FWD deflection data: from TMH 13 to TMH 18 and into an investigation

FWD deflection data are the load, temperatures and peak surface deflections that a falling weight deflectometer (FWD) records at a test point. In South African practice the record is made and positioned under TMH 13 Parts B and F (Committee Draft Final, May 2016), filed under TMH 18 (Draft Version 4, October 2014, or Committee Draft 5.0, February 2016), stored under TMH 22 Part D (Draft, March 2013), the road asset management manual, and used in an investigation under TRH 12 (Draft, 1997), the flexible pavement rehabilitation manual, and SAPEM Chapter 6 (2nd edition, October 2014). The record changes meaning at four points: its position, its normalisation (scaling to a reference load and temperature), its unit label and its file extension. This guide maps those points and does not interpret bowls. TMH (Technical Methods for Highways) is the series of South African road manuals issued under COTO, the Committee of Transport Officials; TRH (Technical Recommendations for Highways) is the companion series; SAPEM is the South African Pavement Engineering Manual from SANRAL, the South African National Roads Agency.

Key facts

• TMH 13 Part F (Committee Draft Final, May 2016) describes normalising falling weight deflectometer (FWD) deflections for load in section F.4.4.1 and for temperature in section F.4.4.4 and Appendix F-2.
• In TMH 18, the .fwd extension means normalised FWD bowls in Draft Version 4 (October 2014) and raw FWD data in F25 format in Committee Draft 5.0 (February 2016).
• TMH 18 labels FWD deflection fields nanometres in both versions, while TMH 13 Part F and TMH 22 Part D (Draft, March 2013) work in micrometres.
• TMH 13 Part F lists kilometre, lane and differential GPS coordinates as location items, and TMH 22 Part D, section D.5.4, stores one deflection record per test point.
• SAPEM Chapter 6 (2nd edition, October 2014), the South African Pavement Engineering Manual, says FWD data help divide rehabilitation projects into uniform sections and describes remaining-life estimates as crude.
• A deflection bowl is a response, not a diagnosis, because TMH 13 Part A, section A.2.5.4, says deflection used in isolation cannot give a reliable estimate of structural condition.

The concept map

The connections in text

1. Test point km, lane and GPS (TMH 13) → FWD drop: load and deflection bowl (TMH 13): locates the test
2. FWD drop: load and deflection bowl (TMH 13) → Raw F25 peak record (TMH 18): peak load, deflections
3. Raw F25 peak record (TMH 18) → Bowl scaled to reference load and temperature (TMH 13): scaled to reference
4. Bowl scaled to reference load and temperature (TMH 13) → Bowl file with its unit stated (TMH 18): unit and version
5. Bowl file with its unit stated (TMH 18) → Condition data reported in micrometres (TMH 22): files received
6. Condition data reported in micrometres (TMH 22) → Initial assessment of the pavement (TRH 12): roads for investigation
7. Condition data reported in micrometres (TMH 22) → Pavement investigation and FWD analysis (SAPEM): deflection records
8. Initial assessment of the pavement (TRH 12) → Pavement investigation and FWD analysis (SAPEM): uniform sections

What happens to FWD deflection data after the drop?

After the drop, the record passes through five documents: it is positioned, normalised, filed, stored and then used in an investigation. TMH 13 covers the device and normalisation, TMH 18 the file, TMH 22 reporting and storage, and TRH 12 and SAPEM Chapter 6 the investigation.

Deflection is the elastic movement of the road surface under a load, and a deflection bowl is the set of peak deflections measured at fixed offsets from the centre of the load (COTO, 2016g).

How does one FWD drop become a deflection bowl record?

One FWD drop becomes a bowl record when the device stores a load, two temperatures, a test point location and one peak deflection per sensor (COTO, 2016g). A weight falls onto a rubber buffer that loads a plate, usually 300 mm in diameter, while geophones (velocity transducers) at fixed offsets record the peak deflection at each one. South African practice typically targets 40 kN, simulating an 80 kN axle, with a 20 to 30 ms pulse (COTO, 2016g).

Narrated film · 1:32

Where an FWD record changes meaning

A map follows one drop from its test point through normalisation and the TMH 18 file to the investigation, then shows a synthetic bowl scaled to a reference load, what .fwd means in each TMH 18 version, and the nanometre label against micrometres.

