Strength Class in Concrete Pipes: How Much Load Does “Class 90” Mean?

Before getting into the subject, let us clarify four abbreviations that are constantly encountered in specifications and on product labels.

• DN — nominal diameter. The nominal dimension, in millimetres, used to designate the product. The expression “DN 600” indicates that the nominal diameter of the pipe is 600 mm. Because it is a nominal size, it does not have to be exactly the same as the actual internal diameter; it is the label that identifies the product.
• kN — kilonewton. A unit of force. 1 kN corresponds approximately to the weight of a mass of 100 kilograms; a figure of 50 kN can be thought of as a load of approximately 5 tonnes.
• F_n — minimum crushing load. The lowest load the pipe must carry, given per metre (kN/m). This is the requirement for load per unit length under the standard test arrangement.
• F_c — crack load. For reinforced pipes, the load level at which the crack width is measured.

Once these four are in place, what the class number says becomes self-evident.

The performance of concrete pipes under transverse loading is assessed using the support and loading arrangement defined in the standard. F_n denotes the minimum crushing load that must be achieved for the class concerned; its unit is kN/m. The result obtained in the test is compared with this requirement and with the other acceptance conditions.

The strength class, in turn, is the ratio of this load to the nominal diameter:

Class = F_n ÷ (DN / 1000)

Inverting the formula gives F_n = Class × (DN / 1000). This value gives the minimum test load corresponding to the class; it is not used as the permissible load on site or directly as the load-bearing capacity.

Let us consider a DN 400, Class 120 pipe:

F_n = 120 × 0.40 = 48 kN/m

For a DN 1200 pipe of the same class:

F_n = 120 × 1.20 = 144 kN/m

Both carry the same class label; the minimum test load required for the larger-diameter product is three times as high per metre. This relationship follows from the definition of the class; on its own, it does not demonstrate the suitability of the products on site.

Different diameters should not be compared by class number alone. F_n can be used to compare test load requirements; product selection is made by considering hydraulic requirements, loads, bedding and installation conditions together.

Returning to the example in the title, for a DN 600, Class 90 pipe:

Fn = 90 × 0.60 = 54 kN/m

For a pipe to be accepted, it is not enough for it simply to carry the load without breaking. Depending on what the cross-section contains, a second criterion comes into play.

Unreinforced, reinforced and steel-fibre-reinforced concrete pipes have different test stages and acceptance conditions. Crack control, crushing load and the other required checks are assessed in accordance with the standard applicable to the product type concerned and the project requirements.

The crack limit used in a particular test is not a general acceptance criterion that can be applied to all products or to all cracks on site. The load level, the test stage and the measurement method are considered together.

The class number is not an invention of the catalogue; it is the outcome of the project. The load on the pipe arises from the burial depth, the soil conditions, the trench and fill conditions and the traffic passing overhead. The required test strength is determined by considering together the calculated site loads, the bedding conditions and the safety principles of the design method used. The appropriate strength class is selected accordingly.

Two practical conclusions follow from this:

• Asking for a higher class at the same diameter is not always “better”; a class higher than necessary is a cost with no corresponding benefit.
• The class cannot be considered independently of the laying conditions. The calculation is based on a specific bedding and fill assumption; if the site does not match this assumption, even a product of the correct class will not deliver the expected performance. Laying conditions are covered by TS EN 1610.

On a pipe manufactured in accordance with the standard, the nominal diameter, strength class, manufacturer information and production date must be legible on the product. When taking delivery on site, comparing this marking with the value in the specification is the simplest and most effective check that can be carried out.

A similar logic applies to inspection manholes; the strength and acceptance criteria for manhole units are defined in TS EN 1917.

• The class is the ratio of the crushing load to the diameter in metres: Class = F_n ÷ (DN/1000).
• Multiplying the class by the diameter in metres gives the minimum test load; suitability for the site is calculated separately.
• The test load requirements of different diameters can be compared using F_n.
• Crack and strength acceptance conditions are assessed according to the product type and the relevant test method.
• The load and laying conditions of the project are decisive in selecting the required strength class.

In determining the appropriate class for your project, information such as diameter, burial depth, soil, bedding, fill and traffic loads is assessed together. You can share your project data with our technical team.