The strength of concrete
is normally specified by strength class,
that
is the 28-day characteristic compressive strength of specimens
made
from the fresh concrete under standardized conditions. The
results of strength tests are used routinely for both control of
production and contractual conformity purposes.
Characteristic strength is defined as that level of strength
below which a specified
proportion of all valid test results is expected
to fall. Unless otherwise
stated, this proportion
is taken to be 5%. Test cubes - either
100 mm or 150 mm - are the specimens normally used in the UK and most
other European countries, but cylinders
are used elsewhere. Because their shapes are different, the strength test results, even from the same concretes of the same
ages, are also different, cylinders being weaker
than
cubes. For normal-weight aggregates, cylinders are about 80% as strong as cubes, whereas cylinders made from lightweight aggregates have
90%
of the corresponding cube strength.
Accordingly the strength classes
recognized in BS EN 206-1 / BS 8500 are classified
in terms of both values, with the cylinder
strength followed by the cube strength.
The standard compressive
strength classes are listed in Table 9.
Table : Concrete compressive strength classes taken from
BS EN 206-1 .
|
Concrete compressive strength
classes
|
|
|
Concrete made with normal-weight aggregates
|
Concrete made with lightweight aggregates
|
|
C8/10
C12/15
C16/20
C20/25
C25/30
C28/35
C30/37
C32/40
C35/45
C40/50
C45/55
C50/60
C55/ 67
C60/75
C70/85
C80/95
C90/105
C100/115
|
LC8/9
LC12/13
LC16/18
LC20/22
LC25/28
LC30/33
LC35/38
LC40/44
LC45/50
LC50/55
LC55/60
LC60/66
LC70/77
LC80/88
|
In principle, the compressive strength may be determined
from cores cut from the hardened concrete.
Core tests are normally
made
only when there is some doubt about the quality of concrete placed, for example,
if cube strengths have been unsatisfactory - or to assist in determining the strength and quality of an existing structure for which records are not available.
Great care needs to be taken in the interpretation of the results of core testing: core samples drilled from the in-situ concrete
are expected to be lower in strength than the cubes made, cured and
tested under standard laboratory conditions.
For more information see Test cores on page 58. The standard
reference for core testing is BS EN 12504-1 and a useful guide is given
in Concrete Society Digest No. 9
Flexural and indirect tensile
strength
The tensile strength of concrete is generally taken to be about one- tenth of its compressive strength, but different aggregates cause
this proportion to vary and a compressive test is therefore only a
very general guide to the tensile strength
The indirect tensile strength (cylinder
splitting) is seldom specified
nowadays. Flexural testing of specimens, to measure the modulus of rupture, may be used on some airfield
runway contracts where the method of design is based on the modulus of rupture, and for
some
precast products such as flags and kerbs.
Durability of concrete
Concrete has to be durable and resistant
to various environments
ranging from mild to most severe, including weathering, chemical
attack, abrasion, freeze/thaw attack and fire. In addition,
for reinforced and prestressed concrete, the cover concrete
must provide protection against the ingress of moisture and air, which
would
eventually cause corrosion of the embedded steel.
The strength of the concrete alone is not necessarily a reliable
guide to the durability of concrete; many other factors also have to be taken into account. Of all the factors influencing the durability
of concrete the most important is that of impermeability. The
degree of impermeability is mainly dependent on:
- Constituents of the concrete, and in particular the free water/cement ratio
- Compaction, to eliminate air voids
- Curing, to ensure continuing hydration.
Table : Typical relationships between
free water/cement ratio, aggregate type, consistence class and Portland cement content.
|
Free w/ c ratio
|
Type of aggregate
|
Consistence/slump class
|
||
|
Low
S1(1
0 - 4 0 mm )
|
Medium
S2 (5 0 - 9 0 mm )
|
High
S 3 (100-15 0 mm )
|
||
|
Free water Cement
demand content (litres/m3 ) (kg/m 3 )
|
Free water Cement demand content (litres/m3 ) (kg/m 3 )
|
Free water Cement demand content (litres/m3 ) (kg/m 3 )
|
||
|
0.7
|
Uncrushed
Crushed
|
160 230
190 270
|
180 260
210 300
|
195 280
225 325
|
|
0.6
|
Uncrushed
Crushed
|
160 265
190 315
|
180 300
210 350
|
195 325
225 375
|
|
0.5
|
Uncrushed
Crushed
|
160 320
190 380
|
180 360
210 420
|
195 390
225 450
|
|
0.4
|
Uncrushed
Crushed
|
160 400
190 475
|
180 450
210 525
|
195 490
225 565
|
|
NOTES
1. 20 mm maximum aggregate size.
