Functions of Various Ingredients

Functions of Various Ingredients

Cement is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.

Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin. They should be clean, sharp, angular and hard. They give mass to the concrete and prevent shrinkage of cement. Fine aggregate consists of river sand. It prevents shrinkage of cement. When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete, since it fills the voids. Denser the concrete higher is its strength.

Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete. The variation of strength of concrete with water cement ratio. To achieve required workability and at the same time good strength a water cement ratio of 0.4 to 0.45 is used, in case of machine mixingand water cement ratio of 0.5 to 0.6 is used for hand mixing.

Concrete Compaction - Methods

Concrete Compaction - Methods

Concrete should be placed and compacted immediately after mixing. The concrete should be placed within 30 to 40 minutes to prevent the danger of concrete getting its initial set, before laying the concrete, the shuttering should be cleaned of all of dust or debris. Crude oil or grease etc is usually applied to the shuttering before concreting to prevent the shuttering absorbing the water from the concrete or getting stuck to it. In placing the concrete, care should be taken to see that it should not be thrown from heights. Concrete should be laid in layers 15 to 30 cm (6” to 12”) in thickness and each layer should be properly compacted before laying the next one.

Compaction of concrete should be proceed immediately after placing. The function of compaction of concrete is to expel the air bubbles in the mass and make it impermeable in addition to its securing the desired strength. The concrete mass should be consolidated or compacted till the cream of the cement starts appearing on the surface. Over compaction may lead to segregation of concrete while-under-compaction may leave air voids in concrete and results in honey combing. Compaction may be done by hand or mechanical device.

Hand compaction : The hand compaction may be done by rodding, tamping or hammering. Tamping is usually adopted for compacting concrete for slabs or other such surfaces. Rodding is done for thin vertical members. Hammering is done for massive plain concrete works and for compacting an almost dry concrete the surface is beaten with heavy flat bottom rammers till the thin film of mortar start appearing on the surface.

Mechanical compaction: Mechanical compaction is done by the use of vibrators. Vibrators are of three types 1. Internal 2. External 3. Surface. Internal vibrators are commonly used in large works for flat surface compaction. In this the vibrator is immersed in the full depth of concrete layer. The vibrator should be kept in one position for about 3 minutes and then removed and placed another position. External vibrators are placed against the form work and are only adopted for thin section of members or in places where internal vibrators cannot be used with ease. Surface vibrators are generally employed in concrete road construction. Compaction of concrete by use of vibrators permits the use of stiff concrete mix of high strength and ensure better compaction than that obtained by the method of hand compaction

Preparation of concrete mix

Preparation of concrete mix

There are two types of concrete mixing

 (i)Hand mixing

(ii)Machine mixing

Hand Mixing: This method of mixing concrete is resorted to when the quantity if concrete to be used in a work is insufficient to warrant the necessity of machine. This is used with advantage in places where machinery cannot be used on account of their non-availability or in works near a hospital where the noise of machine is not desirable. Hand mixing is done on a clean, hard and impermeable surface. Cement and sand are first mixed dry with the help of shovels until the mixture attains uniform color. Aggregative are then added to this mixture and the whole mixture is then turned by shovels until the stone pieces uniformly spread throughout. After this, desired are quantity of water is poured into the heap from a can fitted with a rose. The mass is then turned until a workable mixture is obtained. It is advised to add 10% extra cement to guard against the possibility of inadequate mixing by this method.

Machine Mixing: The machine used for mixing concrete is termed as concrete mixer. Two types of concrete mixers are in common are

1. Continuous mixers

2.Batch mixers

Continuous mixers are employed in massive construction where large and continuous flow of concrete is desired. The process of feeding the mixing is more or less automatic. The machine requires careful supervision so as to obtain the concrete mix of desired consistency.

In batch type of concrete mixer. The desired proportion of materials are fed into the hopper of a drum in which the materials get mixed by the series of blades or baffles inside the mixer. Batch mixers are further two types 1. Tilting drum type 2. closed drump type. In the first type, components are fed in the revolving drum in a tilted position and after sometime the concrete mix is discharged by tilting the drums in the opposite direction. In the latter type the drum remains rotating in one direction and emptied by means of hopper which tilts to receive the discharge.

