Chloride Diffusion of Concrete on Using GGBS as a Partial Replacement Material for Cement and Without and With Superplasticiser
Increase in environmental awareness over the past decade, resulted in increasing attention to individual pollution and waste management control. The use of recycled waste cementitious materials is becoming of increasing importance in construction practice.
In India, we produce about 7.8 million tonnes of blast furnace slag, which is a by-product of steel. The disposal of GGBS as a landfill is a problem, which leads to serious environmental hazards. GGBS can be incorporated in cementitious materials to modify and improve certain properties for specific uses.
An attempt is made to replace partially GGBS for cement in concrete of M20 & M25 grades and study its Chloride diffusion. GGBS is replaced for cement in the level of 10%, 20%, 30%, 40%, 50% and 60%. The study results showed that, with the increase in percentage of GGBS, the Chloride diffusion of concrete decreases. Also it is found that the Chloride diffusion in the M25 concrete is less than M20 concrete.
The partial replacement of GGBS for cement in concrete has great potential economical benefits in all areas of construction industry. The GGBS will also make a significant contribution to sustainable development.
IntroductionIn recent years there is an increasing awareness regarding environmental pollution due to domestic and industrial waste. Now pollution control board is formed to regulate environmental degradation due to industrial waste. The development and use of blended cement is growing in Asia, mainly due to considerations of cost saving, energy saving, environmental protection and conservation of resources.
Ground Granulated Blast furnace Slag is a by-product obtained in the manufacturing of pig iron in the blast furnace. It is a non-metallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching in water to form a glassy sand like granulated material. GGBS is recognized as a desirable cementitious ingredient of concrete and as a valuable cement replacement material that imparts some specific qualities to composite cement concrete.
In India, we produce about 7.8 million tonnes of blast furnace slag and it is available separately as GGBS. The disposal of such slag even as a waste fill is a problem and makes serious environmental hazards with the projected economic growth and development in the steel industry, the amount of production is likely to increase many folds and environmental problem will thus pose a larger threat.
It is seen that high volume eco-friendly replacement by such slag leads to the development of concrete, which not only utilizes the industrial wastes but also saves a lot of natural resources of energy. While using the GGBS in concrete, it reduces heat of hydration, refinement of pore structure, permeability and increase the resistance to chemical attack.
Chloride Permeability of concrete is the relative ease with which chloride ion can penetrate into the pores of concrete. The study of chloride permeability in concrete is of importance when concrete is subjected to chlorine atmosphere such as saline nature, chlorine-manufacturing plants etc. The penetration of chlorine ions into concrete may lead to the corrosion of reinforcement and hence weaken the structures and also adversely affect durability of concrete. Therefore a detailed study has been required to find the chloride permeability of concrete.
The factors affecting chloride permeability are as follows
- Physico-chemical properties of the mass transport system
- Chloride source concentration
- Addition of mineral and chemical admixtures
- Water binder ratio
Ordinary Portland cement of 53 grade was used, which has the fineness modulus 1.5, Specific gravity 3.08, Consistency 37%, Initial setting time 2hrs 30 min and Final setting time 3hrs 30min.
Coarse aggregateAngular shape aggregate of size of 20 mm was used and it has the following properties: Specific gravity 2.935, Flakiness index 100%, Abrasion value 20.4%, Crushing value 30.02%, Impact value 23.6%, Bulk density 1.42 x 103 Kg/m3 and Water absorption 1.01%.
Fine aggregateRiver sand conforming to zone III of IS: 383 – 1970 was used and its properties are found as follows: Specific gravity 2.68, Moisture content 0.71 and Fineness modulus 2.75.
GGBSPhysical properties of GGBS are: Specific gravity 3.44 and Fineness modulus 3.36, and the chemical composition of GGBS is Carbon (C) 0.23%, Sulphur (S) 0.05%, Phosphorous (P) 0.05%, Manganese (Mn) 0.58%, Free silica 5.27% and Iron (Fe) 93.82%.
Chloride PermeabilityFor reinforced concrete bridges, one of the major forms of environmental attack is chloride ingress, which leads to corrosion of the reinforcing steel and a subsequent reduction in the strength, serviceability, and aesthetics the structure. This may lead to early repair or premature replacement of the structure. A common method of preventing such deterioration is to prevent chlorides from penetrating the structure to the level of the reinforcing steel bar by using relatively impermeable concrete. The ability of chloride ions to penetrate the concrete must then be known for design as well as quality control purposes. The penetration of the concrete by chloride ions, however, is a slow process. It cannot be determined directly in a time frame that would be useful as a quality control measure. Therefore, in order to assess chloride penetration, a test method that accelerates the process is needed, to allow the determination of diffusion values in a reasonable time.
PrincipleThis test method consists of measuring the amount of electrical current passed through 50 mm thick slices of 100 mm nominal diameter cores or cylinders during a 6-h period. A potential difference of 60-voltage dc is maintained across the ends of the specimen. One of which is immersed in a sodium chloride solution, the other in a sodium hydroxide solution. The total charge passed, in coulombs, found to be related to the resistance of the specimen to chloride ion penetration.
Significance and useThis test method covers the laboratory evaluation of the electrical conductance of concrete samples to provide a rapid indication of their resistance to chloride ion penetration. The test method is suitable for evaluation of materials and material proportions for design purposes and research development.
