Correlation between Actual and Estimated W/C Ratio of Fresh Concrete

IJSTE - International Journal of Science Technology & Engineering | Volume 2 | Issue 06 | December 2015 ISSN (online): 2349-784X

Correlation Between Actual and Estimated W/C Ratio of Fresh Concrete Yadav Neha M.E. Scholar(Structure Engineering) Department of Civil Engineering Jabalpur Engineering College, Jabalpur

Chandak Rajeev Professor Department of Civil Engineering Jabalpur Engineering College, Jabalpur

Dubey A.K. Professor Department of Civil Engineering Jabalpur Engineering College, Jabalpur

Abstract The quality of cement-based material is most important parameter in construction industries. To decide whether the quality of concrete is good enough or not, it is the first most priority to determine the water cement ratio of concrete. To determine the water cement ratio of fresh concrete in less time, a new method named as “Electrical Resistivity” by wenner‟s array and the same method is adopted in this work. A total number of three mixtures were designed and prepared with varying water cement ratio (0.4,0.5 &0.6) with replacement of 25% of fly ash according to IS 456-2000 code recommendations and experiments were conducted on these mixtures for determination of actual and accurate water cement ratio instantaneously. The fresh mixtures with different w/c ratio may have very similar (porosities) or overall water content.

Keywords: Water-cement ratios, electrical resistivity, fresh concrete, fly ash, resistivity probe & quality control ________________________________________________________________________________________________________

I. INTRODUCTION Due to the lack of practical knowledge and practice, it is not possible to accurately mix the ingredients of concrete together at site. As a result of that, many practical problems occur due to improper w/c ratio. The actual w/c ratio and estimated w/c ratio has been determined and found same with an accuracy of ±0.01 by archie‟s law.

A. Electrical Resistivity and W/C Ratio of Wet Mixtures: In the present wenner array probe setup, cement concrete was prepared in a cubical shaped non-conductive container. The distance between wenner probes are constant i.e., 3.5cm and also get separated by using a non-conductive material. As shown in figure given below, the current flows and get the bulk electrical resistivity of fresh concrete which has a direct correlation with water content (w/c ratio)

Fig. 1: Schematic Diagram of Electrical Resistivity Probe

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Correlation Between Actual and Estimated W/C Ratio of Fresh Concrete (IJSTE/ Volume 2 / Issue 06 / 019)

II. LITERATURE REVIEW Ozkan Sengul and Odd E. Gjørv[1] “Electrical Resistivity Measurements for Quality Control During Concrete Construction” investigated the relationship between chloride diffusivity and electrical resistivity of the given concrete based on the NernstEinstein equation. Then the chloride diffusivity can indirectly be controlled by routine-based measurements of the electrical resistivity during concrete construction. The test program was mainly based on the four-electrode (Wenner) testing of the electrical resistivity and included different probe spacing and different geometry of the test specimens. M. Mancio, J. R. Moore, Z. Brooks, P. J. M. Monteiro, and S. D. Glase[2] “Instantaneous In-Situ Determination of Water-Cement Ratio of Fresh Concrete” described the importance of w/c ratio of fresh concrete for quality control for the construction industries and a new technique to determine the same. Experiments were conducted on eight concrete mixtures designed according to the ACI 211.1 procedure, with varying w/c (0.30, 0.40, 0.50, and 0.60) and fly ash percentages (0 and 25%). They also validated their estimated w/c ratio(by Archie‟s law) results with the actual w/c ratio.

III. OBJECTIVE OF STUDY 1) 2) 3) 4)

To prepare a correlation between estimated w/c ratio with actual w/c ratio.

To establish a relationship between actual w/c ratio & bulk electrical resistivity.

To develop a new method of determining w/c ratio for quality control.

To investigate the instantaneous w/c ratio of fresh concrete.

IV. METHODOLOGY The experiment were conducted on three concrete mixtures with varying w/c ratio (0.4, 0.5 & 0.6) with fly ash (25% with the replacement of cement) according to IS concrete mixture design procedure and guidelines. The resistivity for all ingredients are determined separately. In this wenner‟s array ( four probe) setup, the spacing between probes are constant i.e., 3.5cm. Based on the IS procedure, a non-air entrained concrete with a maximum aggregate size 20mm and slump(25-50mm), as used in this research, will have a mixing water content of approximately 186 Kg/m3.

