The Clegg Impact Tester, also known as the Clegg Hammer, is used to measure and control the strength of soils, aggregates, and synthetic materials for sports surfaces and roads. It also verifies uniform compaction over large ground areas and identifies poorly compacted areas.
Sports surfaces, including hard track surfaces and artificial turf surfaces, put athletes at risk for injury. Hard track surfaces are even known to affect equine athletes with fractures or injuries to the bones, limbs, and tendons (Munoz-Nates et al., 2017; Forrester & Tsui, 2014). Mechanical testing devices, such as the Clegg Impact Tester, effectively measure the hardness and strength of various surfaces, which helps prevent potential injuries. The Clegg Impact Tester is also an inexpensive and easy way of determining surface hardness compared to obtaining direct biomechanical measurements of athletes under sports conditions.
The Clegg Impact Tester consists of a 2.25 kg hammer, an accelerometer (mounted on the hammer), and a tablet-based app. The hammer is guided in a tube and dropped to the soil surface from a height of 45 cm. The accelerometer sends the output of the hammer striking the ground (measured in gravities) to a hand-held digital readout unit. The readout device scans and filters the signal before registering the deceleration in Impact Value units (IV). The Impact Value is a measure of the soil’s tensile strength.
A Clegg Impact Test (CIT) comprises a single hammer drop or a series of drops. The CIT can track variations in a material’s strength/stiffness as moisture content, density, and compaction effort change. In general, three to five hammer drops on a specific spot are recommended to get an accurate result. The Clegg Impact Tester’s mass and impact velocity (about 3 m/s) is comparable to the effective mass and vertical velocity of the equine forehoof at impact under training conditions.
The Clegg Impact Tester consists of a 2.25 kg hammer, a tube that guides the hammer, and an accelerometer mounted on the hammer. The guide tube is metal, making it durable.
The soil impact output by the hammer can also be read on a tablet-based app. The accelerometer will send a wireless signal to the app after impact with the ground.
The 2.25 hammer is used for testing sports surfaces. Other hammers are also available for other surfaces, including the 4.5 kg hammer, which is popular for testing road surfaces. Other versions, including a 10 kg and 20 kg hammer, are also available for testing harder materials, such as aggregate roadbeds, trench reinstatement, flexible pavements, and bell holes.
Comparison between hoof impact shock measurements and Clegg Impact Soil Tester
Munoz-Nates et al. (2017) compared measurements between the Clegg Impact Soil Tester and vertical deceleration peak on horses’ hooves impact on thirteen surfaces under various exercise conditions. The study aimed at analyzing the reliability of the Clegg Impact Soil Tester to assess equestrian surfaces. Thirteen different sports surfaces were analyzed – eight racing trotters and five show jumping and dressage horses on five different sites. The sand was the major top layer component in all cases; however, the surfaces for the latter horses differed in granulometry and origin. Moreover, some sites had fiber mixed with the sand. The underlayer also varied across the different surfaces. The thickness or preparation of surfaces of the top layer for the sports horses was analyzed. Biomechanical measurements were retrieved using a piezoelectric accelerometer fixed to the dorsal hoof wall of the subject. The Clegg Impact Tests were performed on the same day as the biomechanical tests and on the same surfaces. The results indicated that the vertical deceleration peak at the hoof’s impact had greater variability than the Clegg Impact Tester. Moreover, inter-individual variability was also large in the biochemical tests than in the Clegg Impact Tests.
Analysis of surface hardness of a third-generation artificial turf pitch
Forrester and Tsui (2014) analyzed spatial and temporal variations of a five-year-old third-generation (3G) artificial turf pitch over a year. A 2.25 kg hammer Clegg Impact Tester was used to measure surface hardness over ninety-one locations of the turf pitch over thirteen months. The peak deceleration of the Clegg hammer’s impact with the surface was expressed in gravities. Harder surfaces were recorded with higher peak decelerations. Five consecutive impacts were performed on each test location. The hardness for the location was determined by taking the average of the last four impacts. The ground temperature and infill depth were also measured in each location using a mechanical gauge meter and a temperature probe. It was observed from the results that spatial variation exceeded temporal variation in surface hardness. Surface hardness variation was moderately associated with shock pad thickness and slightly associated with infill depth in terms of spatial variation (both negative).
The Clegg Impact Tester can test a wide range of natural grass and synthetic turf fields, as well as all sports surfaces, infill materials, pads, and amendments, for safety and playability purposes. It can also test a wide variety of soil and stone found in flexible pavements and earthworks. Moreover, it can be used to swiftly assess variations during construction and track changes over time due to seasonal environmental changes or road traffic, and test natural and constructed conditions. It requires minimal training to be utilized. Furthermore, it provides results rapidly, with each test taking only around 30 minutes.