Curing Concrete

Curing Concrete

Concrete must be kept moist for several days after it is placed to allow the portland cement in the mix to cure and harden properly. The most common methods of curing concrete are as follows:

• Cover the surface completely with large sheets of plastic. Be sure to keep the plastic flat on the surface of the concrete, or it will cause uneven coloring. Weight down edges and joints with pieces of lumber.
• Cover the surface with roofing felt. Tape the joints and edges or weight them down with pieces of lumber to help seal moisture in and retard evaporation.
• Cover the surface with burlap bags, using a garden hose to keep the bags wet.
• Sprinkle or fog the concrete with a garden hose or sprinkler.
• Apply a chemical curing compound.

Plastic sheeting and roofing felt can cause uneven discoloration of the concrete surface if they are not kept flat. On large surfaces, it is difficult to smooth out all of the wrinkles in a covering, so if the concrete will be exposed to view and its appearance is important, use another method for curing. Wet burlap curing should not be used on colored concrete surfaces because it can cause the color to become splotchy. Keep the concrete moist for seven days.

After concrete slabs have cured for 24 hours, and concrete walls and footings for three days, remove the forms, but do not pry or hammer against the concrete itself. The concrete will continue to cure slowly for another month until it reaches full strength, but slabs are safe to use for foot traffic after the first day and for light rubber-tired vehicles after the first week. Heavy traffic areas should be protected with plywood. Foundation walls and footings should cure for at least two weeks before substantial framing loads are added.

Equipment Required in Ready Mix Concrete

Equipment Required in Ready Mix Concrete

Following are the equipments required in Ready Mix Concrete

1.Batching plant

2.Transit mixer

BATCHING

Batching plants are classified as

1.Manual

2.Semiautomatic

3.Fully automatic 

STORAGE

Storage of the raw materials is done by following methods: –

INLINE BINS Inert raw materials like fine & coarse aggregates are stored in bins called as

“Inline Bins” where the trucks carrying fine & coarse aggregate can dump the material easily.

The aggregates required are fed by the means of aggregate belt conveyer. On the aggregate belt conveyer the aggregates are weighed automatically by means of computer form the computer room presents on the plant.

SILOS

Cement & Flash are stored in airtight container called as “Silos”. The required quantity of cement & fly ash is extracted by the silos. There are two cement silos and one silo of fly ash.

Concrete Curing Period

Concrete Curing Period

The curing period must be designed so that the areas near the surface achieve the structural strength and impermeability required for durability of the concrete, and corrosion protection of the reinforcement.

Strength development is closely connected to the concrete composition, fresh concrete temperature, ambient conditions, concrete dimensions and the curing period required is influenced by the same factors.

As part of the European standardization process, standardized European rules are being prepared for concrete curing.

The principle of the European draft is incorporated in E DIN 1045-3. Its basis is that curing must continue until 50% of the characteristic strength fck is obtained in the concrete component. To define the necessary curing period, the concrete producer is required to give information on the strength development of the concrete. The information is based on the ratio of the 2 to 28 day average compressive strength at 20°C and leads to classification in the rapid, average, slow or very slow strength development range. The minimum curing period prescribed according to E DIN 1045-3 is based on these strength development ranges. The table below shows the minimum curing period as a factor of the strength development of the concrete and the surface temperature.

Retardation/Hot Weather Concrete

Retardation/Hot Weather Concrete

The concrete should be protected from drying out during handling.

Concreting is only possible at high temperatures if special protective measures are provided. These must be in place from the start of concrete production to the end of curing. They are dependent on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development and dissipation and the dimensions of the pour.

The fresh concrete must not be hotter than +30°C during placement and installation without these protective measures.

Possible problems

Working with non-retarded concrete can become a problem at air temperatures over 25°C.

• Hydration is the chemical reaction of the cement with the water. It begins immediately on contact, continues through stiffening to setting (initial set) and finally to hardening of the cement paste.
• Each chemical reaction is accelerated at a higher temperature.

This can mean that correct and complete compaction is no longer possible.

The normal counter measures are the use of retarded super plasticizers or super plasticizers combined with a set retarder.

Retardation terms and dosing tables

Purpose of retardation: To extend the working time at a specific temperature.

