PLACING AND FINISHING
There is little or no difference in the techniques required for placing lightweight concrete from those used in properly placing normalweight concrete. The most important consideration in handling and placing concrete is to avoid segregation of the coarse aggregate from the mortar matrix.
The basic principles required for a good lightweight concrete placement are:
- A workable mixture using a minimum water content
- Equipment capable of expeditiously handling and placing the concrete
- Proper consolidation and
- Good workmanship.
A well-proportioned lightweight concrete mixture can generally be placed, screeded, and floated with less effort than that required for normalweight concrete. Over vibration
or overworking of lightweight concrete should be avoided. Over manipulation only serves to drive the heavier mortar away from the surface where it is required for finishing and to bring the lower-density coarse aggregate to the surface. Upward movement of coarse lightweight aggregate may also occur in mixtures where the slump exceeds the recommendations.
Satisfactory floor surfaces are achieved with properly proportioned quality materials, skilled supervision, and good workmanship. The quality of the finishing will be in direct proportion to the efforts expended to ensure that proper principles are observed throughout the finishing process. Finishing techniques for lightweight concrete floors are described in ACI 302.1R.
Slump is an important factor in achieving a good floor surface with lightweight concrete and generally should be limited to a maximum of 5 in. (125 mm). A lower slump of about 3 in. (75 mm) imparts sufficient workability and also maintains cohesiveness and body, thereby preventing the lower-density coarse particles from working to the surface. This is the reverse of normalweight concrete where segregation results in an excess of mortar at the surface. In addition to surface segregation, a slump in excess of 5 in. (125 mm) may cause unnecessary finishing delays.
Surface preparation before troweling is best accomplished with magnesium or aluminum screeds and floats, which minimize surface tearing and pullouts.
A satisfactory finish on lightweight concrete floors can be obtained as follows:
- Prevent segregation:
- Using a well-proportioned and cohesive mixture;
- Requiring a slump as low as possible;
- Avoiding over-vibration;
- Time the placement operations properly;
- Use magnesium, aluminum, or other satisfactory finishing tools;
- Perform all finishing operations after free surface bleeding water has disappeared;
- Cure the concrete properly.