BS 8500 FAQs
The purpose of this Frequently Asked Questions (FAQs) page is to address questions that have been received either at events or through The Concrete Centre helpline. This page will be regularly updated as more queries come through. Please also see ‘How to design concrete structures using Eurocode 2: BS 8500 for building and civil structures’ which provides specifiers comprehensive information on the revision.
Some of the questions on this page may fall outside the scope of BS 8500 but will be affected by the changes to the standard. For clarity and ease, when the following words are used in this FAQ page the definition provided below is to be used:
- Cement –refers to cements produced by the cement supplier and equivalent combinations which are produced by the ready-mixed concrete supplier by combining a cement with supplementary cementitious materials (SCMs) in the mixer.
- CEM I – refers to a cement with >95% Portland cement clinker content.
- Clinker – the end product from production of cement. It is nodular in appearance, normally 3-25mm in diameter and grey in colour. The BS 8500 How to Guide [link] provides further detail on the process of producing clinker.
General
Q: What are the principal changes in BS 8500-1:2023 and BS 8500-2:2023?
A: On 30 November 2023, BS 8500-1:2023 and BS 8500-2:2023 were published. This change to the standard increases the range of lower carbon concretes that can be specified.
Since the 1980s, the UK experience of using supplementary cementitious materials (SCMs) has mainly been combining Portland cement (CEM I) with either fly ash (FA) or ground granulated blast-furnace slag (GGBS). In 2021, the cement standard EN 197-5, was published and allowed cements with up to 65% of the Portland cement clinker to be substituted with two SCMs, so providing multi-component equivalents to the binary combinations that have become well established in the UK. Extensive testing was carried out on these new multi-component cements, which allowed the update in BS 8500:2023 that will enable the specification of these lower carbon concretes.
With the new standards now available, the CEM I content in concrete can be replaced with up to 20% of limestone fines (powder) , an SCM that can be sourced locally across the UKand another SCM such as GGBS. For every 5% of limestone powder used, approximately a 5% CO2 reduction can be delivered per tonne of cement.
With the potential to use these new concretes across all mainstream applications, the associated carbon saving could add up to an annual saving of 1 million tonnes of carbon dioxide.
Research commissioned by the cement members of the Mineral Products Association found that the inclusion of limestone fines in a CEM I and GGBS ternary system improved material efficiency, demonstrating that the limestone fines could be used as a replacement for both the CEM I and GGBS portions of the blend. This results in concrete with both a lower embodied carbon and a reduced use of GGBS, whilst producing an equivalent performance.
Q: How will the new cements affect visual concrete?
A: The recommendation for visual concrete is that a minimum of 350kg/m3 of fine material (less than 125µm) is needed within the concrete. The fine material would include cement, fine material within the sand and any fine filler material used such as limestone fines or pigments. The reason the required amount of cement is higher for visual concrete in comparison to an equivalent concrete not used for visual applications is due to the need for a greater proportion of fine material near the face of the concrete. The fine material will have the smallest particle size within a concrete mix in comparison to the other constituent materials. To create the smooth finish on the face of the concrete, the smallest particle sizes should be at the face. Therefore, having a higher fine content increases the number of smaller particles at the face and reduces the risk of imperfections when striking the concrete.
It is also worth noting, that because the limiting value for visual concrete tends to be the cement content required to achieve the finish, this will drive the final strength. Therefore, consideration should be given to the anticipated strength that will be achieved based on the cement content needed for the visual concrete, as it may be possible to use a lower strength cement (which will have a lower embodied carbon associated with it) and still achieve the strength requirements for that concrete.
However, a key consideration with visual concrete is ensuring the colour meets the requirements of the specification and is consistent across the project. The cement type will influence the colour of the concrete and therefore, when planning a project for use of visual concrete, trials should be carried out using the desired cements.
Exposure classes/limiting values
Q: Why have the limiting values been changed for carbonation and chlorides?
A: In previous versions of the British Standards for concrete (BS 5328 and before), it was the responsibility of the contractor to test compliance of a concrete to the specification. The water/cement (w/c) ratio tends to be the limiting value that controls the durability of the concrete, however, to check the w/c ratio on site isn’t straight forward. Therefore, to test for this the site teams would carry out slump tests, and by checking if this is within the allowable range alongside knowing the batched cement content, it was assumed that the w/c content was correct. The site team would then also produce cubes to check the strength. This led to using minimum cement content, maximum w/c ratio and strength as the limiting values within the standard. As part of the revision to BS 8500, to allow the industry to move toward using lower clinker cements and therefore lower carbon concrete, the British Standards committee agreed that the limiting values within the standard needed to be decoupled. This has led to a couple of changes detailed below:
Carbonation
As part of the testing carried out by BRE it was seen that there was a good relationship between carbonation depth and 28-day strength for all cements, a relationship that had also been noted previously. As a result, the committee(?) decided to allow for easier specification for concrete exposed to carbonation, by specifying only 28-day characteristic strength and allowing for any cement to be specified. The standard does however state that any concrete that is reinforced with carbon steel should have a minimum cement content of 200kg/m3.
Chloride
As mentioned above, the w/c ratio is the key limiting value when looking at durability for chlorides and strength was previously included to check compliance to a specification onsite. It was noted that the inclusion of strength as a limiting value in this revision may unintentionally preclude the use of lower clinker cements with lower strengths but adequate durability. It was agreed by the committee(?) that removing the strength requirement and only using maximum w/c ratio and minimum cement content for the limiting values for chloride exposure classes (both XD and XS) would meet the durability requirements and allow for the maximum use of lower carbon concretes within the standard.
