This article walks through a real job step by step, from the building owner's brief, the test load calculation and the readings taken on site to the pass/fail decision, to show where the numbers in the report come from and how they are used. For general principles, see What Is a Floor Load Test, and for choosing the test load, see How to Load a Floor Load Test.
What Was the Brief for This Job?
The brief was to prove that the existing slab can actually carry a live load (L) of 400 kg/m² for its new use. The building is a 2-storey RC building used for commercial and residential purposes in Bang Mot subdistrict, Thung Khru district, Bangkok. The owner was preparing drawings to apply for a building alteration permit, and the design engineer specified this value of L.
SPN was engaged to test 1 location (TP-1) on the first-floor slab using a monotonic load test per ACI 318-19 Chapter 27 (Strength Evaluation of Existing Structures). The key details are as follows.
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| Item | Details |
|---|---|
| Building | 2-storey RC building (commercial–residential), alteration/extension works, Thung Khru, Bangkok |
| Test location | TP-1, cast-in-place RC slab, first floor (slab-on-ground), 3.75 × 4.00 m |
| Slab thickness (h) | 32 cm |
| Design span (ℓt) | 3,750 mm (short span) |
| Design live load (L) | 400 kg/m², confirmed by the client and the design engineer before testing |
| Loading method | Water ponding over the full slab panel between beam edges |
| Instruments | Automatic level + leveling staffs at 3 points (D1, D2 at mid-panel, D3) |
| Test day | Loading started 25 August 2026 · Load removed 26 August 2026 |
How was the test load calculated, and why 64 cm of water?
The 64 cm water depth comes from the portion of the test load that has to be added, namely 1.6L = 640 kg/m². The Total Test Load (Tt) is the entire load the structure carries during the test, including the slab's self-weight. ACI 318-19 Section 27.4.6.2 requires three cases to be calculated and the largest value used. On this job there was no superimposed dead load to be added later (Ds = 0) and no roof, snow or rain load, so case (a) reduces to 1.0Dw + 1.6L.
Tt = 1.0Dw + 1.6L = 768 + 1.6(400) = 1,408 kg/m² Governs over case (c) 1.3(Dw + Ds) = 998 kg/m²
The slab's self-weight of 768 kg/m² is already acting on the structure, so only the 1.6L portion needs to be added, i.e. 640 kg/m². 1 cm of water weighs 10 kg/m², so the maximum water depth is 64 cm.
With a 32 cm slab, Dw is larger than the added load. If the actual thickness differs from the assumed value, Tt changes accordingly, so always confirm the actual slab thickness before calculating.
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| Parameter | Value | Remarks |
|---|---|---|
| Added load (W) | 640 kg/m² | W = 1.6L |
| Maximum water depth (Hmax) | 64 cm | 1 cm of water = 10 kg/m² |
| Load steps | 16 / 32 / 48 / 64 cm | 25% / 50% / 75% / 100% |
| Full-load hold time | 24 hours | Per ACI 318-19 Chapter 27 |
| Deflection limit exempting recovery measurement | 1.88 mm | Greater of 1.27 mm and ℓt/2000 |
How was the site prepared and deflection measured?
This job used a ponding pool covering the full slab panel as the load, with deflection measured from above at 3 points using an automatic level. Before filling, the team photographed and recorded the existing condition of the slab and surrounding beams to confirm there were no cracks or damage indicating an unsafe condition. The site was then prepared in 4 steps.
- Build the ponding pool (water bund) along the beam edges to cover the full slab panel, with walls high enough for 64 cm of water plus freeboard.
- Lay a single continuous waterproof sheet (PE sheet) with no joints, so no water leaks during the 24-hour hold.
- Set up the automatic level on a tripod outside the loaded area and define 3 measuring points, D1, D2 (mid-panel) and D3, read to 0.01 cm (0.1 mm).
- Fix a staff in the pool to gauge the water level, confirming the actual water level at every reading and recording air temperature with each data set.
The datum before filling read 2.00 cm at all three points. Net deflection is calculated as (reading − 2.00) × 10 in millimeters. Measuring from above with a level suits a slab-on-ground, where a dial gauge cannot be installed under the slab.
The two figures below summarize the site preparation and the load calculation above. The first is a 2D section of panel TP-1 during the four ponding steps; the second is a 3D model of the ponding pool, measuring points and the deflection result against the criterion.
How was the load applied in steps and recorded?
Loading was applied in 4 steps of about 1 hour each. At every step, readings were taken at 0, +5, +15, +30 and +60 minutes. Once the water reached 64 cm, readings were taken at +5, +15, +30 and +60 minutes, then every hour until 24 hours.
Because it is a slab-on-ground, the underside could not be observed. During the test the team watched for cracking and concrete spalling on the slab surface around the pool and along the beams, checked the pool for leaks, and used the level deflection readings as the main indicator, ready to drain the water immediately at any sign of distress.
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| Time | Water level (cm) | Added load (kg/m²) | D1 / D2 / D3 (cm) | Max. Δ (mm) |
|---|---|---|---|---|
| Datum | 0 | 0 | 2.00 / 2.00 / 2.00 | 0.00 |
| 50% · +60 min | 32 | 320 | 2.00 / 2.00 / 2.00 | 0.00 |
| 75% · +30 min | 48 | 480 | 2.00 / 2.00 / 2.01 | 0.10 |
| 75% · +60 min | 48 | 480 | 2.01 / 2.01 / 2.02 | 0.20 |
| 100% · +30 min | 64 | 640 | 2.03 / 2.03 / 2.04 | 0.40 |
| 100% · +60 min | 64 | 640 | 2.04 / 2.04 / 2.05 | 0.50 |
| 100% · +2 h | 64 | 640 | 2.05 / 2.05 / 2.05 | 0.50 |
| 100% · +12 h | 64 | 640 | 2.05 / 2.05 / 2.05 | 0.50 |
| 100% · +24 h (unloaded) | 64 | 640 | 2.05 / 2.05 / 2.05 | 0.50 |
The table above shows only the intervals where readings changed. The full report records every interval, 44 data sets in total. Temperature during the test ranged from 28–33 °C.