Read the film transcript

After the falling weight deflectometer, the FWD, makes its drop, where does that record go? The record changes meaning at four points: position, normalisation, unit label and file extension. TMH 13 describes how a test point is located by kilometre, lane and GPS. The raw F25 record is kept, and TMH 13 describes scaling the bowl to a reference load and temperature. TMH 18 sets out the file, TMH 22 describes its storage, and TRH 12 and SAPEM describe its use. In this synthetic example, scaling a forty-two kilonewton drop to forty kilonewtons turns seven hundred and fourteen micrometres into six hundred and eighty. In TMH 18 Draft Version 4, the dot f w d extension holds normalised bowls. In Committee Draft 5.0, Table 5 lists raw F25 data under that extension, so name the version. TMH 18 labels deflections nanometres, but TMH 13 and TMH 22 work in micrometres. Read literally, the label shrinks every deflection a thousand times, so state the unit in the header. In an investigation, deflection data help divide the pavement into uniform sections. SAPEM Chapter 6 recommends testing every two hundred metres for a network, every fifty for a project. Even then, a bowl is a response, not a diagnosis. Before a record moves on, check its position, normalisation, unit and TMH 18 version. Next, open TMH 18 for other file families, and SAPEM Chapter 10 for deflection analysis.

Part F describes initial seating drops, usually not recorded (section F.4.4.3), and TMH 18 carries a drop number so that a row says which recorded drop it holds (COTO, 2016g)(COTO, 2014). Section F.4.2 lists the typical items: operator, date and time, test point location, temperatures, applied load, sensor deflections and visual-condition comments (COTO, 2016g). TMH 13 explained walks through its seven parts.

How is an FWD test point positioned by kilometre, lane and GPS?

TMH 13 Part F lists section information, kilometre, lane, wheelpath (the strip of the lane under the wheels) and differential GPS (DGPS) coordinates as location items, and Part B describes the GPS, inertial and distance-measuring equipment behind them (COTO, 2016g)(COTO, 2016c). Part B, section B.4.3, lists control checks of start, end, distance and survey direction against the road definition (COTO, 2016c).

TMH 18 then stores the test point as a road identifier, a lane code and a survey-point kilometre referring to the road's network record, and values outside the road's start and end kilometres are discarded (COTO, 2014). TMH 22 Part D, section D.5.4, stores a deflection record per test point, unlike the 100 m records of roughness and rutting, and names the survey direction, lane, wheelpath and test interval (COTO, 2013). A kilometre measured from a different origin than the network record is an error that later stages cannot recover from the bowl alone.

How are FWD deflections normalised for load and temperature?

Normalisation converts a measured bowl to the bowl that would have been recorded at a reference load and temperature, so that tests from different days can be compared. For load, TMH 13 Part F, section F.4.4.1, describes multiplying each deflection by the target load divided by the measured load, with a target within 10 percent of the applied load (COTO, 2016g). For temperature, section F.4.4.4 recommends a reference temperature of 25 °C at network level, using the air and surface temperatures recorded during the survey, and Appendix F-2 gives the method (COTO, 2016g).

Synthetic example, every value invented: road S1, km 12.400, lane P1, left wheelpath. A drop applies 42 kN against a 40 kN target, so each deflection is multiplied by 40 / 42 = 0.952.

Offset (mm)	Measured (µm)	Load-normalised (µm)
0	714	680
300	378	360
600	252	240
900	189	180
1,200	126	120

Check value: 714 × 40 / 42 = 680 µm. The temperature step is not worked here: Appendix F-2 needs the asphalt thickness, surface temperature, previous day's mean air temperature and time of day. Section F.4.2 leaves it to the specification whether raw files are delivered and whether normalisation is done before delivery (COTO, 2016g).

What does a TMH 18 .fwd file hold, and what changed between versions?

The TMH 18 version decides what .fwd means. Draft Version 4 uses .fwd for normalised bowls, while Table 5 of Committee Draft 5.0 lists F25 data, the raw FWD data format, under fwd (COTO, 2014)(COTO, 2016i).

TMH 18 section 5.7 sets out the normalised bowl as a semicolon-delimited UTF-8 record: road identifier, lane code, survey-point kilometre, date, air and surface temperature, applied load, drop number, a deflection per offset, pulse duration, three derived bowl parameters (BLI, MLI and LLI, the base, middle and lower layer indices defined in TMH 13 Part F, section F.2.6) and start coordinates; Committee Draft 5.0 adds an authority identifier (COTO, 2014)(COTO, 2016i). The table compares the two versions:

Item	Draft Version 4 (October 2014)	Committee Draft 5.0 (February 2016)
Normalised bowl	.fwd in Table 5, section 5.7	Section 5.7; Table 5 has no row for it
Raw F25 data	.f25 in Table 5, section 5.8	Table 5 lists F25 under fwd; section 5.7.1 still says .f25
Units in the F25 appendix	Header flags choose microns or milli-inches	Header flags choose microns or milli-inches

The .fwd extension alone therefore does not say what the file holds. In the manual's examples an F25 file is a set of comma-separated lines whose header lines begin with record numbers such as 5001, unlike the one-line normalised record (COTO, 2014)(COTO, 2016i).