2. Uncrushed - natural
gravels and natural sands.
Crushed - crushed gravel or rock and crushed
sand.
.
. water demand
3. For a given consistence class, cement content
= free w/c ratio
4. Where concrete
contains a water-reducing admixture the relationship will be different.
S. Actual free water demands
may vary from the above values by ±10 litres/m3 and corresponding adjustments to the cement
contents may be required.
|
||||
Table : Exposure classes.
|
Class designation
|
Class description
|
|
XO
|
Concrete without reinforcement or embedded metal
Concrete with reinforcement or embedded metal in very dry conditions
All exposures with no freeze/thaw, abrasion or chemical attack
|
|
xc
XC1
XC2
XC3
|
Corrosion induced by carboration
Dry or permanetly wet
Wet, rarely dry
Moderate humidity
or cyclic wet and dry
|
|
xs
XS1
XS2
XS3
|
Corrosion induced by chloride
from seawater
Exposure to airborne salt but not in direct
contact with seawater
Permanently submerged
Tidal, splash and spray
zones
|
|
XD
XD1
XD2
XD3
|
Corrosion induced by chloride
other than seawater
Moderate humidity
Wet, rarely dry Cyclic wet and dry
|
|
XF
XF1
XF2
XF3
XF4
|
Freeze/thaw attack
Moderate water saturation without de-icing
agent Moderate water saturation with de-icing agent High water saturation without de-icing agent
High water saturation with de-icing
agent
|
|
XA
|
Chemical attack. Sulfate classification is given in BS 8500
|
Although free water/cement ratio is the main factor affecting impermeability, and hence durability, it cannot easily be measured
either in the fresh or hardened concrete.
However, for a particular
aggregate type and grading, the water demand for the same
consistence class is more or less constant
and is independent of the cement content.
Therefore, by knowing the water demand for a particular consistence class the cement content can be evaluated
for
the required and specified
free water/cement ratio. This is illustrated in Table 10.
Exposure Classes
It is recommended that exposure
classes be given 'X' codes, ranging from XO for mild exposure through the following codes
for exposure to different causes of deterioration:
- XA for exposure to chemical attack
- XC for risk of corrosion induced by carbonation
- XS for exposure to the sea and sea spray
- XD for exposure to chlorides from sources other than the sea
- XF for risk of freeze/thaw attack (with and without salt present.
Each group (apart from X0, mild exposure class) has a ranking system from 1 to 3 or 4 depending on the severity of the exposure. The exposure classes and their descriptions are listed in
Table 11. Guidance on limiting values recommended
as
being suitable for resisting
these exposure classes is given in BS 8500.
With increasing severity
of exposure the free water/cement ratio
needs to be decreased since durability is related to the concrete's
impermeability. It should also be noted that requirements
for exposure classes tend to include requirements for lowest strengths
of
concrete. In the past, specified
strengths tended to be lower than the minimum recommended for durability because the earlier
specifications were largely related to structural rather than durability requirements.
Compaction
In addition to the capillary voids (pores),
which are dependent on the water/cement ratio, air pockets or voids and even large cavities
or
'honeycombing' may also be present if the concrete
has not been fully compacted.
Concrete that has not been properly
compacted because of bad workmanship or because the mix design made compaction
difficult can result in a porous concrete,
which may, for example,
allow water seepage as well as easy ingress of air and chemicals
harmful to concrete.
Well-compacted concrete should not contain more than 1 % of entrapped air.
This subject is considered
in more detail in the section entitled Placing and compaction on page 34.
