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

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.

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.

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.

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.

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).

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.

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.

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.

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.