The bowl record has no unit flag and carries load in kN, where the F25 appendix gives peak load as pressure, in kPa. Its field list holds the applied load and two temperatures but no reference load, reference temperature or method, so these travel with the file, for example in the definition sheet (Part F, section F.4.2) (COTO, 2016g). TMH 18 explained covers the other file families.

Is a TMH 18 deflection in nanometres or micrometres?

TMH 18 labels deflections nanometres, but TMH 13 Part F and TMH 22 Part D use micrometres, so a deflection file needs its unit stated (COTO, 2014)(COTO, 2016g)(COTO, 2013). The label appears on fields D200 onward and on every value in its printed example, in both versions (section 5.7; Table 20, which Draft Version 4 captions as Table 19) (COTO, 2014)(COTO, 2016i). TMH 13 Part F states sensor resolution and range in micrometres (Table F.1), TMH 22 Part D, section D.4.5, reports deflections in micrometres, and the F25 appendix of TMH 18 uses microns (COTO, 2016g)(COTO, 2013)(COTO, 2014).

The printed example gives a central deflection of 1,120 under 40 kN. Read as nanometres that is about 1 µm, whereas SAPEM Chapter 6 plots an example bowl on an axis in microns running to 700 (SANRAL, 2014k). The label and the magnitude disagree, so a receiving system that takes the label literally reads every deflection as 1,000 times smaller than the sensor recorded.

TMH 18 files need no header row (section 1.1), so write the unit in the header of every working table and in the definition sheet delivered with the file, then check one record against an expected magnitude before loading.

How does an investigation use FWD deflection data?

An investigation uses FWD data to help divide the pavement into uniform sections, lengths that are alike in their functional and structural parameters (Department of Transport, 1997)(SANRAL, 2014k). TRH 12 section 1.3.2 says a network-level study identifies roads for project-level investigation, and sections 2.2 and 2.3 set out an initial assessment, which collects enough surveillance data, including deflection and bowl parameters, to divide the pavement into uniform sections, and a detailed one (Department of Transport, 1997). SAPEM Chapter 6, section 7.3.4, recommends testing every 200 m for network surveys and every 50 m for project surveys, in the left wheelpath (SANRAL, 2014k). A network record may therefore be too sparse for a project.

Interpretation is a separate step. Horak's benchmarking paper places bowl parameters within relative structural benchmarking (Horak, 2008), and SAPEM Chapter 6, section 7.3.4.1, describes remaining-life estimates as crude and says the correlations need great care (SANRAL, 2014k). TRH 12 Appendix 1, section 1.3.2, notes that parameter magnitudes depend on the instrument as well as the pavement (Department of Transport, 1997), and TMH 13 Part A, section A.2.5.4, says deflection used in isolation cannot give a reliable estimate of structural condition (COTO, 2016d). A bowl is a response, not a diagnosis; the insights on bowl interpretation and repeatability explain why.

What do the manuals leave undecided, and what is often misread?

The manuals in this guide do not decide where normalisation happens, which TMH 18 version a receiver accepts, or what a bowl means for a layer. The specification and the client's terms of reference decide the first two; the investigation decides the third.

• A .fwd file always holds normalised bowls. In Draft Version 4 it does; in Committee Draft 5.0, Table 5 lists F25 data under that extension (COTO, 2014)(COTO, 2016i).
• Nanometres means nanometres. The label and the magnitude disagree, so state the unit.
• TMH 20 is a separate manual. TMH 22 Part D and TMH 13 Part B send readers to TMH 20 for formats. No published TMH 20 was found; read those references as TMH 18, whose Draft Version 4 running head reads COTO Standard for Road Network Data (COTO, 2013)(COTO, 2016c)(COTO, 2014).
• Normalised means comparable. Part F states that no formal procedure has been developed to date for seasonal variation in bowl parameters (section F.4.4.5) (COTO, 2016g).

What should I keep with each FWD deflection record?

Keep eight items with every FWD deflection record:

• the raw F25 file or peak record, beside the normalised file;
• the TMH 18 version, and the extension rule agreed with the receiver;
• the deflection unit, in the definition sheet;
• the reference load, reference temperature and normalisation method;
• applied load, drop number, pulse duration, and air and surface temperature;
• road identifier, lane code, survey-point kilometre, coordinates and survey direction;
• calibration, repeatability and control-test records for the device (Part F, section F.3) (COTO, 2016g);
• operator comments on flagged files.

How to check a TMH 18 file set turns this list into checks.

