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IP Online First, published on July 28, 2017 as 10.1136/injuryprev-2017-042335 Original article

Shock-absorbing aggregates beneath playground equipment: grain properties and moisture content Tapani Jäniskangas, Kari P. K. Pylkkänen, Pauli Kolisoja Abstract 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 Correspondence to and gravels was tested using a guided headform with Tapani Jäniskangas, a uniaxial accelerometer inside. The result for impact Department of Civil Engineering, attenuation was the acceleration value of the headform Earth and Foundation measured from four different drop heights and the Head Structures, Faculty of Business and Built Environment, Tampere Injury Criterion (HIC) calculated from it. The acceptable University of Technology, PL HIC value of a shock-absorbing layer is 5.6 mm

 23

2.9

4.29

2.29

2.6

0.4

1.8

1%

 24

3.5

5.49

2.43

5.4

1.1

1.9

9%

 25

3.4

6.03

2.83

5.3

0.9

1.7

12%

 26

1.8

7.12

3.12

4.0

0.2

1.7

22%

2.7

4.75

2.98

1.6

0.4

1.9

18%

Fine gravel  27

Grain size 4–8 mm

 28

2.0

3.98

2.07

1.5

0.0

1.7

9%

 29

2.3

7.25

3.52

2.2

0.1

1.7

36%

 30

2.6

7.09

3.89

3.4

0.7

2.0

46%

 31

2.3

7.26

4.69

2.0

0.1

1.9

67%

 32

3.6

7.36

4.76

2.2

0.7

1.9

67%

 33

3.3

7.79

5.65

8.8

2.2

2.2

76%

7.78

4.35

8.2

2.5

1.6

 33  34  34

Dry 3.2

2.4

Dry

49%

1.8

 35

2.6

7.89

5.66

6.9

0.5

1.8

64%

 36

1.9

7.56

5.79

2.0

0.7

1.7

84%

 37

1.7

7.77

5.92

1.5

0.0

1.8

95%

The material was irrigated so that excess water exited through the bottom of the frame. The structure was covered with plastic sheeting to prevent evaporation and tests were conducted the next day. Four materials were also tested as dry. In this study, a guided headform (figure 2) was used to measure the impact attenuation of the shock-absorbing layer. To recognise the influence of the compaction of the material, the headform was Jäniskangas T, et al. Inj Prev 2017;0:1–8. doi:10.1136/injuryprev-2017-042335

dropped three times in a row from the same height onto the same test spot. The layer was not disturbed in any way between drops. The HIC value of the third drop is the measured result for a specified drop height (HIC1 and H1 in figure 2). An HIC value is calculated for each time/acceleration curve according to Formula (1). A series of three drops was repeated from at least three other drop heights (H2, H3, H4 in figure 2) to determine 3

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Original article

Figure 2  Drop test on tested safety sand and a typical HIC value curve versus drop height.7 HIC, Head Injury Criterion.

the critical drop height corresponding to an HIC value of 1000 (Hc in figure 2). Between drop height changes, the test frame was emptied and refilled.

Results Grain properties of tested aggregates

Table 1 presents the moisture content, maximum grain size, median grain size, uniformity coefficient, fines content and some extra comments on grain size for each test material. Table 1 also shows the critical fall heights determined for the tested safety sands and gravels.

Impact of grain size distribution

Uniformity coefficients, Cu, for medium sands were under 4. Their critical fall heights in wet conditions were 1.9 m, 2.0 m and 1.7 m. The amount of grain size 0.125–0.25 mm was the most influential factor concerning the critical fall height of medium sands—as it increased from 11% to 24%, the critical fall height increased 0.3 m. The fines content of the medium sands was 0%–2.0%. The examined medium sands fell for the most part within the grading envelope for safety sand shown in figure 1. Critical fall heights of wet coarse sands were 1.7–2.1 m. Their fines contents were 0%–2.0%. Concerning studied coarse sands the increase in the amount of gravel size grains (>2 mm) increased the critical fall height. Seven coarse sands having 10%–29% gravel size particles had critical fall heights of 1.9–2.1 m. Two very uniformly graded (Cu=1.8 and 2.7) coarse sands containing only 0%–4% gravel size grains had critical fall heights of 1.7 m and 1.8 m. 4

Wet gravelly sands’ critical fall heights varied between 1.9 and 2.1 m with one exception the critical fall height of which was 1.7 m. Compared with the other sands, this gravelly sand contained more grains larger than 5.6 mm (12%). Wet sandy gravels’ critical fall heights ranged from 1.7 m to 1.9 m. The two sandy gravels with a critical fall height of 1.7 m were more coarse grained than the others: one contained 12% grains larger than 5.6 mm and the other 22%. Of the fine gravels tested in wet conditions, the critical fall heights of three very uniformly graded ones (Cu=1.5–2.2) were 1.7 m. The critical fall heights of one very uniformly graded gravel (Cu=1.5) and one mixed graded fine gravel (Cu=6.9) were 1.8 m. The critical fall heights of three very uniformly graded (Cu=1.6–2.2) fine gravels were 1.9 m. One uniformly graded (Cu=3.4) fine gravel had a critical fall height of 2.0 m. Figure 3 presents the grain size distributions of the mixed graded fine gravel (No. 33, Cu=8.8) with the best impact attenuation properties (critical fall height 2.2 m) and the one with the worst properties (No. 34, Cu=8.2; critical fall height 1.6 m). Figure 3 reveals that the grading curve of the best safety gravel falls mainly (80%) in the 4–5.6 mm and 5.6–8 mm grain size classes as to the gravel fraction and mainly in the fine/medium sand grading envelope (0.063–0.63 mm) (7.8%) as to the sand fraction. The significance of the uniformity coefficient of gravels was examined by washing the sand fraction (3

200 mm

Compressed depth

Dry

Bullen and Jambunathan14

200 mm

Uncompressed depth

SFS-EN 1176-11

≤2

≤2

300 mm

Uncompressed depth

SFS-EN 1176-11

≤3

≤3

200 mm

Uncompressed depth

Wet

This research

1.6–2.2

1.7–2.1

200 mm

Uncompressed depth

Dry

This research

1.8–2.4

2.6–3.0

>3

CPSC, Consumer Product Safety Commission.

get consistent results for river gravel, and determination of a safe fall height was thus impossible. Ramsey and Preston16 also reported varying behaviour of aggregates in an impact attenuation test. They also stated that the impact attenuation of an aggregate is not directly proportional to grain size. In the case of gravels, the uniformity coefficient, Cu, is not enough to explain the suitability of gravels as a shock-absorbing loose-fill layer. The amount of sand in the interstices between adjacent gravel grains affects the movement of the grains. According to the grading curves drawn on the basis of the measurement data, one might conclude that certain type of gap-gradedness is advantageous for a safety gravel. Sand grains (about 10%) function as sort of ball bearings in the interstices between gravel grains. The sand is mainly fine and medium sand—the coarse sand fraction is almost non-existent (discontinuity of grading curve). The studied safety sands contained only a small amount of fines (0.0%–2.5%; 

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

To assess the influence of grain size distribution and moisture condition on aggregates' performance as an impact attenuation layer beneath playground...
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