Quality control - Concrete Mix Design, Quality Control and Specification - page 243

curing boxes were used on site and at what age the specimens were
typically collected).
5. Unusually, strength shows little reverse correlation with slump
(possibly suggesting that slump tests were not well performed).
Direct plot graphs
Direct plot graphs are not as informative as cusum graphs but do show
where a few obvious errors have occurred. These graphs should also be
briefly examined daily and an attempt made to determine the cause and
significance of any errors. The results showing high 7- to 28-day gain can
be caused by bad operator performance (lab or field) but are unlikely to be
a property of the concrete.
Multigrade statistics table
The “Multigrade Statistics” are in fact all of the same 32 MPa grade,
covering the three months December 2011 and January and February
2012. They show:
1. A slightly inadequate strength overall, arising in January, with
December and February both acceptable. An overall SD of 3.89,
surprisingly slightly greater at 28 days than at 7 days. This is a situ-
ation where a very minor cash penalty might be appropriate rather
than any further investigation.
2. The problem arose due to slightly increased variability rather than
a lower strength. Of course a slightly higher strength margin should
have been used, and the cash penalty should desirably be about double
the cost of providing the higher margin, and applied to all concrete
supplied in January.
Use of the program
Download the program from the website http://www.kenday.id.au and
key “Export Results” at the bottom of the screen. This provides an Excel
spreadsheet. Copy your own results a column at a time into the spread-
sheet. Only the data columns are required, as the calculated columns will
automatically recalculate, deleting existing calculated items. It is important
to delete any data not replaced at the foot of the spreadsheet or if you only
have one 28-day result as, if only one column were replaced, the program
would average the remaining column data with your own data.
Now return to the downloaded program and key “Import Results” at the
bottom of the screen. It does not matter whether your results are in strict
date order or sorted into grades, as the program will attend to this.