In the past few decades, monolithic strip-type foundations cast of commercial concrete directly at construction sites have become very popular in private construction. The standard foundation blocks that used to be real classics have begun to give ground to the more practical and efficient monolith. It is worth noting that the term “monolithic” can refer not only to strip foundations but also to various combinations such as bored piles or pile-and-beam foundations. The abundance of design solutions leads to the need of choosing the proper concrete grade which, according to its technical characteristics, will be suitable for a specific type of foundation.
If you have ready-made design documentation available which specifies the concrete mix grade, its freeze-thaw resistance, water tightness, and mobility, then this problem is already solved. If there is no such design documentation, you will have to choose the concrete yourself. You don’t need to know everything about concrete, but you will still need some basic information.
Choosing a Concrete Grade for Your Foundation
Regardless of what kind of foundation structure you prefer for a house under construction, you will have to decide on a concrete composition which will be suitable for all tasks. What parameters does foundation concrete have? There are only three of them:
- total weight, ultimate structural load
- geotechnical parameters of the site (groundwater level, soil type, etc.).
- type of the underground parts of the structure (piles, strip, basement).
All of the above factors will, in a certain way, affect your choice of the concrete grade and its special parameters. Let’s consider each of these parameters in more detail. See also how to choose concrete for a foundation.
Ultimate Structural Load
It’s one of the most important factors to consider when choosing a concrete grade. A prefabricated panel house will weigh much less than a reinforced concrete or stone structure. Accordingly, lower-grade concrete is recommended for a prefabricated house. Here is an example. If you build a prefabricated panel house, the M15 concrete grade will be good enough. For a log house foundation, it is better to use concrete grades M20 or M25. The same grades will be suitable for the foundation of gas silicate, expanded clay concrete, and foam concrete blocks since their specific weights and densities are not too high and are almost the same as the density of wood. When building houses of bricks or prefabricated structures of reinforced concrete (or similar heavy wall building materials), you need to choose a concrete mix of grade M30 and higher, since the load will be much higher.
Geotechnical Parameters of the Site
It’s an equally important factor that influences the choice of the proper concrete grade for a foundation. If there is rocky or sandy soil at your site, then for a strip foundation, you can choose low grade (M20 or M25) concrete without any negative consequences. Unfortunately, such soils are rarely encountered. Most often, you have to deal with loams, clays, and other soils that builders hate so much.
Why is clay soil so problematic? The reason for that is its high heaving characteristics. When clay freezes, its volume increases greatly. If the foundation at a clay area is laid shallowly, it can rise by 3 cm to 4 cm in winter. It rises unevenly depending on the water saturation of the soil. That is, along the perimeter of the strip, the strip foundation will be affected by uneven loads that may result in deformations. A solution is to lay foundations at a greater depth, below the depth of soil freezing. Laying foundations in soils that are not subject to freezing helps avoid frost heaving. That is, the tangential heaving forces acting on the foundation can be neglected.
Clay soils are very demanding to the grade of concrete for a foundation. If you want to avoid trouble, spare no expense to choose the concrete one grade above the design grade. For example, a cubic meter of the M25 grade concrete is only about $2 more expensive than the M20 grade (in Russia). It’s no big deal when compared to all the other construction materials. But you get a tangible increase in water tightness, strength, and freeze-thaw resistance of the structure.
Here is an example. The M20 grade concrete has a water tightness grade of 4 bars and a freeze-thaw resistance of F100. For grade M25, these values are 8 bars and F200, respectively. As a result, grade M25 has twice the service life of grade M20, plus double resistance to groundwater penetration, which is especially important if you have a basement. This is worth the $2 that you spend on a higher grade, isn’t it?
Type of the Underground Part of the Foundation
This parameter also affects the choice of the concrete grade. The main choice criterion is if you have a basement or not. This seemingly insignificant factor may affect the water tightness grade (coefficient W) of the concrete that you will need to use. As can be seen from the above example, grade M25 has significantly greater water tightness than grade M20. In addition, to protect a concrete structure from water, you can use a coating or adhesive waterproofing — a practical, reliable, and inexpensive method. Other options are expensive, not so reliable, and unpredictable in service. In general, first of all, pay attention to the groundwater level when choosing a concrete grade for a foundation with a basement.
- Buy any grade of concrete, plus use outer surface or roll waterproofing (bituminous mastics, hydroglass waterproofing, etc.). It’s one of the cheapest options.
- Use concrete of medium grades but ensure additional protection using special mud injections. It’s slippery, dangerous, unpredictable but, according to many, worthy of consideration as a working option.
- Use high grades with maximum water tightness. There are two cons here: first, it’s expensive and, second, you need to work really quick. The higher the grade of the concrete, the faster the curing.
- Use special concrete mixes designed for hydraulic structures. They add special waterproof compounds to concrete mixes of this type. This additionally increases the density of the structure.
Which of these options to choose? It’s up to you to decide. If you are a perfectionist, you will use grade M45 concrete featuring a water tightness of 16 bar to 20 bar along with outer waterproofing, injecting mud into the walls and ensuring drainage around the house under construction. If you don’t worry much, you will use grade M7.5. The right choice is somewhere in the middle.
Sulfate Resistance of Concrete
Not all builders take account of this parameter. If your foundation is in contact with aggressive media (groundwater with various impurities, salts, and minerals), the sulfate resistance of the structure should be as high as possible. How to increase this parameter? Firstly, you can use modifying additives. Secondly, when you mix the concrete, you can use a special sulfate-resistant type of cement. Anyway, you are unlikely to find any ready-made sulfate-resistant concrete mixes, so you have to buy modifying additives and add it to your concrete mix yourself.