What were the deflection results?
Maximum slab deflection was 0.50 mm, 27% of the 1.88 mm criterion, and remained constant throughout the final 22 hours of the hold. The slab's behavior can be read in 3 stages.
- 25–50% steps (16–32 cm of water) — no change at all, meaning deflection was smaller than the level's 0.1 mm resolution.
- 75% step to the first hour at full load — deflection began at 48 cm of water and increased in 0.1 mm increments to 0.50 mm at +60 minutes at full load (mid-panel point D2 reached 0.50 mm at +2 hours).
- +2 to +24 hours — constant at 0.50 mm for 22 hours, showing no significant time-dependent deflection (creep).
A deflection of 0.50 mm over a 3,750 mm span is about ℓt/7,500, which is very small. The behavior was linear, with no break point indicating loss of stiffness or cracking of the section.
How is pass/fail decided under ACI 318-19 Section 27.5.3?
ACI 318-19 Section 27.5.3 judges on two aspects: visible signs of damage and deflection. For deflection, recovery normally has to be measured after unloading, with residual deflection (Δr) not exceeding Δl/4, but Section 27.5.3.6 grants an exemption if the maximum deflection is very small.
ℓt/2000 = 3,750/2,000 = 1.88 mm → Δl = 0.50 mm ≤ 1.88 mm Meets Section 27.5.3.6, so residual recovery per Section 27.5.3.5 need not be checked
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| ACI 318-19 criterion | Finding | Result |
|---|---|---|
| 27.5.3.1 No spalling or crushing of concrete | Visual check of slab surface and beam lines — none found | Pass |
| 27.5.3.2 No shear cracks indicating imminent failure | Visual check — none found | Pass |
| 27.5.3.3 / 27.5.3.4 No inclined cracks in regions without transverse reinforcement, and no cracks along reinforcement at anchorages and lap splices | Visual check — none found | Pass |
| 27.5.3.5 Residual recovery Δr ≤ Δl/4 | Not measured (exempt) | Not required |
| 27.5.3.6 Δl ≤ greater of 1.27 mm and ℓt/2000 | 0.50 ≤ 1.88 mm | Condition met |
After the 24-hour hold, the team drained all the water and inspected the slab surface and beam lines again (being a slab-on-ground, the underside could not be inspected). No new cracks, shear cracks or concrete spalling were found in the loaded area. The overall result therefore passes the strength evaluation criteria of ACI 318-19 Chapter 27.
ACI 318-14 and earlier editions used a deflection criterion of ℓt²/(20,000h). ACI 318-19 revised Chapter 27 to align with ACI 437.2 and changed the recovery exemption to the greater of 0.05 in. and ℓt/2000. When reading a load test report, check which ACI edition it cites, because the two formulas give very different numbers.
In addition to the load test, the team drilled a concrete core from a nearby slab to check thickness and concrete quality. If the concrete compressive strength is also needed, use a Concrete Core Test; for non-destructive slab thickness measurement, see Impact Echo.
What does this result confirm, and what does it not cover?
The test confirms that slab TP-1 can adequately carry the design live load of 400 kg/m², and the result can be applied to slabs with identical details in the same area, but there are 3 limits.
- It does not cover beams, columns, foundations or other slabs that were not tested. If other panels need confirming, additional tests should be done at locations specified by the design engineer.
- It confirms a live load of 400 kg/m² only. If the service load is to be increased beyond this value in future, the structural capacity should be re-evaluated first.
- The final decision on how this result is used in the alteration design rests with the structural design engineer or the inspecting engineer appointed by the client.
Lessons from this job for building alterations and extensions
This job offers 5 lessons that apply to testing existing building floors before alteration or extension.
- Confirm L with the design engineer before testing — this value sets both the water level and the report's conclusion. This job used 400 kg/m² for the new use.
- Know the actual slab thickness before calculating Tt — with a 32 cm slab, self-weight accounts for more than half of Tt.
- Choose a representative test location — the panel that will take the heaviest use or has the longest span.
- Instrument resolution must be much finer than the criterion — the level reads to 0.1 mm against a 1.88 mm criterion, so pass/fail is clearly distinguished.
- Record the actual water level and temperature every time — this helps separate temperature effects from true deflection when reviewing data later.
If you are planning to alter a building or change its use, see the overview of structural checks at Before Extending or Changing the Use of an Old Building: What Structural Checks Are Needed. For factory or warehouse floors, see How Much Load Can a Factory or Warehouse Floor Carry, or see SPN's Non-Destructive Structural Testing (NDE) service.
The cost of a Floor Load Test depends on the number of test locations, the slab panel size, the water depth required and site access, so it is assessed per project. Request a free quotation within 24 hours on the SPN contact page. Prices do not include travel and equipment transport, which are charged extra according to the distance to site.
The figures and photos in this article come from an actual SPN test (Job No. SPP6908022), without naming the building owner or house number. The graphs were redrawn from field-recorded values. The conclusions are explanatory and are not design advice for other buildings.