Mix Design For Concrete Roads As Per IRC

Transcription

1 Mix Design For Concrete Roads As Per IRC ABSTRACT: The stresses induced in concrete pavements are mainly flexural. Therefore flexural strength is more often specified than compressive strength in the design of concrete mixes for pavement construction. A simple method of concrete mix design based on flexural strength for normal weight concrete mixes is described in the paper. INTRODUCTION: Usual criterion for the strength of concrete in the building industry is the compressive strength, which is considered as a measure of quality concrete. however, in pavement constructions, such as highway and airport runway, the flexural strength of concrete is considered more important, as the stresses induced in concrete pavements are mainly flexural. Therefore, flexural strength is more often specified than compressive strength in the design of concrete mixes for pavement construction. It is not perfectly reliable to predict flexural strength from compressive strength. Further, various codes of the world specified that the paving concrete mixes should preferably be designed in the laboratory and controlled in the field on the basis of its flexural strength. Therefore, there is a need to design concrete mixes based on flexural strength. The type of aggregate can have a predominant effect, crushed rock aggregate resulting in concrete with higher flexural strength than uncrushed (gravel) aggregates for comparable mixes, assuming that sound materials are used. The strength of cement influences the compressive and flexural strength of concrete i.e. with the same water-cement ratio, higher strength cement will produce concrete of higher compressive and flexural strength. MIX DESIGN DETAILS IRC: specified that for concrete roads OPC should be used. This code also allowed PPC as per IS: 1489 may also be used. Accordingly OPC + fly ash may be used in concrete roads. However, IS: specified that fly ash conforming to grade-1 of IS-3812 may be used as part replacement of OPC provided uniform blended with cement is essential. The construction sites where batching plants are used this may be practicable. In ordinary sites where mixer or hand mixing are done uniform blending of fly ash with cement is not practicable. At such construction sites, PPC may be used. 1 Characteristic Flexural Strength at 28 days 4.5N/mm 2 2 Cement Three mixes are to be designed MIX-A With PPC (Flyash based) conforming to IS:1489-part-I days strength 37.5N/mm 2. Specific Gravity: 3.00 MIX-B With OPC-43- Grade conforming to IS: days strength 40.5N/mm 2. Specific Gravity : 3.15 MIX-C With OPC of Mix-B and Fly ash conforming to IS:3812 (Part-I)-2003 Specific Gravity : 2.20 Note Requirements of all the three mixes are the same. Fine Aggregate, Coarse Aggregate and Retarder Super plasticizer are the same for all the three mixes. 3 Fly ash replacement 25% Fly ash is required to be replaced with the total cementitious materials. 4 Maximum nominal size of aggregates 20 mm Crushed aggregate 5 Fine aggregate River sand of Zone-II as per IS: Minimum cement content 350 kg/m 3 including Fly ash 7 Maximum free W/C Ratio Workability 30 mm slump at pour the concrete will be transported from central batching plant through transit mixer, at a distance of 20 Km during June, July months. The average temperature last year during these months was 40 0 C. 9 Exposure condition Moderate 10 Method of placing Fully mechanized construction 11 Degree of supervision Good 12 Maximum of cement content (Fly ash not included) 425 kg/m 3 13 Chemical admixture Retarder Super plasticizer conforming to IS: With the given requirements and materials, the manufacturer of Retarder Super plasticizer recommends dosages of 10 gm per kg of OPC, which will reduce 15% of water without loss of workability. For fly ash included cement dosages will be required to be adjusted by experience/ trials. 14 Values of J ax o x 0.5 N/mm 2

