Ready Mix Concrete

General Introduction

Ready Mix Concrete is a specialized material in which the cement aggregate

and other ingredients are weighed at batch. Ready-mix concrete is concrete

that is manufactured in a factory or batching plant, according to a set recipe,

and then delivered to a work site, by truck mounted in–transit mixers. This

results in a precise mixture, allowing specialty concrete mixtures to be

developed and implemented on construction sites. The first ready-mix

factory was built in the 1930s, but the industry did not begin to expand

significantly until the 1980s, and it has continued to grow since then.

Ready-mix concrete is sometimes preferred over on-site concrete mixing

because of the precision of the mixture and reduced work site confusion.

However, using a pre-determined concrete mixture reduces flexibility, both

in the supply chain and in the actual components of the concrete.

Ready-mix concrete is also referred as the customized concrete products for

commercial purpose. Ready-mix concrete, or RMC as it is popularly called,

refers to concrete that is specifically manufactured for delivery to the

customer's construction site in a freshly mixed and plastic or unhardened

state. Concrete itself is a mixture of Portland cement, water and aggregates

comprising sand and gravel or crushed stone. In traditional work sites, each

of these materials is procured separately and mixed in specified proportions

at site to make concrete. Read-mix concrete is bought and sold by volume -

usually expressed in cubic meters (cubic yards in the US). Ready-mix

concrete is manufactured under controlled operations and transported and

placed at site using sophisticated equipment and methods.

Storage Materials

Cement:- A cement is a binder, a substance that sets and hardens and can

bind other materials together. The word "cement" can be traced back to the

Roman term opus caementicium, used to describe masonry resembling

modern concrete that was made from crushed rock with burnt lime as binder.

The volcanic ash and pulverized brick supplements that were added to the

burnt lime, to obtain a hydraulic binder, were later referred to as cementum,

cimentum, cäment, and cement.

Cements used in construction can be characterized as being either hydraulic

or non-hydraulic, depending upon the ability of the cement to set in the

presence of water.

Non-hydraulic cement will not set in wet conditions or underwater; rather, it

sets as it dries and reacts with carbon dioxide in the air. It can be attacked by

some aggressive chemicals after setting.

Hydraulic cements (e.g., Portland cement) set and become adhesive due to a

chemical reaction between the dry ingredients and water. The chemical

reaction results in mineral hydrates that are not very water-soluble and so are

quite durable in water and safe from chemical attack. This allows setting in

wet condition or underwater and further protects the hardened material from

chemical attack. The chemical process for hydraulic cement found by

ancient Romans used volcanic ash.

Aggregates:- Construction aggregate, or simply "aggregate", is a

broad category of coarse particulate material used in construction, including

sand, gravel, crushed stone, slag, recycled concrete and geosynthetic

aggregates. Aggregates are the most mined materials in the world.

Aggregates are a component of composite materials such as concrete and

asphalt concrete; the aggregate serves as reinforcement to add strength to the

overall composite material. Due to the relatively high hydraulic conductivity

value as compared to most soils, aggregates are widely used in drainage

applications such as foundation and French drains, septic drain fields,

retaining wall drains, and road side edge drains. Aggregates are also used as

base material under foundations, roads, and railroads.

In other words, aggregates are used as a stable foundation or road/rail base

with predictable, uniform properties (e.g. to help prevent differential settling

under the road or building), or as a low-cost extender that binds with more

expensive cement or asphalt to form concrete.

Preferred bituminous aggregate sizes for road construction are given in EN

13043 as d/D (where the range shows the smallest and largest square mesh

grating that the particles can pass). The same classification sizing is used for

larger armor stone sizes in EN 13383, EN 12620 for concrete aggregate, EN

13242 for base layers of road construction and EN 13450 for railway ballast.

The American Society for Testing and Materials publishes an exhaustive

listing of specifications including ASTM D 692 and ASTM D 1073 for

various construction aggregate products, which, by their individual design,

are suitable for specific construction purposes. These products include

specific types of coarse and fine aggregate designed for such uses as

additives to asphalt and concrete mixes, as well as other construction uses.

State transportation departments further refine aggregate material

specifications in order to tailor aggregate use to the needs and available

supply in their particular locations.

