Troubleshooting - Concrete Mix Design, Quality Control and Specification - page 270

or high concrete temperature. If water is not the cause, then the basic
cause is likely to be in the area of poor testing (including sampling,
compaction, curing, capping [if cylinders], defective or badly cleaned/
assembled molds [if cubes], centering, load rate, etc.), or of cement
quality or quantity.
8. Use the AASHTO T318 Standard method of test for water content of
freshly mixed concrete using microwave oven drying to help confirm
the actual total water content from all sources.
12.1.2 Poor workability/pumpability
Generally the causes are an excess or deficiency of fine material, a gap in
the grading, or an excess or deficiency of fluidity.
1. Does the concrete bleed? If so, there is either a gap in the grading, a
deficiency of fine material, or excessive fluidity. If the concrete pumps
reasonably at the start, but will not restart after a delay, this is often
due to bleeding.
2. Using Ken Day's MSF, the value of this must be at least 24 to 25 for
pumping to be possible. The higher the desired fluidity, the higher the
MSF value will have to be; however, values in excess of 32 will exhibit
excessive friction unless superplasticised to high slump.
3. Draw a graph or produce a table of individual percentage retained
on each standard sieve. Ideally all sieves below the largest will have
similar percentages of around 7% to 10%. One size missing may not
be fatal if those on either side are normal. Any two consecutive sieves
with a combined total retained of less than 7% would be a potential
problem. More than 20% on a single sieve finer than 4.76 mm might
also create a problem in pumping.
4. Is there at least 300 kg/m 3 of material passing the 0.15 mm sieve
(including cement)? If not, additional fines may be needed as fine
sand, crusher fines, fly ash, or cement.
5. If the (single) sand is so coarse that more than 55% (perhaps 50%) of
it is necessary to provide an MSF of 25 there is likely to be a problem
with bleeding, segregation, and pumpability. Additional fines as in
item 4 are necessary.
6. Air entrainment, fly ash, and silica fume (in increasing order of
effectiveness) are effective suppressors of bleeding and so assist pump-
ability. The authors have witnessed huge foundations up to 4.5 metres
deep filled with flowing concrete and containing 40 kg/m 3 of silica
fume, which exhibited no bleeding whatever.
7. Although nothing to do with mix design, it should be borne in mind
that it is pressure that causes a problem in pumping and faster pump-
ing requires higher pressure. Also a delay caused by a gap in deliveries