Workability of concrete mix: regulatory documents, brands,

It is well known that concrete is divided into different grades of compressive strength and cold resistance. The workability of the concrete mix is ​​a parameter that is rarely paid attention to; Meanwhile, it strongly influences the process of monolithic concreting. Let's get acquainted with the relevant terminology and classification methods.

What it is

The workability of concrete is a property that affects its ability to fill a formwork of arbitrary shape under its own weight. With reference to fluid mixtures, it is called mobility; for mixtures that cannot flow under their own weight, another term is used - stiffness.

Curiously: the spreading of concrete, the spontaneous filling of the mold is prevented by the adhesion of the filler particles between themselves and with the walls of the mold; at the same time, the larger the particle, the greater the resistance. Contribute to him as a large amount of water, cement and special additives - plasticizers.

The presence or absence of pores in the thickness of the monolith depends on the mobility of the solution. The lack of mobility is partly leveled by baying and / or vibro-laying; However, they do not always allow to get rid of cavities. Meanwhile, 2 percent of the volume occupied by the pores will reduce the strength of the structure by 10%; increasing the percentage of cavities to 5% will result in a 30 percent decrease in strength.

It would seem that the obvious solution would be to produce only and exclusively high-mobility mixtures: after all, they flow perfectly, do not require tedious whiping or energy-intensive vibrating. It was not there: sometimes excessive mobility is harmful.

Let's give a couple of simplest examples:

1. Too mobile mixture will be poured out through the cracks of the plank formwork.
2. When laying on the bedding of crushed stone, mobile concrete will not remain on its surface, but will go inside.

Regulations

The parameter we are interested in is affected in two normative documents:

1. GOST 7473-94 contains general requirements for concrete mixtures and describes their classification.
2. GOST 10181.1-81 регламентирует методику испытаний смесей на удобоукладываемость и применяющиеся для этих испытаний инструменты.

Let's take a look at the highlights of both documents.

GOST 10181.1-81

Let's start with the test methods. Their description will help to understand the classification of concrete by workability.

According to GOST, it is assumed that the mixture is tested in one of two ways, depending on the expected mobility or stiffness.

Now - a little more specifics.

Devices

Прибор для испытаний на осадку конуса - усеченный металлический конус с воронкой, упорами и ручками (читайте также статью «Алмазная чашка по бетону – ее виды и особенности»).

To test the stiffness of the mixture requires two devices, each of which is much more complicated. The vibrotable is a platform that oscillates at a speed of 3000 vibrations per minute and with an amplitude of 0.5 millimeter. The measuring device itself is a cylinder with a tripod and a massive metal disc with six holes descending on a rod.

Test method

The test procedure instructions contained in GOST prescribes first testing the mixture with unknown characteristics of the cone draft.

The technique is extremely simple:

1. The device is installed on a flat steel sheet.
2. Through the funnel, it is filled with concrete for testing.. The mixture is placed in three layers of equal thickness with the obligatory splicing of each layer. The method of splicing and the instrument used for this are strictly regulated: the concrete should be spun 25 times per layer with a bar with a diameter of 16 and a length of 600 millimeters with rounded edges.

It is useful: to be honest, neither the material of the surface nor the length of the rod have almost no effect on the result. When testing a mixture with your own hands, you can do, for example, with a piece of hardboard or plywood and any smooth piece of reinforcement.

1. The funnel is removed.
2. Excess concrete at the level of the upper cutoff of the device is removed with a trowel.
3. The device gently, smooth movement without shaking removed and placed next. The standard even stipulates the time that the device removal process should take: from 3 to 7 seconds.
4. The ruler with a division of not more than 0.5 cm measures the difference between the top of the settled cone of the concrete mix and the upper slice. The resulting difference will characterize the mobility of the mixture.

For greater certainty, the test is repeated twice. The result is an arithmetic average between two measurements. At the same time, the spread between the obtained measurement results must be within certain limits, depending on their absolute value:

Nuance: for concretes with coarse-grained (over 40 mm) aggregates, an increased cone is used. Obtained by measuring with the help of the result is multiplied by a factor of 0.67.

If the draft of the cone is zero, the sample is tested with a device of the second type.

1. The device is rigidly attached to the vibroplatform. According to GOST, for this purpose it must be supplied with a flange.
2. As described above, a concrete cone is placed in the hollow cylinder of the device.
3. The tripod is rotated so that the disk takes up a position strictly above the cone, and is fixed by the clamping screw.
4. The disc is lowered to the sample surface, after which the vibration is simultaneously turned on and the stopwatch is started. The countdown stops as soon as the cement paste is pressed through any two of the six holes in the disc. The result in seconds characterizes the stiffness of the mixture.

GOST 7473-94

The key information contained in this document is the table according to which the grade of concrete is determined by its workability.

In addition, the standard sets out the requirements for water used for mixing (it must comply with GOST 23732), the composition of the mixture and the maximum error percentage of its components (for cement and plasticizers - 1%, for aggregates - 2%).

Application

How does the grade of concrete mix for workability affect typical areas of its application?

Useful stuff

• The easiest way to increase the mobility of concrete is to add water to it. However, its strength will suffer; such a solution is suitable only for unloaded monoliths.

• In addition to special additives - plasticizers, the price of which sometimes turns out to be quite immodest, to increase the mobility of the solution, they often add a little (about a tablespoon to a bucket) of liquid soap or dish detergent.
• Processing bearing structures with dense reinforcement (in particular, cutting or drilling) is extremely undesirable. Among other things, at the same time the reinforcement is violated, which is supposed to be anchored at the edges of the opening. If, due to some reasons, failure of the reinforcement cannot be avoided, the use of corrugated reinforcement, diamond drilling of holes in concrete and cutting of reinforced concrete with diamond circles can be a compromise.

The corrugation of the reinforcement cage allows you to do without additional anchoring; diamond cutting and drilling do not break the bond between the reinforcement cage and the concrete due to the absence of shock vibration.

In addition: the edges of the holes and openings when using a diamond tool will be much more accurate than in the case of using a puncher and a jackhammer.

Conclusion

Надеемся, что изучение нормативных документов и таблиц не показалось читателю слишком скучным. Как обычно, в прикрепленном видео в этой статье можно найти дополнительную полезную информацию (см.также статью «Заливка и упрочнение бетона на примере устройства пола: последовательность работ и правила технологии»).

Successes!