- A head form is released from a series of four heights and the HIC for each impact is recorded, the critical fall height (CFH) at which HIC would equal 1,000 is then estimated using a regression model
- This test is undertaken in the laboratory under various conditions including pre-and post-simulated wear as well as a field test.
- A 4.6kg (±0.05kg) hemispherical head form with a 160mm (±5mm) diameter with an accelerometer is used. The impacting part of the head form between the lower boundary and accelerometer shall be homogenous and free from voids. For wired head forms the mass of the wires and any connections should be accounted for in the determination of For the purpose of clarity, this head form has the same specification as the head form defined in EN 11771.
- A means of suspending the head form statically prior to dropping it at various heights with the rounded surface (contact surface) facing towards the surface. Where required, it must be possible to reproduce the drop height for subsequent drops. The drop height must be measured using one or both of the two following options:
- A direct measurement device from the bottom most part of the head form to the top of the infill or
- A combination, where the two results must differ from each other by no more than 1%, of:
- Using Δt, the time between release and contact of the head form with the surface, and the formula s = (1/2)a(Δt)2 where s is the drop height and a is the acceleration due to gravity.
- Using the impact velocity (v) and the formulas = v/2a where s and a have the same meanings as (i) above.
- Note: Attention should be paid to the potential for lag to be created by the release system
- A means of measuring the impact including:
- an accelerometer measurement system capable of measuring, recording and displaying the acceleration and time duration for each complete impact. It should have a frequency range of 20-1,000Hz (-3dB).
- Amplitude errors below 5%, in accordance with ISO 64873 Channel Class It shall be capable of measuring, recording and displaying the acceleration and time duration of each complete impact.
- A recording device capable of measuring the acceleration/time signals of the accelerometer with a sampling rate of at least 5kHz and a means of displaying the recorded signal
- A means of calculating the HIC for each drop, as per Section 3 below, and a means of completing a regression analysis on the results achieved
- The apparatus must be calibrated by a competent laboratory in accordance with ISO 170252 requirements
- Signal conditioning and filtering shall be compatible with the accelerometer and the data channel specified. This process shall conform to ISO 6487. NOTE: According to ISO 64873, the analogue anti-aliasing filters should have an attenuation minimum of 30dB at half the sampling rate.
- Validate the apparatus using one drop at a known drop height which must be ≥1,000mm (to be physically measured and compared (±5%) with that calculated from the time between release and contact with the surface) on a reference against the expected HIC performance (±5%) for the sample before undertaking a series of lab tests or field tests. It is recommended that the reference sample be tested at a range of temperatures to allow for environmental variances on site at a frequency of at least once every 25 field tests or once a month, whichever is more frequent
3. Test Procedure
- For laboratory testing, testing shall be carried out on a flat, rigid, concrete substrate of sufficient mass, density and thickness that its deformation during the test makes no significant contribution to the test result. The minimum requirements for this surface are:
- Thickness of 100mm
- Concrete hardness of 40MPa, verified according to EN 12504-24
- For each location to be tested the characteristic HIC for four separate and distinct drop heights must be calculated.
- Calculate the characteristic HIC for a given drop height by one of the following procedures:
- PROCEDURE A
- Perform three drops of the head form from the same height onto the same location on the surface so that the impact centres for the three drops are as close as possible to the same point on the surface.
- Calculate the HIC for each individual impact using the formula below, where:
- Where t1 and t2 are the initial and final times (in seconds) of the interval during which HIC attains maximum value, and acceleration a is measured in terms of g.
- If t2 - tl < 3ms then the impact will be considered invalid and the test will be restarted for that drop height on an area which has had no previous testing completed on it for that location
- Use the largest HIC value of the three calculated as the characteristic HIC value for that drop height.
- The drop height (in mm) and the three HIC values calculated must be recorded
- PROCEDURE B
- Perform one drop of the head form from the drop height onto a point on the surface
- Calculate the HIC for this impact using the equation specified in 3.3.1 above
- Use this HIC value calculated as the characteristic HIC value for that drop height
- The height and HIC calculated must be recorded
- PROCEDURE A
- Calculate the characteristic HIC for four separate heights ensuring that the following has been achieved:
- The lowest and highest drop height are not more than 1,000mm apart
- There is at least 150mm difference between all individual drop heights
- There must be two drop heights with a characteristic HIC value greater than 1,000 and two drop heights with a characteristic HIC value less than 1,000.
- There must be no characteristic HIC values between 975 and 1,025.
- The impact centre for a given drop height test must be at least 200mm from the impact centre of any other drop height test, in instances where there is significant displacement of infill, this should be increased at the operator’s discretion
- All impact centres for a given test location must be within a square area of side 1,000mm.
- No impact centre must be closer that 150mm from the edge of the surface
- When the characteristic HIC for all four drop heights have been calculated plot the drop height (x-axis) against the characteristic Using a linear regression model (with the y-intercept calculated normally) determine the drop height at which the HIC value is predicted to be equal to 1,000.
- This drop height is the Critical Fall Height (CFH).
4. Laboratory testing:
- For laboratory testing on a sample size smaller than 0.64m2 it is permissible to use the following method:
- Using a drop height equal to the minimum CFH requirement specified use Method A in 3.3.1 above to calculate the characteristic HIC value for that drop height
- If this characteristic HIC value is less than 1,000 then the test is deemed to have passed the requirement.
- If this characteristic HIC value is ≥1,000 then the test is deemed to have failed.
5. Expression of results
- The following information must be reported for each test location:
- The four drop heights used (in mm)
- The HIC calculated for each drop at each of the four drop heights
- The characteristic HIC for each of the four drop heights
- The Critical Fall Height (in mm) determined
6. Normative references
EN 1177 - Impact attenuating playground surfacing — Methods of test for determination of impact attenuation
ISO 17025 - General requirements for the competence of testing and calibration laboratories
ISO 6487 – Road vehicles – measurement of impact velocity instrumentation
EN 12504-2 – Testing concrete in structures -Part 2: Non-destructive testing – Determination of rebound number.