Curing
The importance of curing in relation to durability
is seldom fully
appreciated. It is essential that proper curing techniques are used
to reduce the permeability of concrete by ensuring
the continued hydration
process. The formation of the reaction
products, which
fill
up the capillary
voids, ceases when the concrete
dries to below
80% relative humidity.
Detailed information is given in Table 17 on page 46, from which
it
should be noted that longer curing periods are required
when cements containing
additions are used.
Cover
Many defects
in reinforced concrete are the result of insufficient cover, leading
to reinforcement corrosion. Too often, not enough
care
is given to the fixing of reinforcement to ensure that the specified minimum cover is achieved. The position of the reinforcement, and its cover, should be checked
before and during concreting, and may need to be checked after the concrete
has hardened. Further
information about cover is given in the section
titled Reinforcement on page 41 .
Carbonation
Reinforcement embedded in good concrete
with an adequate depth of cover is protected against corrosion
by the highly alkaline
pore water in the hardened
cement paste. Loss of alkalinity of the
concrete can be caused by the carbon dioxide in the air reacting with and neutralizing the free lime. This reaction is called
carbonation and if it reaches
the reinforcement, corrosion
will occur in moist environments.
Carbonation is a slow process progressing from the surface and
dependent on the permeability of the concrete and the humidity
of the environment. Provided that the depth of cover and quality of concrete are correctly
specified and achieved to suit the
exposure conditions, corrosion
due to carbonation should not
occur during the lifetime of the structure.
Resistance to freezing
and thawing
The freeze/thaw resistance of concrete depends on its
impermeability and the degree of saturation when exposed to frost;
concrete with a higher degree of saturation is more liable to damage. The use of salt for de-icing roads greatly increases the risk
of
freeze/thaw damage.
The benefits of air-entrained concrete have been referred to on
page
16 under Air-entraining admixtures where it was
recommended that all exposed horizontal
paved areas, from
motorways to garage drives, footpaths and marine structures, should be air-entrained. Alternatively, the strength of concrete
should be SO N/mm2 or more. Whilst C50 concrete is suitable for
many situations, it does not have the same freeze/thaw resistance as air-entrained concrete. Similarly, those parts of structures adjacent to highways
and in car parks, likely to be splashed
or come into contact with salt or salt solution used for de-icing,
should also be air-entrained.
Particular care needs to be taken to ensure that the concrete is
properly cured (see the section on Curing
on page 45).
Resistance to chemical
attack
Portland cement concrete is attacked by acids and by acid fumes, including organic acids, which are often produced when foodstuffs
are
being processed. Vinegar,
fruit juices, silage effluent,
sour milk and sugar solutions
all attack concrete.
In general, concrete made
with
Portland cement is not recommended for use in acidic conditions where the pH is 5.5 or less without careful
consideration of the type of exposure
and the intended construction. Alkalis have
little effect on concrete.
For construction exposed
to made-up ground, including
contaminated and/or industrial material,
specialist advice should be
sought so that the Design Chemical (DC) class can be correctly determined and a suitable
concrete specified.
The most common form of chemical attack that concrete has to resist is the effect of solutions of sulfates that may be present in
some
soils and groundwater.
In all cases where concrete is subject to chemical attack, resistance
is related to the free water/cement ratio, cement content, the type of cement and the degree of compaction. Well-compacted concrete will always be more resistant to sulfate attack than one which is less well compacted, regardless of cement
type.
BS 5328 and BS 8500 incorporate a primary set of
recommendations specific
to concrete exposed to sulfate-
containing groundwater and chemically-contaminated Brownfield
sites.
Alkali-silica reaction
Alkali-silica reaction (ASR) in concrete
is a reaction
between certain
siliceous constituents in the aggregate and the alkalis - sodium and
potassium hydroxide - that are released during the hydration
of cement. A gelatinous product is formed, which imbibes pore fluid
and
in so doing expands, inducing an internal
stress within the
concrete. The reaction
will cause damage to the concrete only when the following three conditions occur simultaneously:
- A reactive form of silica is present in the aggregate in critical quantities
- The pore solution contains sodium, potassium and hydroxyl ions and is of a sufficiently high alkalinity
- Water is available.
If any one of these factors
is absent, then damage from ASR will not occur and no precautions need be taken.