Concrete

The classification of concrete grades is based on their compressive strength. Each concrete grade e.g. C30/37 is characterized by two equivalent strengths, which in this specific example are 30 MPa and 37 MPa. The first is the characteristic strength f_ck of a standard concrete cylinder and the latter is the characteristic strength of a standard concrete cube.

C12/15	C16/20	C20/25	C25/30	C30/37	C35/45	C40/50	C45/55	C50/60
Secondary Uses				Usual Uses

C55/67	C60/75	C70/85	C80/95	C90/105
Special Uses

	Environmental Conditions Category
	Usual Conditions (XC2/XC3)	Extreme Conditions (XC4)	Sea side conditions (XD1/XS1)	Pools (XD2)
Suggested Concrete Grade	C30/37	C30/37	C30/37	C30/37
Minimum Cover	25 mm	30 mm	35 mm	40 mm
Suggested Favorable Concrete Grade	C35/45	C40/50	C40/50	C40/50
Minimum Cover	20 mm	25 mm	30 mm	35 mm

The protection of reinforcement steel against corrosion depends on the porosity, the quality and the thickness of the concrete cover. The density and quality of the concrete cover is assumed using the stipulated concrete grade.

Exceeding a defined limit, the more water the concrete mixture contains, the less its strength and the higher its porosity. Apart from its strength, concrete is also characterized by its workability. Workability defines the amount of the water to be used for the production of the mixture. The slump height is the workability criterion of concrete.
Depending on the use of the concrete mixture a specific value of slump height is recommended.

Exceeding a defined limit, the more water the concrete mixture contains, the less its strength and the higher its porosity. Apart from its strength, concrete is also characterized by its workability. Workability defines the amount of the water to be used for the production of the mixture. The slump height is the workability criterion of concrete. Depending on the use of the concrete mixture a specific value of slump height is recommended.

When ordering a pre-mixed concrete the supplier must be given at least the following data:
(a) Quantity: e.g. 60 m
(b) Concrete Grade e.g. C30/37
(c) Slump Class: e.g. S3
(d) Super-plasticizer: e.g. YES (placed on order at the building site)
(e) Pump’s size: e.g. 36 m
(f) Time between truckloads: e.g. every 30 minutes
(g) Environmental exposure category e.g. S1 (sea side environment).
(h) Concrete strength category e.g. ΙΙ / 42.5.
(i) Maximum nominal value of the largest aggregate grain e.g. π.χ. 31.5 mm.
(j) The maximum ratio of Water/ Concrete (Ν/Τ), e.g. Ν/Τ ≤ 0.55.
(k) Compliance to ΕΝ 206-1 form.

For an amount of concrete lesser than 20m^³, 3 test samples must be taken from different mixing drums. This means either 3 samples from the same mixing drum or 3 samples from each drum, in cases that 2 mixing drums are needed for the concreting process.

For an amount of concrete ranging from 20m^³ to 150m^³, 6 test samples must be taken from 6 different drum mixers.

For an amount of concrete greater than 150m^³, 12 samples must be taken from 12 different drum mixers.
Samples must be taken from different concrete mixers and must be transferred to a certified laboratory for special curing within 20 to 32 hours.

Adding water to an already made concrete mixture may increase its workability but at the same time it lowers its strength. However, concrete must be workable enough to efficiently pass between densely spaced steel reinforcement. This is achieved by use of a super-plasticizer added (1.5 to 2.0 kg/m^³) inside the drum containing the wet concrete mixture before its discharge and mixed for 3 to 5 minutes in high rotating speed (8-12 R.P.M.). The addition of water or waterproofing admixtures on site is forbidden.
Generally, the concrete used for the casting of foundation footings does not require a high level of workability and therefore adding a super-plasticizer is not necessary. On the other hand, in strip foundation, usage of super-plasticizer may be required when casting the beam’s web, always done after the completion of the flange’s (the footing’s) concreting. Finally when casting staircases (or sloped slabs) super-plasticizers should be avoided because the workable concrete flows towards due to the important slope of the formwork.

The first thing prior to casting is to thoroughly clean the formworks from dust, oil, etc. This should be done not only to ensure the building’s strength but also to set a technical-social example, in other words to set an example of professional behavior for all people working in the building site. Concrete’s casting must be done directly from the pump’s hose end. The concrete must be poured in the slabs formworks in vertical and not in horizontal layers since, in case concreting has to be stopped for a long period of time, when it is resumed the new concrete will not bind with the old one and a horizontal joint will be created whereas, when concrete fills the formwork parallel to the thickness axis, a potential stop in casting will lead to the creation of a vertical joint.
The boom’s hose end should be around 0.50 m above the formwork of the element to be casted. An exception can be made for usual columns and shear walls, where it can be up to 2.50 m above the column’s bottom. In cases where columns or shear walls have a relatively large height e.g. 5.0 m, concreting must be done by one of the following ways:

(a) The boom’s hose end is positioned in an adequate depth inside the column’s formwork,

(b) Extra pipes are placed from the upper part of the column to an adequate depth and concrete is poured through them with the use of a special funnel.

