Troubleshooting - Concrete Mix Design, Quality Control and Specification - page 269

1. Restore strength to a safe level so work can continue while
investigating. Cement content adjustments should always overshoot
when increasing and undershoot when reducing. Use 8 to 10 kg per
MPa to adjust upward or equivalent water reduction, 4 kg per MPa
to adjust downward. If adjustment gives cement content over 500 kg
use 500 kg plus 2 kg of fly ash (for each 1 kg of cement not added), or
0.5 kg silica fume, or 100 ml superplasticiser.
2. Start casting at least four, perhaps six, test specimens per sample.
Test at 2, 3, 7, 28 and perhaps 56 days. Assume gain in megapascals
will remain the same with the revised mix. In default of prior data,
conservatively assume that strength will increase 33% from 2 to 3
days, another 33% from 3 to 7 days and 10 MPa from 7 to 28 days.
Substitute actual figures as soon as available. Testing one specimen at
a particular test age can be a problem if testing quality is an issue.
3. Draw cusum graphs of strength (at all available ages), density, concrete
temperature, slump, 7- to 28-day gain (for example). If data is avail-
able, cusum graphs of sand silt content or specific surface should also
be drawn on the same presentation. A cusum of average pair differ-
ence between pairs of specimens from the same sample will show
whether there has been a deterioration in quality of testing (an average
pair difference in excess of 1.5 MPa is an indication of poor testing
quality). Such graphs will usually show when the problem started and
what caused it.
4. Examine batching records (assuming a computer-operated plant that
records actual batch quantities) before and after the downturn for
signs of cement shortfall or aggregate, especially sand, overbatching.
5. Calculate MSF (mix suitability factor) using formulas in Chapter 8.
MSF is a measure of the sandiness of the mix taking into account
sand grading, sand percent, cementitious material content, and
entrained air. Calculate water content using formulas in Chapter 8. Is
actual water content really known? An MSF in excess of 30 represents
oversanding and high water requirement unless for flowing, super-
plasticised concrete.
6. Calculate strength according to one or more of formulas in Chapter 8.
If this agrees with strength obtained/being investigated, then high
water content is the explanation and the reason and cure are obvious
(may be any combination of high MSF, silt in sand, concrete tempera-
ture, high slump).
7. If calculated water or strength does not agree with actual, recheck
sand silt percent and grading. Check concrete density, as this will
confirm water, air content, or compaction of test specimens. The water
content is the major separating factor between alternative directions
of investigation. If water is the end cause, then the basic cause is likely
to be in the area of dirty or finer sand, high sand content, high slump,