CONCRETE MIXING.

CONCRETE MIXING.

INTRODUCTION.

Concrete is a composite material usually made up of three main components: water, aggregate (fine and coarse aggregate) and Portland cement. Fine aggregates fill the spaces between the coarse aggregates while cement in the presence of water forms cementitious material, which binds the aggregates together. Concrete exists in two states, fresh concrete, and hardened concrete. The properties and characteristics of concrete are functions of the raw materials used as well as the mix proportions.

CONCRETE MIX DESIGN

A concrete mix design is used to achieve mixes on an empirical basis since it’s not possible to determine the strength of concrete correctly using properties and proportions of the mix components alone(Day 2003). Mix design ensures that the concrete bears the specified features both in the fresh and hardened state. The weight ratio between water and cement is the most critical design variable and is commonly known as a water /cement ratio (W/C). Typically, the w/c ratio should vary between 0.4-0.6 because excess water reduces the strength of concrete, and too low w/c ratio lowers the workability of concrete. Mix design also yields cement-to- aggregate ratio and fine-to-coarse aggregate ration. Different classes of concrete are achieved by varying the volume of one component relative to the others. Various classes of concrete have different strengths, as shown in Table 1 below.

Table 1: Concrete classes

SLUMP TEST

The slump test is conducted on fresh concrete to assess the consistency of concrete (Clayton, 2003). Slump test checks whether the correct amount of water has been added to the mix. A 300mm diameter slump cone is placed on a solid impermeable base and filled with fresh concrete in 3 equal layers while rodding each layer to achieve compaction. The third layer is leveled with the top of the cone. Cone is lifted carefully, leaving a heap of concrete that settles or ‘slumps’ slightly. Cone is placed next to the slump and used to measure the height of the slump to the nearest 5mm. Figure1 shows different types of slumps.

Figure1: Different types of the slump.

REFERENCES.