Transcript Objective in Concrete Production

P2P
WHAT DOES IT
MEAN?
Ken W. Day
Consultant, Australia
Two Alternatives
PRESCRIPTION TO PERFORMANCE
(true meaning, as intended by USA
initiators)
PURCHASER TO PRODUCER
(my interpretation of significance as
to who designs and controls)
The Current Situation
USA is almost the last major country
where specifiers/purchasers of concrete
nominate actual mix details rather than
the properties required of the concrete.
As a consequence of this there is no
financial incentive for producers to
acquire expertise in mix design or
quality control
The Current Situation
So any producer who spends money
on better plant and materials than
he can get away with, or employs
more or more expensive staff,
prejudices his chances of obtaining
a contract under competitive
tendering.
Difficulties to be Overcome
Specifiers and producers eye the overseas
situation enviously:
Specifiers would like to allow more
freedom to producers if only the
producers were more expert
Producers would like to develop more
expertise if enabled to profit by it
Difficulties to be Overcome
The real difficulty seen by US
specifiers is how to ensure
durability as opposed to strength.
They are leaning towards RCP tests
(Rapid Chloride Permeability) but
this is both too delayed and has
too high variability for use as a
primary acceptance test.
Difficulties to be Overcome
My solution to this is to use strength
for primary acceptance and to
insert a bonus/penalty clause
based on several RCP results over
a period.
Standard Mixes
A major benefit of a performance
specification is that it permits a
producer to develop and control a
range of standard mixes of which a
range of properties have been
established in advance of a
particular contract.
Reaction to Change
A quick reaction to any change in
concrete quality is very important.
A purchaser cannot demand a
reaction until there is clear
evidence (usually taking weeks)
but a producer can take action at
the first sign of change.
Costs of Inaction on P2P
A prescription specification must
necessarily be more conservative
and allow for greater variability.
It is likely also to increase the
producer’s costs by requiring him
to use or not use particular
materials.
Costs of Inaction on P2P
A prescription specification would also
increase the purchaser’s costs
substantially if he really exercised
control over his concrete.
However few specifiers have the expertise
to do more than pick on isolated items
in a submission and react to an
unfavourable report from a commercial
testing lab.
Costs of Inaction on P2P
Other substantial worries are that
few specifiers have the expertise
to know what really matters,
particularly in respect of
supplementary materials such as
fly ash, silica fume, metakaolin,
limestone fines and high fines
contents in crusher fines used as
fine aggregate.
Inept Specifications
Favourite items to specify are minimum
cement contents and/or water/cement
ratios.
W/C ratio is an important factor but is
very difficult to measure and enforce.
Excess water is more deleterious than
less cement at a given W/C, so, at a
given strength, the mix with the lowest
cement content is the best mix.
Benefits of Producer Control
Under producer control, any substantial
producer finds it worth while to operate
his own high standard laboratory
A good specification clause, after
specifying required properties, would be
“the producer shall operate an approved
effective QC system”