While using the mixer, coarse aggregates should be fed first, sand and cement should be put afterwards. In this revolving state, the components get mixed while water is poured with the help of can. The concrete should be for atleast 2 minutes, the time being measured after all the ingredients including water have been fed into the drum.

Concrete

concrete

Cement concrete is a mixture of cement, sand, pebbles or crushed rock and water. When placed in the skeleton of forms and allowed to cure, becomes hard like a stone. Cement concrete is important building material because of the following reasons.

·     It can be moulded into any size and shape of durable structural member.

·     It is possible to control the properties of cement concrete.

·     It is possible to mechanize completely its preparation and placing processes.

·     It possesses adequate plasticity for mechanical working.

The cement concrete has the following properties

·     It has high compressive strength

·     It is free from corrosion

·     It hardens with age and continues for a long time after concrete has attained sufficient strength

·     It is proved to be economical than steel

·     It binds rapidly with steel and it is weak in tension, steel reinforcement is placed in cement concrete at suitable places to take up tensile concrete or simply R.C.C.

·     It forms a hard surface, capable of resisting abrasion stresses. This is called reinforced cement.

·     It has tendency to be porous to avoid this proper grading & consolidation of the aggregates, minimum water-cement ratio should be adopted.

Sand

Sand

Sand is an important building material used in the preparation of mortar, concrete, etc.

·     Sources of Sand: Sand particles consist of small grains of silica (Si02). It is formed by the decomposition of sand stones due to various effects of weather. The following are the natural sources of sand.

·     Pit Sand: This sand is found as deposits in soil and it is obtained by forming pits to a depth of about 1m to 2m from ground level. Pit sand consists of sharp angular grains, which are free from salts for making mortar, clean pit sand free from organic and clay should only be used.

·     Rive Sand: This sand is obtained from beds of rivers. River sand consists of fine rounded grains. Color of river sand is almost white. As the river sand is usually available in clean condition, it is widely used for all purposes.

·     Sea Sand: This sand is obtained from sea shores. Sea sand consists of rounded grains in light brown color. Sea sand consists of salts which attract the moisture from the atmosphere and causes dampness, efflorescence and disintegration of work. Due to all such reasons, sea sand is not recommendable for engineering works. However be used as a local material after being thoroughly washed to remove the salts.

Shrink Mixed Concrete

Shrink Mixed Concrete

Concrete that is partially mixed in a plant mixer and then discharged into the drum of the truck mixer for completion of the mixing is called shrink mixed concrete. Central mixing plants that include a stationary, plant-mounted mixer are often actually used to shrink mix, or partially mix the concrete. The amount of mixing that is needed in the truck mixer varies in these applications and should be determined via mixer uniformity tests. Generally, about thirty turns in the truck drum, or about two minutes at mixing speed, is sufficient to completely mix shrink-mixed concrete.

Checks Needed at Site During Concreting

Checks Needed at Site During Concreting

Proper co-ordination between the Ready Mix Concrete supply and placing and compacting gangs.

·     Proper signaling or communication at site is necessary.

·     Workability of concrete within accepted limits.

·     Adequacy of cohesiveness of concrete for pump ability.

·    Ensure that water addition or chemical admixtures are not added during transportation by RMC unauthorized persons and without the knowledge of the site in charge of the consumer.

·     Temperature of concrete at the time of receipt at site (if specified).

·     Continuous and steady supply at site and speedy unloading of the Monitor speed and progress of placing to avoid formation of cold joints transit mixers.

·     Monitor proper  placement without segregation.

·     Monitor placement of concrete at the closest possible point to its final location.

·     Arrange for curing as soon as finishing is completed. This is specially required in case of slabs, pathways and roads in hot/warm weather.