The initial resistivity of the mixing water was measured and kept constant at 64 Ω-m. Immediately after mixing, a portion of each concrete was poured into a non-conducting chamber (25x 25 cm) to hold the sample as resistivity measurements were conducted over time. Standard for all measurements, the probe was vertically immersed in concrete and centrally located within the glass chamber. Before initial setting of the cement, resistivity was measured at 10-minute intervals for approximately 2 hours.

The electrical resistivity of the concrete ρ, a material property independent of sample geometry, is given by ρ = kR, where k is the geometric factor (a parameter determined by the geometry of the probe used).the probe geometry factor k determined analytically(k = 4πa), and experimently determine the value of this constant, the probe was submerged in aqueous solutions of known resistivities (ρs ≈ 0.1-0.4 Ω-m, 1-4 Ω-m, and 10-40 Ω-m), and the expression k = ρs /Rs as well. Rs is given by Vs/Io, that is,the potential measured across the two inner electrodes divided by the current driven through the circuit. The purpose of this work is to correlate the estimated and actual w/c ratio through Archie‟s law According to Archie‟s law, the bulk electrical resistivity of porous media is a function of two parameters: 1) resistivity of the pore fluid; and 2) porosity

where ρb is bulk electrical resistivity, ρf is the resistivity of the pore fluid, φ is the porosity (by volume), and a and m are empirical constants.

V. RESULT AND GRAPHS A. Concrete Mixing Proportions: Table – 1 Concrete Mixing Proportions UNIT PROPORTIONS MIXTURE NO.

W/C RATIO

CEMENT

FLY ASH

FINE AGG

COARSE AGG

CEMENT (KG)

4 5 6

0.40 0.50 0.60

1.00 1.00 1.00

0.25 0.25 0.25

1.257 1.759 2.304

2.410 3.096 3.73

348.75 279 232.5

AMOUNT KG/M3 FLY FINE ASH AGG (KG) (KG) 116.25 619.01 93 690.55 77.5 740.482

COARSE AGG (KG) 1186.81 1214.32 1198.37

Note: specific gravity of material: cement =3.15, fine aggregate =2.60, Fly ash = 2.77, coarse aggregate=2.804.

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Correlation Between Actual and Estimated W/C Ratio of Fresh Concrete (IJSTE/ Volume 2 / Issue 06 / 019)

B. Probe‘s average geometric factor (K) 2(A) . Empirical determination of probe„s average geometric factor (K) Table – 2 Probe „S Average Geometric Factors SOLUTION TYPE Low ρ=0.1-0.4 (Ω-m)

Medium ρ=1 - 4 (Ω-m)

High ρ=10 -40 (Ω-m)

NO.

Ρ (Ω-M)

R (Ω)

K1

K2

1

0.2380

0.50503

0.471

0.471

2

0.2387

0.5065

0.472

0.472

3

0.23951

0.50805

0.471

0.471

4

2.427

5.148

0.472

0.472

5

2.42736

5.1487

0.4714

0.4714

6

2.42394

5.1478

0.4712

0.4712

7

24.53

52.0514

0.4723

0.4723

8

24.65

52.2880

0.4715

0.4715

9

24.6462

52.2801

0.4721

0.4721

Average geometric factor

0.471

Standard deviation

0.022

Probe geometrical factor was based on 18 measurements conducted with aqueous solution of low-medium and high electrical resistivity (refer table 2). The average measured value for K was 0.471 2(B). ANALYTICAL DETERMINATION OF PROBE „S AVERAGE GEOMETRIC FACTOR (K) k = 4 ; k =0.439; Geometrical factor for resistivity probe immersed in solution has been analytical determined as given above where „a‟ is spacing between electrode i.e., 3.5cm. error = 0.471-0.439 =0.032 The error between empirical & analytical geometrical factor for resistivity probe is 0.032.