Late Entry of RMC in India

RMC is highly mechanized activity and entails initial high cost.

Especially due to import of basic equipment and machinery.

Smaller size of construction in unorganized sector highly competitive

and cost conscious.

Availability of abundant cheap labour for making and transporting

concrete.

Differential taxation between RMC and SMC. Especially before 1997

when excise duty @16% also existed.

Factors that promoted entry of RMC

Increasing stakes in the reliability and durability of construction of

emerging users.

Increasing stakes in the reliability and durability of construction of

emerging users.

Decreasing share of construction cost in overall cost of the facility.

Increasing awareness on environmental factors and convenience.

Globalization – adoption of best practices across the globe.

Bigger size of projects and Time is recognized as a cost factor.

RMC in India

In India RMC was first initially used in 1950 during the constrction of

dams like Bhakra Nangal, Koyna. At the construction site the transportation

of concrete is done either manually or mechanically using ropeways or

conveyor systems. RMC Readymix (India) is one of the largest ready-mixed

concrete manufacturer in India. Set-up way back in 1996, it is one of the

pioneers of ready-mixed concrete technology in India. RMC currently

operates 88 ready-mixed concrete plants in 35 cities and towns across the

country. RMC also ventured into Aggregates business and operates large

quarries and crushers. At present, RMC has 8 quarries and crushers across

the country. RMC has been at the forefront in setting high standards for

plant and machinery, production and quality systems and product services in

the ready-mixed concrete industry. RMC plans to scale-up its capacity from

3.87 million m3 (2008-09) to 11 million m3 by 2014-15.

RMC Readymix (India) has the best of the technical abilities and systems in

the ready-mixed concrete and aggregate business, capable of delivering the

right quality products to the customer in the most cost-effective manner.

The Company tracks the latest technological developments occurring in the

sphere of concrete technology and adapts them to the needs of the Indian

market. It is one of the few companies in the country which has successfully

produced and delivered latest-generation special concretes such as high-

strength, high-performance concrete, self-compacting concrete, fiber-

reinforced concrete, temperature-controlled concrete, etc.

RMC employs state-of-the-art plants and equipment and can boast of having

the best technical expertise and manpower in the Indian concrete and

aggregate industries.

RMC strictly adheres to all statutory requirements. It has a well-defined

policy on Health, Safety and Environment and follows strict norms to ensure

compliance.

Merits of RMC

Speedy construction through continuous mechanized operations

leading to economy.

Lower labor and supervisory costs assured.

Multi- transportation of raw materials for concrete is eliminated.

Controlled W/C ratio.

Close quality control of batching reduces variability of the desired

properties of hardened concrete.

Use in congested sites or in highway construction where there is little

space for a mixing plant and aggregate stocking.

Use of agitator trucks to ensure care in transportation, thus preventing

segregation and maintaining workability.

Minimization of cement wastage in view of bulk handling of storage.

It can be supplied with required slump and consistency

Useful additives like fly ash, plasticizers, and retarders can be added

to enhance the properties under controlled condition.

Demerits of RMC

As the Ready Mixed Concrete is not available for placement

immediately after preparation of concrete mix, loss of workability

occurs. In addition, there are chances of setting of concrete if transit

time involved is more. Therefore, generally admixture like

plasticisers/ super plasticisers and retarders are used. Addition of

retarders may delay the setting time substantially which may cause

placement problems. In addition, it may also affect the strength of

concrete. Therefore, it is necessary that the admixtures i.e. plasticisers

and super plasticisers/ retarders used in Ready Mixed Concrete are

properly tested for their suitability with the concrete. In case loss of

strength is observed, the characteristic strength may have to be

enhanced so that after loss of strength, required characteristic strength

is available.

Because of large quantity of concrete available in short span, special

placing and form work arrangement are required to be made.

