CHAPTER 3 MATERIAL PROPERTIES AND MIX DESIGN

Transcription

1 50 CHAPTER 3 MATERIAL PROPERTIES AND MIX DESIGN 3.1 GENERAL Cement concrete is a composite material comprises of cement, fine aggregate, coarse aggregate and water. Nowadays, concrete is made with several types of cement and also contains pozzolana, fly ash, blast furnace slag, sulphur, admixtures, polymers, fibers etc. These concretes can be heated, steam-cured, autoclaved, vacuum-treated, hydraulically pressured, shockvibrated, extruded and sprayed. Making good concrete components is a skill, since the ingredients of good concrete and bad concrete are exactly the same. Bad concrete refers to concrete with the consistence of soup, hardening into a honeycombed, non-homogeneous and weak mass. Concrete which exhibits excellent behaviour in the fresh and hardened state refers to good concrete. In the fresh state, good concrete offers consistence of mix such that it can be transported, placed and compacted in a good manner without segregation. In the hardened state it should offer satisfactory compressive strength. Since many properties of concrete such as density, impermeability, durability, resistance to abrasion, resistance to impact, tensile strength, resistance to sulphates, relates to the compressive strength of concrete. To make concrete of desired strength, it is necessary to analyse the basic properties of cement and aggregates. In the case of reinforced cement concrete, the study on the chemical composition and

2 51 mechanical properties of steel reinforcement is an important aspect. Since these data are useful while designing a structure and gives an idea about the corrosion resistance behaviour. 3.2 MATERIAL PROPERTIES Coarse aggregate comprises of 19 mm down graded aggregate procured from local source and sieved through 20 mm sieve. Natural river sand obtained from local source sieved through 4.75 mm sieve forms the fine aggregate. Ordinary Portland cement of 53 grade (brand name: Coromandel King) confirms to IS and obtained from a single source was used. Reinforcing steel consists of 12 mm, 16 mm and 20 mm diameter high yield strength Cold Twisted Deformed (CTD) and Thermomechanically Treated (TMT) rebars. Fe 415 grade, branded TMT rebars and IS grade CTD rebars were procured locally from the same batch to maintain uniformity. The following tests were conducted to study the properties of concrete constituent materials and reinforcement bar. Standard consistency of cement Initial and final setting time of cement Specific gravity of fine and coarse aggregate Water absorption of fine and coarse aggregate Fineness modulus of fine and coarse aggregate Dry rodded density of coarse aggregate Chemical compositions of steel Mechanical properties of steel

3 Standard Consistency of Cement The consistency test was conducted as per IS 4031-Part Table 3.1 shows the observation on consistency test. The consistency of the cement was found to be 28%. Table 3.1 Consistency test observation Trial no. Weight of cement (gm) Water content (ml) Water content (%) Final reading (mm) (from mould bottom) Setting Time of Cement The initial setting time of cement was determined as per IS 4031-Part Initial setting time of cement : 1 h 45 min Final setting time of cement : 6 h 30 min Specific Gravity and Water Absorption The specific gravity of fine and coarse aggregates were determined as per IS 2386-Part Tables 3.2 and 3.3 show the specific gravity and water absorption test observation for fine aggregate and coarse aggregate. Weight of saturated and surface dry sample = A Weight of pycnometer with sample and water = B Weight of the pycnometer with water Weight of cooled sample = C = D

4 53 Specific gravity = Water absorption (%) = D A (B C) 100 (A D) D Table 3.2 Specific gravity and water absorption test observation for fine aggregate Description Sample A Sample B Weight of saturated surface dry sample (kg) Weight of pycnometer + sample + water (kg) Weight of pycnometer + water (kg) Weight of dry sample (kg) Specific gravity Water absorption 0.80 % 1.01 % Table 3.3 Specific gravity and water absorption test observation for coarse aggregate Description Sample A Sample B Weight of saturated surface dry sample (kg) Weight of pycnometer + sample + water (kg) Weight of pycnometer + water (kg) Weight of dry sample (kg) Specific gravity Water absorption 0.4 % 0.4 % The following results were derived Specific gravity of fine aggregate : 2.56 Specific gravity of coarse aggregate : 2.67

5 54 Water absorption of fine aggregate : 0.90% Water absorption of coarse aggregate : 0.40% Fineness Modulus Fineness modulus of fine aggregates and coarse aggregates were determined by conducting sieve analysis as per IS 2386-Part I Table 3.4 shows the observation on sieve analysis test for fine aggregate and Table 3.5 shows the sieves analysis test observation for coarse aggregate. Fineness modulus was obtained by adding the cumulative percentages of aggregate retained on each sieve divided by an arbitrary number 100. The fineness modulus of fine aggregate was calculated as According to IS , the aggregates conforms to Grading Zone II. The fineness modulus of coarse aggregate was calculated as Table 3.4 Observation on sieve analysis of fine aggregate IS sieve no. Weight retained (gm) Weight retained (%) Cumulative % weight retained Cumulative % passing 4.75 mm mm mm µm µm µm The fineness modulus of fine aggregate = 279/100 = 2.79