·     Retempering should be prohibited as experiments shows the addition of water to RMC truck at the construction site may result in substantial reduction in strength. The reduction in strength was found to be proportional to the increase in slump. Large increase in slump means higher reduction in strength. When the amount of water added is not controlled, reduction of strength may be as high as 35%. In cases where controlled amount of water is added to restore the slump within the specification’s limits (100 ±25 mm), the reduction of strength may be below 10%.

Use of Fly Ash Based in Concrete

Use of Fly Ash Based in Concrete

To have a better performance characteristics in terms of durability of concrete, fly ash is been used successfully in concrete. The improvement in gel structure caused by pozzolanic action of fly ash leads to a very impervious concrete. These factors improve the resistance of concrete against external aggression. Since Fly ash is an environmental hazard, therefore by effectively using it in concrete it can be consumed constructively and thus contribute to the ecological balance. Many prestigious and critical structures have been built using either PPC or  by blending fly ash directly in concrete. The names of the famous Petronus towers & Eurotunnel can be definitely quoted in this regard. However , the  construction industry has offered some resistance in using PPC or fly ash in structural concrete. With advent of RMC the doubts regarding controls in using this material are slowly dispelled and engineers have become more open to the idea of using fly ash as pozzolan for partial replacement of cement in concrete.

“Limestone is the raw material for the manufacture cement & limestone is limited resource.” It cannot last long it ever. We need to realize the importance of this fact. We need to use our cement rationally. Whereas we talk about huge figure describing development and building needs of our country, concrete still remains the most widely used and environmental friendly construction material to achieve this.

Use of Ready Mix and Concrete

Use of Ready Mix and Concrete

RMC is generally looked upon as a costly product rather than a facility to get an appropriate quality product on site as and when required.The first cost of RMC may seem higher . However, there are several hidden advantages which can cause considerable reduction in cost to the owner. Since they cannot be accurately determined, they are ignored while evaluating the cost of RMC over site mixed produced concrete.

The following points answer the above question:

·     Generally speaking, the quality of concrete will be superior than site mixed concrete. However ,it will greatly depend on the controls and checks exercised at site and at RMC producer's plant.

·     There is a considerable wastage of materials on site due to poor storage conditions and repeated shifting of the mixer location. This is prevented if RMC is used.

·     In most cities, the plot area is barely sufficient to store reinforcement steel, formwork, concrete and other construction materials. Using RMC can cause less congestion and better housekeeping on the site resulting in efficient working environment.

·     Obtaining RMC at site can reduce supervision and labor costs which would otherwise be required for batching and mixing of concrete at site.

·     Many sites in cities, house their work force on the site itself to reduce the time and cost of daily travel. This creates unsafe and unhygienic conditions on the site as well as for the surrounding areas. This will reduce to a certain extent if RMC is utilized.

·     Fluctuation of raw material prices and their availability has always caused delays and problems of inventory and storage for site producers of concrete. This is totally avoided when RMC is used.

·     RMC plants have proper facilities to store and accurately batch concrete admixtures (chemical and mineral). To improve properties of concrete both in plastic and in hardened stage this accuracy is useful.

·     In general, RMC plants have superior and accurate batching arrangements than the weigh batchers used on site.

·     RMC plants have superior mixers than the rotating drum mixers generally used for mixing concrete materials at site.

·     RMC plants have efficient batching and mixing, facilities which improve both quality and speed of concrete production.

·     Temperature control of concrete in extreme weather conditions can be exercised in a much better manner than done at site.

·     RMC helps encourage" mechanization and new technologies like pumped concrete bulk transportation of cement production of self-compacting concrete and high strength high performance concrete.

·     New materials like micro silica and fibers can be safely used in RMC which in conventional concrete may pose problems.

·     Introduction of RMC improves the rate of supply of concrete in the formwork and thereby automatically improves quality of formwork, layout of reinforcement steel and its detailing and safety / strength of scaffolding and staging.

Intelligent building systems

Intelligent building systems

Developments in intelligent buildings have not been limited to advances in technology in the areas of computers, communications and building engineering. Changes in societal attitudes that reflect a higher standard of living have highlighted issues associated with the provision of a healthy working environment. This is being reflected in an increasing demand for high quality office space, spread across all classes of buildings, and a need for advanced information processing and communications systems. The technologies and services that are part of the intelligent building infrastructure.

