4 Factors Affecting The Surface Deflection Of Pavements
Deflection testing is a critical tool used in evaluating the structural adequacy of pavements and their ability to withstand traffic loads. It involves measuring surface deformations caused by applied loads, providing valuable insights into the condition of the pavement layers, subgrade, and their response to environmental factors. By analysing deflection measurements, engineers can estimate remaining pavement life, assess load-carrying capacity, and make informed design decisions. Deflection testing also helps to identify areas that may need further investigation, as well as making decisions on load restrictions, and monitoring pavement quality during construction.
But here’s where things can get a little complicated. There are several factors that influence the magnitude of these measured deflections which can make the interpretation of the results more complex. These factors include:
1. Pavement structure
2. Pavement loading
3. Climate
4. Testing seasons
Understanding these factors and considering them prior to commencing deflection testing is essential for obtaining meaningful and representative deflection data. Let’s discuss each of these factors in further detail.
Pavement Structure
The deflection of a pavement is influenced by its surface, base, subbase layers, and subgrade. Factors such as the thickness and stiffness of these layers affect the measured deflections. Weaker systems tend to deflect more than stronger ones under the same load, and the deflection shape depends on the stiffness of each layer. Other pavement-related factors that can impact deflections include testing near joints, edges, or cracks, variations in pavement layer thickness, and differences in subgrade properties.
Structure of flexible pavement
2. Pavement Loading
The magnitude of the applied load is a significant factor in pavement deflections. Modern deflection equipment can apply loads ranging from 13 kN to 245 kN, and it's important to choose appropriate load levels for specific applications. The type of loading, whether slow and static or rapid and dynamic, also affects deflections. Generally, more rapid loading with shorter load pulses leads to smaller deflections.
Falling Weight Deflectometer (FWD) conducting structure testing.
3. Climate
Temperature plays a crucial role in pavement deflection testing. In asphalt pavements, the stiffness of the asphalt layer decreases as temperature rises, resulting in larger deflections. To interpret the data accurately, deflection measurements need to be corrected to a standard temperature, typically 21 °C (70 °F). Temperature variations also impact Portland Cement Concrete (PCC) pavements, causing the slab to curl upward or downward depending on temperature differences between the top and bottom. Conducting deflection tests when the slab is curled downwards or upwards can lead to unsupported conditions and potentially higher deflections. Additionally, temperature affects joint and crack behaviour in PCC pavements, with warmer temperatures causing expansion and potential joint locking. Testing at locked joints may show lower deflections and higher load transfer efficiencies, which can be misleading regarding the overall load transfer capabilities of the joint.
4. Testing Season
Seasonal variations in temperature and moisture conditions also influence pavement deflection response. Deflections are often greatest in the spring due to saturated conditions and reduced pavement support, while they are lowest in the winter when the underlying layers and subgrade are frozen. Seasonal changes have less of an impact on PCC pavements in support conditions.
Our recommendations
When conducting structural testing, we recommend deploying either the FWD or HWD. The FWD is designed for network and project level assessment of roads subject to daily vehicular and commercial transport. The FWD is normally used for local government network and project level assessment, while the HWD is used for rigid pavements, airports and port authorities. Before commencing any pavement structural testing, please consult with our pavement engineers. Click here to contact us or get a quote.
Conclusion
Deflection testing plays a crucial role in evaluating the structural adequacy of pavements and their ability to withstand traffic loads. By measuring pavement surface deformations caused by applied loads, engineers can gain valuable insights into the condition of the pavement layers and subgrade, as well as their response to environmental factors. Deflection data allows for estimating remaining pavement life, determining load-carrying capacity, and making informed design decisions. Additionally, deflection testing helps identify areas that require further investigation, establish load restrictions, and monitor pavement quality during construction. With its significant contributions to assessing pavement structural integrity, deflection testing is essential for ensuring the safety and durability of road infrastructure.
References
FHWA. (2017). Chapter 2. Deflection Testing Guidelines - Using Falling Weight Deflectometer Data With Mechanistic-Empirical Design and Analysis, Volume III: Guidelines for Deflection Testing, Analysis, and Interpretation , December 2017 - FHWA-HRT-16-011. [online] Available at: https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/16011/002.cfm#:~:text=The%20major%20factors%20that%20affect [Accessed 7 Jun. 2023].