#Bonded anchor#Mechanical anchor

What is the minimum distance between an anchor and the edge of a concrete slab?

12 min

Whether you are about to use bonded or mechanical anchors, you really need to maintain a correct distance from the anchoring point to the base material edge. This way you will avoid unnecessary stresses in the material and make sure that the fixing is ultimately secure.

Chemical anchors (frequently referred to as bonded anchors) can be used in a wide variety of substrates and in demanding conditions, even in wet environments. Resins are characterised by high resistance to corrosion, they are perfect for deep anchoring and, above all, they can withstand considerable loads. Metal fasteners, on the other hand, are primarily used in concrete. Conventional mechanical anchors form joints with the substrate by way of expansion forces, while a chemical anchor owes its properties to adhesion, i.e. surface bonding of materials.

What is an anchor’s minimum edge distance?

The minimum edge distance of an anchor is a vital parameter, irrespective of the type of the chosen fixing solution (chemical or mechanical). Its value depends on a number of aspects, including:

  • anchor type,
  • base material,
  • load applied, and
  • specifics of the element to be fixed.

Before choosing an adequate fixing, you should always pay attention to the recommended minimum edge distances and anchor spacing information manufacturers are expected to state in their respective installation manuals or technical documents. It is the information provided in such documents that one should consider as the most crucial guideline. When in doubt, it is advisable to consult an expert or installation professional.

Minimum edge distance of mechanical anchors

Mechanical anchors work by producing expansion in the base material. This characteristic enables them to provide secure embedment. In order to ensure installation durability and reliability, it is important to check the anchor’s minimum distance from the substrate edge and from adjacent anchoring points (i.e. anchor spacing).

If anchors are set too close to one another or to the edge, the stress cones produced by neighbouring anchors can overlap, causing the load bearing capacity and durability of the installation to decline. Additionally, setting a mechanical anchor in close proximity of the edge can lead to weakening of the base material structure which, in turn, increases the risk of material failure and loss of the fastener’s load bearing capacity. Therefore, it is usually recommended that mechanical anchors are to be set at a certain distance from the edge as well as from one another to ensure adequate fixing strength and structural safety.

R-HLX Concrete Screw Anchor
Rawlplug's Concrete Screw Anchor R-HLX

Concrete screws feature a unique design and setting mechanism, which distinguishes them from other steel fasteners and makes them suitable for fixing with a relatively small edge distance and in moderately dense spacing. Their specific design enables them to penetrate the base material with much less risk of the structure weakening. The penetration depth and thread angle of concrete screws are key factors affecting the mechanical strength and load bearing capacity of the installation.

Must-know fact!

With a thread of suitable design and adequate length, these screws make it possible to achieve optimised expansion effect in the base material which, consequently, contributes to the increased stability and durability of fixing. It is the reason why concrete screws are typically chosen in cases where one is forced to fix in close proximity to edges or to other anchoring points. Concrete screws can also be used for temporary installation as they can be easily removed. This specific feature is what bonded anchors cannot do not provide. Once applied, resin cannot be removed from the hole.

Minimum edge distance of bonded anchors

Where the distance between individual anchors or between fixing points and the edge is relatively small, using mechanical anchors can cause a risk of excessive stresses, which makes it impossible to achieve the intended installation effect. In such cases, bonded anchors represent a viable alternative, as their operating principle is based on adhesion rather than material expansion. A bonded anchor is characterised by application flexibility, making it suitable for fixing in close proximity to edges as well as in dense anchor spacing.

Rawlplug Chemical Anchors
Rawlplug's Bonded Anchors offer

Resins can be used in a variety of environmental conditions, including sat extreme temperatures, in wet and flooded base materials, in seismic zones as well as fire-prone areas. On account of its versatility and effectiveness, a bonded anchor is often the preferred – and frequently the only available – fixing solution for installation in concrete.

Minimum distance between anchors and the edge of a concrete slab

Concrete is a structural material offering high compressive strength and widespread use in construction. Its tensile strength, on the other hand, is relatively low. The most crucial aspect affecting the effectiveness of anchoring in concrete is the specific substrate type. The distinctive types of concrete are:

  • non-reinforced concrete,
  • reinforced concrete,
  • non-cracked concrete,
  • cracked concrete.

Additionally, concrete can also be broken down into the following classes according to its weight by volume:

  • plain and heavy concrete (e.g. classes C20/25 or C50/60),
  • and lightweight concrete (e.g. classes LC25/28 or LC55/60).