(c) Openings with adequate size are created e.g. around the middle of the formwork and the concrete is casted through them (technically challenging procedure).

The procedures in (a) and (b) are more practical but they require a certain amount of space. For example when using the boom’s hose end this space must have a diameter around 180 mm and when using the extra pipes this diameter is decreased to 160 mm. The required space can be provided only if columns’ casting is performed separately from the rest of the elements. This means that concreting should not be done simultaneously in columns, beams and slabs with the use of one unified formwork. In earthquake prone regions simultaneous concreting should be avoided for one more reason: columns have a large amount of stiffness and therefore they are used for securing the position of formworks and scaffolds in cases of seismic events both during the casting and the curing process of the beams’ and slabs’ concrete.

Generally, concrete compaction should be carried out with the use of a vibrator. Two vibrating devices must be used regardless of the concrete to be casted. Based on the fact that one vibrator, serves around 5 to 10 m³ of concrete per hour, we calculate the required number of devices. For instance, when casting 100 m³ of concrete in 6 hours time, we will need from 100/10/6=1.6 up to 100/5/6=3.2 vibrators, i.e. practically 3 vibrators are needed. When casting a small quantity of concrete e.g. 20 m³, in 4 hours, theoretically one vibrator is enough, however having a backup device is mandatory for safety reasons. Nowadays, the best solution is to use light-weight electric vibrators that ensure both high production rates due to their ease of operation and a high vibration quality which is very important especially in visible concrete surfaces (off-form concrete, architectural concrete).

Concrete curing is mandatory. As a matter of fact, the higher the ambient temperature and the wind velocity are, the more meticulously it has to be carried out. In every case, the concrete surface has to be hosed as needed to remain moistened throughout the entire day for at least the first week after casting. The curing process however will last for 28 days.
Concrete curing in extremely high temperatures can be done in three ways:

(a) Right after finishing the concrete’s surface, we cover it with special sheets (burlaps). These sheets must be kept wet 24 hours a day for at least one week. Special attention must be paid to hold them down and prevent them from being blown away.

(b) By ponding. We form a dam a few centimeters high (4 to 5 centimeters) around the perimeter of the slab right after concreting. We fill this dam with water thus creating a pond and we make sure to replace water loses due to vaporization. The circumferential dam may be constructed with bricks cut in half or simply with fast curing cement mortar. This solution has two disadvantages: it is costly and it hinders works on the top of the slab for at least 7 days.

(c) We spray the concrete’s wet surface with a special chemical fluid that becomes a membrane thus preventing concrete from drying out.

This is the simplest procedure but in order to be effective, the concrete’s surface must be free from grooves created by a manually operated screed board. This can be achieved only with the use of a mechanic screed board that compacts the concrete as it vibrates. Also, “bleeding” water should be removed. Evidently, concrete casting should be done under optimal conditions i.e. very early in the morning or late at night, with concrete being as ‘cool’ as possible, aggregates stored in shadow etc.
Concrete curing in frost:
Normally concrete must not be casted in extremely low temperatures, however when this is unavoidable e.g. sudden drop of the temperature below zero, its free surface must be covered with concrete curing blankets. These are made out of thermo insulating materials like rolls or plates of rockwool, glasswool with aluminium covering, polystyrene boards to be later used in insulations. In that way, we can make use of the concrete’s own heat. The blankets must be secured from wind up-turnings with e.g. rafters and balks. If the temperature drops too low, heaters like the ones used in out-door coffee shops may be used with their reflectors in an inverted position. In the past, barrels with fire were usually placed underneath the formwork; they contained sand wet with diesel oil.
In areas exposed to extremely low temperatures, the use of air–entraining admixtures or additives is mandatory in order to protect the concrete from the catastrophic results of frost.

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Constructional elements	Cement strength category
	32.5	42.5	52.5
Lateral sides of beams, slabs, columns, shear walls formworks	3 days	2 days	2 days
Slabs formworks and beam span formworks when the span is lesser than 5 m	8 days	5 days	4 days
Slabs formworks and beam span formworks when span is equal to or greater than 5 m	16 days	10 days	8 days
Safety columns of frame beams and slabs with a span greater than 5 m	28 days	28 days	22 days

The science dealing with construction materials has created a new type of concrete, the self-compacting or self-consolidating concrete which is ideal for earthquake resistant structures that have narrow spaced reinforcement. It is a kind of ‘gravel-concrete’ (with aggregates ranging from 12 to 16 mm). It contains 4^th generation plasticizers and has a strength equal or greater than C25/30. Its slump height is far greater than the S₅ class (therefore its workability is determined by the spread test). It literally flows inside the formwork and it does not need vibration!