Dispelling Creep Confusion

“The power to question is the basis for all human progress.”

- Indira Gandhi

Written by: Brian Majeska, Joseph Drbohlav, Joseph Lorenc, Paul Noring, Geoff Rowe, PhD

Introduction

In the concrete and asphalt paving industry, the real problems and cost effective solutions are often hidden in what is being developed in material testing.? Let’s turn our attention to Creep.? And, no we are not talking about a childhood trouble maker who repeatedly or constantly made fun of someone.? Although the constant or repetition may be a tie into the consequences of stress loads over time for people.? An example of Creep would be the repeated loading and unload that occurs on a ports transfer slabs.? No one container being loaded onto the pavement is the cause of the concrete cracking or the asphalt rutting or fatigue cracking; it is the number of times this occurs.? In roads, we will notice more extensive fatigue related cracking in turn lanes from a collector to an arterial, this is due to increasing load and higher concentrations of load in those areas versus the ongoing lanes.

Understanding Creep:

Creep:? Creep is the slow continuous deformation that According to Wikipedia, “…Creep (Sometimes called Cold Flow) is the tendency of? a solid material to undergo slow deformation while subject to persistent mechanical stresses….? Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress.? Instead strain accumulates as a result of long-term stress.”? [Recall stress is a load that an object can manage whereas strain is a force load beyond the ability of the objects ability to withstand.].? ?

Consider Creep to be the consequences of sustained load stress on a concrete slab over time - creep impacts, soils, concrete, asphalt, and steel [basically everything in a road].

Types of Creep and Force Considerations:

• Time-Dependent: Creep is a gradual form of stress that leads to material strain or failure.? The manifestation of creep in pavements typically takes place over years.
• Material Specific: Soils, asphalt, concrete, and steel all experience creep, but the forces/mechanisms or rates are different for each material
• Temperature Sensitivity:? As described, the creep can also be called cold flow, creep becomes more pronounced as materials approach (or surpass) their “melt points.”? This is significantly more of a concern for asphalt cement, and may be one of the reasons ring and ball softening points were used in asphalt modification prior to the advent of Performance Grade (PG) testing methods.

Creep is a method of describing the consequences of prolonged force on a material.? For example a slow moving heavily loaded fork lift on the same pavement segment over the span of years.? To provide context of forces and creep, consider:

• Tensile Stress - this is when materials are place into a “tug of war” or stretching force.? Tensile Creep example:? a metal cabler that is under constant stress will elongate over time.
• Compressive Stress - This is when a material is being “squeezed” or compressed by a force.? Compressive Creep example: concrete columns? under constant compressive loads can shorten over time.
• Shear Stress - this occurs when a force is applied on a parallel plane in the opposite direction where the material slides past on each plane.? Shear creep example:? a truck only lane ruts over time due to the load associated with the trucks relative to the stability of the asphalt pavement.

Creep Made Simple:? Creep is the consequence of repetitive forces (compressive, shear, and tensile forces) on a pavement over years which results in cracking and rutting.

Considerations of Creep in Asphalt Pavements

In 2010 a Creep test method was accepted by AASHTO (American Associate of State and Highway Officials) for asphalt binder testing.? This test is the Multiple Stress Creep Recovery Test (MSCR). ?

This test method tests four elements of an asphalt binder:

1. Applies repeated loads onto an asphalt binder to develop the repetitions necessary to introduce stress and ultimately strain onto the binder to Crete deformation which is creep.
2. Measures the non-recoverable performance of the binder during each loading and loading (designated Non-recoverable Creep Compliance ~ Jnr).
3. Determines likelihood of rutting over the life of the asphalt binder in the mixture.? This defines the long-term stiffness performance of the binder.? This characteristic can be validated with increasing temperature of the binder.
4. Traffic simulation through repeated loading and unloading of the binder.

Types of Distresses in Asphalt Pavements

Rutting:? is the wheel path depressions that occur over time due to higher load relative to the pavement design. This uneven pavement is a significant safety risk during rain or snow events.

Asphalt Shoving or Flow: This is the lateral flow of the pavement in high temperature and slow traffic movement.? Often occurs at intersections and heavy load terminals.? Thinning of the pavement with larger bumps in other areas occur.

Fatigue Cracking:? As the pavement ages and the impact of creep accumulates, alligator cracking or longitudinal cracking in our around the wheel path area will appear. This cracks allow the intrusion of water into the pavement.

Types of Distresses in Concrete Pavements

Increased Deflection: Creep in concrete can cause increasing deflection of pavement slabs over time.? This will lead to loss of pavement structure due to the loading and unloading of the slab.

Joint Deterioration:? Increasing stresses over time stresses the joints.? This will result in joint spalling which then allows the flow of water into the pavement and compromises the pavements ability to smoothly carry traffic.

Reduced Load-Bearing Capacity:? Over time, the concrete pavement losses its ability to carry the traffic load.? This lets to premature failure of the pavement.

Conclusion: “The power to question is the basis for progress.”

Asking the simple question of how does Creep apply to pavements?? Creep is the consequence of repetitive forces (compressive, shear, and tensile forces) on a pavement over years which results in cracking and rutting.?

This understanding of Creep is an important consideration for maintaining a pavement.? Specifically, upon the development of joint or crack distresses in the pavement, it is vital to have options that have strong adherence and cohesion to “weld” and waterproof the pavement. .

Acknowledgements & Sources

The authors extend gratitude to:

• Ryan Brown, PE
• Dan Wegman, PE
• Concrete Pavement Preservation Guide (Second Addition) - FHWA Publication No. FHWA-HIF-14-014
• Rheological Performance of Non-Asphalt Crack Sealing Materials for Concrete Pavements - Geoffrey Rowe, et al.
• Tech Brief - Joint Sealing FHWA-HIF-18-019
• Chat GPT 4.0

#PublicWorks #Concrete #Asphalt #PavementCracks #PavementPreservation

Connect with us for more insights on pavement engineering and infrastructure development. Let’s build stronger, more resilient roadways together!