Given the effect of the stresses caused by the expansion of the mounting pieces being anchored and the loads for which such anchors have been engineered, one must take specific parameters into account when defining the technical data used to establish the load bearing capacity of individual products. Consideration should be given to factors such as:

  • minimum substrate thickness, determined by the effective embedment depth (hef),
  • minimum anchor spacing (s)
  • distance from anchors to the edge of a concrete slab or structural element (c1, c2) as well as to corners (c3).

The overlapping of the stress cones of adjacent mounting pieces embedded in concrete reduces the load bearing capacity of the fixing due to the effect of tensile forces.

Effect of edge distance on the load bearing capacity of anchors

In certain situations, for instance when installing a handrail, anchors need to be set in close proximity of edges, both for aesthetic reasons and to maximise the use of the available space. Such decreased distances to edges and corners can adversely affect the load bearing capacity of anchors, which requires adequate reduction coefficients to be applied in order to account for these factors.

Various reduction coefficients appear to be relevant in the context of the anchor load capacity calculations, including:

a) anchor spacing coefficient s (fs);

b) coefficient c1 associated with the distance between the anchor and the base edge, where no load is transferred in the direction of a free edge (fc1);

c) coefficient c2 associated with the distance between the anchor and the element edge, where there is load transferred in the direction of a free edge (fc2);

d) coefficient c3 associated with the distance between the anchor and the slab corner (fc3);

e) and f) applicable where one must take into account the anchor located in the most unfavourable position within a group of anchors.

Odległości kotew od krawędzi

Characteristics of concrete screws

Concrete screws are highly specialised mechanical anchors enabling permanent fixing of structural elements in concrete substrates. One of their characteristics is that they can be screwed into the base material and, if necessary, removed freely. For small edge distances or dense spacing of fixings, concrete screws prove particularly effective, and often the only viable solution.

Concrete screws are designed specifically to provide secure fixing in hard substrates. The perfect design and geometry of the thread are the key aspects behind the high functionality of this anchor type. For example, the R-HLX concrete screws feature a thread of novel design, terminated with additional undercutting teeth for fast and easy installation, also in reinforced concrete of grades C20/25–C50/60. It is the combination of the above features that makes concrete screws perfect for fixing close to edges or near other mounting pieces. This makes them an ideal choice for the installation of railings or balustrades, ensuring safety and visual appeal at its highest, especially when the use of bonded anchors is not possible.

R-HLX Concrete Screw Product Information
R-HLX Concrete Screw Product Information

Did you know that…?

Screw-in anchors also offer the possibility of fixture adjusting, e.g. when it has originally been installed crooked. According to the applicable technical approval, such an anchor can be loosened by up to 10 mm, the fixing corrected, and the concrete screw re-tightened without impairing its load bearing capacity. The approval allows this to be done twice.

Edge distance vs embedment depth

The following table provides a technical specification of the R-HLX concrete screws. The data include: nominal anchoring depth (hnom ≥), minimum hole depth (h0 ≥), minimum anchor spacing (Smin), minimum edge distance (Cmin), and effective anchoring depth (hef). All these values have been stated relative to the hole diameter (d0) and the corresponding screw sizes (O10, O12).

R-HLX Concrete Screw Anchor Table

Characteristics of bonded anchors

When used for installation in concrete, bonded anchors represent the highest standard of durability and versatility of fixing in this substrate. Owing to its unique properties and operating principle, resin comes as an effective alternative where traditional mechanical anchors are insufficient. However, on account of the characteristics of bonded anchors, one should still consider the matters of minimum distances from edges and other anchoring points, since these parameters are paramount to the safety and durability of installation.


The engineering method applied when designing bonded anchors in line with the ETA guidelines draws on the experience related to their mechanical strength. Bear in mind that the European Technical Assessment is a normative document which takes precedence over other engineering methods should any discrepancies arise!

There are many types of bonded anchors available on the market, some of which are designed for use in concrete only, while others, such as polyester resin-based anchors, can also be used in other substrates. The choice of the right bonded anchor for concrete applications should be relative to a number of factors, such as the type and magnitude of the load (i.e. the capacity to carry heavy loads), the ambient temperature in which the installation is to be conducted, and some environmental conditions such as exposure to earthquakes or fires. One should also strictly follow the manufacturer’s recommendations concerning the minimum distances from bonded anchors to edges and other anchoring points.

