The M-E Design Guide software starts by characterizing the climatic effects of the location, including daily and annual temperature cycles, precipitation, etc. The layer types, thicknesses and properties are entered and in conjunction with the proposed traffic (including the axle load distributions), the stresses and strains (material responses and damage) are calculated over a short time period. The damage is summed up over the time period and the quantity of distress accumulated over the time period is reported. If the distress is more than the amount allowed by the pavement designer, the layer types, thicknesses or properties are changed.
(College Station)—When the Texas Transportation Institute (TTI) completed work on the review of a revised mechanistic-empirical (M-E) pavement design guide (see figure), a new question emerged: What will it take to implement the new procedure in Texas?
The Texas Department of Transportation (TxDOT) had the same question and enlisted the help of TTI in a project aimed at successfully finding a means to implement the M-E Design Guide in its districts.
"The M-E Design Guide represents a substantial leap forward in the way we analyze pavement performance," says Tom Freeman, engineering research associate at TTI. "But there are also many more inputs versus the old method."
The M-E Design Guide is capable of evaluating pavement designs for both rigid and flexible pavements. It also accounts for local environmental conditions, local highway materials, and highway traffic distribution and payload.
"We analyzed existing data and developed trends on how pavement performed and the attributes of these pavements, such as thickness and stiffness of layers. This design program is based on actual in-service pavements, and the inputs we enter will tell us how the pavement is expected to perform under certain conditions," says Freeman.
With these improvements, the Design Guide is more complex to use compared to the previous design method, which required only six or seven inputs. The revised procedure has room for approximately 150 inputs. The challenge is identifying which inputs are important for applications in Texas and how to gather that information.
To help find a solution, TTI researchers developed two case studies in varying climatic regions within Texas. Because the Design Guide was developed for national use, it was important for researchers to note which inputs were necessary for designing pavement on Texas roadways.
"This new procedure allows TxDOT to account for the effects of various materials in the design process as well the environmental effects over time," says Gregory Cleveland, the project director and technical operations manager at TxDOT.
The implementation recommendations from TTI involve advanced training for TxDOT engineers. The trained engineers can then calibrate the program for their area in conjunction with the current design method.
"With this program, we now have the capability of analyzing various combinations of materials for a specific project to achieve the best pavement performance which in turn will translate into fewer maintenance costs down the road," says Cleveland.
