‘CODE OF PRACTICE FOR PLAIN AND REINFORCED CONCRETE -
Materials
Cement (Cl. 5.1) :
All three grades of OPC viz. 33, 43 & 53, Low Heat Portland cement and
Sulphate resisting Portland cement have been included under various
types of cement. In the manufacturing of concrete, the code also permits
other combination of Portland cement with mineral admixtures of quality
listed in Cl. 5.2.
2.1.2 Mineral Admixtures (Cl. 5.2) :
The mineral admixtures, which may be used as part replacement of
cement, listed in the code are Pozzolanas (such as fly ash, silica fume,
rice husk ash and metakaoline) and Ground Granulated Blast Furnace
Slag.
2.1.3 Aggregates (Cl. 5.3) :
(i) Preference has been given to the use of natural aggregates as far
as possible. Most of the zonal railway specifications permit use of
only crushed aggregate for RCC structures. It is considered that
this should be reviewed to generate greater use of natural
aggregates.
(ii) The limit of sulphate content in other types of aggregates such as
slag etc. has been reduced from 1% to 0.5%.
(iii) According to the revised code, 40 mm or larger size aggregates
may be permitted where there is no restriction to the flow of
concrete in the section.
(iv) The code recommends considering use of 10 mm nominal
maximum size aggregate for thin section, closely spaced
reinforcement and smaller cover.
2.1.4 Water (Cl. 5.4)
(i) From durability considerations, permissible limits of solids in water
have been reduced as under:
• Sulphate - from 500 mg/l to 400 mg/l
• Chloride - from 1000 mg/l to 500 mg/l for RCC works
(ii) The 1978 edition of the code allowed use of seawater under
unavoidable circumstances in such RCC structures, which are
permanently under seawater. But the present revision prohibits this.
2.1.5 Admixtures (Cl. 5.5)
The 1978 edition contained a single sentence permitting use of admixture
conforming to IS-9103. Considering increase in the usage of several types
of admixtures available in the market, the new revision has given following
detailed guidelines on their use under sub clause 5.5.1 to 5.5.6.
• Admixtures should not impair durability of concrete.
• Workability, compressive strength and slump of concrete should be
checked for both with and without admixtures in trial mix.
• Relative density of liquid admixtures to be checked.
• Chloride content to be independently tested.
• If two or more admixtures are used simultaneously, data should be
obtained to assess their interaction to ensure their compatibility.
2.1.6 Reinforcement (Cl. 5.6)
i) Use of hot rolled deformed bars conforming to IS 1139 has been
withdrawn.
ii) Use of structural steel conforming to Grade A of IS 2062 has been
introduced in place of IS 226 as the latter code is superceded by
the former.
iii) Cleaning of reinforcement by sand blasting or other treatment has
been recommended.
iv) Reference to specialist literature has been recommended for
special precautions like coating of reinforcement in exceptional
cases.
2.2 Concrete (Cl. 6)
2.2.1 Grade of Concrete (Cl. 6.1)
i) Higher grade of concrete up to M 80 has been permitted (earlier
revision was permitting maximum grade M 40). This is a
progressive provision in line with recent trend for using high
strength concrete in India and abroad. However, the code has
cautioned that for high strength concrete (compressive strength
greater than M 55), the design parameters given in this standard .
may not be applicable and the values may be obtained from
specialist literatures and experimental results.
ii) The grades of concrete have been classified in following three
groups:
a) Ordinary Grade M 10 to M 20
b) Standard concrete M 25 to M 55
c) High strength concrete M 60 to M 80
iii) The minimum grade of concrete for plain and reinforced concrete in
various exposures conditions have been revised as under:-
Exposure condition Minimum grade of concrete
Plain Cement
Concrete
Reinforced Cement
Concrete
Mild - M 20
Moderate M 15 M 25
Severe M 20 M 30
Very Severe M 20 M 35
Extreme M 25 M 40
2.2.2 Properties of concrete (Cl. 6.2)
(i) IS 456 : 1978 allowed increase in compressive strength of concrete
up to 20% depending upon the age of concrete where it could be
shown that a member would not receive its full design load/stress
within a period of 28 days. The new revision though acknowledges
the increase in compressive strength with age but recommends
design based on 28 days strength only unless there is evidence to
justify higher strength for concrete of a particular structure. This is
because the increase in strength depends upon the grade and type
of cement, curing, environment conditions, etc. For concrete of
Grade M 30 and above, the rate of increase of compressive
strength with age should be based on actual investigation.