2 TEST DATA FOR MATERIALS AND OTHER DETAILS 1. The grading of fine aggregate, 10 and 20 mm aggregates are as given in Table. 1 ( given in the end). Fine aggregate is of zone-ii as per IS: and 20 mm crushed aggregate grading are single sized as per IS: Properties of aggregates Tests Fine aggregate 10mm aggregate 40mm aggregate Specific Gravity Water Absorption % Target average flexural strength for all A, B and C mixes S = S + J ao- = x 0.5 = 5.3 N/mm 2 at 28 days age 4. For Mix A and B free W/C ratio with crushed aggregate and required average flexural target strength of 5.3 N/mm 2 at 28 days from Fig. 1 Curve D ( Figure shown in the end) found to be This is lower than specified maximum W/C ratio value of 0.50 Note: In absence of cement strength, but cement conforming to IS Codes, assume from Fig. 1 Curve A and B For OPC 33 Grade Curve C and D For OPC 43 Grade Take curves C and D for PPC, as PPC is being manufactured in minimum of 43 Grade of strength. 5. Other data s: The Mixes are to be designed on the basis of saturated and surface dry aggregates. At the time of concreting, moisture content of site aggregates are to be determine. If it carries surface moisture this is to be deducted from the mixing water and if it is dry add in mixing water the quantity of water required for absorption. The weight of aggregates are also adjusted accordingly. DESIGN OF MIX-A WITH PPC a) Free W/C ratio for the target flexural strength of 5.3 N/mm 2 as worked out is 0.42 b) Free water for 30 mm slump from Table 2 for 20 mm maximum size of aggregate. 2/3* /3*195 = 175 kg/m 3 From trials it is found that Retarder Super plasticizer at a dosages of 15gm/kg of cement may reduce 15% water without loss of workability Then water = 175 (175 x 0.15) = kg/m 3 For trials say 149 kg/m 3 c) PPC = 149/0.42 = 355 kg/m 3 This is higher than minimum requirement of 350 kg/m 3 d) Formula for calculation of fresh concrete weight in kg/m 3 U M = 10 x G a (100 A) + C M(1 G a/g c) W M (G a 1) Where, U m = Weight of fresh concrete kg/m 3 G a = Weighted average specific gravity of combined fine and coarse aggregate bulk, SSD G c = Specific gravity of cement. Determine actual value, in absence assume 3.15 for OPC and 3.00 for PPC (Fly ash based) A = Air content, percent. Assume entrapped air 1.5% for 20 mm maximum size of aggregate and 2.5% for 10mm maximum size of aggregate. There are always entrapped air in concrete. Therefore ignoring entrapped air value as NIL will lead the calculation of higher value of density. W m = Mixing water required in kg/m 3 C m = Cement required, kg/m 3 Note:- The exact density may be obtained by filling and fully compacting constant volume suitable metal container from the trial batches of calculated design mixes. The mix be altered with the actual obtained density of the mix. U m = 10 x G a (100 A) + C m (1 G a/g c) W m (G a 1) = 10 x 2.65 ( ) + 355(1-2.65/3.00) 149 (2.65-1) = kg/m 3 Say 2405 kg/m 3 e) Aggregates = = 1901 kg/m 3 f) Fine aggregate = From Table 3 for zone-ii Fine aggregate and 20 mm maximum size of aggregate, W/C ratio = 0.42, 30 mm slump found to be 35%.

