Bond Strength Testing for Overlays on Exposed Concrete
UP. PPP is a technical testing and measurement unit operating under Jakarta’s Dinas Bina Marga public works department. It provides accredited laboratory and field investigation services for road and bridge projects, covering asphalt, soil, concrete, elevation and infrastructure assessment. The practical task is testing the bond strength of overlay materials to existing concrete, exposing whether the interface has been prepared adequately.
An overlay may appear complete while remaining poorly attached. A thin repair layer can then separate from an exposed concrete slab, creating hollow areas, loose patches and trip hazards on footpaths. On bridge decks and busy roads, detachment can also allow water to reach reinforcement, accelerating corrosion and reducing service life.
Bond strength depends on more than the strength of the new material. Compatibility with the existing surface, preparation depth, moisture condition, primer selection, curing time and contamination can determine whether the interface survives everyday loading. Testing therefore examines the complete bonded system rather than the overlay in isolation.
Different systems fail in different ways, so the test method and acceptance criteria must suit the material and project. UP. PPP applies SNI ISO/IEC 17025 laboratory requirements and ISO 45001 safety management to provide dependable results for planning, construction, maintenance, quality assurance and safer public infrastructure.
Choose the overlay and the right test
The first step is to identify what the overlay is expected to do. A repair mortar restores damaged concrete, a polymer-modified screed creates a wearing surface, and an epoxy system provides a durable adhesive layer. The expected service conditions and failure mechanisms then guide the locations, equipment and interpretation of each bond test.
| Overlay system | Main bond mechanism | Suitable preparation | Frequent failure concern |
|---|---|---|---|
| Cementitious repair mortar | Hydration and mechanical interlock | Remove loose material, roughen, clean and condition the surface | Shrinkage, inadequate saturation or a weak substrate |
| Epoxy repair coating | Adhesive bond to prepared concrete | Produce a clean, sound profile with controlled moisture | Contamination, trapped moisture or incorrect mixing |
| Polymer-modified screed | Cement hydration with polymer modification | Prepare and prime according to the system specification | Incompatible primer or unsuitable surface moisture |
| Sprayed cementitious overlay | Mechanical keying and progressive build-up | Obtain sound concrete, adequate profile and controlled application | Inadequate bonding, rebound or poorly controlled thickness |
The project specification should define the required pull-off value, permitted failure patterns, number of tests and treatment of individual results. A generic threshold is rarely adequate because the overlay, substrate strength, exposure and service load differ between a footpath, a bridge deck and a commercial floor.
Prepare and document the substrate
Before placing a doll or testing an overlay, inspect the existing concrete. Map cracks, hollow areas, spalls, laitance, oil, paint and previous repair materials. Hammer sounding can identify suspicious areas, although it should be combined with visual examination and relevant laboratory data. A sound-looking surface can still conceal weak concrete beneath a thin skin.
Surface preparation must expose sound material without damaging reinforcement or the concrete section that will remain in service. Grinding, scarifying, shot blasting or other specified methods remove contaminated and weakened layers. Loose dust must then be removed, and the surface must be allowed to reach the moisture condition required by the overlay manufacturer. Cementitious products may require a saturated surface with no standing water, whereas some resin systems demand a dry substrate.
Foundation movement and drainage deserve equal attention. Where settlement is suspected, soil testing should establish whether the support is stable rather than relying on a visual check. The UP. PPP guide on why soil compaction matters explains how density, moisture and load response affect long-term performance, helping teams avoid spending a repair budget on a surface that lacks stable support.
Execute the pull-off test with control
Pull-off testing applies tension perpendicular to the overlay until separation occurs. The maximum load is divided by the cross-sectional area of the test dolly to express bond strength in megapascals. ISO 4624 and ASTM C903 provide recognised frameworks for coating pull-off testing, while a repair specification may prescribe another method for the particular concrete or repair system.
Prepare representative test locations before placing the overlay. A trial area can establish whether the proposed preparation, primer and application procedure produce acceptable adhesion. The trial should use the same materials, mixing method and curing conditions planned for the main work, rather than an easier laboratory sample.
The tester attaches a compatible dolly to the surface with an adhesive capable of exceeding the expected failure load. After the adhesive cures, excess material is removed from around the dolly. A calibrated instrument then applies tension at the rate specified by the governing procedure, while the operator records the maximum load, failure location and visible condition of the exposed interface.
