Whilst strength testing is not intended to be used to judge conformity for standard or standardized prescribed concrete, the characteristic compressive strength, as shown in Table 12, may be assumed for the purposes of design.
For these concretes
it is necessary to specify:
n The mix title (ST1, ST2, ST3, ST4 or ST5)
n The class of concrete as reinforced or unreinforced
n The maximum aggregate size
n The consistence class.
Admixtures are not permitted in standard
mixes but are permitted
in
standardized prescribed concretes and, whilst numerous cement types are permitted, it is not intended that properties normally associated with some of those cements - such as low heat or
sulfate resistance, for example
- will be produced in the concrete.
The concrete producer is responsible for
ensuring that the
materials used conform to those specified and that the batched
weights are based on the proportions given in the appropriate standard. A guide to the correct selection
of standard/standardized prescribed
concretes is reproduced
in Table 12.
Conformity with the specification for standard mixes and standardized prescribed concretes is judged against supply of
concrete with the correct materials and proportions as defined in
BS 5328 : Part 2, or BS 8500 : Part 2. Strength
testing does not form part of the assessment
of standard mixes, or standardized
prescribed concretes. Because the proportions of standardized
prescribed concretes have been selected to take into account different types of aggregate
and variations in cement strengths,
cube
compressive strengths would be likely to exceed by as much
as 12 N/mm2 the assumed
characteristic strengths associated with
Table : Guide to the selection of standard/standardized prescribed concrete in housing and general applications.
|
Standard/
standardized prescribed concrete title
|
Assumed
characteristic cube strength (N/mm2 )
|
Application
in conditions where Design
Chemical class 1 concrete
is appropriate
|
Recommended consistence
class
|
|
ST1
|
8
|
Kerb bedding and backing
|
S1
|
|
ST2
|
10
|
Pipe bedding and drainage works to give immediate support
Other drainage works
Strip footings
Mass concrete foundations
Trench fill foundations
Blinding and mass concrete
fill
Oversite below suspended slabs
House floors with no embedded
metal - permanent finish
(e.g. screed) to
be
added
|
S1
S3
S3
S3
S4
S3
S3
S2
|
|
ST3
|
15
|
House floors with no embedded
metal - direct finish
|
S2
|
|
ST4
|
20
|
Garage floors with no embedded
metal
Wearing surfaces - light foot and trolley traffic
|
S2
S2
|
|
ST5
|
25
|
House floors with embedded metal
|
S2
|
|
NOTES
1. See Resistance to chemical attack on
page 23.
2. See BS 8500
for details of Design
Chemical classes.
3. Concrete containing embedded metal should be regarded as reinforced.
|
|||
the respective strength
classes. This compensates for the lack of
strength testing and the fact that standardized prescribed concretes
are
intended for site production with basic equipment and control.
At the time of publication, the use of Form C in BS 5328 : Part 2 is
recommended when specifying standard concretes. A copy is
reproduced in Appendix 1c, which can be used for this purpose by ringing the appropriate items.
Checking conformity
BS 5328 and BS EN 206-1 / BS 8500 give options for checking
the conformity of prescribed, standard and standardized prescribed
concretes, indicating that they may be assessed
by one of the
following methods:
- Observation of the batching
- Examination of the records of batch weights used
- Analysis of the fresh concrete in accordance with procedures defined in British Standards.
Designated
concretes
This group of wide-ranging concretes provides for almost every
type
of concrete construction. They have developed from the
original designated mixes introduced in 1991 to BS 5328 and, being
specific to the UK, are perpetuated in BS 8500. Divided into four sub-groups, designated concretes are deemed to be fit for the following specific purposes:
- General purpose low grade applications (GEN concretes)
- For use as foundations in sulfate-bearing ground conditions (FND concretes)
- Air-entrained concretes for pavement quality concrete (PAV concretes)
- Normal structural classes for reinforced concrete applications (RC concretes).
Full specifications for all designated concretes are given in BS 8500 : Part
2.
The use of the designation e.g. RC35, is an instruction to the
producer to conform to the specification in BS 8500 : Part 2.
The producer of designated concretes must operate a recognized accredited, third-party certification system, and ensure that the
concrete conforms to the specification given in BS 8500 : Part 2,
including:
n Characteristic strength
n Minimum cement content
n Maximum free water/cement ratio.
For these concretes
it is assumed that the nominal maximum aggregate size will be 20 mm; it is necessary simply to state that the concrete is required to conform to BS EN 206-1 / BS 8500 :
Part
2 and to specify the designation. The consistence class is
selected by the user of the concrete and this information
is passed to the specifier for inclusion
in the specification.
Aggregate sizes other than 20 mm may be specified, of course, but
this
detail would be given along with any further additional
requirements such as the use of fibres or a higher than normal air
content to allow for any loss of air during pumping, for example.