3 Fine aggregate = 1901 x 0.35 = 665 kg/m 3 Coarse aggregate = = 1236 kg/m 3 10 and 20 mm aggregate are single sized as per IS: Let they be combined in the ratio of 1.2:1.8 to get 20 mm graded aggregate as per IS: mm aggregate = /3 = 494 kg/m 3 20 mm aggregate = /3 = 742 kg/m 3 g) Thus for 4.5 N/mm 2 flexural strength quantity of materials per cu.m. of concrete on the basis of saturated and surface dry aggregates: Water = 149 kg/m 3 PPC = 355 kg/m 3 Fine Aggregate (sand) = 665 kg/m 3 10 mm Aggregate = 494 kg/m 3 20 mm Aggregate = 742 kg/m 3 Retarder Super Plasticizer = kg/m 3 MIX- B WITH OPC a) Water = 175 (175 x 0.15) = 149 kg/m 3 say 149 kg/m 3 b) OPC = 149/0.42 = say 355 kg/m 3 c) Density: 10 x 2.65 ( ) (1 2.65/3.15) 149 (2.65 1) = kg/m 3 say 2420 kg/m 3 d) Total Aggregates = = 1916 kg/m 3 Fine Aggregate = 1916 x 0.35 = say 670 kg/m 3 Coarse aggregate = = 1246 kg/m 3 10 mm Aggregate = /3 = 498 kg/m 3 20 mm Aggregate = /3 = 748 kg/m 3 e) Thus for 4.5 N/mm 2 flexural strength quantity of materials per cu.m of concrete on the basis of SSD aggregates are given below: Water = 149 kg/m 3 OPC = 355 kg/m 3 Fine Aggregate (sand) = 670 kg/m 3 10 mm Aggregate = 498 kg/m 3 20 mm Aggregate = 748 kg/m 3 Retarder Super Plasticizer = kg/m 3 MIX. C WITH OPC + FLYASH With the given set of materials increase in cementitious materials = 10.7% Total cementitious materials = = 393 kg/m 3 Materials Weight (kg/m 3 ) Volume (m 3 ) OPC = 393 x / Flyash = 393 x / Free Water = 149 x / Retarder Super Plasticizer = 6.2 kg 6.2/ Air = 1.5% Total Total Aggregates = Coarse Aggregate 1246/

4 Fine Aggregate = = = x 2650 = 607 kg Note:- 1. Specific gravity of Normal Superplasticizer = Addition of Flyash reduces 5% of water demand. For 4.5 N/mm 2 flexural strength quantity of material per cu.m of concrete on the basis of saturated and surface dry aggregates of Mix A, B and C are given below: Materials MIX. A with PPC Mix. B with OPC Mix. C with OPC+Flyash Water kg/m PPC kg/m OPC kg/m Flyash kg/m 3 98 Fine Agg. kg/m mm Agg. kg/m mm Agg. kg/m Retarder Super- plasticizer kg/m W/Cementations ratio Note:- 1. For exact W/C ratio the water in admixture should also be taken into account. 2. The W/C ratio of PPC and OPC is taken the same assuming that the strength properties of both are the same. If it is found that the PPC is giving the low strength then W/C ratio of PPC have to be reduce, which will increase the cement content. For getting early strength and in cold climate the W/C ratio of PPC shall also be required to be reduced. 3. PPC reduces 5% water demand. If this is found by trial then take reduce water for calculation. 4. If the trial mixes does not gives the required properties of the mix, it is then required to be altered accordingly. However, when the experiences grows with the particular set of materials and site conditions very few trials will be required, and a expert of such site very rarely will be required a 2 nd trial. 5. It may be noted that, for the fly ash concrete the total cementation material is greater but the OP cement content is smaller, the coarse aggregate content is deliberately, the same, the water is reduced and the density is reduced, because of the lower density of fly ash compared with OPC. CONCLUSION 1. For 4.5 N/mm 2 flexural strength concrete having same material and requirement, but without water reducer, the PPC and OPC required will be 175/0.42 = 417kg/m 3 2. With the use of superplasticizer the saving in cement is 62 kg/m 3 and water 26 lit/m 3 for PPC and OPC. 3. In the Fly ash concrete the saving in cement is 122 kg/m 3 and water 33 lit/m 3 including utilization of 98 kg/m 3 of fly ash witch is a waste material. 4. In the financial year India has produces 200 million tonnes of cement. In India one kg of cement produce emitted 0.93 kg of CO 2. Thus the production of 200 million tonnes of cement had emitted 200 x 0.93 = 186 million tonnes of CO 2 to the atmosphere. 5. If 50 million tonnes cement in making concrete uses water reducers tonnes of cement can be saved KL of potable water will be saved and the saving of Rs crores per year to the construction Industry tonnes of CO 2 will be prevented to be emitted to the atmosphere. The benefits in the uses of water reducers not limited to this. When water reduces shrinkage and porosity of concrete are reduces which provides the durability to concrete structures. 6. India is facing serious air, water, soil, food and noise pollution problems. Every efforts therefore are necessary to prevent pollution on top priority basis. 7. As the stress induced in concrete pavements are mainly flexural, it is desirable that their design is based on the flexural strength of concrete. The quality of concrete is normally assessed by measuring its compressive strength. For pavings, however, it is the flexural strength rather than the compression strength of concrete which determine the degree of cracking and thus the performance of road, and it is imperative to control the quality on the basis of flexural strength.

