4. INTRODUCTION
The first ready mix factory which was built in 1930’s.
The consumption of cement in the form of RMC is about 5% in
India, where as in developed countries it’s about 70%.
The leading RMC supplier worldwide is the mexican concrete and
cement company cemex.
5. DEFINITION
A concrete which is specially manufactured for the delivery to the
costumer construction site in a freshly mixed, plastic and
unhardened state.
6. BACKGROUND
RMC industry is about 12 years old in India, while it was introduced
in other countries much earlier
- PATENTED IN GERMANY
• 1913- FIRST DELIVERY OF RMC (BALTIMORE)
• 1926- BIRTH OF TRANSIT MIXER
• 1931- ERECTION OF FIRST COMMERCIAL PLANT IN LONDON
• 1950-1974 REMARKABLE GROWTH OF RMC – 31 MILLION
CUBIC METRE OF CONCRETE PER YEAR
9. CEMENT
Cement is a binder material which sets and harden
independently, and can bind other materials together.
Cement is made up of four main compounds tricalcium
silicate (3CaO SiO2), dicalcium Silicate (2CaO SiO2),
tricalcium acuminate (3CaO Al2O3), and tetra-calcium
aluminoferrite (4caco Al2O3 Fe2O3).tetra-calcium
aluminoferrite (4CaO Al2O3 Fe2O3).
10. AGGREGATES
Aggregates are the important constituents in concrete. They give body to
the concrete, reduce shrinkage and effect economy. Earlier, aggregates
were considered as chemically inert materials but now it has been
recognised that some of the aggregates are chemically active and also
that certain aggregates exhibit chemical bond at the interface of
aggregate and paste.
Aggregates are divided into two categories from the consideration of size
Coarse aggregate Fine aggregate The size of the aggregate bigger than
4.75 mm is considered as coarse aggregate and aggregate whose size is
4.75 mm and less is considered as fine aggregate.
11. ADMIXTURES
A substance added to the basic concrete mixture to alter one or
more properties of the concrete; i.e. fibrous materials for
reinforcing, water repellent treatments, and colouring
compounds.
Role of Admixture in Ready Mix Concrete: The role of admixture is
ready mixed of concrete is same as that in normal concrete.
However, admixture used in RMC is modified to meet the
requirement of pumpable concrete and other properties of
concrete. The types of admixture used in RMC are generally
termed as Super plasticizers.
12. FLY ASH
Fly ash is a by-product from coal-fired electricity generating power
plants. The coal used in these power plants is mainly composed of
combustible elements such as carbon, hydrogen and oxygen
(nitrogen and sulphur being minor elements), and non
combustible impurities (10 to 40%) usually present in the form of
clay, shale, quartz, feldspar and limestone.
Fly ash use improves concrete performance, making it stronger,
more durable, and more resistant to chemical attack. Fly ash use
also creates significant benefits for our environment. The size of
fly ash ranges from 1.0 to 100 micron & the average size is around
20 microns.
13. WATER
The pH value of water should be
in between 6.0 and 8.0 according to IS 456-2000.
Effect of Mixing Sea Water in Concrete:
The sea Water generally contains salinity of about 3.5% in which about 80% is
sodium chloride. Many researchers have been conducted to study the
corrosion problem of steel Embedded in concrete where sea water is used as
mixing water in concrete nevertheless the Indian standard is adamant & do
not permit using sea water for mixing or curing in reinforced Concrete
constructions, but allows for using of sea water only for PCC work that too
under unavoidable circumstances.
14. JOURNEY OF RMC
Ready – mix concrete has cement, aggregates, water and other
ingredients, which are weighed and batched at a centrally located
plant.
This is then delivered to the construction site in transit mixers and
can be used straightaway without any further treatment.
Ready Mixed Concrete is manufactured under computer-controlled
operations and transported and placed at site using sophisticated
equipment and methods.
15.
