Modification of Mix Proportion for Flowing Concrete
Now more or less we are using ready mix concrete where concrete is mixed in plant and transported to construction site or concrete may be produced in in-situ plant and pumped to considerable distance and or height. Flowing concrete offers excellent concrete pumping performance which can be easily realized by its name.
Flowing concrete exerts less resistance in pumping system than exerts a normal slump mix. This offer a better rate of pumping and concrete can be transported to greater distance.
We know water-cement ratio is an important factor for both strength and economy in concrete production. To produce a greater slump we have to increase water content and water content is related to cement content. Again greater cement content produces more heat of hydration and accompanied by greater shrinkage too.
Flowing concrete is desired in large concrete pours as there slump is enhanced by using superplasticizers which permits us to achieve concrete of desired workability with low cement content, obviously, in combination of low water content. Thus development of both heat and shrinkage is possible to kept low. Inclusion of superplasticizers offering retardation is beneficial in this purpose.
Few special comments about flowing concrete will be made here; at first we should remember that concrete having slump more than 7.5 in (190 mm) showing cohesive nature is known as flowing concrete according to ASTM C1017.
Generally flowing concrete should have following properties
• Slump = 8 in (200 mm)
• Or flow of 20 to 14.5 in ( 510 to 620 mm)
• Or compacting factor =0.96 ~ 0.98
In mix proportioning, at first concrete is designed to have slump of 3 in (75 mm), the additional requirement of slump is achieved by application of suitable type and quantity of superplasticizers.
Flowing concrete, if properly proportioned, shows little segregation or bleeding or segregation within limit. To achieve these properties, coarse aggregate should be free from highly angular, elongated or flaky materials. Our purpose is here to produce a cohesive mix.
An increase in fine aggregate content by 5% more than usual, a cohesive mix can be achieved. The increase in fine aggregate content must be accompanied by reduction of coarse aggregate content.
When fine aggregate available for the mix is found very coarse, more increase (>5%) in fine aggregate content is required. In calculating yield, decrease in water content should be considered.Alternatively we can choose fine aggregate of smaller size, at least can maintain smaller size aggregate to some extent to ensure cohesion in flowing concrete. To achieve this, for maximum aggregate size of the mix of 20 mm, mass comprising fine aggregate of size <300µm and mass of cement or cementitious materials should be more than 450 kg/m3 of concrete.
When maximum aggregate size is 40 mm, mass of ultra fines content must be 400kg/m3. Notice that ultra fines content is not related, here, with cement content, rather are expressed as a function of maximum aggregate size.
According to UNI 7163 (Italian Standard for Ready Mix Concrete) for maximum aggregate size of 15 mm, materials <250 µm in the mix as a whole should be 450kg/m3, for that of 20 mm maximum aggregate size, this content should be 430 kg/m3.