5 REFERENCES: 1 IS : Specifications for coarse and fine aggregates from natural sources for concrete (second revision) BIS, New Delhi 2 IS: Code of practice for plain and reinforced concrete (fourth revision), BIS, New Delhi 3 IS: Specification for admixtures for concrete (first revision) BIS, New Delhi 4 IS: Specifications for 43 Grade ordinary portland cement (first revision) BIS, New Delhi 5 IS: 2386 (Part-III) 1963 method of test for aggregate for concrete. Specific gravity, density, voids, absorption and bulking, BIS, New Delhi 6 IS: 3812 (Part-I) 2003 Specification for pulverized fuel ash: Part-I for use as pozzolana in cement, cement mortar and concrete (second revision) BIS, New Delhi 7 IS: 1489-Part-I 1991 Specifications for portland pozzolana cement (Part-I) Flyash based. (Third revision), BIS, New Delhi 8 IRC: Standard specifications and code of practice for construction of concrete road (third revision) 9 Kishore Kaushal, Concrete Mix Design Based on Flexural Strength for Air-Entrained Concrete, Proceeding of 13 th Conference on our World in Concrete and Structures, 25-26, August, 1988, Singapore. 10 Kishore Kaushal, Method of Concrete Mix Design Based on Flexural Strength, Proceeding of the International Conference on Road and Road Transport Problems ICORT, December, 1988, New Delhi, pp Kishore Kaushal, Mix Design Based on Flexural Strength of Air-Entrained Concrete. The Indian Concrete Journal, February, 1989, pp Kishore Kaushal, Concrete Mix Design Containing Chemical Admixtures, Journal of the National Building Organization, April, 1990, pp Kishore Kaushal, Concrete Mix Design for Road Bridges, INDIAN HIGHWAYS, Vol. 19, No. 11, November, 1991, pp Kishore Kaushal, Mix Design for Pumped Concrete, Journal of Central Board of Irrigation and Power, Vol. 49, No.2, April, 1992, pp Kishore Kaushal, Concrete Mix Design with Fly Ash, Indian Construction, January, 1995, pp Kishore Kaushal, High-Strength Concrete, Bulletin of Indian Concrete Institute No. 51, April-June, 1995, pp Kishore Kaushal, Concrete Mix Design Simplified, Indian Concrete Institute Bulletin No. 56, July-September, 1996, pp Kishore Kaushal, Concrete Mix Design with Fly Ash & Superplasticizer, ICI Bulletin No. 59, April-June 1997, pp Kishore Kaushal. Mix Design for Pumped Concrete, CE & CR October, 2006, pp Table. 1: Grading of Aggregates IS Sieve Designation Percentage Passing Fine Aggregate Crushed Aggregate 10 mm 20 mm 40 mm 100

6 20 mm mm mm mm mm mm Micron Micron Micron 5 Table. 2: Approximate free-water content (kg/m 3 ) required to give various levels of workability for non-air-entrained (with normal entrapped air) concrete. Maximum size of aggregate(mm) Type of aggregate Slump(mm) Uncrushed Crushed Uncrushed Crushed Note:- When coarse and fine aggregate of different types are used, the free water content is estimated by the expression. 2/3W f+1/3w c Where, W f = Free water content appropriate to type of fine Aggregate W c = Free water content appropriate to type of coarse aggregate. Table. 3: Proportion of fine aggregate (percent) with 10mm and 20mm maximum sizes of aggregates and slump mm. Grading Zone of F.A W/C Ratio 10 mm aggregate 20 mm aggregate I II

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