What the concrete grade means for your Bristol project

A ready-mix quote often lists a code such as C25 or GEN1 against each item, and the code can look like a reference number rather than a description of the concrete. In fact each code tells the supplier what strength the mix must reach and, for designated mixes, what sort of job it is meant for. Understanding the grade makes it easier to check that the concrete is right for a path, a slab or a footing, and to ask the supplier a precise question when something on the quote does not match the work.

What a strength class describes

Structural concrete is classified by its compressive strength. In the common notation, a class such as C25 refers to the characteristic compressive strength measured on a standard test specimen, which is the value the mix must reach with a high degree of confidence. The concept is set out in the European standard for concrete, and the BSI's standards publications explain how the classes are defined. A higher number means a stronger mix, which generally needs more cement and therefore costs more.

The number on its own does not describe durability. A mix can reach the required strength yet still be unsuitable for a site that is frequently wet and frozen, because durability depends on the water to cement ratio and on the exposure the concrete will face. That is why a quote should give both the strength class and the exposure, rather than the strength alone. The Concrete Centre explains the relationship between strength and durability in its guidance on concrete, and it is a useful reference when a quote seems to specify a grade that does not fit the job.

Designated mixes and GEN grades

Designated mixes are pre-defined mixes that carry a name and a known performance. The GEN series, which runs from GEN 0 to GEN 3 in the British standard, covers general-purpose concrete for work that does not carry significant load, and GEN1 is one of the common choices for domestic jobs such as paths, patios and some floor slabs. The designation tells the supplier which mix to produce, which saves the buyer from specifying every property individually.

A designated mix is sensible for many garden projects, because its performance has already been defined for typical uses. The question to ask is whether the job is a general-purpose application or whether it carries a load that needs a higher class. A path designed for pedestrians can usually use a GEN mix, while a driveway that takes the weight of a car may need a stronger grade, and the supplier should confirm which applies.

The mineral products sector describes how these designations are supplied, and the Mineral Products Association's homepage is a helpful starting point for understanding the ready-mixed range.

Typical uses for each grade

For a footing that supports a fence or a low wall, a general-purpose mix is usually sufficient, provided the ground is firm and the footing is deep enough. A slab that supports a shed or a patio is commonly specified in a GEN mix, with the depth and reinforcement chosen to suit the load. A driveway that carries cars needs more careful thought, and a higher strength class is often specified, particularly where the drive is near a turning area or a point where vehicles brake.

Structural elements such as a retaining wall, a suspended floor or a foundation for a building are different. They need an engineer to specify the concrete, and the grade is chosen to match the design. A quote that offers a general-purpose mix for a structural element should be questioned before it is accepted.

Exposure matters as well. A slab in a wet, frost-prone location, or one that will receive road salt, needs a mix that resists those conditions. That requirement is usually written as an exposure class, and the grade and the exposure class should be stated together on the order.

Reading a grade on a quote

When a quote lists a grade, check that it matches the use described in the scope. A path quoted in a high-strength mix may be unnecessarily expensive, while a driveway quoted in a low-strength mix may fail early. The scope should name the job, the load it carries and the exposure, and the grade should follow from those. If the grade is not explained, ask the supplier to state it in words as well as code.

Look also for consistency. The volume on the quote should match the area and depth stated in the scope, and the grade should match the sub-base and the finish. A slab on a poorly prepared sub-base may crack whatever the grade, so the grade alone does not ensure performance. The guide on ground preparation for a Bristol slab or drive explains how the sub-base affects the result.

Asking the supplier the right question

A useful question for any supplier is: which mix would you recommend for this use, and why? A knowledgeable supplier will explain the grade, the exposure and the reason for any admixture, and will flag where the chosen mix is unsuitable. A supplier who simply repeats the code without explanation has not shown that the mix has been considered for the particular job.

Ask also whether the mix is a designated or a designed mix, and whether a higher grade is being quoted as a default. Some suppliers offer a higher class to cover the risk of a mistake, which raises the cost without necessarily improving the result. The question is a fair one, and the answer shows whether the quote reflects the work or the convenience of the supplier.

Where this fits in the wider plan

The grade is one decision among several. The volume, the access, the sub-base, the reinforcement and the finish all affect whether the slab performs. The standards behind these classes and designations are explained in the guide on BS EN 206 and BS 8500 concrete standards in Bristol, which complements the grade explained here.

For a comparison of quotes, the same grade and the same exposure class should be requested from every supplier, so that the figures can be read side by side. The quote page on this site helps to set out the information each supplier needs, and the services page describes the concrete work covered. A grade chosen for the job, and explained clearly, is a sound basis for a Bristol project that will last.

Matching the grade to the risk

The grade is also a question of risk. A grade that is too low for the load will show cracking, edge breakage or surface wear sooner than expected, and the repair of a failed surface costs far more than the difference between two grades would have cost at the start. A grade that is higher than necessary costs more without adding useful life, and it does not compensate for a poor sub-base or a thin slab. The practical approach is to choose the grade that matches the use, to check that the depth and the sub-base are consistent with that grade, and to ask the supplier to confirm that the combination is sound.

Bristol has many older houses with shallow or uneven front gardens, where a new slab is laid over ground that has been altered several times. In those situations the grade alone cannot solve a problem that begins in the ground, and the contractor should describe how the base will be prepared before the grade is discussed at all. Where a new slab forms part of a vehicle crossing, the council's requirements may also set out the grade and thickness, and the guide on how a Bristol dropped kerb application is handled explains how the consent process works before the concrete is placed.

Finally, the grade should be stated on every document that describes the work, including the quote, the order and the delivery note. A grade that appears on the quote but not on the delivery note leaves room for a substitution, and a substitution is hard to detect once the concrete has been placed. Checking the delivery note against the order on the day is a simple discipline that protects the buyer, and it costs nothing beyond a few minutes at the kerb.

Sources and further reading

1. BSI Group: standards and publications (BSI Group)
2. The Concrete Centre: material, design and construction guidance (The Concrete Centre)
3. Mineral Products Association (Mineral Products Association)