PACKAGE : I Bid Document VOLUME : II - Public Health Engineering

Coarse aggregate shall be piled in layers not exceeding 1.2m in height to prevent
coning or segregation. The aggregates must be of specified quality not only at the
time of receiving at site but more so as the time of loading into mixer.
Wet fine aggregate shall not be used until, in the opinion of the Engineer, it has
drained to a constant and uniform moisture content, unless the contractor with the
knowledge of the Engineer measures the moisture content of fine aggregate and
adds water in each batch of concrete mixed to allow for the water contained in the
fine aggregate.
7.6 Mix design.
Mix design is normally a prerequisite to any concreting job and will be required on
all major works. If so required, an approved testing laboratory shall, at the
Contractor’s expense, design a mix for each class of concrete and shall submit full
details of the mix designs to the Engineer for his approval. The Engineer’s
representative and the contractor shall clearly code each approved mix with a
number and date, and file all details for identifying and reproducing exactly the
same mix.
General: Each mix design shall be such that the aggregate shall comprise fine
aggregate and coarse aggregate of the size specified and the combined aggregate
grading shall be continuous. Aggregate shall be calculated by weight, and batching
procedures shall be established. The cement content by weight shall not be outside
the minimum and maximum limits calculated from the minimum and maximum dry
aggregate to cement ratios specified. The mixes shall be designed to produce
average concrete cube strength at twenty eighth day after manufacture not less than
the trail mix test strength specified. The water/cement ratio shall be in the region of
0.45 to 0.55 and shall never exceed 0.60.
Preliminary Mix: The proportions of cement, aggregate and water determined by
the Contractor in his mix design shall be used in preliminary mix of concrete made
and tested for strength and workability under laboratory conditions observing the
appropriate requirements. These preliminary mixes shall be repeated with adjusted
proportions as necessary until concrete mixes meeting the requirements of the
preliminary and trial mix tests specified and with the workability defined herein
have been produced. If at any time during construction of the works, the source of
cement or aggregates is changed, or the grading of the aggregate alters, then further
preliminary mixes shall be undertaken.
GTS : General Technical Specification. Page 57