabulated can be used in this equation!!!!!!

OVERLAY EXAMPLE PROBLEM The following Dynaflect Sensor 1 readings were recorded along a 4-lane section of highway in October. If the present AADT is vpd with 20% heavy vehicles and an annual growth rate of 2.0%/year, design an asphalt concrete overlay for a design period of 12 years if the subgrade in the area is a light clay. STA Sensor BBR First step is to convert Dynaflect Sensor 1 readings to Benkelman Beam rebounds: Next calculate mean and standard deviation: Then calculate the mean plus two standard deviations: Finally, multiply by spring/fall ratio to find Design Deflection: For a light clay, S/F = 1.8 according to Table Therefore, DD = ”

OVERLAY EXAMPLE (Continued) Now to find the DTN from the traffic data given: Ti = 0.20; Tf = 0.20 Since AADTi =16,000, LDFi = 0.75 Since AADTf =20,292, LDFf = TFi = 0.90TFf = 0.96 and finally the MTD… Then find the DTN:

OVERLAY EXAMPLE (Continued) MTDab ab Since MTD is between the 0.02” and 0.03” curves, evaluate T0.02” and T0.03” and interpolate For T 0.02” : For T0.03”: a = b = a = b = The proportion that ” divides the interval from 0.02” to 0.03”: The interpolated thickness of extra GBE required: And finally,the overlay thickness in millimetres of new AC: