I am working on a rail project in Toronto and the partner architect is concerned that higher levels of cement replacement (beyond 25%) will problematically reduce strength of “rail carrying structure”. This structure should have 100 year durability. Does anybody have a references that might help me respond to this concern? Thank you.
Hi @aaron.vaden-youmans - my understanding is if anything the opposite is true: mixes with cement replacement tend to have a higher long-term strength. The issue is more with early strength, which can be an issue for elements like PT slabs where you need to strip forms fast to keep schedule.
If long-term durability is a driving factor, you may also want to consider concrete cover on the rebar depending on exposure and environmental factors. I’m not familiar with rail projects, but maybe others have more to contribute on this topic as well.
Concrete mixes conforming to CSA A23 can be considered suitable for the exposure conditions. Unfortunately AFAIK the Canadian standards do not define a service life, so you may want to engage a concrete specialist to demonstrate to the client that the mix is suitable. In general I would say the following applies:
- higher cement replacement increases durability of the concrete against chlorides, typically this is the limiting deterioration mechanism for concrete in Canada thanks to deicing salts.
- the initial pH of high SCM mixes is reduced compared to OPC, so resistance to carbonation can be somewhat reduced.
- air entrainment is required for freeze / thaw resistance in cold climates (e.g., Toronto), the admixture dosing may need to be adjusted for high SCM.
- hydration rate and thus strength gain is generally delayed by use of SCMs, hence for thermal crack control we will normally use a higher SCM mix, however ultimate strength gain of the mix will be designed to meet the characteristic strength. I don’t believe there are any issues with long term strength of high SCM mixes, but would be very interested to hear about this.
- resistance to cement paste attack is improved by slag.
For references I would start with CSA A23, ACI 201, and ACI 222. As always speak to your concrete supplier as early as possible to talk through the requirements of the mix and what options are available!
As a final point the ultimate cement replacement mixes are alkali activated materials / geopolymers, and although the mix design is a little trickier, their long term strength and durability are proven in the field since at least the 1950s.
SCM cement replacement changes the chemistry of the concrete, some properties are less desirable, and others are beneficial to long term durability. I agree with @Luke-Lombardi and @will.nash, in general the good outweighs the bad when it comes to durability and the inclusion of SCM’s (supplementary cementitious materials).
However, we should not be looking at this in binary terms. SCM’s are just a single variable in the mix and other factor such as water-cementitious ratio and reinforcing cover play a significant role in durability of the solution. Your approach to a durable design will depend on the environment you are in and the properties you are trying to promote.
I would not fear a high volume SCM. Early strength is a real limitation, but even this can me mitigated by blending in more reactive SCM’s or by using more finely ground Portland Cement.
I would recommend you hire a concrete mix professional that is familiar with concrete durability to help maximize cement reduction while achieving the projects durability requirements.
The National Ready Mix Concrete Association (NRMCA) has a great series of resources promoting the concept of performance-based concrete specs (specs that would not limit SCM content, among other restrictions). This series incorporates technical resources, including resources to understand the impact of SCMs on durability. My personal takeaway from the material: set strong performance requirements and don’t place limits on SCMs. Encourage low carbon, and consult with your local concrete providers to see what options they may be able to provide to meet low carbon and performance criteria.