Working time: The time after mixing during which the concrete can be correctly vibrated.

Free retardation: The initial set is certain to start only after a specific time.

Targeted retardation: The initial set is started at a specific time.

Monolithic Concrete

Monolithic Concrete

Wear resistant, level concrete floors or decks ready for use quickly. Monolithic concrete has the same high quality throughout and these floor designs are extremely economic.

Composition

The concrete mix must be adapted to any special requirements (waterproof concrete, frost resistant concrete etc.)

Placing

Standard placing and compaction with immersion vibrators. Smooth off with vibrating beam. After the stiffening process begins, the surface is finished with power floats.

Curing

Start as early as possible, by spraying with Curing agent (Attention !What coating is to follow?) and cover with sheeting.

Notes

• Check the potential for the use of steel fibers when forming monolithic concrete slabs
• To improve the finished surface, we recommend the use of Mineral, synthetic and metallic grades which are spread into the surface during the finishing operation
• Concrete admixtures for extended workability are not generally suitable for monolithic concrete

Physical requirements for aggregates

 

Physical requirements for aggregates

Aggregates into categories covering:

• Resistance to splitting
• Resistance to wear
• Resistance to polishing and abrasion
• Particle density and water absorption
• Bulk density
• Durability

Durability

This is mainly associated with the frost and freeze/thaw resistance of coarse aggregates, which must be adequate for the specified purpose and must be verified if necessary.

 

Alternative aggregates (recycled material)

Large natural gravel and sand deposits are often valuable, non-renewable resources. It is becoming increasingly impossible to obtain and use gravel from these natural areas.

Possible substitutes are:

• Crushing and processing of old concrete to form concrete granules
• Reuse of micro fines from concrete wash water installations The suitability of recycled material should preferably be checked in every case.

 

Standard aggregates

Standard aggregates

In Europe aggregates are defined in standard EN 12620. This standard is very comprehensive and to give more details than in the list below would be outside the scope of this document.

Important terms from the standard (with additional notes)

• Natural aggregate

Comes from mineral deposits; it only undergoes mechanical preparation and/or washing.

• Aggregate mix

Aggregate consisting of a mixture of coarse and fine aggregates (sand). An aggregate mix can be produced without prior separation into coarse and fine aggregates or by combining coarse and fine aggregates (sand).

• Recycled aggregate

Aggregate made from mechanically processed inorganic material previously used as a building material (i.e. concrete).

• Filler (rock flour)

Aggregate predominantly passing the 0.063 mm sieve, which is added to obtain specific properties.

• Particle size group

Designation of an aggregate by lower (d) and upper (D) sieve size, ex-pressed as d/D.

• Fine aggregate (sand)

Designation for smaller size fractions with D not greater than 4 mm. Fine aggregates can be produced by natural breakdown of rock or gravel and/or crushing of rock or gravel, or by the processing of industrially produced minerals.

• Coarse aggregate

Designation for larger size fractions with D not less than 4 mm and d not less than 2 mm.

• Naturally formed aggregate 0/8 mm

Designation for natural aggregate of glacial or fluvial origin with D not greater than 8 mm (can also be produced by mixing processed aggregates).

• Fines

Proportion of an aggregate passing the 0.063 sieve.

• Granulometric composition

Particle size distribution, expressed as the passing fraction in percent by weight through a defined number of sieves.

Precast Concrete Applications

Precast Concrete Applications

Precast concrete has been used extensively in recent years. We have already discussed the advantages to this type of repair process. Here are some of the types of concrete structures where precast concrete can be used:

■ Navigation locks

■ Dams

■ Channels

■ Floodwalls

■ Levees

■ Coastal structures

■ Marine structures

■ Bridges

■ Culverts

■ Tunnels

■ Retaining walls

Preplaced Aggregate Concrete

Preplaced Aggregate Concrete

Preplaced-aggregate concrete is more resistive to shrinkage and creep than conventional concrete is. This is due to the aggregate. The net result is more protection against cracking. This type of concrete can be used on numerous types of structures. The cost involved with using preplaced-aggregate concrete is likely to be more, but it may be worth it in the long run.

When longitudinal reinforcement is required by design, the welding of stirrups, ties, inserts, and similar elements is not allowed.