What is plum concrete?/ All about plum concrete.

 Let us go through some of the FAQs related to plum concrete in construction.

1. What is plum concrete?

Plum concrete is a normal concrete grade, containing 30 to 40% of plums as inert fillers. The term plum means the natural stone aggregates having a size not less than 160mm. In making plum concrete, clean & hard stones that have a size of up to 300mm. are utilized.

2. Why do we use plum concrete?

Plum concrete is used to fill up the uneven pockets beneath the footings or foundations. If the foundation strata have a steep slope, it becomes necessary to make a leveled base for the structure.

 Making a foundation PCC beyond a certain thickness is unnecessary  & uneconomical. For heavier structures like bridges, dams, & multi-storeyed buildings, to save the extra cost of constructing a leveled base, plum concrete is used.

3. What is the mix ratio of plum concrete?

As per IS-456 (2000), clause 5.3.3, the size & percentage of plums are defined as given below.

Size of the plum 👉 160mm. to 300mm.

Concrete grade   👉  Normal PCC grade 

Usually, the M15 grade is used for heavier structures.

Percentage of plum within the concrete 👉   20% to 40%

Usually, 1/3rd volume (33%) of concrete is filled by the plum.

4. What is the procedure for doing plum concrete?

1. Stone selection:

The selected stones shall be free from dirt, chemical, & other organic impurities. The size of the stone should be above 160mm. & nearer to 300mm. It should be hard & sound without any cracks or fissures over the surface. The shape of the stone should be angular rather than round or flat for good bonding with the concrete. 

2. Preparing the strata:

The surface of the strata should be cleaned & washed with water to remove the dirt & loose materials. If the strata contain any deep pockets or cracks, the same shall be cleaned to remove unwanted soft soils or any organic matters adhered to them. 

3. Pouring the concrete:

A layer of concrete is placed over the strata to cover all the required areas. A formwork is done up to the needed level to hold the plum concrete.  

 

4. Laying boulder stones:

The boulder stones are laid over the concrete bed at regular intervals from each other. The minimum distance between the two adjacent stones should be 50mm. & it shall not exceed 150mm. The stones should not touch the formwork & you should maintain a minimum gap of 50mm. in between them.

5. Extending layers:

The second layer of concrete should be poured all over the laid stones. Care should be taken to fill all the voids or gaps in between the stones. If necessary, you can use needle vibrators to compact the concrete layers. 

The thickness of each layer should be ideally between 400mm. to 500mm. In any case, the thickness of each layer of boulder & concrete shall not exceed 900mm.

6. Finishing the top:

The same process should be repeated layer by layer until we reach the required top level of the plum concrete. The top layer should be finished in such a way that it should not contain any protruded boulder surface above the level. The boulder should be embedded at least 50mm. beneath the top concrete layer. 

7. Curing:

The plum concrete should be cured for a minimum period of 7 days or as specified.

5. How do you calculate plum concrete?

Let us consider 10 cum. of plum concrete for calculation purposes.

a. Calculation by volume:

Let us fill 33% of the concrete volume with boulder stone (plum).

The volume of stone = 33% of the total volume

i.e. (33 ➗ 100) Χ 10cum = 3.3 m³

Let us take 30% extra for the voids while measuring volumetrically.

Therefore,

The total volume of stone required

= [3.3m³ + 30% of 3.3m³.]

= [3.3 +{ (30 ➗ 100) Χ 3.3}]

= [3.3 + 0.99]

= 4.29m³

The volume of cement concrete

 = [100% -33% ]of the total volume

 = 67% 

i.e. (67 ➗ 100) Χ 10cum = 6.7 m³

Note: This is an approximate method, as the voids in between the irregular plum stones vary depending on the size. However, we can consider 30% voids in rubble stones while adding them volumetrically.

 

b. Calculation by weight:

Quantity of concrete required

= [required volume Χ density of concrete]

= [6.7m³ Χ 2400kg/m³.]

= 16,080kg

= 16 tons.

Quantity of  stone required

= [required volume Χ density of stone]

= [6.7m³ Χ 1850 kg/m³.]

= 12395kg

= 12.4 tons.

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Thank you for going through this article❤. Have a good day 😄.

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