Download PDF version (226.9k) The full article is available as a free PDF document.
Producers have measured concrete's surface hardness with rebound hammers, also called Swiss or Schmidt hammers, since the 1950s. When producers follow ASTM C 805-94, "Standard Test Method for Rebound Number of Hardened Concrete," they can use rebound hammers to determine strength-gain uniformity in a structure or compare one concrete with another. Monitoring uniformity is also important when manufacturing concrete products. While simple to use, manual rebound hammers have not been widely used because interpretation of test data can be difficult, and record keeping is tedious. New digital and automated rebound hammers ease the reading of test results and reduce field paperwork. Impact hammers, both manual and automatic, are rated by the energy levels at which internal springs impel the hammer mass toward the surface. In theory, the harder the surface, the greater the hammer mass' rebound distance. Matching energy level to the surface type is important. Hammer manufacturers recommend that concrete producers select hammers that have energy levels of 1.6 foot-pounds (225 kg). Hammers designed for geological exploration use higher impact energies, while hammers with lower impact energies are used in the paper industry. Automated rebound hammers operate the same way as manual hammers. Both styles of impact hammers are portable, weigh only about 3 to 5 pounds and require no external power source, so they are well suited to field testing. The main difference between manual and automated impact hammers is how they display the rebound numbers. Both hammer styles measure the hammer mass' rebound distance and report it with an arbitrary "rebound number." Using a manual unit, the technician determines the rebound number by judging where the sliding indicator has landed on a scale marked from 10 to 100 on the device's side. Documenting test results can become tedious. For example, to develop a test area's rebound number under ASTM C 805, the technician must conduct at least 10 separate readings in a 6-inch-diameter area. On large structures where readings must be taken at many test areas, a technician might have to record hundreds of numbers. Automated hammers replace the mechanical indicator and scale with electronics. Basic automated units are equipped with easy-to-read LCD displays that reduce the chance of readout error. Manufacturers also offer a way to prevent errors in transcribing test results. Hand-held automated units are available with built-in recorders that inscribe impact results on pressure-sensitive chart paper. The printout displays rebound numbers, average rebound values, impact angles and bar graphs of the test results. Some manufacturers offer advanced automated impact hammer systems that are cable-connected to small computers. The dedicated software allows the user to create separate data files by downloading field sample results. The software then calculates an average rebound number for each test sample. Some software will also correct for impact direction (horizontal vs. vertical). Some software provides methods for the technician to graph the relationship between rebound numbers and compressive strength samples. Manual impact hammers may cost less than $700. Automated hammers cost about $100 more than manual units. Automated hammers with printout capability cost $900 to $1,000. The article includes the proper way to conduct a rebound test. KEYWORDS: rebound impact hammer, Swiss hammer