The technologies and services that form part of the intelligent building infrastructure

Infrastructure	Services
Enclosure	·     load balancing ·     solar control ·     heat loss control ·     day lighting ·     passive and active ventilation ·     passive and active solar heating
Interior	·     spatial quality ·     thermal and air quality, visual quality ·     acoustic quality in the individual workstation ·     new workgroup concepts ·     shared services and amenities
Telecommunications	·     external connectivity and command centers ·     vertical chases and satellite closets/rooms ·     horizontal networks and horizontal plenums ·     service hubs and shared equipment ·     conference hubs, connectivity
Site	·     transport ·     streetscape, public access and thoroughfares ·     relationship with the community

Developers of building systems of all types have been challenged to deliver on these demands. The successes to date have created a new opportunity for adding value to the capabilities already inherent in the building. Intelligence has become ‘distributed’ enabling micro zones to exist independently of the rest of the building. For organizations, this is an important consideration. Organizations modify space to suit business needs. People, by their nature, tend to modify their work environment to suit personal tastes and corporate identities. The ability of building systems to interact using distributed intelligence enables a successful outcome to be achieved.

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.

Concrete Curing Methods

Concrete Curing Methods

Protective measures against premature drying are:

·     Applying liquid curing agents

·     Leaving in the forms

·     Covering with sheets

·     Laying water-retaining covers

·     Spraying or “misting” continuously with water, keeping it effectively submerged and

·     A combination of all of these methods

Liquid curing agents can be sprayed onto the concrete surface with simple tools (e.g. low pressure, garden type sprayers). They must be applied over the whole surface as early as possible: on exposed concrete faces immediately when the initial “shiny” surface of the fresh concrete becomes “matt”, and on formed faces immediately after striking. It is always important to form a dense membrane and to apply the correct quantity (in g/m²) as specified, and in accordance with the directions for use. Several applications may be necessary on vertical concrete faces.

Leaving in the form means that absorbent timber formwork must be kept moist and steel formwork must be protected from heating (i.e. by direct sunlight) and from rapid or over-cooling in low temperatures.

Careful covering with impervious plastic sheets is the most usual method for unformed surfaces and after striking of formwork components. The sheets must be laid together overlapping on the damp concrete and fixed at their joints (e.g. by weighing down with boards or stones) to prevent water evaporating from the concrete.

Sprayed Concrete with increased Fire Resistance

Sprayed Concrete with increased Fire Resistance

A sprayed concrete has increased fire resistance if it is improved with polypropylene fibers. In the event of fire, the PP fibers melt and leave pathways free for the incipient vapour diffusion, preventing destruction of the cement matrix due to the internal vapour pressure. Suitable aggregates are essential for increased fire resistance. Their suitability must be verified by preliminary tests.

·     Granulometry                         0–8 mm

·     Cement type                           CEM III / A-S

·     Cement content                               425 kg/m³

·     Polypropylene fibers                2.7kg/m³, according to type

Semi-dry Concrete for Precast Manufacture of Concrete Products

Semi-dry Concrete for Precast Manufacture of Concrete Products

General

Semi-dry concrete is used for the manufacture of small precast concrete products.

·     Concrete paving stones

·     Kerbs

·     Flags and paving slabs

·     Garden products

·     Pipes

The main application is now concrete paving stones.

The special characteristics of semi-dry concrete are

·     Instantly demouldable

·     Stability of unhydrated concrete

·     Dimensional accuracy immediately after compaction (green strength)

The advantages of this process are

·     Only one form per product shape (low capital investment)

·     A compacting installation for all the products

·     Production flexibility due to rapid changing over of the formwork for a new product type

Semi-dry concrete technology

How are these different fresh concrete characteristics obtained technologically?

