Shock-absorbing aggregates beneath playground equipment: grain properties and moisture content

Objectives To assess the influence of grain size distribution and moisture condition on aggregates’ performance as an impact attenuation layer beneath playground equipment.Setting and methods Impact attenuation of sands and gravels was tested using a guided headform with a uniaxial accelerometer inside. The result for impact attenuation was the acceleration value of the headform measured from four different drop heights and the Head Injury Criterion (HIC) calculated from it. The acceptable HIC value of a shock-absorbing layer is <1000, that is, less than the critical fall height. Tests were conducted with both dry and wet materials.Results The impact attenuation of a shock-absorbing layer made of loose aggregate is determined mainly by the following material factors: maximum grain size, median grain size, uniformity coefficient, fines content, grain shape and moisture content. In addition, the moisture content of aggregate, especially sands, has a major impact on its impact attenuation capacity. Of the studied sands, coarse and gravelly ones had the best impact attenuation properties. Most of the examined sands were uniformly graded. None of the grain properties of gravels proved more significant than the others. Yet, open-graded gravel (dominant grain size 4–8 mm) containing a small amount (about10%) of sand fraction 0.06–0.6 mm, but with hardly any coarse sand (0.6–2 mm), was found to have good impact attenuation.Conclusions This study shows that the aggregates for shock-absorbing layer should always be tested wet in laboratory conditions corresponding the most critical situation. Standards should include a procedure for wet testing of aggregates.