Recommendations are available for minimizing the risk of damage
from
ASR in new concrete
construction, based on ensuring that at
least one of the three
factors listed above is absent.
Two essential properties of hardened concrete are durability and strength. Both properties are affected by the voids and capillaries in the concrete, which are caused by excessive water or by incomplete compaction.
In principle
the lower the free water/cement ratio the stronger and more durable the concrete will be. The concrete should be fully
compacted if it is to retain or exclude water and provide corrosion protection to reinforcement.
Within the UK, the producer is normally required to take action to prevent
damaging alkali-silica reaction and therefore
provisions in
the specification are not normally
required.
The required consistence needs to be known at the time of
specification so that the concrete can be proportioned to give the
required strength and durability.
High-strength concretes can be designed and proportioned to a very high or self-compacting
consistence so overcoming conditions that make placing or vibration difficult.
The methods of specification and what to specify are given in
BS
8500-1. Three types of concrete - designed, prescribed
and standardized prescribed concretes - are recognized by BS EN
206-1 , but BS 8500 adds two more: designated and proprietary
concretes.
Designed
concretes
These are concretes
for which the producer is responsible for
selecting the mix proportions to meet the required performance as communicated by the specifier. Therefore it is essential that the specifier, in compiling the specification, takes account of:
n The uses of the fresh and hardened
concrete
n The curing conditions
n The dimensions of the structure; this affects heat development
n The environmental exposure conditions
n Surface finish
n Maximum nominal aggregate size
n Restrictions on suitability of materials.
The most common form of designed concrete is that defined by the characteristic compressive strength at 28 days and identified by
the strength class. For example,
strength class C25/30 concrete is
one
having a characteristic compressive cube strength of 30 N/mm2 at 28 days. (The same concrete would have a characteristic cylinder strength of 25 N/mm2 at 28 days if cylinders were used for testing,
as in certain European countries.) To understand the meaning of the term 'characteristic' see Strength on page 26.
However, strength alone does not necessarily define the required
durability, and for structural
concrete BS 8500 indicates
minimum strength
class, the maximum free water/cement ratio and minimum cement content that are required
for different degrees of exposure. The maximum free water/cement ratio, minimum cement content and types of constituent materials are the main
factors influencing durability.
If a specification for designed concrete
is to be compiled correctly the following
details need to be included:
- A requirement to conform to BS 5328, or BS EN 206-1 and BS 8500-2
- The compressive strength class
- The limiting values of composition e.g. maximum free water/cement ratio, minimum cement content or the design chemical class where appropriate
- Type of cement or combination
- The maximum aggregate size
- The chloride class
- The consistence class.
Optional items may be included such as the target density of
lightweight concrete, heat development or other technical requirements listed in BS 8500 :
Part 1.
At the time of publication, the
use
of Form A in BS 5328 : Part 2 is
recommended when specifying designed
concretes. A
copy is reproduced in this publication in Appendix 1 a, which can be used for this purpose by ringing the appropriate items.
Conformity of designed concretes is usually determined by strength testing of 100 mm or 150 mm cubes and in BS 8500 this is the responsibility of the producer. Recommendations about the required rate of sampling are given in BS 5328 and BS EN 206-1 .
The producer will respond to the specification by producing a mix design that satisfies all of the specified requirements. Mix design methods are described in several
publications and the subject will not be dealt wit h in any great detail here.
Prescribed concretes
These are concretes
where the specification gives the mix
proportions in kilograms of each constituent in order to satisfy
particular performance requirements. Such concretes seldom need
to
be used but may be required for special surface finishes or where particular properties are required. The specifier should include details of the cement content, the type and strength class of
cement and either the free water/cement ratio or consistence class.