(ii) The value of Modulus of elasticity has been reduced from 5700√fck
to 5000√fck, which means that the deformation of the structure will
be more.
2.3 Workability of concrete (Cl. 7)
IS 456 : 1978 specified workability in terms of compacting factor, vee-bee
time and slump. But the new revision specifies workability only in terms of slump;
except in case of “very low” and “very high” degree of workability, where
compaction factor and flow determination method respectively have been specified. This is probably because of the absence any correlation between the three test methods.
2.4 Durability of concrete (Cl. 8)
Major thrust/emphasis has been given on durability aspects. The durability
clause has been enlarged to a great extent to include guidance on concerning
factors. Detailed clause covering various requirements for durability of concrete
structures has been incorporated.
2.4.1 Shape and size of member (Cl. 8.2.1)
For the first time, importance has been given to shape and design
detailing to enhance durability of exposed concrete structures. Specific
mention has been made regarding good drainage arrangement, adequate
curing, cover to steel, chamfering of corners, surface coating, member
profiling and design detailing of member intersections to ensure easy flow
of concrete.
2.4.2 Exposure conditions (Cl. 8.2.2)
2.4.2.1 General Environment (Cl. 8.2.2.1)
Table 3 on environmental exposure conditions has been modified to
include “very severe” and “extreme” exposure conditions. Five environmental
exposure conditions (viz. Mild, moderate, severe, very severe and extreme) have
been defined in this table.
2.4.2.2 Abrasive action (Cl. 8.2.2.2)
Reference to specialist literature has been recommended for durability
requirements of concrete surface exposed to abrasive action.
2.4.2.3 Freezing and thawing (Cl. 8.2.2.3)
Use of suitable air entraining admixtures has been suggested for
obtaining enhanced durability in case of freezing and thawing actions under wet
conditions. For concrete lower than grade M 50, the mean total air content has
been specified for such cases. Since air entrainment reduces the strength of
concrete, suitable adjustment in the mix design may be required.
2.4.2.4 Exposure to sulphate attack (Cl. 8.2.2.4)
Table 4 (on requirements for concrete exposed to sulphate attack) has
been modified to include two more classes of sulphate attack viz. Class 4 and 5.
For very high sulphate concentration in class 5, use of lining with polyethylene or
polychloroprene sheet or suitable surface coating has been recommended.
The cover to embedded steel -
The nominal cover to embedded steel required from durability
consideration has been related with exposure conditions vide Table 16.
Tolerances for concrete cover have also been specified. In addition, minimum
cover is also specified to meet different specified period of fire resistance from
0.5 hour to 4 hours.
Nominal cover has been defined as the depth of concrete cover to all
steel reinforcement, including links.
2.4.4 Concrete mix proportions (Cl. 8.2.4)
From durability considerations appropriate values for minimum cement
content and the maximum free water-cement ratio applicable to 20 mm
nominal maximum size aggregate have been specified in Table 5 for
different exposure conditions. Adjustment for minimum cement content
for other aggregate size has been given in Table 6.
2.4.5 Maximum cement content (Cl. 8.2.4.2)
A new clause 8.2.4.2 has been added specifying that cement content
not including fly ash and ground granulate blast furnace slag in excess
of 450 kg/m3
should not be used unless special consideration has
been given to the increased risk of cracking due to drying shrinkage in
thin sections or to early thermal cracking and to increased risk of
damage due to alkali silica reactions.
This provision may lead to increased use of mineral admixtures as a
part replacement of cement particularly for higher grade of concrete
leading to likely enhancement of durability.
2.4.6 The type and quality of mix constituents (Cl. 8.2.5)
For concrete to be durable, careful selection of the mix and materials is
necessary so that the presence of deleterious constituents do not
exceed the prescribed limits.
2.4.6.1 Chloride in concrete (Cl. 8.2.5.2)
Chloride in concrete is harmful and there is an increased risk of
embedded steel being corroded. To minimize the chances of
deterioration, the maximum total acid soluble Chloride content in the
concrete at the time of placing for different type/use of concrete has
been limited vide Table 7. As per the new code, the maximum Chloride
content is 0.6 kg/m3
for RCC works; while the earlier edition limited the
chloride content to 0.15% by mass of cement. Thus the new provision .
is more lenient in this respect for concrete having cement content up to
400 kg/m3
.