Must-know fact!

Each type of anchor requires adequate substrate preparation and precise installation to ensure maximum fixing efficiency and structural safety. Thorough hole cleaning prior to installation and careful removal of all debris and dust from inside the hole are both important steps towards full effectiveness and durability of anchorage.

Another important factor to take into account is the difference in the parameters of bonded anchors relative to the type of the element to be fixed. These parameters may vary due to the installation specifics and the load bearing capacity requirements of a given joint. Some examples of the bars and rods one should take into consideration when selecting appropriate bonded anchor parameters include:

  • Threaded rods: They feature an external thread which makes them easy to attach to structural members. When choosing a bonded anchor for threaded rods, one should consider the thread diameter and the rod length. Being particularly versatile and convenient in handling, they are commonly used in a variety of construction and installation jobs.
  • Internally threaded rods: These rods feature an internal thread, which makes them suitable for installation using nuts or other fasteners. When choosing a bonded anchor for internally threaded rods, one should consider the diameter and length of the internal thread. They are intended for a variety of applications, especially where it is necessary to attach fixtures directly to the rod rather than to the hole surface.
  • Reinforcing bars: Reinforcing bars are used in concrete structures to reinforce them and increase their load bearing capacity. When choosing a bonded anchor for rebars, one should consider the bar diameter and the method of its installation in concrete.

It is important to note that the parameters of bonded anchors matching different types of bars may vary due to the bar diameter, length, material (steel grade and strength), and fixing method. Therefore, it is important to carefully match the parameters of the bonded anchors to the specific installation conditions and load bearing requirements of a given joint.

Minimum edge distance of anchors intended for other substrates

When using anchors in substrates of different properties than concrete, the matter of the minimum required edge distance is just as important as for concrete applications. Only by determining the right safety margins can one ensure the durability and stability of the joint to be developed as well as prevent damage to the substrate and maintain structural integrity. The variety of the material properties to be taken into account requires a carefully tailored installation process, which only emphasises how important it is to understand the very nature of minimum distances and correct application techniques (e.g. use of the right drill bit).

Function of mesh sleeves in the application of bonded anchors

Mesh sleeves play an important role when using bonded anchors in hollow or crumbling substrates, such as certain grades of concrete or hollow masonry units. They represent a key component of bonded anchor systems, providing a stable and lasting base for fixing in these specific substrates. Mesh sleeves are made of materials resistant to both chemical compounds and the environment, which makes them durable and effective in diverse installation conditions.

Tuleja siatkowa

When fixing in hollow materials, one needs mesh sleeves to be inserted into drilled holes and then filled with resin. In concrete, holes should be filled to 75% of their depth, while in hollow substrates (e.g. brick or hollow masonry units), they are to be filled in 100%. This not only guarantees secure and durable bonding between the sleeve and the substrate, but also an effective transfer of mechanical loads through the bonded anchor.

Characteristics of polyester anchors

Polyester anchors, also referred to as universal anchors, are resins that can be used in most substrates.

The main advantage of the polyester anchor is its high load bearing capacity and corrosion resistance, making it ideal for application in a wide range of environmental conditions. Unlike epoxy resin-based anchors, which are mainly used in concrete, polyester anchors are widely used in other substrates such as brick, stone, and ceramic ware. They are also more flexible than epoxy resin anchors, which can prove advantageous when working with materials of variable structure or prone to dynamic movement.

The polyester anchor also offers the possibility of quick curing, reducing the time needed to attain full post-installation strength. Its versatility and mechanical strength make it a popular solution for fixing of structural elements in various built features as well as for internal and external installation jobs. 

In the case of bricks and hollow clay blocks, this bonded anchor type offers far more possibilities than traditional fixings, such as expansion plugs. The relevance of installation parameters is essentially limited to the quality of the substrate material, rather than the capability of the bonded anchor itself, as the latter can handle considerable loads without much problem.

With reference to specific examples, the following tables define key installation parameters of bonded anchors against different substrates, both hollow and solid, such as aerated concrete, solid brick, and hollow construction materials. These include: thread diameter, substrate hole diameter, minimum substrate hole depth, minimum embedment depth, minimum anchor spacing, and minimum edge distance. For hollow substrates, mesh sleeve sizes have also been defined.

Aerated Concrete Table
Solid Brick Table
Hollow Substrates Table

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