Anil. K. Sharma, Chief Engineer, CPWD

Concrete Deterioration

Concrete is a porous material. This is the intrinsic property of concrete. Once the pores within structure of concrete are interconnected, this leads to creation of pathways (i.e. capillary porosity) for migration of aggressive chemicals from surface of concrete to deep inside concrete. The aggressive chemicals react with chemical constituents of cement paste and alter its character from highly alkaline towards acidic. This chemical deterioration of reinforced concrete, which is age related includes:
  • Carbonation of cover concrete
  • Corrosion of reinforcement
  • Effect of aggressive chemicals present

    • Inside concrete
    • In external Surroundings of concrete
Capillary Porosity

Mechanical Water Dozer for Durable Concrete Making
 
Mechanical Water Dozer for Durable Concrete Making
Porous Concrete Without Inter Connectivity Durable
 
Porous Concrete (Pores Inter Connected) Leading to Surface Non -Durable

It is this capillary porosity, which can control durability of concrete. Capillary porosity is the major route by which gases and liquids can permeate in the concrete. All these factors lead to deterioration of chemical characteristics of concrete leading to corrosion of reinforcement and cracking of concrete. Higher porosity leads to interconnectivity of pores and makes it lesser durable reinforced concrete. Lower is the capillary porosity, likelihood of interconnected pores are reduced and leads to more durable reinforced concrete.

Water Cement Ratio

The primary factor responsible for porosity in concrete is excessive quantity of water used in its manufacture than its actual need for hydration & hardening. The quantity of water needed for hydration is only 0.23 by weight of cement, whereas water/cement (w/c) ratio in excess of 0.23 is needed from workability consideration for placement and compaction of concrete. Higher is the w/c ratio, higher would be the resultant porosity of concrete. Higher is the concrete porosity, lesser durable is the concrete. Thus, key for getting durable concrete is to effectively control w/c ratio during concrete making.

There are also other secondary factors like improper compaction and/or micro-cracks due to flexure, weathering, heat of hydration etc, which also add to enhancement in capillary porosity and early deterioration of concrete.

Fixed w/c Ratio Assures Proportioning of Constituents

For desired workability, the quantity of water needed is almost fixed and it is in the range of 175 to 180 litres per cubic metre of concrete. The effective control on w/c ratio results in fixed quantity of cement per unit volume of concrete. Thus, with workability of concrete also being fixed, fixed w/c ratio assures automatic control on the proportioning of its constituents [i.e. cement: fine aggregate(sand): coarse aggregate(stone)]. With fixed w/c ratio, even by mistake, any error in proportion of constituents would lead to un-workable concrete and its rejection by the technicians themselves who are actually responsible for placing and compacting concrete in position.

Thus, if we are able to develop an effective control on w/c ratio during concrete making, cement/aggregate ratio gets controlled automatically for a desired workability by the local work force of technicians actually manufacturing and placing concrete. This results in uniform quality of concrete in each batch.

Achieving Fixed w/c Ratio

Mechanical Water Dozer for Durable Concrete Making

Control on w/c ratio in traditional manufacture of concrete had always been a major challenge. It is most difficult to have a check on actual quantity of water added in each batch of concrete manufactured using traditional single bag concrete mixer. It solely depends on the judgment of concrete mixer operator regarding water need to achieve desired workability by technicians actually placing and compacting concrete. If labor force commits any error in ratio of sand and stone aggregate, quantity of water could be varied by the mixer operator to achieve the desired workability. Most engineers have failed to tame the mixer operator responsible for adding water during concrete making.

Mechanical Water Dozer (fabrication drawing given below) has been developed to overcome the difficulty of taming concrete mixer operator and ensure a fixed quantity of water in each batch of concrete made with one bag of 50 Kg cement.

Mechanical Water Dozer

Mechanical Water Dozer for Durable Concrete Making
The calibrated container (10) with controlled & calibrated water storage capacity varying from 17 Litres to 34 litres can yield concrete with w/c ratio from 0.34 to 0.68 using 50 Kg cement bag in each batch. This container capacity can be adjusted up to 4 ml accuracy using flexible overflow pipe (7) by adjusting level with water leveling (9) device using clamp (8) sliding over the Idle Overflow pipe (12). Water is continuously pumped from storage tank (1) using 1/8 HP pump (4) through three-way valve (6) with valve direction from pump towards container. As soon as water level in container reaches the predetermined level, the excess pumped water is overflown through overflow pipe and is received back in water tank. The overflow of water is an indication that the Water Dozer is ready for delivery of fixed quantity of water to concrete mixer.

For adding the aforesaid fixed quantity of water to concrete mixer, the Three Way Valve (6) is to be operated with its direction changed for water flow from calibrated container to concrete mixer (15). During this period of water flowing from calibrated container to concrete mixer, the pumped water entry into calibrated container is blocked and pumped water is diverted to Idle Overflow Pipe (12) with its out fall at highest level.

Conclusion

By the use of Mechanical Water Dozer in concrete manufactured using traditional concrete mixer, following advantages are accrued to concrete and its durability is assured:
  1. Consistently uniform quality of concrete in every batch as per design mix parameters by effectively controlling on w/c ratio and other constituents.
  2. Any error in cement-aggregate ratio results in unworkable concrete and its rejection by workforce themselves.
  3. Quality of concrete is assured with a low cost appliance with fabrication cost less than '7,500-, which can be fabricated locally at site. All raw materials used in its fabrication are locally available.
  4. The Doser has been tested and used at several sites of construction and has been found to be very effective in ratio control which is an essential component of quality control for concrete.
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