No fines concrete is, as the name indicates, concrete consisting of coarse aggregate, cement and water- fines being omitted entirely. Some of the advantages are impressive.
One of the main advantages of no fines concrete results from its relatively low hydrostatic pressure when wet. The hydrostatic pressure of no fines concrete is on average only about one third that of dense concrete. This circumstance, coupled with the even grading, eliminates segregation even when the material is discharged from quite a high level, and allows very large formwork units to be used. Some builders cast no fines in lengths of 60 feet and form heights up to 25 feet.
The nature of no fines concrete is such that it has a relatively high proportions of interconnected voids but practically no fine capillary pores. This means that transfer of moisture by capillary attraction does not occur and that with an external rendering to prevent wind pressure form forcing water through the wall, the construction is waterproof. With no fines concrete the danger of dampness is remote unless there is bad detailing, or a concentration of cement grout in the wall as a result of using too wet a mix. Cracking of the rendering is another obvious danger.
Because of the low hydrostatic pressure, no fines concrete permits the use of very large, lightweight forms. Frequently, because of the absence of cement-sand slurry, the form may consist of only open-steel mesh or expanded metal on a light timber framing. The strength of the forms and the bracing applied must be sufficient to withstand the pressure of the concrete without deformation. Usually, however, it is found that the main design factor is the need to make the forms sufficiently strong to withstand repeated use, transportation, and handling. Experience has shown that expanded metal will normally withstand at least 25 placings and the timber frame about 100 placings. For economic and strength considerations the formwork should rarely be less than a full story in height, so that placing of a complete wall can proceed as one continuous operation.