C. Resistivity (Ω-m) for different mixtures (25% fly ash): Table – 3 Resistivity (Ω-M) For Different Mixtures RESISTIVITY(Ω-M) FOR DIFFERENT MIXTURES TIME (MINUTES) 0.40 0.50 0.60 0 29.649 35.312 38.65 10 31.469 37.702 41.6574 20 29.42 36.2184 38.8465 30 32.082 38.4221 39.817 40 34.815 37.640 42.347 50 33.7826 35.139 38.145 60 39.337 40.979 43.587 70 36.785 39.665 42.5291 80 37.554 38.890 39.241 90 46.5474 42.652 42.648 100 41.2211 43.102 45.689 110 43.6599 44.385 46.2741 120 45.453 47.559 48.392

Resistivity measurements were made at 10-minute intervals for each of the three different samples during the first 2 hours after mixing. Two measurements were made at each time interval, and average values for each mixture were calculated including all readings made during the testing period. All measurements were made using the probe apparatus described previously.

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Correlation Between Actual and Estimated W/C Ratio of Fresh Concrete (IJSTE/ Volume 2 / Issue 06 / 019)

Fig. 2: Graph between Ρ & Time (Minutes) for W/C Ratio 0.4

Fig. 3: Graph between Ρ & Time (Minutes) for W/C Ratio 0.5

Fig. 4: Graph between Ρ & Time (Minutes) for W/C Ratio 0.6

Relationship between electrical resistivity and w/c ratio for fresh concrete (25% fly ash) S.NO. 1 2 3

W/C 0.40 0.50 0.60

Table – 4 Electrical Resistivity and W/C Ratio for Fresh Concrete AVERAGE Ρ(Ω-M) STANDARD DEVIATION COEFFICIENT OF VARIATION, % 37.059 5.52 0.304 39.820 3.80 0.144 42.140 3.167 0.101

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Correlation Between Actual and Estimated W/C Ratio of Fresh Concrete (IJSTE/ Volume 2 / Issue 06 / 019)

Fig. 5: Graph between Ρ & W/C Ratio Table – 5 Comparison between Actual and Estimated Value of W/C

w/c actual value 0.40 0.50 0.60

w/c estimate 0.40 0.50 0.61

Standard deviation 0.008 0.0081 0.011

Average error % 2.01 1.60 1.83

Comparison between actual and estimated value of w/c (25 % fly ash)

VI. CONCLUSION A relationship between resistivity and the w/c was established and tested, and two equations were developed to correlate the electrical resistivity of fresh concrete with its w/c.

The most significant findings are as follows: 1) The average w/c estimates based on the electrical resistivity of fresh concrete were virtually identical to the actual w/c values for all mixtures tested.

2) The use of fly ash tends to increase the electrical resistivity of fresh concrete.

3) Time did not have a statistically significant effect on the electrical resistivity of fresh concrete before initial setting.

4) There is a strong direct relationship between the w/c of fresh concrete and the material‟s electrical resistivity.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

Ozkan Sengul and Odd E. Gjørv “Electrical Resistivity Measurements for Quality Control During Concrete Construction “ACI Material journal, vol. 105, issue 6, pp. 541 -547 (2008) M. Mancio, J. R. Moore, Z. Brooks, P. J. M. Monteiro, and S. D. Glase “Instantaneous In-Situ Determination of Water-CementRatio of Fresh Concrete” ACI Material journal, vol. 107 issue 6, pp .586 -592 (2010) Monteiro, P. J. M., and Helene, P. R. L., “Designing Concrete Mixtures for Desired Mechanical Properties and Durability,” Concrete Technology: Past, Present, and Future, SP-144, P. K. Mehta, ed., American Concrete Institute, Farmington, Hills, MI, 1994, pp. 519-543.

ACI Committee 211, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) (Reapproved 2009),” American Concrete Institute, Farmington Hills, MI, 1991, 38 pp.

Lyse, I., “Tests on Consistency and Strength of Concrete Having Constant Water Content,” ASTM International, Proceedings, V. 32, Part 2, Dec. 1932, pp.

629-636.

IS 383-1970 (reapproved in 1997), “Specifications for coarse and fine aggregates from natural sources for concrete”, Bureau of Indian Standards, New Delhi, India.

IS 8112-1989 (re-affirmed in 2000), “Specifications for 43 grade Portland cement”, Bureau of Indian Standards, New Delhi, India.

IS 456-2000 (fourth revision), “code of practice for plain and reinforced concrete (with amendment No. 2)”, Bureau of Indian Standards, New Delhi, India.

IS 383-1970 (re-affirmed in 1999), “Recommended Guidelines for Concrete Mix Design”, Bureau of Indian Standards, New Delhi, India.

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