Making a “good” concrete for an experienced builder is a simple matter when a beginner in this area is confronted with a lot of information that needs to be taken into account when selecting a compound. The main mistake is to think that, having high-quality components, any recipe will produce a high-class mixture. But it is not.
Such a factor affects the success of only 40%. Approximately 10% can be given to environmental conditions in which concrete is prepared and stood.
And yes, 50% – “the lion’s share” falls on the competent selection of the composition. Why it is so important, and how to independently determine the correct ratio of sand, cement and rubble in concrete, let’s take a closer look.
Properties of components and their influence on the selection of the composition
The number of components in concrete depends not only on its brand. Also, the characteristics of raw materials, which can vary the ratio of cement, sand and rubble in concrete in the manufacture of a mixture of the same class, have a great influence.
Portland cement – the main binder. It is from its quantity that they are repelled in determining the ratio.
And the calculation of cement on concrete depends on the following characteristics:
Mark. The higher the grade of portland cement, the more durable it is. Accordingly, the higher the grade, the smaller the amount of cement in the cube of concrete. Immediately a thought arises: “And if I buy the best cement, for example, M500D0, I will add its minimum quantity, thereby saving on the most expensive component of concrete.” It is difficult to call such an act thrift, because the price for such a binder is decent.
Well, let’s go the other way, we will buy the M400D20, which is cheaper – but this is also a dubious savings, because due to the lower grade strength, you need to add it to the concrete much more, respectively, the cost of the cube will increase. However, such a replacement is justified in many cases.
For example, in the manufacture of concrete, the brand is not higher than M400, and only if the strength of the sample at 28 days of age will satisfy the GOST requirements.
Steaming activity is the most important characteristic of cement. It shows how quickly the concrete will pick up the dismounting strength. It may be different even in the cement of a single supplier for different batches. This parameter is determined only under laboratory conditions. There are 3 groups of activity in steaming. The best is the first, and accordingly, the worst is the third.
What exactly is needed in a particular case depends on the specifics of production and the fabricated structure. This does not particularly affect the ratio of cement and sand in concrete, it usually prolongs the heat-moisture regime or the standing time, if the activity is absolutely useless. But, if the budget allows and a faster hardening of the concrete is required, then a larger volume is taken.
Date of manufacture of cement as important as the day of the release of the food product. The fresher the binder, the better it is. Therefore, manufacturers do not buy it in advance at a lower cost in the winter. After the “performance” of the month of cement, a countdown is conducted. His brand strength gradually decreases. This process is much faster if the cement is stored in open bags and in a humid environment. Therefore, the “old” binder is added to concrete above the norm.
Attention! If you caught lumpy cement – do not feel sorry for it and throw it away, since nothing good can be done from it.
Water-cement ratio is a sore subject for any concrete worker.
As many know, the addition of excessive amounts of water to the working solution leads to a decrease in brand strength.
- It must be dosed in a ratio of exactly as much as the cement goes to 1 cubic meter of concrete, which is approximately 30% of its mass. That is why the hard semi-dry mixtures have good brand strength, frost resistance and durability.
- But with this B / C, completely non-plastic solutions are obtained. Therefore, it is not always appropriate to make a non-flowing mixture.
- Mark on the rigidity of concrete is usually indicated in the projects, but if one is not available, especially at construction sites, water is dosed “by eye”, which is not good.
- Therefore, before letting the nominal recipe of the mixture into mass production, test batches are made at which the selection of the composition is adjusted.
- When testing samples in the laboratory, the most suitable compounding for workability and strength is revealed. That is why plasticizers are so loved by builders.
To the note: With the help of plasticizers, it is possible to reduce w / c, and even the amount of cement, and at the same time get “good” concrete. But, again, a dilemma arises – how profitable it is, because the price of plasticizers, oh, how to “bite”! Therefore, “miracle-wizards” appear, advising to add cheap detergents to concrete, or “not to bother at all” and prepare solutions in the old manner.
- I would also like to touch on the subject of cement water demand. This characteristic shows the water-cement ratio, at which the normalized plasticity of Gostovsky cement slurry is obtained.
- It does not have a huge impact on the proportions of cement sand and crushed stone for concrete. But it happens that some cements require a little more water, some a little less than the set value.
- To determine this property do not conduct any additional tests. This is found out in the process of preparing cement paste for beams, which, after steaming, will be samples for testing the strength and activity of the binder.
Note! Knowing that the cement used in production has a great water requirement, the selection of the composition does not change in advance, but they have in mind that if the process of laying the mixture is complicated, an adjustment of the water-cement ratio will be required. In this case, be sure to take samples of batches for detailed laboratory testing.
And now we will plunge headlong into the abyss of information on large and small aggregate. When designing a certain brand of the mixture, they try to minimize the consumption of cement for concrete, but at the same time to match the specific grade strength and ductility.
To achieve this, use large and small aggregate. It not only reduces the amount of cement paste, but also adds strength characteristics (this applies especially to rubble).
If we talk about heavy concrete, then the selection of the composition, the amount of sand is taken with some excess in comparison with the volume of rubble. In the case when the recipe is calculated for concrete made from sand and cement, then under the condition of non-use of coarse aggregate, the voidness of the solution increases and is approximately 27%, which leads to greater use of cement paste. And this is almost 2.5 times more than when dosing gravel.
Interesting fact! On building sites, when large areas are monolithic, for example, when casting a slab foundation, they use “raisins” – cobblestones, in order to spend as little expensive concrete as possible.
The size of the aggregates also plays an important role. Its module size is taken into account when designing concrete.
For example, crushed stone with a fraction of 20-40 mm is usually taken for foundations, and for the production of floor slabs, aggregate of 5-10 mm is used. At the same time, the ratio of rubble in concrete varies from the design mark. But, on average, about 5 parts of crushed stone per 1 part of cement are taken.
Concrete recipe – everything is already invented for us
None of the normative documents having legal force regulates the concrete proportions of sand and cement for concrete. Too many factors affect this.
We play with the selection of the composition
The proportions of sand, cement, crushed stone in concrete, carefully verified and approved by the construction laboratory, were observed, but did the working solution turn out to be “somehow different”? Construction is in full swing, but there is no time to wait for the test reports?
Let us examine the main problems and possible solutions:
- In the process of making the working solution, too much water was added — how much cement, sand, rubble in the concrete, you need to increase their quantity. We strive to achieve the “ideal” of the project water-cement ratio. Start by dispensing cement and sand in small pieces.
- Cracking concrete after stripping, a glossy surface, peeling on the side of smoothing – a lot of water. It is necessary to reduce the w / c and increase the presence of a plasticizer, check the sand moisture – perhaps it is this which increases the amount of water in the solution, thereby reducing the quality of the hardened stone.
- Cracking of the concrete during the aging process may not be enough moisture. Add more water to prevent this.
- The low density of foam concrete, the working solution turns out to be “fluffy” – the proportion of sand and cement in concrete has nothing to do with it. All the fault of improperly prepared or dosed concentrate.
- Added more rubble than planned – do not worry, there is not a lot of such aggregate in concrete! Here, as they say, you cannot spoil the porridge with butter. The only thing you need to add a little water, so that the gravel does not take away the moisture needed to hydrate the cement. And get ready for a longer process of vibrating.
The above small instruction is not relevant for all situations. To better navigate the selection of the composition, try, as often as possible with your own hands to do test batches with different numbers of components. Only by experience can one understand how the ratio of sand, cement and rubble in concrete is built.