Need huge investment

Not affordable for small project

Need effective transportation system

RMC should be placed within 90 min. (approx.) of batching at the

plant

Difference between Ready Mix and Site Mix

Concrete

Ready Mix Concrete Site Mix Concrete

Consistent quality concrete is made

in high tech batching plant in

computerized system

Quality is inconsistent because

mixing is manual

Construction is double quick time Manual mixing is time consuming

Raw materials are chosen after

quality check

Quality of materials is manually

checked

Large quantity of material can be

ordered for large project

Repeated mixing is to be done for

large quantities in project

Low wastage of Raw Material High Wastage of raw material

Mixing Process

There are three types of process of RMC

Transit Mixed Concrete:- In transit-mixed concrete, also called truck

mixed or dry-batched, all of the raw ingredients are charged directly in the

truck mixer. Most or all water is usually batched at the plant. The mixer

drum is turned at charging (fast) speed during the loading of the materials.

There are three options for truck mixed concrete:

Concrete mixed at the job site. While travelling to the job site the drum is

turned at agitating speed (slow speed). After arriving at the job site, the

concrete is completely mixed. The drum is then turned for 70 to 100

revolutions, or about five minutes, at mixing speed.

Concrete mixed in the yard. The drum is turned at high speed or 12-15 rpm

for 50 revolutions. This allows a quick check of the batch. The concrete is

then agitated slowly while driving to the job site.

Concrete mixed in transit. The drum is turned at medium speed or about 8

rpm for 70 revolutions while driving to the job site. The drum is then slowed

to agitating speed.

Central Mixed Concrete:- Central-mixing concrete batch plants include a

stationary, plant-mounted mixer that mixes the concrete before it is

discharged into a truck mixer. Central-mix plants are sometimes referred to

as wet batch or pre-mix plants. The truck mixer is used primarily as an

agitating haul unit at a central mix operation. Dump trucks or other non-

agitating units are sometimes be used for low slump and mass concrete

pours supplied by central mix plants. About 20% of the concrete plants in

the US use a central mixer.

Principal advantages include:

Faster production capability than a transit-mix plant

Improved concrete quality control and consistency and

Reduced wear on the truck mixer drums.

There are several types of plant mixers, including:

Tilt drum mixer

Horizontal shaft paddle mixer

Dual shaft paddle mixer

Pan mixer

Slurry mixer

The tilting drum mixer is the most common American central mixing unit.

Many central-mix drums can accommodate up to 12 yd3 and can mix in

excess of 200 yd3 per hour. They are fast and efficient, but can be

maintenance-intensive since they include several moving parts that are

subjected to a heavy load.

Horizontal shaft mixers have a stationary shell and rotating central shaft with

blades or paddles. They have either one or two mixing shafts that impart

significantly higher horsepower in mixing than the typical drum mixer. The

intensity of the mixing action is somewhat greater than that of the tilt drum

mixer. This high energy is reported to produce higher strength concrete via

to thoroughly blending the ingredients and more uniformly coating the

aggregate particles with cement paste. Because of the horsepower required

to mix and the short mixing cycle required to complete mixing, many of

these mixers are 4 or 5 yd3 units and two batches may be needed to load a

standard truck or agitator.

Pan mixers are generally lower capacity mixers at about 4 to 5 yd3 and are

used at precast concrete plants.

Shrink Mixed:- 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.

Grade of Concrete

Specified

Grade

Mean of the group of 4 Non-

Overlapping

Consecutive Test results in

N/sq.mm

Individual Test Results in

N/sq.mm

(1) (2) (3)

M 15 > or equal to fck+0.825 x

established standard deviation

> or equal to (fck-3)

N/sq.mm

Grade of Concrete Assumed Standard Deviation

N/sq.mm

M 10 3.5

M 15

M 20 4.0

M 25

M 30 5.0

M 35

M 40

M 45

M 50

(rounded off to nearest 0.5

N/sq.mm) or fck+3 N/sq.mm

whichever is greater

M 20 or

above

> or equal to fck+0.825 x

established standard deviation

(rounded off to nearest 0.5

N/sq.mm) or fck+4 N/sq.mm

whichever is greater

> or equal to (fck-4)

N/sq.mm

Reference

http://www.pellresearch.com/us/ready-mix-concrete-

manufacturing.html

R.S. Agarwal “Concrete Technology”

http://www.nrmca.org/aboutconcrete/howproduced.asp

http://elearning.vtu.ac.in/12/enotes/Adv_Conc_Stru/Unit5-RVR.pdf

http://www.nrmca.org/sustainability/RMCREF%20LEED%20Guide

%20Revised%2001-10.pdf