- “Home” reinforced concrete
- Concrete quality
- Iron in concrete
- All about vibration
- Concrete laying and maintenance
Reinforced concrete structures are reliable and durable. Their quality depends on adherence to the technology during manufacture. In this article we will tell you how to achieve the quality of reinforced concrete close to the factory one in home (field) conditions..
“Home” reinforced concrete
Concrete is one of the most common building materials known since ancient Rome. The principle of reinforced concrete structures (RC) and reinforced concrete products (RC) has not changed since then. However, today the variety of concrete is huge, and we will focus on those structures and products that can be manufactured and used in private construction..
Consider the most common reinforced concrete structures, which it would be reasonable to make yourself.
There are the following types of monolithic foundations: pile (with or without grillage), tape (economy version, only under the walls), solid slab (with floating soil, on sands), prefabricated foundations.
They are used in stone houses. Extremely reliable and durable, but require time-consuming formwork preparation.
Walls, piers, parapets
Parapets can be either semi-decorative (sides, railings), or constructive (an emphasis for the ground or embankment). Monolithic walls are quite complex in execution, so they are rarely made at home.
Platforms, blind areas
Concrete is not afraid of precipitation; on the contrary, it hardens in a humid environment. Withstands high loads.
Stairs, stair flights
They make it possible to fit a reinforced concrete staircase to local conditions and dimensions (something that the plant will not do).
An integral element of a stone (foam and aerated concrete) house. In fact – the same strip foundation, only thinner and on top of the wall.
Concrete goods that can be made by hand:
- Jumpers. Window and door lintels are filled in place.
- Sidewalk slabs, well covers, curbs, other small items. Here you may need forms, a vibrating table, which you can also make yourself. However, you can get by with an ordinary vibrator..
Sometimes “concrete” is called randomly thrown stones, slightly doused with mortar on last year’s cement and slag, but such a solution is far from high-quality concrete. The properties of concrete depend on three factors: the grade of cement, the proportion of components and the presence of additives.
For load-bearing and self-supporting elements and critical structures (lintels, ceilings, staircases), we recommend using grade 500 cement. In other cases, grade 400 is suitable..
Crushed stone of 5–20 mm fraction is optimal for reinforced concrete structures. For small concrete products – 5-10 mm.
Attention! Crushed stone must be conditionally clean, without significant admixture of organic matter.
The quality of concrete is also influenced by the composition and origin of the sand: angular sea grains of sand are better kept in solution than smooth river.
Attention! Even a small percentage of clay admixture in sand can dramatically reduce the quality of concrete..
The presence of clay can be distinguished by its characteristic brownish tint (clean sand has a light yellow color) and stickiness (the cobbled “snow” does not crumble). Pay attention to this when unloading sand from a dump truck – how much it sticks to the body.
Classic proportions of concrete: 3 parts of sand, 3 parts of crushed stone, 1 part of cement, water – to the desired consistency.
Attention! Too “tough” (dryish) concrete may not fill all the sinuses, but too thin – it will crack (“cement milk” will flow out through the formwork cracks).
A common mistake: more cement, more strength. Cement is a chemically active binder. With its increased concentration and lack of sand, it simply reacts with water, breaks down into its constituent elements and “does not work.” In case of “overdose” with cement, small and medium cracks also appear on the surface, the product has a whitish tint.
Bulk additives: slag, granulated slag, expanded clay, vermiculite and others – are used only to improve the thermal insulation properties. At the same time, the load-bearing capacity and resistance to atmospheric phenomena are sharply reduced (such surfaces must be protected). Do not add slag to the foundation to save money! The main function of the foundation is to carry the load from the walls. Viscose will help improve the properties of concrete (when installing floors).
There are basically three types of liquid supplements:
- plasticizers (make concrete more plastic)
- winter (accelerate hardening to a temperature loss)
- hardeners (increase strength up to 15%)
For the manufacture of small concrete products, be sure to use plasticizers according to the instructions.
Iron in concrete
Common misconceptions: “why spend money on reinforcement at all – concrete is already heavy and durable” and “reinforcement is needed only so that the concrete holds on to it”.
