SPECIFICATION FOR READY MIX CONCRETE SUPPLY (RMC)..WHAT IS THE REASONS BEHIND THE COLLAPSE SLUMP OF CONCRETE??
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A) Technical Specifications.
➢ Max. cement content shall not exceed 450 Kg/Cum. And W/B ratio shall not be less than 0.32. Use of Fly ash is must for all grades of concrete.
➢ No Mahad Sand, Crusher waste product, stone dust to be used. Only Vaitarna sand or Crushed sand to be used.
➢ Cement slurry in starting will be brought by RMC supplier. Cement slurry shall not be pumped into pouring place since w/c ratio is higher than required.
Acceptance criteria for concrete at the time of arrival on site.
Casting of concrete cubes.
Cubes will be cast by Lab Technician at Site. As well as RMC Lab. Tech. from the same batch of concrete. It has been observed that even though Site lab technician and RMC person are casting cubes from same batch, variation is found in cube results. Some time our cubes passes and RMC cube fails, and vice a versa. Hence following points shall be followed to minimize chances of variation/ error in cube results.
1) Cubes shall be kept in site curing tank instead of taking them to RMC plant.
2) Cube testing of both will be tested on site lab. In presence of site engineer and RMC representative.
3) Min. 13 cubes shall be cast. 3 Companion Cubes will be taken 3 times + 4 Cubes for 28 days testing.
Cubes cast on site & cubes cast by RMC will be tested on site lab in presence of site Engineer & representative of RMC, & if any one of them passes, then it will be considered acceptable & no penalty, to be put. If we do not get the required compressive strength of concrete, we will impose a penalty on RMC supplier, which will go on doubling, if the cube results continue to fail.
Compressive strength (Kg /sq.cm) requirement is as follows.
Grade of Concrete |
5 days strength |
15 days strength |
28 days strength |
Min. Cube to be tested |
3 nos |
3 nos. |
4 nos. |
M30 |
248 |
248 |
330 |
M35 |
285 |
285 |
380 |
M40 |
322 |
322 |
430 |
Concreting can be supplied on intimation within 24 hrs. Tentative Schedule for 1 week will be given by site in advance. Concreting will start at 8.00 a.m. & should get over by 9.00 p.m.
C) General terms and conditions.
1) Labour - 6 Nos. for shifting Pipes, ghodas etc. also will have to be supplied by RMC supplier.
2) RMC supplier to ensure that concrete Pump should not fail. If concrete pump fails, there will be heavy penalty on RMC supplier.
3) Pipes, including bending radius pipes also to be sent for Pouring concrete on slab. Proper arrangements for supporting pipes shall be made by RMC in such a way that they do not disturb shuttering of slab.
4) Supervisor - 2 Nos. to be available at Concrete Pouring Point and communication with Plant and pouring plant with Mobile
5) There should be no interruption in concreting supply, for slab. Minimum pour - 50cum Maximum pour - 300cum.
6) RMC supplier shall submit Concrete Mix Design of concrete mix, at the time of every pour.
7) Washing and cleaning of transit mixers shall be done by RMC supplier, in such a way that sites cleanliness is not affected.
8) All transit mixers must have cleaner.
9) All basic safety norms like wearing helmet etc shall be observed by RMC personels, supervisors and labours etc.
10) During rains, mouth of transit mixer shall be covered to prevent rain water from getting inside transit mixer.
11) If due to any delay from the RMC side, if concrete is not pumped for more than 3 hrs. after arrival on site, then same will be rejected.
Some guidelines for slump adjustment and to avoid pump choke up.
Basically the collapse slump can be avoided if the following points listed below are taken care :
1. Number of times RMC concrete pumping is stopped, due to various reasons at site, including changing of direction in pipes supplying concrete on top of slab being concreted. If pump is stopped for any reasons, keep the pump working, sucking backwards i.e. 2 strokes back and 1 stroke forward, again wait for 1 or 2 min. and if pumping does not start, again repeat the same & so forth up to regular pumping starts. Cycle - Suck 2 push 1, suck 2 push 1, suck 2 push 2, up to the RMC concrete pumping restarts.
2. A operator observing at the Pump hopper level should ensure that concrete level does not go below, the blade level so that air is not sucked or place a mirror in position, so that operator can see the level of concrete in hopper & ensure that concrete in hopper does not go below the blade level.
3). Slump for pumping should be 125mm.
4). Concrete will be sent on site, with a slump of less than 125mm and if required, to adjust for making it pumpable, we will add (1:4) super plasticizer: water, at site itself, thereby ensuring that case of collapse slump of RMC concrete does not occur at all.
4. The temperature of 34-degree centigrade will be allowed, for the concrete supplied at site, when the transit mixer actually arrives & not at placing the concrete, since there can be delays at placing point due, to site conditions.
6. Wetted Hessian cloth, to be placed on RMC concrete supply pipes & transit mixer, in order to control the temperature of concrete.
Reasons for additional water in concrete, which makes the concrete with Collapse Slump is:
a) There can be fault with valve supplying water in the drum of transit mixer, for cleaning the transit mixer drum, after concreting is completed .
b) After every supply of concrete, the mixer drum is washed. This water may not be removed and fresh concrete is loaded in the transit mixer, at the RMC plant.
Hence during mixing of concrete in pan mixer at the RMC plant, in the first 2 mixes of 0.5 cu.m, the operator to see the workability of concrete. If required, then in the balance 10 batches of 0.5 m3, water to be added in lesser quantities. (Assuming one batch of concrete is 0.5 cu.m)
Hope the above process, will solve the problems of collapse slump & we can work in more harmonious manner, achieving Quality, Speed & Economy.
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