·     Fine particle size distribution curve (max. particle size 8 mm, high water requirement)

·     Low w/c ratio (0.35 to 0.40)

·     Low binder content

·     High strength cement (42.5 R)

·     Cement substitutes (fly ash, powdered limestone)

This gives a low adhesive matrix content and therefore a granular consistence in the fresh state. The results:

·     Difficult to compact

·     Low air entrainment

·     Susceptible to early water removal drying

The strength develops according to the general laws of concrete technology and is then similar to high strength concretes.

Frost and Freeze/Thaw resistant Concrete

Frost and Freeze/Thaw resistant Concrete

Frost and freeze/thaw resistant concrete must always be used when concrete surfaces are exposed to weather (wet) and the surface temperature can fall below freezing.

·     Fair-faced concrete façades

·     Bridge structures

·     Tunnel portal areas

·     Traffic areas

·     Retaining walls

By adding air entrainers, small, spherical, closed air voids are generated during the mixing process in the ultra-fine mortar area (cement, finest grain, water) of the concrete. The aim is to ensure that the hardened concrete is frost and freeze/thaw resistant (by creating room for expansion o any water during freezing conditions).

·     Type, size and distribution of air voids

Air voids contained in a standard concrete are generally too large (>0.3mm) to increase the frost and freeze/thaw resistance. Effective air voids are introduced through special air entrainers. The air voids are generated physically during the mixing period. To develop their full effect, they must not be too far from each other. The “effective spacing” is defined by the so-called spacing factor SF.

·     Production/mixing time

To ensure high frost and freeze/thaw resistance, the wet mixing time must be longer than for a standard concrete and continue after the air entrainer is added. Increasing the mixing time from 60 to 90 seconds improves the content of the air voids by up to 100%.

·     Quantity of air voids required

To obtain high frost resistance, the cement matrix must contain about 15% of suitable air voids. Long experience confirms that there are enough effective air voids in a concrete if the results of the test (air pot) show the following air contents:

-       Concrete with 32 mm maximum particle size 3% to 5%

-       Concrete with 16 mm maximum particle size 4% to 6%

Fresh concrete with an air void content of 7% or over should only be installed after detailed investigation and testing.

Concrete Additives

Concrete Additives

Concrete additives are fine materials which are generally added to concrete in significant proportions (around 5–20%). They are used to improve or obtain specific fresh and/or hardened concrete properties.

Types of inorganic concrete additive:

Type I

Virtually inactive materials such as lime fillers, quartz dust and color pigments.

·     Pigments

Pigmented metal oxides (mainly iron oxides) are used to color concrete. They are added at levels of 0.5–5% of the cement weight; they must remain color-fast and stable in the alkaline cement environment. With some types of pigment the water requirement of the mix can increase.

·     Rock flours (quartz dust, powdered limestone)

Low fines mixes can be improved by adding rock flours. These inert materials are used to improve the grading curve. The water requirement is higher, particularly with powdered limestone.

Type II

Pozzolanic or latent hydraulic materials such as natural pozzolans (trass), fly ash and silica dust.

Fly ash is a fine ash from coalfired power stations which is used as an additive for both cement and concrete. Its composition depends mainly on the type of coal and its origin and the burning conditions.

Silica dust (Silicafume) consists of mainly spherical particles of amorphous silicon dioxide from the production of silicon and silicon alloys. It has a specific surface of 18–25 m² per gram and is a highly reactive pozzolan.

Standard dosages of silica dust are 5% to 10% max. of the cement weight.

Different types of cement

Mastou Ready Mix, we understand that our ready mix concrete and cement products, are the most unique and versatile building materials.

Same type of cement may not be suitable for different locations and climatic conditions. Therefore various types of cement have been developed as per the actual requirements. The necessary changes have been achieved by different methods like :

(a)   Changing oxide composition

(b)   Changing fineness

(c)   Using additives or mineral mixtures like slag, fly-ash or silica fumes etc.

The various types of cements generally used in various locations are as given below:

1. Ordinary Portland Cement (OPC)

2. Rapid Hardening Cement (RHC)

3. Sulphate Resistant Cement (SRC)

4. Blast furnace slag cement

5. Portland Pozzolana Cement (PPC)

6. Air entraining cement

7. Quick Setting Cement

8. Expansive Cement

9. High alumina cement