Benkelman Beam A Simple Tool to Understand Road Strength for road and highways

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Roads appear to be strong and fixed, but in fact, they behave like a flexible system. When heavy vehicles pass over a road, the road surface deflects slightly and then returns to its original position. This deflection is not noticeable to the naked eye, but it is very important in determining the lifespan of a road. To determine this deflection, engineers employ a simple yet effective tool called the Benkelman Beam.
The Benkelman Beam has been employed for many years in pavement engineering, particularly in the assessment of flexible (bituminous) roads. Even with modern electronic instruments available today, this tool is still valuable because of its simplicity, accuracy, and low cost.

What is a Benkelman Beam?​

A Benkelman Beam is a mechanical tool used to determine the deflection of a road surface under a standard wheel load. In simpler terms, it is a tool used to determine how much a road surface deflects and returns to its original position when a heavy vehicle passes over it.

The tool comprises:

• A long, thin steel beam
• A pivot system
• A probe (or sensing tip) that touches the road surface
• A dial gauge to measure the deflection

The Benkelman Beam operates on the basis of leverage, which enables the measurement of very small deflections of the pavement. The deflection measured by the Benkelman Beam gives engineers an indication of the structural status of the existing road.

Why Pavement Deflection Matters​

Each time a truck passes over a road, the pavement deflects under the weight of the truck. When the deflection becomes too high, it indicates that the road's layers are not strong enough and will not last long. High deflection can result in:

• Cracks
• Rutting
• Potholes
• Road failure

Engineers use deflection measurements to determine:

• If the road is still strong enough
• If it needs to be maintained
• How thick an overlay needs to be applied

This is where the Benkelman Beam is applicable.

Working Principle of the Benkelman Beam​

The Benkelman Beam is quite simple to understand and operate.

Step-by-step working of Benkelman Beam​

1. Positioning the Vehicle​

A loaded truck of standard weight (with a standard axle load) is placed on the road at the testing site. The rear wheel load is used to apply pressure on the road.

2. Placing the Beam​

The probe of the Benkelman Beam is positioned between the dual rear wheels, directly below the point of expected deflection. The beam is placed properly.

3. Initial Reading​

The initial reading of the dial gauge is taken when the truck is stationary.

4. Truck Movement​

The truck is moved forward slowly by a predetermined distance (usually 2.7 m).

5. Rebound Measurement​

When the truck moves ahead, the pavement rebounds. This rebound is magnified by the beam and measured by the dial gauge.

6. Deflection Calculation​

The rebound deflection is adjusted for temperature and seasonal variations to get the actual deflection of the pavement.
This rebound deflection reading is a measure of the strength of the pavement.

Applications of the Benkelman Beam​

The Benkelman Beam is primarily employed in the field of highway engineering, specifically in flexible pavement.
The major applications of the Benkelman Beam are:

1. Assessment of existing highways
2. Structural analysis of bituminous roads
3. Design of pavement overlays
4. Maintenance and rehabilitation planning
5. Pavement performance analysis
6. Information for IRC overlay design procedures

It is widely employed by road agencies and consulting firms, especially in developing nations, owing to its low cost and simplicity.

Advantages of the Benkelman Beam​

The Benkelman Beam has several advantages:

1. Simple Design​

It lacks complex electronics, which makes it simple to comprehend and employ.

2. Low Cost​

It is significantly less expensive compared to sophisticated equipment such as Falling Weight Deflectometers (FWD).

3. Reliable Results​

It yields accurate deflection measurements if employed properly.

4. Easy Maintenance​

Being a mechanical device, it requires little maintenance.

5. Field-Friendly​

It can be employed in rural areas where sophisticated equipment is not accessible.

Limitations of the Benkelman Beam​

Though the Benkelman Beam is very helpful, it has some drawbacks:

1. Slow Data Collection​

The process involves point-to-point data collection, which is quite time-consuming.

2. Traffic Disruption​

The test involves halting and moving a truck, which leads to traffic congestion.

3. Manual Errors​

The results are operator-dependent and require experience.

4. Limited Coverage​

It only measures deflection at certain points, not continuously.

5. Less Advanced​

It cannot generate dynamic traffic loads like modern devices.

Due to these disadvantages, it is now often used in conjunction with modern testing equipment instead of being used alone.

Case Study: Strength Evaluation of a District Highway​

A 10-year-old bituminous road, which was constructed to connect two towns, started developing surface cracks and slight rutting. The traffic on the road was dominated by heavy trucks carrying goods. The need was to determine whether the road needed simple resurfacing or strengthening.

Testing Method​

Benkelman Beam tests were performed at regular intervals on the road using a standard loaded truck. Deflection measurements were taken and adjusted for road pavement temperature.

Observations​

• Low deflection areas indicated good structural condition
• Areas around intersections and truck parking areas indicated high deflection
• Average deflection exceeded permissible limits in some areas

Decision​

From the Benkelman Beam test results:

• Thin resurfacing was proposed for low-deflection areas
• A thicker bituminous overlay was planned for high-deflection areas

Outcome​

The specific strengthening work improved the road's performance and minimized maintenance expenses. The road continued to function well for several years after the treatment.

Conclusion​

The Benkelman Beam test may appear to be very simple, but it is an extremely effective tool for road performance analysis. The test measures the deflection of the pavement, which is essential for engineers to make informed decisions regarding road maintenance and strengthening. Although new technologies have been developed, the Benkelman Beam test remains a reliable, cost-effective, and efficient tool, particularly for flexible pavement analysis under actual road conditions.