Lean Concrete (DLC) is an important part of modern rigid pavement. It is plain concrete with a large ratio of aggregate to cement than conventional concrete and generally used as a base/subbase of rigid pavement. The compaction of DLC is done under 10 to 12T vibratory roller in the field. Further DLC is mostly made with Ordinary Portland Cement.
DLC is generally manufactured with ordinary Portland cement (OPC) as per IRC: SP-49: 1998 specification.
Mix proportion of Dry Lean Concrete
The mix design for DLC is not similar to the conventional concrete mix. Unlike conventional concrete mixes the water to- cement ratio is not the criteria for DLC mix design but it is the optimum moisture content (OMC) to ensure full compaction of concrete under rolling. The mix should not be too wet, as it gets stuck to the roller drums. Hence, it is important to determine the optimum moisture content for adequate compaction and the mix proportions i.e. aggregate-to-cement ratio to yield required compaction and compressive strength for the concrete.
Optimum moisture content
Optimum moisture content for a DLC mix is determined to ensure full compaction under rolling. The optimum moisture content is determined to achieve full compaction and maximum dry density. It is commonly determined by compacting DLC cubes with the help of a vibratory hammer. Vee Bee Consistometer Test can be adopted in the laboratory to know approximate OMC of DLC mixes.
Cement content in DLC
As per IRC SP-49, 1988, the minimum cement content in a DLC shall not be less than 150 kg/m3 of concrete. If this minimum cement content is not sufficient to produce concrete of the specified strength, it shall be increased as necessary. However, this specified amount of cement is for OPC. In the case of PPC nothing is quantified about the amount of cement.
The average compressive strength of a DLC mix shall not be less than 10 MPa at 7 days.
7-day cube compressive strength is the main acceptance criteria for the dry lean concrete mixes. Therefore, standard cubes prepared from DLC mixes were tested for strength development at 7-day.
All You Need to know about Dry Lean Concrete
1- Dry lean concrete is to provide a firm base to support traffic over pavement quality concrete (pqc) pavement.
2- There should be no bond between pqc and dry lean concrete.
3- After laying dry lean concrete with the help of paver and compacted with roller passes.
4- Minimum eight times up and down i.e total 16 passes. It should be sprayed with MC0 grade bitumen primer.
5- Cracks will appear from 10m to 15 distance depending upon cement quantity, mix grading of aggregate, temperature and wind speed, before laying pqc 500-micron anti static polythene sheet shall be laid over dry lean concrete.
6- Cube strength of dry lean concrete shall be 5 N/mm² and above. Higher cement depends upon the strength required for sub base.
7- Max size of aggregate shall not be more than 20 mm.
8- Good mix falls in the middle portion, weaker mix falls at the ends. Take a sample from the end and compare with the middle portion of concrete.
9- The difference in strength shall not vary more than 6%.
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