Frequently asked

Does a .fwd file hold normalised bowls or raw F25 data in TMH 18?

A .fwd file holds normalised bowls or raw F25 data, depending on the TMH 18 version. TMH 18 (Draft Version 4, October 2014), the South African road data exchange manual, lists .fwd for normalised bowls from a falling weight deflectometer (FWD) and .f25 for raw F25 data. In Committee Draft 5.0 (February 2016), Table 5 lists F25 data under fwd and has no .f25 row, so name the version and open the file to check.

Which unit should I state for FWD deflection in a TMH 18 file?

State the unit in the definition sheet delivered with the file and agree it with the receiver. TMH 18 (Draft Version 4 and Committee Draft 5.0) labels falling weight deflectometer (FWD) deflection fields nanometres, but TMH 13 Part F and TMH 22 Part D (Draft, March 2013), the South African road manuals for deflection measurement and asset data, use micrometres, and the central deflection in the TMH 18 example reads sensibly only as micrometres.

Is falling weight deflectometer (FWD) data normalised before or after it is filed under TMH 18?

TMH 13 Part F (Committee Draft Final, May 2016), the South African road manual on surface deflection, leaves the timing of normalisation to the specification: section F.4.2 says the specification should state whether raw files are delivered and whether pre-processing such as normalisation is done. TMH 18 calls its bowl record normalised but has no field for the reference load, reference temperature or method, so record those details with the file, for example in the definition sheet.

Is TMH 20 a different manual from TMH 18?

No separate TMH 20 was found to cite. TMH 22 (Draft, March 2013) and TMH 13 Part B (Committee Draft Final, May 2016), two South African road manuals, send readers to TMH 20 for data formats, but the Standard for Road Network Data content is published as TMH 18, whose Draft Version 4 running head reads COTO Standard for Road Network Data. Cite TMH 18 and name its version.

Which manual decides how to interpret an FWD deflection bowl?

No single manual decides how to read a falling weight deflectometer (FWD) bowl. TMH 13 Part F describes measuring, checking and normalising, and TMH 18 sets out the file, while interpretation belongs to the investigation. SAPEM Chapter 6 (2nd edition, October 2014), the South African Pavement Engineering Manual, describes FWD remaining-life estimates as crude, and TRH 12 (Draft, 1997) notes that parameter magnitudes depend on the instrument as well as the pavement.

What file format does FWD data take under the South African road manuals?

TMH 18, the South African road data exchange manual, sets out a normalised falling weight deflectometer (FWD) bowl as a semicolon-delimited UTF-8 record. In its examples, raw F25 data are comma-separated lines whose header lines begin with record numbers such as 5001. The extension depends on the TMH 18 version (Draft Version 4 or Committee Draft 5.0), so name the version with the file.

Where this connects

The next manuals to open are TMH 18, for the file families around the deflection file, and SAPEM Chapter 10 (2nd edition, October 2014), which SAPEM Chapter 6 names for further discussion of FWD analysis (SANRAL, 2014k). Last reviewed on 21 October 2026 against the editions named in the opening paragraph.

References

1. COTO (2016c). TMH13: Automated Road Condition Assessments. Part B: Positioning. Committee Draft Final, May 2016. Original source.Positioning and distance measurement guidance
2. Department of Transport (1997). TRH12: Flexible Pavement Rehabilitation Investigation and Design. Draft TRH12, Pretoria: Department of Transport for COLTO. Original source.Chapter 1: rehabilitation design process; investigation, evaluation, analysis and economic appraisal
3. Horak (2008). Benchmarking the structural condition of flexible pavements with deflection bowl parameters. Journal of the South African Institution of Civil Engineering, 50(2), pp. 2–9. Original source.Relative structural benchmarking with deflection bowls
4. COTO (2016d). TMH 13: Automated Road Condition Assessments. Part A: General. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
5. COTO (2016g). TMH 13: Automated Road Condition Assessments. Part F: Surface Deflection. Committee Draft Final, May 2016. Original source.Sections, tables and clauses are cited inline in each guide
6. COTO (2014). TMH 18: Road Asset Data Electronic Exchange Formats. Draft Version 4, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
7. COTO (2016i). TMH 18: Road Asset Data Electronic Exchange Formats. Committee Draft 5.0, February 2016. Original source.Sections, tables and clauses are cited inline in each guide
8. COTO (2013). Draft TMH 22: Road Asset Management Manual. Draft, March 2013. Original source.Sections, tables and clauses are cited inline in each guide
9. SANRAL (2014k). South African Pavement Engineering Manual, Chapter 6: Road Prism and Pavement Investigations. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide

Use the manual edition and project requirements that apply to your work. Worked examples use synthetic values and are not site-specific design advice.