BS 8500 : Part 1 lists the options that may be exercised
by specifiers for these special cases.
Because designated concretes are quality-assured, there is no
necessity for the purchaser of the concrete to make test cubes.
Product conformity is ensured through accredited third-party
inspection of the quality procedures.
At
the time of publication, the use of Form D in BS 5328 : Part 2 is recommended when specifying designated concretes. A copy is
reproduced in Appendix 1d, which can be used for this purpose by ringing the appropriate items.
Proprietary concretes
A new sub-group
of concrete is proposed for UK practice to
provide for those instances when a concrete
producer would give
assurance of the performance of concrete without being required
to
declare its composition. This class of concrete is termed
proprietary concrete. However,
because the producer is nominated, specification of these concretes may be unsuitable for use by public authorities.
Strength
The strength of concrete is usually defined by the crushing strength of 100 mm or 150 mm cubes at an age of 28 days.
However, other types and ages of test and other sizes and shapes
of specimen are sometimes
used.
Test procedures are described
under Testing of hardened
concrete on pages 57 - 59. The strength of a concrete
will
usually be specified
as a characteristic strength. This is the strength below
which
not more than a stated proportion
of the concrete
falls. In BS 5328 and BS EN 206-1 this proportion
is defined as 5% (1 in 20). To protect the user, an absolute minimum strength of any
batch is specified.
The variability in results needs to be considered statistically, and a
detailed discussion on this subject is outside the scope of this
publication. However, it is briefly mentioned
to clarify the consideration of concrete strengths.
Because of the variability of test results, and the inherent variability
of the constituent materials, the concrete must be designed
to have a mean strength high enough above the characteristic strength to ensure that not more than the expected percentage of results fall below the characteristic strength. The difference
between this 'target mean' and characteristic strength is known as
the 'margin'. The spread of results from concrete strength tests has
been found to follow what is known, in statistics, as a 'normal' distribution, which enables it to be defined by the 'standard deviation of the results. The standard deviation
is a measure of the control that has been exercised over the production of the concrete. Where the spread of results and the standard
deviation are large, the margin also must be large, but where control over materials, mixing and testing procedures is good, the standard
deviation will be smaller and the margin may be reduced, leading to economies in materials. In practice,
the margin will usually be about 7-1 2 N/mm2.
To use this statistical method reliably for judging conformity
to the specification, a large number of test results is needed. Yet
conformity is commonly judged by examining
the results of smaller numbers of results as outlined below.
BS 5328 conformity rules
In BS 5328, groups
of
four test results are used, each result being the average of two results of cube tests on concrete from the same batch.
For the highest classes
of concrete (C20 and higher) to meet the BS 5328 specification requirements the average strength of a group of four consecutive test results must exceed the
characteristic strength by 3 N/mm2, and the strength
of any individual result must not be less than the characteristic strength minus 3 N/mm2.
In the case of the lowest
classes (C15 and below) the BS 5328
specification requirements are deemed to have been met when the
average strength of a group of four consecutive test results exceeds
the
specified characteristic strength by 2 N/mm2 and the strength of any individual
test result is not less than the characteristic
strength minus 2 N/mm2.
Additionally, for all strength
classes, the rules for the very
first sets of test results for a particular concrete on a new project permit the
average of the first two and the first three test results to be lower than the requirements for the mean of four by 2 N/mm2 and 1 N/mm2 respectively.
EN 206-1 conformity rules
During the initial stages of production,
that is, until at least 35 test
results have been obtained, the results are assessed in overlapping
or non-overlapping groups of three results. The mean strength of
each
group of three test results must be not less than 4 N/mm2 greater than the specified characteristic strength whilst the
occasional individual test result is permitted to be 4 N/mm2 less than the specified
characteristic strength.
After 35 test results have been generated within a period of not more than 12 months the initial production period is over and
continuous production is achieved.
The standard deviation
is calculated, the test results are assessed in groups of at least 15 and
the
minimum requirement is that the mean strength of each group
of results must be not less than the specified
characteristic strength
plus 1.48 x standard
deviation. As with the initial production period, the occasional individual test result is permitted to be 4 N/mm2
less than the specified
characteristic strength.
Conformity may be established using individual concretes or
defined concrete families.
The 'members' of each family
would typically be concretes that use the same type and strength class of
cement from a single source, their aggregates would be
demonstrably similar and they would all either contain an admixture or not
contain one. Test results
are collected over the
full range of consistence classes and a limited range of
strength classes, enabling
statistical evaluation to be made in determining whether a concrete remains within its family or must be removed from it.
During any contract
the materials will vary, and by keeping
continuous records of test results it is possible to vary the margin so
as
to make the best use of the materials while conforming
to the specification. Any changes that are made must not conflict with the
specific limiting values. The cement content, for examples, must not be reduced below the specified
minimum figure.