ScanAxis · Quebec
Voids beneath slabs and concrete delamination
A floor that sounds hollow suggests the same problem in both cases, yet these are two different defects. A void sits under the slab, between the concrete and its support. A delamination sits inside the slab, between two planes of concrete that have separated. Confusing them leads to the wrong question, therefore the wrong survey and sometimes the wrong repair. This page is an entry point and links to the pages that develop each method.
Request a quoteTwo defects that get mixed up
A void is a cavity under a ground-supported slab, between the concrete and the material carrying it. It appears when the support settles, washes out or shrinks while the slab keeps its shape. The concrete is intact; what has gone missing, in places, is its bearing.
A delamination is an internal separation in concrete, often parallel to the surface. It develops within the slab itself, frequently at the level of the reinforcement. Corrosion of the steel, or freeze and thaw cycling, are the causes most often cited.
The practical consequence is clear. A void is addressed from the support side, by injection or by rebuilding the bearing. A delamination is addressed in the concrete, by removal and reinstatement. Different trades, different costs, different schedules.
Where each one forms
Voids are found mostly under ground-supported slabs, in industrial buildings, warehouses, grade-level garages and exterior surfaces. Poorly compacted fill, a leaking pipe or drainage that carries fines away creates the cavity, sometimes long after construction.
Delamination is found mostly on exposed structures: parking decks, balconies, sidewalks and bridge decks, wherever salt-laden water reaches the reinforcement. It shows first as an area that sounds hollow, then as a spall once the surface concrete detaches.
One structure can present both, for reasons unrelated to each other. That is one reason it is worth naming the defect you are looking for before requesting a survey.
What a survey establishes and what it leaves open
A non-destructive survey locates indications and delimits areas. It shows where the behaviour of the concrete or of its bearing departs from its surroundings, and lets those areas be placed on a drawing. That is usually what is missing to scope an intervention.
What it does not settle deserves saying in advance. An indication gives neither the cause of the defect, nor its age, nor the choice of repair. It does not say whether the process is stable or ongoing either. Those questions call for an engineer’s interpretation, often with complementary verifications.
Each method is developed on the concrete scanning page for GPR locating, and on the structural assessment page for evaluating a structure in place. The terms used here are defined in the glossary.
Families of methods and what they observe
No single method answers both questions. The report ACI 228.2R-13, Report on Nondestructive Test Methods for Evaluation of Concrete in Structures reviews the families of non-destructive methods applicable to in-place concrete, including visual inspection, stress-wave methods, magnetic and electrical methods, infrared thermography and ground-penetrating radar.
The choice follows from the question asked, the access available and what the deliverable has to show. An area to delimit on a drawing and a single doubt before an opening do not call for the same survey organisation.
- What is being sought, a void under the slab or a separation within its thickness
- The accessible face, the top only or both faces of the element
- The condition of the surface, covering, membrane, moisture and obstructions
- The expected deliverable, floor marking, sketch, map of areas or report
The case of parking garages
Parking garages concentrate the conditions that favour delamination: water, de-icing salts and freeze and thaw cycling. They are also subject to verification obligations in Quebec, described by the Régie du bâtiment du Québec.
In that context, a map of areas mainly serves to follow how a sector evolves between two verifications and to target what will be examined more closely. It supports the engineer’s judgement, it does not replace it.
Framing the question before the survey
A few items gathered beforehand change the quality of what you receive, and avoid a second visit.
Name the defect you are looking for
State whether the question is about bearing giving way under the slab or about concrete separating within its thickness.
Describe what has been observed
An area that sounds hollow, a crack, a spall, water ingress, a settled floor slab or distress that appeared after works.
Gather what already exists
Drawings, earlier reports, repair history and photographs of the areas concerned.
Check access
Cleared surface, covering in place, reachable faces and the hours when the premises are available.
Say what the result will be used for
Scoping a repair, supporting a decision, documenting a condition or preparing an opening call for different deliverables.
What you can ask for
Describe the area concerned, what has been observed and the intended use of the result on the contact page. The survey scope and the deliverable are set from those elements, and stated in the quote.
Frequently asked questions
How do you tell a void from a delamination without opening up?
Both can sound hollow, which is not enough to decide. The position of the defect, under the slab or within its thickness, and the context of the structure guide the interpretation. A survey adds indications, which the engineer cross-checks with what is observed on site.
Does a survey price the repair?
It delimits areas and places them on a drawing, which helps prepare an estimate. The choice of repair and its cost rest with the contractor and the engineer, based on the whole file.
Does the covering have to be removed before the survey?
That depends on the covering and on the method selected. Mention what it is in your request, since it can affect readings or require partial clearing. The decision is made when the scope is set.
Is an area without indications a sound area?
No. No indication means no departure was recorded under the conditions of the survey, which is not a conclusion about the condition of the structure. The report should state which areas were examined and which were not.