Benkelman beam

The Benkelman beam is the simplest and the oldest deflection test device, developed in the United States in the mid-1950s, that has been extensively used worldwide for many years because of its simplicity in taking measurements and its lowest purchasing cost. Its use today is limited only to the project-level structural evaluation of the pavement. 

The device is composed of an aluminium beam approximately 3.60 m in total length, having a 2.44 or 2.5 m long probe. The cross section of the probe is such to fit in the space between the dual tyres. The device is placed behind a truck between the dual tyres of the 

Type of deflection measuring device Type of equipment Type of loading Typical speed at testing (km/h) 

Semi-static (moving LaCroix deflectograph and Rolling wheel 2.5 or 3.5 

Moving (moving Rolling dynamic deflectometer (RDD) Vibrating mass 5 

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Benkelman beam rebound deflection measurements have been obtained during each of these testing phases. The SDHPT Dynaflect Unit has been used to obtain dynamic stiffness measurements. These results also indicate that the sulfur-asphalt mixes and the asphalt mixes are displaying comparable characteristics. 

The back-calculation module used to compute layers elastic modulus requires data obtained from a Benkelman beam, a Lacroix deflectograph, or a FWD. 

The deflection procedures for asphalt overlay design are based on pavement deflection data collected by dynamic, static or vibratory non-destructive testing (NDT) devices. Examples of such devices are as follows Benkelman beam, deflectograph, falling weight deflectometer (FWD), Dynaflect or Road Rater for details of the testing devices, see Section 16.4. 

Two of the most known deflection procedures requiring the use of the Benkelman beam or the deflectograph will be described below. 

Note JRC, jointed concrete pavements CRCP, continuously reinforced concrete pavement Benkel.. Benkelman beam Dynafl., Dynaflect RR4 S,Road Rater 400 and S10 measurements were made at 4.5 kN and 8-10 Hz RR 2000. Road Rater 2000 measurements were made at 36 kN and IS Hz FWD, falling weight deflectometer measurements were made at 40 kN. 

Figure 16.39 Diagrammatic representation of the principle of the Benkelman beam. (Adapted from Norman, P.J. et al.. Pavement Deflection Measurements and Their Application to Structural Maintenance and Overlay Design, TRRL Report LR 571. Transport and Road Research Laboratory. Crowthorne, UK Transport Research Laboratory, 1973.)...

Figure 16.40 Benkelman beam device. (Courtesy of Controls Sri.)...

The aim of Benkelman beam testing, like any other deflection testing procedure, is to estimate the integrity, or remaining life, of the pavement and then to determine the thickness of the overlay, if required. This is obtained by correlating measured deflection with remaining life, expressed as number of standard loads. 

Similar to the Asphalt Institute MS-17 procedure for temperature adjustment of deflection measured by Benkelman beam and estimation of remaining life is the UK procedure. Details can be found in TRRL Report LR 571 (Norman et al. 1973). 

The Benkelman beam measurements can also be used to determine the adjusted structural number (SNP), a useful parameter for estimating pavement deterioration in pavement management studies, used in WDM-4. The SNP is basically the structural number (SN) used by AASHTO as a measure of pavement strength (AASHTO 1993) but adjusted for the reduced contribution of each layer with depth (Morosiuk et al. 2004). 

Figure 16.41 Benkelman beam deflection adjustment factors owing to temperature variation for full-depth and three-layered asphalt concrete pavement. (From Asphalt Institute 3rd Edition, Asphak Overlays for Highway and Street Rehabilitation. Manual Series No. 17 [MS-17]. Lexington, US Asphalt Institute. With permission.)...

Figure 16.43 Relationship of Benkelman beam deflection and SNP. (From Morosiuk, G. et al.. Modelling Road Deterioration and Works Effects, Version 2, HDM-4. The Highway Development and Management Series, Vol. 6. Paris World Road Association, 2004.)...

Correlation between Benkelman beam and other devices... 

Numerous comparisons have been made between Benkelman beam and other deflection devices in order to develop correlation equations. This offers the possibility to determine the overlay thickness required by following the Asphalt Institute procedure based on the RRD, outlined above. 

The approximate correlation equations proposed by Asphalt Institute MS-17 (Asphalt Institute 3rd Edition) between Benkelman beam deflection and other deflection devices such as Dynaflect, Road Rater and falling weight deflectometer (FWD) are listed below. A short description of the above deflection devices is given in the following paragraphs. 

Dynaflect and Road Rater are vibrating load devices developed in the United States in the 1970s. They measure dynamic deflection while they are stationary, using a sinusoidal force. Unlike the Benkelman beam and semi-static devices, they are also capable of measuring the deflection basin since deflection is recorded by velocity transducers (usually five). Both devices comply with ASTM D 4695 (2008) and ASTM D 4602 (2008). 

The determination of the overlay thickness is usually performed by analytical methods similar to those applied in the analytical pavement design methodologies. However, by converting deflection measurements obtained by FWD to Benkelman beam, or to deflectograph deflection, the established methodologies described in Section 15.17, may also be used. 

The deflectograph is a two-axle truck carrying a beam assembly and an associated recording system between its axles. Deflection measurements are based on the same principle as in Benkelman beam measurements. The deflectograph provides a more rapid method of measuring pavement deflection than the Benkelman beam since setting arrangements are much faster. 

The rear axle is loaded to a standard weight similar to the weight used in the Benkelman beam test. The maximum deflection is recorded by electrical transducers located near the beam pivots. When measurements are taken, the beam assembly is pulled forward at approximately twice the speed of the vehicle, to a new position ready for the next measurement. The working speed of the deflectograph is approximately 2 km/h (walking speed) and measurements are taken every 3.8 to 4.0 m. A deflectograph vehicle is shown Figure 16.50. 

Correlations have been established between curviameter results and FWD, Benkelman beam or deflectograph results. Relevant details for the developed correlation equations over the last years can be found in Van Geem (2010). 

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