Bridge - Coring

Target of investigations

Coring with an increment borer is considered the most dependable technique for investigating internal decay and estimating the age of a tree.(1,2) This approach results in small diameter (i.e., 0.169 inch) but relatively long (i.e., 10 inch) cores.

The cylindrical core sample is another type of core not discussed in this document which has a larger diameter but shorter length (approximately 2 inches). Cores are typically used to determine moisture content, specific gravity, and type of species and grade when performing a bridge load rating analysis. Cores are extracted using a portable drill and a specialized drill bit. Treated wood plugs are then glued into the holes after the core is removed.


Through examining a solid sample from a questionable member, coring enables identification of the presence of voids, the depth of preservative penetration and retention, and the type of wood species.

Physical Principle

Coring involves the visual inspection of a solid sample from a suspect member to investigate the presence of defects, depth of decay, depth of preservative penetration, or type of wood species

Data Acquisition

The incremental borer is a precision instrument consisting of three parts: handle, auger, and extractor (figure 1). An operator uses the handle to apply torque to the auger, which is threaded at its tip for cutting into the wood. After cutting, the core sample is handled by the extractor. The borers are available in various lengths between 4 inches (100 mm) and 28 inches (700 mm), in 2‑inch (50 mm) increments.(3) The borers are also available in two-thread and three-thread designs. A two-thread design cuts at a slower speed and, thus, it is more appropriate for cutting into hardwood. On the other hand, a three-thread design cuts at a faster rate and is more appropriate for softwood. The common internal diameters of the borer (which determines the width of the core) are 0.169 inch (4.35 mm) and 0.2 inch (5.15 mm), although 0.32 inch (8 mm), 0.4 inch (10 mm), and 0.48 inch (12 mm) sizes are also available. The larger diameter borers require more physical effort to cut into the wood.(3)

For coring, place the incremental borer at the core location, and hold the shaft of the auger with one hand while putting the other hand at the center of the handle.(3) Apply enough pressure for the auger tip to penetrate the wood member and rotate the handle. Continue rotating the handle until the auger threads are fully engaged. The hammer should not be used to force the borer to enter the wood.(3)

Boring can be accomplished using both hands and at half-turn (better for hardwood) or full-turn (better for softwood) rotations. At the start of boring, keep the handle as steady as possible without wobbling to the sides to obtain a straight core.(3)

When the desired core length is achieved, insert the extractor with a flash and move all the way into the auger. Turn the handle counterclockwise for one or two full turns to separate the core from the rest of the wood, and then pull the extractor. The core can be stored in a tube, such as a paper or plastic straw, where one end is closed, and straws should be labeled with a pen or marker.

After coring, the borer may be removed from the wood by turning the handle counterclockwise. If the auger is not engaged with the wood, the handle should be pulled and turned to remove the incremental borer.

Boring tools should be cleaned after extracting each core to avoid transferring microorganisms.(1) The bored-out holes should be filled with wood plugs treated with preservatives such as copper naphthalene.(2) When stored, the borer components should be lubricated to prevent the corrosion of components.(3)

Source: U.S. Department of Agriculture, Forest Service
Figure 1. Photo. Boring a core from a timber pile.(2)

Data Processing

No data processing is required

Data Interpretation

An inspector performs a visual inspection of cores to confirm the presence of decay, depth of preservative penetration, and type of wood species.


Some advantages to coring are as follows:

  • Coring is low cost.
  • Coring provides solid samples from the subsurface of wood members.


Limitations with coring include the following:

  • Coring can be time-consuming and labor-intensive.
  • Coring does not provide quantitative information about the physical and mechanical properties of wood members unless mechanical testing is performed on the core samples.
  • Training is required for coring.


  1. Ryan, T. W., J. E. Mann, Z. M. Chill, and B. T. Ott. 2012. FHWA Bridge Inspector’s Reference Manual (BIRM). Publication No. FHWA NHI 12-049. Washington, DC: Federal Highway Administration., last accessed January 28, 2022.
  2. White, R. H., and R. J. Ross, eds. 2014. Wood and Timber Condition Assessment Manual2nd ed. General Technical Report No. FPL-GTR-234. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.
  3. Grissino-Mayer, H. D. 2003. “A Manual and Tutorial for the Proper Use of an Increment Borer.” Tree-Ring Research 59, no. 2: 63–79.