Prediction of target mean strength of concrete mixes.

In practice, cured concrete strength may exhibit strength variation from batch to batch
and within batch despite the mixes being of the same proportions and with quality control.
On the other hand, structural concrete elements are designed to meet specific
characteristic strength. In order to ensure compliance to the specified characteristic
strength, concrete mix designers target strengths higher than the specified characteristic
strengths, commonly known as Target Mean Strength. This study aimed at establishing
the margins between characteristic strength and Target Mean Strength for various local
concrete mixes. In order to achieve this overall objective, a semi-structured questionnaire
was used to identify the popular concrete mix design models in the country. The identified
model was then validated through experimental mix designs, concrete mixes, cube
casting, curing and strength testing. The mixes were designed for normal concrete classes;
C20, C25 and C30. The sample size for each of these classes was guided by the British DOE method of mix design that demands for standard deviation to be calculated from at
least 20 results and the Indian Standard IS 456:2000 that demands a sample size of at
least 30 for each concrete class. This study utilized 101 concrete mixes; 31 samples for
C30 and 35 for each of the C25 and C20 classes. The mix materials used were Ordinary
Portland cement 42.5R manufactured by Lafarge Zambia, 20mm crushed coarse
aggregates and crushed fine aggregates. Three concrete cube specimens were cast for
each mix, cured for 28 days and tested for compressive strength at the Civil Engineering
Laboratory of the University of Zambia. From the compressive strength results, the
probability density function for each class was generated using Microsoft Office excel.
The determined standard deviations (s) from the distributions were 8.19, 8.00 and 8.27
MPa for concrete classes of C20, C25 and C30, respectively, which implied margins of
13.43, 13.12 and 13.56 MPa, respectively for 95% reliability (k=1.64). Therefore, the
established margins (kxs) can be used for predicting Target Mean Strength of concrete
mixes at 95% reliability for the Zambian concrete mixes provided the BS-DOE model is
applied and constituent materials are similar to those used in the study.
Keywords: Characteristic Strength, Target Mean Strength, Standard Deviation, Margin,
British-DOE