2.4.6.2 Sulphate in concrete (Cl. 8.2.5.3)
Sulphates are present in most cement and in some aggregates.
Excessive amount of water-soluble sulphate can cause expansion and
disruption of concrete. To prevent this, the total water-soluble sulphate
content of the concrete mix expressed as SO3 has been limited to 4%
by mass of cement; which is same as was provided in IS 456 : 1978.
2.4.6.3 Alkali-aggregate reaction (Cl. 8.2.5.4)
Some aggregates containing particular varieties of silica may be
susceptible to attack by alkalies (Na2O and K2O) originating from
cement or other sources and may produce an expansive reaction
which can cause cracking and disruption of concrete. The new code
has suggested taking one or more of the following precautionary
measures when the service records of the particular cement/aggregate
combination is not well established:
• Use of non-reactive aggregate from alternative sources.
• Use of low alkali OPC having total alkali content not more than
0.6% as Na2O equivalent.
• Measures to reduce the degree of saturation of concrete during
service such as use of impermeable membranes.
• Limiting the cement content in the mix and thereby limiting the total
alkali content.
2.4.7 Concrete in aggressive soils and water (Cl. 8.2.6)
The code has suggested that at sites where the alkali concentration
are high or may become very high, the ground water should be lowered by
drainage so that it does not come into direct contact with the concrete. It has also
suggested additional protection like the use of chemically resistant stone facing
or a layer of plaster of Paris covered with suitable fabric, such as jute thoroughly
impregnated with bituminous material.
2.4.8 Concrete in Sea-water (Cl. 8.2.8)
From durability considerations, the minimum grade of concrete in sea-
water or exposed directly along sea-coast has been increased from M 15 to M 20
in the case of PCC and from M 20 to M 30 in the case of RCC works.
Concrete mix proportioning -
Salient revised provisions under this clause are highlighted hereunder:
i) In the list of information required in specifying a particular grade of
concrete, the following new items have been added:
• Exposure conditions as per Table 4 and 5 of the standard.
• Maximum temperature of concrete at the time of placing.
• Method of placing.
• Degree of supervision.
ii) As the guarantor of quality of concrete used in the
construction, the constructor shall carry out the mix design and the
mix so designed (not the method of design) shall be approved by
the employer within the limitations of parameters and other
stipulations laid down in this standard.
This provision is very important as the responsibility of
quality assurance and carrying out the mix design has been rightly
entrusted to the constructor. Also what is more important is that the
results of mix rather than the method of its design has been insisted
upon as the basis for finalising the mix proportion. In this context it
is worth mentioning that the earlier version of this code mentioned
that the procedure given in IS : 10262-1982, Recommended
Guidelines for Concrete Mix Design (which was under preparation
at that time), may be followed. But the new IS : 456 does not refer
to the above standard either in the body of the code or in the list of
referred Indian Standards appearing in Annex A.
iii) The target mean strength of the concrete mix should be equal to
the characteristic strength plus 1.65 times the standard deviation.
iv) The provision regarding necessity for revision of concrete mix has
been modified. The mix design done earlier not prior to one year
has been considered adequate for later works provided there is no
change in source and the quality of the materials.
v) When sufficient test results for a particular grade of concrete are
not available, the value of standard deviation given in Table 8 is to
be taken for design of mix in the first instance. The values of
assumed standard deviation given in the above table correspond to
the site control having proper storage of cement; weigh batching of
all materials; controlled addition of water; regular checking of all
materials, aggregate grading and moisture content; and periodical
checking of workability and strength. Where there is deviation from the above, the values given in the table shall be increased by
1N/mm2
. The revised code further stipulates that as soon as the
results of samples are available, actual calculated standard
deviation shall be used and the mix be designed properly.
Production of concrete (Cl. 10)
2.6.1 Quality Assurance Measures (Cl. 10.1)
Some important aspects on quality assurance measures have been
added. These include:
• Quality assurance to proper design, use of adequate materials and
components to be supplied by producers, proper workmanship in
the execution and timely maintenance and repair during service.