16. MANUFACTURING
Ready mixed concrete is proportioned and mixed off at the project
site and is delivered to the construction area in a freshly mixed
and unhardened state. It can be manufactured by any of the
following methods:
1. Central – Mixed Concrete
2. Truck – Mixed Concrete
19. CENTRALLY MIXED CONCRETE
The drum carrying the concrete revolves slowly so as to prevent the
mixed concrete from segregation and prevents its stiffening due to
initial set
20. TRUCK MIXED CONCRETE
The batched material(sand,gravl, and cement) are carried and water
is added just at the time of mixing. In case the cement remains in
contact with wet or moist material and this phase cannot exceed the
permissible period, which is normally 90 minutes.
21. RMC- MAJOR ADVANTAGES
• Assured And Uniform Quality Of Concrete
• Speedier Construction Through Mechanised Operations
• Need For Ordering And Storing Cement, Aggregates And Sand On
Site Totally Eliminated
• Lower Labour And Supervisory Costs
• Minimisation of cement wastage through bulk handling and
storage
• Cleaner working environment
• Eco-friendly product
22. Factors that Prompted Introduction of RMC
• 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 recognised as a cost factor.
23. APPROXIMATE CONSUMPTION OF CONCRETE IN INDIA
• Approximately 60% cement used for concrete
• 1 ton Cement is needed to produce 3 m3 of concrete
25. Growth of RMC plantsPercentage of RMC in Total
Concrete consumption
26. PRESENT MAJOR CONSTRAINTS IN PROMOTION OF
RMC
• AVAILABILITY OF SUITABLE LAND IN CITIES AT
REASONABLE PRICE
• SALES TAX / VAT ON RMC
• TRANSPORTATION BOTTLENECKS (RESTRICTED HOURS)
• COST COMPARISON WITH SMC, WHILE TWO PRODUCTS
ARE DIFFERENT. CONSUMER STILL COST CONSCIOUS
• LACK OF CODAL PROVISIONS, SPECIFICATIONS AND
REGULATIONS TO SUPPORT RMC.
• QUALIFIED AND TRAINED MANPOWER
28. QUALITY
Quality problems of various natures which may get reported are
listed below:
• Delay in setting of concrete beyond 24 hours.
• Development of cracks when the concrete is still in fresh state.
• Development of cracks in hardened concrete.
• Non-compliance of concrete cube strength (cast at site) to specifications
at 7-days.
• Non-compliance of concrete cube strength (cast at site) to specifications
at 28-days.
• Concrete supplied to site either has more slump than specified or has
become stiff making it difficult to pump.
29. THERMAL CRACKS
1. Early-age thermal cracking is caused by restraint to contraction on cooling from a
temperature peak, which is associated with the release of the heat of hydration of the
binders.
2. Early-age thermal cracking occurs within few days in thin sections, but it may take
several weeks to develop in massive sections.
3. Because of fast track constructions taking place to meet demands of infrastructure
development, the trend towards large continuous pours of concrete with the
requirements of high early strength and shorter striking times and problems of heat
of hydration are mainly responsible for early-age cracking in hardened concrete.
4. This type of cracks occurs when the restrained thermal contraction strain exceeds
tensile strain capacity of concrete.
31. It is necessary to understand the behaviour of each and every
constituent of concrete and construction techniques adopted at
site before arriving at any conclusion regarding quality problems
of RMC.
In many cases, information regarding Concrete Mix Design adopted
for the type of material available in the plant may not be available.
Properties of the constituents may not be available due to lack of
testing facilities at RMC plant
32. R
1. Delay in setting of concrete beyond 24 hours: Delay in setting of concrete is mainly due to
excess dosage of set retarding admixture used to retain pumpable slump for longer
durations. These admixtures are used to prolong the plasticity of concrete. Useful to
counter the effects of high temperature, eliminate cold joints, and reduce cracking
associated with form deflections.
2. Basically, retarding admixtures increase dormant stage in the C3S hydration process. Too
large a dose of admixture than specified by the manufacturer will cause the hydration
reaction never to proceed resulting in cement that will never set.
3. Remedies: It is important to carryout extensive trials while using set retarding admixture to
arrive at optimum dosage without delaying setting of concrete than required. This also
establishes the compatibility between cement and admixture.
4. It is important to note that when a particular brand of cement is found to be compatible
with a particular admixture, results may not be reproducible when cement is from a factory
at a different location even though brand of cement is same. It is necessary to have a good
quality control practices as properties of constituent materials vary to a large extent.