Test positions should cover representative traffic areas, edges, joints and locations with different surface conditions. Testing only an obviously weak patch produces useful diagnostic information but cannot establish the condition of the wider overlay. Record every position against a drawing, chainage system or grid so later investigations can find the same location.
Read failure patterns as evidence
The numerical result alone can conceal the most useful information. Failure may occur cohesively within the overlay, between the overlay and primer, at the adhesive used for testing, or within the existing concrete. Each pattern identifies a different cause and requires a different corrective response.
A cohesive overlay failure can provide a valid indication of the bonded interface strength when the test adhesive remains sound. Adhesive failure usually means the pull-off setup was unsuitable, so the test may need to be repeated. Concrete substrate failure points towards inadequate concrete strength, surface preparation or local deterioration. Mixed failures require careful interpretation because the weakest component governs the result.
Pull-off strength also represents only one aspect of performance. A sound test under tension does not guarantee resistance to shear from vehicle loads, abrasion, thermal movement or repeated wet conditions. Bridge decks, road surfaces and heavily used pedestrian areas benefit from complementary inspection, including crack mapping and sounding.
ISO 4624 requires results affected by substrate or adhesive failure to be reported separately from coating cohesion failure. Project specifications should similarly distinguish acceptable and unacceptable failure modes. Without that distinction, a low reading may be blamed on the overlay when the real concern is weak concrete or an incorrectly prepared surface.
Build a traceable acceptance record
Each report should connect the test result to the work it verifies. Useful records include the test location, surface condition, preparation method, moisture state, overlay material, batch or mix reference, primer, application time, curing period, ambient temperature, instrument identification, applied load and failure classification. Photographs of the surface before preparation and after failure add valuable context.
Measurement uncertainty and calibration status should remain visible in the documentation. Field instruments need periodic verification, while laboratory activities fall within the quality controls required by SNI ISO/IEC 17025. That standard supports technically competent, repeatable testing; it does not replace the project’s acceptance criteria, which must be agreed before results are assessed.
Competence also depends on people being supported. Trainees and young technicians may carry academic, travel and workplace pressures simultaneously; the Cheongju resource on resources for teenagers offers a useful reference for a workplace wellbeing initiative. Clear training, supervision and confidential routes to appropriate advice protect both staff and the credibility of field decisions.
Adapt testing for Australian conditions
Australian projects introduce familiar local realities. Sydney’s coastal exposure, Melbourne’s variable temperatures and Brisbane’s humid summers place different demands on concrete repairs. Busy city streets also restrict testing and repair windows, with early-morning deliveries and night work used to reduce traffic disruption. These conditions make substrate preparation, moisture control and careful access planning essential.
Residential demand forms a substantial part of the local market. Australian households commonly refresh worn concrete driveways, garage slabs and back paths instead of replacing them, creating steady demand for repair mortars, primers and resurfacing systems. These smaller works still require suitable preparation and a documented acceptance basis, even when the testing program is less extensive than on a bridge.
Australian buyers often check whether a testing laboratory holds NATA accreditation against AS/NZS ISO/IEC 17025. State transport authorities, councils and private contractors also work through specifications based on Australian concrete, bridge and road guidance. UP. PPP’s Indonesian accreditation provides a useful benchmark, while Australian project teams should apply the requirements of their own jurisdiction rather than treating overseas credentials as a substitute for local suitability.
Safety and environmental duties remain location-specific. Work health and safety obligations operate through state and territory laws based on the Safe Work Australia model, while cleaning, grinding waste and runoff may be governed by environmental rules administered by the relevant regulator. AS 3600, AS 5100 and applicable Austroads guidance may influence design or assessment. These references should complement the written test method, not obscure who is responsible for acceptance.
Secure the bond before approving the work
Before authorising an overlay, define the acceptance threshold, test locations and acceptable failure patterns in the specification. Require a trial area, calibrated pull-off testing and a report that links each result to substrate condition, overlay batch and curing history. Full-scale work can then proceed against evidence rather than visual confidence.
Contact UP. PPP to scope laboratory and field bond testing for roads, bridges, footpaths or private concrete works. The service can cover substrate assessment, trial patches, pull-off investigation and traceable acceptance reporting, with test methods selected for the overlay system and the project’s governing requirements.