• Development and implementation of a general Quality Assurance
Plan (QAP) to identify the key elements necessary to provide
fitness of the structure and the means by which they are to be
provided and measured. The quality assurance would involve
quality audit of inputs such as materials of concrete; workmanship
in all stages of batching, mixing, transportation, placing, compaction
and curing; and related plant, machinery and equipment.
• The QAP shall define the task and responsibility of all persons
involved, adequate control and checking procedure and
maintenance of adequate documentation, which should generally
include:
* Test reports and manufacturer’s certificate for materials,
concrete mix design details;
* Pour cards for site organization and clearance for concrete
placement;
* Record of site inspection of workmanship, field tests;
* Non-conformance reports, change orders;
* Quality control charts; and
* Statistical analysis
2.6.2 Batching -
The following important additions have been made:
To avoid confusion and error in batching, consideration should be
given to using the smallest practical number of different concrete
mixes on any site or in any one plant.
• Ready-mixed concrete supplied by ready-mixed concrete plant has
been given preference. For large and medium project sites, the
concrete should be sourced from ready-mixed concrete plants or
from on site or off site batching and mixing plants.
• The accuracy of measuring equipment should be within ± 2% of the
quantity of cement being measured and within ± 3% of the quantity
of aggregate, admixtures and water being measured.
• Volume batching may be allowed only when weigh batching is not
practical and provided accurate bulk densities of materials to be
actually used in concrete have earlier been established. The mass
volume relationship should be checked as frequently as necessary.
Mixing -
• The mixers are required to be fitted with water measuring
(metering) devices.
• Dosages of retarders, plasticisers and superplasticisers have been
restricted to 0.5, 1.0 and 2.0 percent respectively by weight of
cementitious materials.
2.7 Formwork -
The minimum period before striking vertical formwork to columns,
walls, beams, etc. has been reduced to 16-24 hours from 24-48 hours.
2.8 Assembly of reinforcement (Cl. 12)
• Re-bending or straightening of high strength deformed bars without
prior approval has been prohibited.
• Bar bending schedule is required to be prepared for all
reinforcement work -
• As per IS 456 : 1978, the reduction in concrete cover was permitted
up to one-third of specified cover or 5 mm whichever is less. But
the new code specifies tolerance of cover as +10 mm and –0 mm.
Thus, no reduction in actual cover from the specified cover has
been permitted.
Types of cover-blocks acceptable -(concrete of same strength or
PVC) have been mentioned. This may pave the way for use of PVC
cover blocks.
2.8.1 Welded Joints or mechanical connections (Cl. 12.4)
• Only up to 12 mm for high strength deformed steel bars and up to
16 mm for mild steel bars are permitted to bend aside at
construction joints which can afterwards be bent back to original
position.
• Reinforcement should be placed and tied in such a way that
concrete placement be possible without segregation and allow
compaction by immersion vibrator.
• Within concrete mass, different types of metal in contact should be
avoided to ensure that bimetal corrosion does not take place.
2.9 Transportation, placing, compaction and curing (Cl. 13)
• Maximum permissible free fall of concrete has been stipulated as
1.5m.
• Construction joints should comply with IS 11817.
• The earlier practice of introducing a cement slurry/mortar layer
between old and new concrete has been discontinued. It has now
been recommended to roughen the surface of the previously
poured concrete to expose the aggregate and the prepared surface
should be in a clean surface dry condition when the fresh concrete
is placed against it. Fresh concrete should be thoroughly vibrated
near construction joint so that mortar from fresh concrete flows
between large aggregates and develops proper bond with old
concrete. Provision of shear keys has been recommended where
high shear resistance is required at construction joints.
• The earlier code made a general statement that the curing period
should not be less than 7 days from the date of placing concrete.
But the new code has specified different period of curing for
concrete where mineral admixtures or blended cements are used
and also for concrete exposed to dry and hot weather conditions.
As per the new code, the minimum curing period of concrete made
with OPC is 7 days for normal conditions and 10 days for dry and hot weather conditions. The same for concrete where mineral
admixtures or blended cements are used, are 10 days for normal
conditions and 14 days for dry and hot weather conditions.
• Impermeable membrane such as polyethylene sheeting to provide
effective barrier against evaporation may be used as an alternative
to moist curing .
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