Q: If strength is used as a proxy for carbonation resistance, what effect does aggregate strength have on that proxy?
A: It is certainly the case in high-strength concretes that compressive strength can plateau as the failure mechanism changes from the matrix and interfaces to a failure of individual particles of the coarse aggregate. Until that failure occurs, the coarse aggregate is simply resisting the applied stress and does not contribute to the strength of the concrete. The point at which this occurs is not defined by geological type and may even vary within individual sources depending on the location of extraction in the quarry.
BS 8500-1:2023 (and all previous versions of the standard) have limiting values for resistance to corrosion by carbonation that are derived independently of the geological source of the aggregate and so no correction is required.
Q: Within tables A.4a and A.5a for corrosion by carbonation in BS 8500:2023, the minimum concrete strengths, especially for XC3/4, seem to have increased from the previous revision. Could you confirm why this is and why the flexibility in setting different w/c ratios and minimum cement contents has been removed?
A: Carbonation of concrete is a chemical reaction where carbon dioxide from the air, and calcium hydroxide in the cement paste react to form calcium carbonate. This lowers the pH of the concrete from 12 or 13 to around 9 which breaks down the protective oxide layer surrounding reinforcing steel and corrosion of the steel may occur.
The reaction between carbon dioxide and calcium hydroxide can only take place in solution so water is required for carbonation to occur. The drier the concrete, the slower the reaction will be, although there are very few situations where the relative humidity of the environment will be low enough for carbonation not to be considered. In low humidity internal environments, the corrosion rate of the reinforcement will be slow as this requires greater humidity than that for carbonation. In concrete, which is saturated, the water filling the pore structure slows the ingress of carbon dioxide, once again slowing the rate of carbonation. The optimal conditions for carbonation of concrete are when there is sufficient moisture to allow the reaction but not enough to prevent the ingress of carbon dioxide.
Carbonation occurs from the surface of the concrete inwards so slows with time. The carbonation rate of the concrete and the expected working life of the element will determine the required nominal cover to the reinforcement.
Specification by limiting values
The specification of concrete durability is determined by the working life, exposure class, the cover, and the cement type. This gives us limiting values of characteristic strength, minimum cement content and maximum w/c ratio. If the concrete meets these minimum requirements, then the concrete is deemed to satisfy the specification. The concrete producer determines the required cement content for each of the limiting values, with the greatest being selected for the mix design. It is common for the mix design cement content to be determined by the minimum cement content as the cement content for both characteristic strength and w/c ratio could be significantly less than this with the use of high-range water reducing admixture (superplasticizers).
Even where these admixtures are not used, the specified minimum cement content or maximum w/c ratio can require a cement content far greater than that required for characteristic strength resulting in inefficient design i.e., the design of the element does not make use of the additional strength.
BS 8500-1:2023 - resistance to corrosion due to carbonation
In previous versions of BS 8500, the limiting values for durability had a relationship which was based on Portland cement and Portland cement based binary combinations. With the introduction of newer lower-clinker composite cements, these assumed relationships no longer hold, and so recalibration was required. It was also decided that in this revision to BS 8500, where possible, unnecessary limiting values should be removed to prevent barriers to the adoption of these lower carbon alternatives and for producers to make use of optimised concrete mix design and modern admixtures.
From the review of the test data (both natural and accelerated carbonation), it was noted that that there was a strong correlation between compressive strength and carbonation rate, and, although this has resulted in some generalisation, that this was a better indicator of carbonation resistance for all cements included in BS 8500-1:2023. This has allowed the removal of minimum cement content and maximum water/cement ratio as limiting values for carbonation resistance and, although it was not the aim of the revision, it has resulted in significantly simplifying the specification process. This change also addresses the effect that these limiting values have in restricting the optimization of concrete mix design to produce concrete of the required strength with the lowest possible cement content which is key in improving the sustainability of concrete.
The recalibration has led to efficiencies in some areas and being more cautious in others. This does not necessarily mean that the rules in the previous version of BS 8500 were insufficient for the concretes they were originally written for, but that the rules were not adequate for the inclusion of lower-clinker ternary combinations to BS EN 197-5. From evidence elsewhere in Europe, it is expected that supply of CEM I (Portland cement) in the UK will reduce significantly, and in a short period of time, to be replaced with CEM II/A-L (Portland limestone cement) so it is likely that ternary combination concretes will be predominant types supplied.
In summary the use of concrete strength alone has been demonstrated to be a better indication of carbonation resistance, than previous methods, for modern concretes likely to be supplied in the future in the UK. For this reason, this is the approach adopted by BS8500-1:2023. In some cases, this approach requires more concrete cover than the previous version of the code but enables lower carbon concretes through more flexibility in the cement options and more opportunity for mix optimisation.
Identity/conformity testing
Q: How confident are we about accuracy and precision of digital monitoring systems, is it premature to rely too heavily on it?
A: Digital monitoring systems may be specified as an alternative method of monitoring the plastic properties of the delivered concrete. These systems can be used instead of, or to supplement identity testing such as for consistence, and by the producer to aid factory production control. These do not replace conformity testing. If there are doubts over the consistence shown by the digital monitoring system, trained and competent site personnel may still carry out the appropriate testing to confirm.
It should be noted that these systems depend on the accuracy of the derivation mechanisms in use, and the level of initial and continuing calibration. As long as the systems are maintained and calibrated, they remain effective for use.