Strength gain contributed by portland
cement occurs very rapidly at early ages up to about seven days, after
which it slows markedly. Strength development contributed by fly
ash occurs through chemical combination of reactive fly ash glass with
calcium hydroxide generated by hydration of portland cement. This process
is called pozzolanic activity.
A fly ash concrete mix, designed for equivalent performance to conventional
concrete at normal ages, will generally gain strength more slowly at early
ages. After about seven days, the rate of strength gain of fly ash concrete
exceeds that of conventional concrete, enabling equivalence at the desired
age. This higher rate of strength gain continues over time, enabling fly ash
concrete to produce significantly higher ultimate strength than can be
achieved with conventional concrete.
Fly ash concrete designed for equivalent performance at seven days or
arlier will yield practically the same strength gain prior to the design age.
At all ages thereafter, fly ash concrete will exhibit much higher strength gain
than conventional concrete.
Concrete made with Class C fly ash (as opposed to Class F) has
higher early strengths because it contains its own lime. This allows
pozzolanic activity to begin earlier. At later ages, Class C behaves very much
like Class F,yielding higher strengths than conventional concrete at 56 and
Statistical analyses of compression tests have shown that the
use of fly ash often lowers the variability of strengths (lower
coefficient of variation). This can result in a reduction in “overdesign”,
yielding a direct cost savings to the concrete producer.
In general, a relationship exists between the compres-
sive and flexural strengths of concrete. Concrete which has a higher
compressive strength will have a correspondingly higher flexural strength.
This holds true for fly ash concrete. However, in many cases, fly ash
concrete has demonstrated flexural strength exceeding that of conventional
concrete when compressive strengths were roughly equal.
High Strength Concrete.
In instances where high strength concrete has
been specified (above 7,000 psi), fly ash has consistently proven its use-
fulness. After a certain amount of cement has been added to a mix (usually
about 700 pounds), the addition of fly ash usually results in higher strengths
than an equal amount of added cement. This is especially true for 56 and 90
day strengths. Production of high strength concrete requires the use of high
quality fly ash at a minimum of 15 percent by weight of total cementitious
Strength of Fly Ash Concrete
F l y a s h c o n c r e t e c a n b e d e s i g n e d t o a c h i e v e a n y l e v e l o f s t r e n g t h o b t a i n a b l e b y c o n c r e t e c o n t a i n i n g o n l y
p o r t l a n d c e m e n t.
F l y A s h C o n c r e t e
P l a i n
C o n c r e t e
7 28Age (Days)
F o r m o r e i n f o r m a t i o n o r a n s w e r s t o s p e c i f i c q u e s t i o n s a b o u t t h e u s e o f f l y a s h,
c o n t a c t y o u r n e a r e s t H e a d w a t e r s R e s o u r c e s t e c h n i c a l r e p r e s e n t a t i v e,
c a l l 1 - 8 8 8 - 2 3 6 - 6 2 3 6, o r v i s i t u s o n l i n e a t w w w.f l y a s h.c o m.