Methodology10%, 20%, 30%, 40%, 50% and 60% of cement was replaced by means of GGBS, which is the by-product of steel. The mix grades used were M20 and M25. For each level of replacement, 3 cylindrical specimens were cast by using thoroughly mixed cement, fine aggregate, coarse aggregate and water in the mixer machine. All the specimens were kept for curing in the water for a period of 28 days and specimens were arranged in RCPT testing machine and test is carried out for 6 hrs. Afterwards, using formulae, total charge passed was found out. The results are tabulated as shown in tables from 1 to 5 and the conclusions are made.
Test specimenThe specimen was cylindrical shape, size of 100mm diameter, 50mm length. Three cylindrical specimens were used for each percentage of replacement of slag for determining chloride ion penetration.
ProcedureThe apparatus consists of two cells. The specimen was mounted as shown in Fig 1 and fixed between the cells in such a way that the round edge surface should touch with the solution. After fixing the specimen, the negative side of the cell was filled with 3% NaCl solution. The positive side of the cell was filled with 0.3M NaOH solution till the top surface of the concrete immerses in the solutions. Leakage was checked. Copper rods were used as electrodes. The wires, electrodes, power supply are connected.
Figure 1: AASHTO T277 (ASTM C1202) test setup
A D.C supplier was used to give electrical potential of 12v. The –ve terminal of D.C.S was connected with electrode of NaCl solution. The +ve terminal of D.C.S was connected with electrode of NaOH solution.
As per electro - chemistry principle, due to the applied voltage, the negative ion i.e. the chloride ion was attracted towards positive terminal i.e. NaOH reservoir. Therefore the chloride ion moves through the concrete specimen. Also the positive ion passes towards the negative terminal i.e. NaCl reservoir through the concrete specimen.
Due to the movement of positive and negative ions current was produced. This current was shown in D.C supplier. Reading was taken immediately after voltage supplied at every 30 minutes. This procedure was done for 6 hours duration. Decrease in charge passed values indicates that the concrete has more resistance to chloride ion penetration
FormulaeThe total charge passed is a measure of the electrical conductance of the concrete during the period of the test. If the current is recorded at 30 min interval, the following formula, based on the trapezoidal rule, can be used with an electronic calculator to perform the integration:
Q= charge passed (Coulombs)
I0 = current (Amperes) immediately after voltage is applied, and
It = Current (Amperes) at t min after voltage is applied.
If the specimen diameter is other than 3.75 inch (95 mm) the value for total charge passed must be adjusted. The adjustment is made by multiplying the value by the ratio of the cross-sectional areas of the standard and the actual specimens. That is:
Qs = Qx x (3.75/X) 2
Qs = charge passed (coulombs) through a 3.75-inch (95-mm) diameter specimen.
Qx = charge passed (coulombs) through X in diameter specimen and
X = Diameter (inch) of the nonstandard specimen.
Mix proportionsMix proportions are calculated for M20 & M25 grade concrete. The mix ratio for M20 grade concrete is 0.5:1:1.6:3.559 & the mix ratio for M25 grade concrete is 0.44:1:1.326:3.11
Test resultsThe experimental procedure is conducted on various types of mix containing partial replacement of cement by GGBS. The values of charge passed are tabulated as shown in Table 1 to 5.
GraphsGraphs (Fig 2 to 9) are plotted by taking % of replacement of GGBS in x-axis and charge passed in Y-axis for M20 & M25 grades.
Figure 2: Chloride Permeability of M20 Grade With GGBS Concrete
Figure 3: Chloride Permeability of M25 Grade With GGBS
Figure 4: Chloride Permeability of M20 Grade Super Plasticiser Added GGBS Concrete
Figure 5: Chloride Permeability of M25 Grade Super Plasticiser Added GGBS Concrete
Figure 6: Comparision of M20 & M25 GGBS Concrete
Figure 7: Comparision of M20 & M25 Grade Super Plasticiser Added GGBS Concrete
Figure 8: Comparison of M20 grade GGBS Concrete & Super Plasticiser Added GGBS Concrete
Figure 9: Comparision of M25 Grade GGBS Concrete & Super Plasticiser Added Ggbs Concrete
Results & DiscussionThe Chloride diffusion tests in M20 & M25 grade concrete were conducted using RCPT testing machine. The results are stated as below:
For conventional concrete, the Charge passed for M20 and M25 grade concrete are 407 Coulombs and 318 Coulombs respectively.
For grade M20 with GGBS the Charge passed values varies from 358 Coulombs to 292 Coulombs and for grade M25 with GGBS the Charge passed values varies from 298 Coulombs to 170 Coulombs.
For grade M20 Superplasticiser added GGBS concrete, the Charge passed values varies from 553 Coulombs to 345 Coulombs and for grade M25 Superplasticiser added GGBS concrete, the Charge passed values varies from 378 Coulombs to 185 Coulombs.
ConclusionFor both the grades of GGBS concrete and Superplasticiser added GGBS concrete, as the replacement level increases, the chloride permeability value decreases which improves the chloride penetration resistance of the concrete and durability of concrete.
By using GGBS as a replacement material for cement, the cost of construction will be reduced. Use of GGBS in concrete also prevents the environment from degradation.
M25 grade concrete has less chloride permeability than the M20 grade concrete. So, the permeability value also depends upon the mix grade of the concrete.
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