Simplifying concrete specification

The Concrete Centre has updated its TCC-BS 8500 tool, a free spreadsheet for helping specifiers to navigate the UK standard on concrete specification. Liam Forde explains how to use it.

The UK standard for the specification of concrete, BS 8500, is a decision-making framework for translating design requirements, environmental conditions and the intended lifespan of a structure into a recipe for sound, durable concrete.

Specifying concrete is complex because it is not a single product, but a blend that must be tailored to meet different applications, exposure classes and carbon targets, and which may include a rapidly expanding range of lower-carbon cements.

The 2023 update to BS 8500 dramatically widened the menu of available options, increasing the complexity. It also changed the relationships between the exposure classes and limiting values of strength, minimum cement content and water/cement ratios, and introduced combined performance categories (CPCs) allowing producers to supply the lowest-carbon mix from many possible combinations.

Specifiers use the methodology in BS 8500 part 1 to determine a set of performance requirements – compressive strength, minimum cement content, maximum water/cement ratio and CPCs – for the concrete producer to meet. This can be a detailed calculation that involves optimising for multiple variables and balancing project constraints, structural design, buildability and carbon.

Fortunately, help is at hand: the TCC-BS 8500 tool was developed by The Concrete Centre to simplify the specification process and support early-stage decision-making on sustainability. Users input exposure conditions, design strength and cover. The tool then shows the limiting values that will satisfy durability requirements, compliant CPCs and indicative carbon values.

Specifying concrete with TCC-BS 8500

The TCC-BS 8500 spreadsheet is free to download from www.concretecentre.com/TCCBS8500. It is regularly updated, so specifiers should always check that they are using the latest version.

There are five tabs: Start, Specification – Concrete, Input Values, BRE and Notes. The key sheet is “Specification – Concrete”. This is where most of the design and exposure data is input, and where the outputs are summarised. The specification itself is shown on the left, with the carbon calculation on the right.
There are three stages to producing a concrete specification that can be shared with a supplier.

1. Determine the exposure class
The first stage is to complete the exposure classes, under the “Exposure Conditions” heading, towards the top of the sheet. This provides the following options:

Carbonation, XC – All concrete reinforced with normal reinforcing steel is subject to carbonation, so this exposure class is the only one that is always assumed to apply. The level of carbonation (XC1, XC2 or XC3/4) can be chosen from the drop-down list.
Freeze-thaw, XF – The user chooses the level of freeze-thaw that the concrete will be exposed to (XF1, XF2, XF3 or XF4), and whether an air-entraining admixture should be added.
Chlorides, either from seawater, XS or not from seawater, XD – Non-seawater chlorides would typically be from de-icing salts. Three levels are available in the drop-down lists (XD1, XD2 or XD3, and XS1, XS2 or XS3).

2. Do you need a Design Chemical class?
Below “Exposure Conditions”, the next section is titled “Aggressive Ground Conditions to BRE Special Digest 1”. When concrete is exposed to sulfates in the ground, it must be specified to a Design Chemical (DC) class. If this is the case, the drop-down box under the “DC” text should be set to “Yes”, otherwise it should be left as “No”.

The spreadsheet can calculate the required DC class from the results of a site investigation, following the guidance in BRE Special Digest 1. The relevant values should be input under “BRE 1”. Background data is shown in the “BRE” tab for information, but cannot be changed.

The user does have the option to select a higher DC class in order to avoid using Additional Protective Measures (APM), such as surface membranes or sacrificial layers. They can also choose to allow the use of stronger mixes, which gives a greater range of options on cover and cement type.

3. Set the design requirements
Once the exposure classes have been completed, the user should return to the “Design Requirements” section at the top of the sheet to input the design strength class, working life (50 or 100 years), maximum aggregate size, minimum cover and permitted deviation in cover.

The adjacent box will show the minimum strength class required for the XC and the XF exposures. The user can change the chosen cover and/or the design strength class to ensure that the minimum required for the exposure is met.

The final specification

The bottom of the sheet will now give the final options: strength class, nominal cover, minimum cement content and maximum water/cement ratio for the compliant CPCs. This can be transferred into the specification for the supplier.

Using the carbon calculator

Alongside the specification itself, the TCC-BS 8500 tool shows indicative carbon values. This enables users to easily see how different parameters affect embodied carbon and to explore ways of lowering it.

The carbon calculator has several functions to simplify the outputs. At the top right-hand side, there is a drop-down menu, from which users can choose to show only the embodied carbon values, or colour-code them according to one of two industry rating schemes:

The Universal Classification by Arup-Innovate UK is intended for setting long-term targets and monitoring progress towards net zero. Concretes are ranked in static bands from A to G, where A is carbon-negative and G represents embodied carbon that is higher than a conservative industry baseline.
The GCCA Classification (UK adaptation), developed by the Global Cement and Concrete Association, uses rating bands based on the current market to allow global comparison of products.

These ratings can help inform carbon targets, although any target should be sense-checked with concrete suppliers to ensure it is feasible.

At this point, users can choose to remove non-compliant cements that do not meet strength and durability requirements. They can also change the water content to alter the fresh properties – this will influence the cement content, and therefore the carbon values. The columns show the options for water/cement ratio and minimum cement content, in order to demonstrate the effect of these limiting values on carbon.

Rows show different cement types, with the cement content required to achieve the specified strength on the far right. This may be the most significant factor in the carbon calculation. From the three limiting values (water/cement ratio, minimum cement content or strength), the carbon calculator will take the highest cement content, as this is how the actual cement content will be determined by the supplier.

Embodied carbon factors

Default embodied carbon factors are listed on the sheet titled “Input Values”. These are typically based on data such as sector environmental product declarations (EPDs), but they can be replaced when more accurate data is available from manufacturers.

The alternative figure should be input into the “User” column. In the adjacent column, “User” should be selected
from the drop-down menu under “Used”, and this will update the carbon calculations in the main specification sheet. The input values should be set back to “Default” for new projects.

This sheet also shows the input values for concrete density, water and admixture content, and the cement content required to meet design strengths. These can also be changed, but any data should be validated with guidance from suppliers.

The TCC-BS 8500 tool is intended to aid specifiers in understanding how using different cements, altering design inputs and changing fresh state properties can affect the final carbon figures for a concrete specification. While it is only indicative, it can help steer early-stage design decisions. As projects progress, working collaboratively with concrete suppliers is essential for unlocking further carbon savings.

Liam Forde is a chartered civil engineer and head of construction and infrastructure at the Mineral Products Association

Photo Moore Concrete, Kilnbridge, Holcim UK