Indeed, in concrete goods and reinforced concrete structures, concrete perceives a compressive load. But in 90% of cases, the load-bearing elements are subjected to combined loads – compression and tension. The reinforcement withstands tension in reinforced concrete, preventing the structure or element from breaking. By itself, concrete holds vertical loads well only in the form of a regular cube and in laboratory conditions.
Selection of the diameter of the reinforcement. The rods in the frame are divided into:
- working (longitudinal)
- mounting (cross, clamps)
In order not to go deep into the calculations, we will describe the principle of choice.
Working reinforcement in load-bearing and self-supporting reinforced concrete structures (floors, stairs, lintels, thrust walls, arches, foundations, columns, pillars, piles) use A3 with a diameter of 16 mm with a step of 150 mm.
Distribution (transverse rods of hanging staircases, additional outlets and reinforcements) use A3 with a diameter of 12 mm with a step of 150-200 mm.
Frame clamps – wire 6-8 mm with a pitch of 150-250 mm (depending on the purpose of the element).
Attention! In ceilings and walls, both reinforcement – both longitudinal and transverse – are working and the frame consists of two “mirror” meshes.
For reinforcement of ground slabs, paths, blind area more than 80 mm thick, use reinforcement of any brand with a diameter of 10–12 mm with a cell of 150–200 mm in one layer. With a smaller thickness, a welded (masonry) mesh 2.5-3 mm with a cell of 50-100 mm is suitable.
When making small concrete elements, also use the specified welded mesh.
All about vibration
Vibration is the “moment of truth” for reinforced concrete. Many neglect this procedure, negating the advantages of reinforced concrete structures and reducing the service life of reinforced concrete products by 2-3 times..
Why do you need vibration and what happens at this moment in liquid concrete? The main bearing component of concrete is crushed stone (coarse aggregate). The cement-sand mortar only distributes the load between the stones. Vibration has the following functions:
- Allows crushed stone to take its place, tightly fit stone to stone, so that after hardening, the load is transferred to it, and not to a weak cement mortar.
- Provides removal of air bubbles so that there are no voids after hardening. This is especially important for small concrete products.
- Fills with concrete all formwork cavities of reinforced concrete structures and small notches (patterns) of reinforced concrete forms.
- Makes liquid concrete mobile, making it easier to work with it (driving off excess, setting the surface “horizon”).
To work with reinforced concrete structures with significant vertical dimensions, use a deep vibrator with a vibrator head with a diameter of 40-50 mm. When concreting slabs and planes, a vibrating screed (an engine with a vibrating device, fixed on a rigid metal rail) is suitable. Such a motor can be fixed on a metal table (you get a vibration table for small forms).
The vibration time is usually about 10 seconds when the vibrator head is fully immersed. Air bubbles should stop escaping, and “cement milk” should ooze through the small cracks in the formwork. The concrete surface will become even and smooth..
Attention! Avoid vibration for too long (more than 20 seconds). This can lead to settling of coarse and fine aggregates and delamination resulting in uneven and unreliable concrete..
Take care of your hands: take breaks between vibration sessions (no more than 10 minutes continuously).
Concrete laying and maintenance
Placing concrete in formwork is not only the end of the work, but also the beginning of the life of a new structure.
Hardening (setting) of concrete is a chemical reaction with the participation of water and a positive temperature. It is recommended to carry out concrete work at an air temperature of at least +5. At lower temperatures, use special liquid additives. In winter, at large construction sites, electric heating is used (the heating cable is woven into the reinforcement cage) using special 380 V power plants.
Attention! Ceilings, stairs and lintels are always filled in one go, without pauses. Dismantling of formwork – not earlier than 10 days.
With a large volume of reinforced concrete, the foundation can be poured several times, but layer by layer with interruptions no more than 10 hours. The laid concrete should be covered to maintain a favorable humid environment.
It is better to make the formwork from oiled film faced plywood – this will facilitate disassembly. Make sure that no oil gets on the fittings. The formwork of hanging flights of stairs and floors is dismantled after 10 days, other reinforced concrete structures – after 12 hours. Concrete sets in 3 hours and within 28 days gains brand strength.
Remember! Compliance with technology is the key to the reliability and durability of any structures.