6 55 Table 3.5 Observation on sieve analysis of coarse aggregate IS sieve no. Weight retained (gm) Weight retained (%) Cumulative % weight retained Cumulative % passing 20 mm mm mm mm mm µm µm µm The fineness modulus of coarse aggregate = /100 = Dry Rodded Density Bulk density indicates the density of coarse aggregate when it was filled in a standard manner. The dry rodded density of coarse aggregate was determined as per IS Table 3.6 shows the observation on dry rodded density test. The dry rodded density of coarse aggregate was found to be 1630 kg/m 3. Table 3.6 Dry rodded density test observation Sample no. Wt. of mould (kg) Wt. of mould + aggregate (kg) Wt. of mould + water (kg) Dry rodded density (kg/m 3 )

7 Chemical Compositions The quantitative determination of chemical compositions of Cold Twisted Deformed (CTD) and Thermomechanically Treated (TMT) rebars were analysed by Optical Emission Spectroscopy (OES). The principle involves the production of light radiation characteristics of different constituents present in the sample and measurement of the spectral intensity. The intensity is directly proportional to the concentration of the element. The rebar samples were polished using a 60 grit Aloxide abrasive paper in an UNIMAT polishing machine prior to testing. Test was carried out in an optical emission spectrometer. Table 3.7 shows the details of chemical compositions of rebars used in experiments. Table 3.7 Details of chemical compositions for CTD and TMT rebars Type of rebar Diameter (mm) Chemical compositions (%) Fe C Si Mn P S Cr Ni CTD TMT CTD TMT Mechanical Properties of Steel Mechanical properties of CTD and TMT rebars were determined by conducting direct tension test as per IS FIE automatic load-rate control electronic Universal Testing Machine of 1000 kn capacity was used to conduct tension test. Figure 3.1 shows the tension test in progress and Table 3.8 shows the mechanical properties of CTD and TMT rebars.

8 57 Figure 3.1 Tension test in progress Table 3.8 Mechanical properties of CTD and TMT rebars Sl. No. Mechanical properties 12 mm CTD Type of rebar 12 mm TMT 16 mm CTD 16 mm TMT 1 Yield stress (N/mm 2 ) Ultimate stress (N/mm 2 ) Nominal breaking stress (N/mm 2 ) Actual breaking stress (N/mm 2 ) Weight / metre length (N/m) Elongation (%) Area reduction (%) MIX DESIGN The mix design was carried out as per American Concrete Institute Method ( ACI Committee ).

9 58 Design Data Characteristic compressive strength = 25 N/mm 2 Specific gravity of cement = 3.15 Specific gravity of fine aggregate = 2.56 Specific gravity of coarse aggregate = 2.67 Fineness modulus of fine aggregate = 2.80 Fineness modulus of coarse aggregate = 6.90 Dry rodded density of coarse aggregate = 1630 kg/m 3 Size of coarse aggregate Shape of aggregate Slump value Degree of control = 19mm down graded = Angular = mm = Very Good Water absorption (coarse aggregate) = 0.40% Water absorption (fine aggregate) = 0.90% Target mean strength, f m = f min + Ks Assuming 5% of results are allowed to fall below the specified design strength, value of Himsworth constant (K) = Standard deviation, (s) for the placing and mixing control = 4.2 N/mm 2 Mean strength = = N/mm 2 From Table 11.5 of ACI , water-cement ratio for the compressive strength of concrete, N/mm 2 = 0.52 From Table 11.8 of ACI , for a slump of 50 mm, 20 mm maximum size of aggregate, for non-air entrained concrete, the mixing water content is 185 kg/m 3 of concrete. The entrapped air content is 2%.

10 59 The required cement content = 185/0.52 = 356 kg From Table 11.4 of ACI , for 20mm coarse aggregate, of fineness modulus 2.80, the dry rodded bulk volume of coarse aggregate is 0.62 per unit volume of concrete. The weight of coarse aggregate = = 1011 kg follows : Finding fine aggregate content by absolute volume method is as Ingredients Weight (kg/m 3 ) Specific gravity Absolute volume (cm 3 ) ( 10 3 ) Cement Water Coarse aggregate Air content (2%) Total Absolute volume of fine aggregate = ( ) 10 3 = cm 3 Weight of fine aggregate = = 777 kg From Table 11.9 of ACI , the first estimate of density of fresh concrete for 20 mm size aggregates and for non-air entrained concrete = 2355 kg/m 3 The weight of all the known ingredient of concrete are as follows Weight of water = 185 kg/m 3 Weight of cement = 356 kg/m 3 Weight of coarse aggregate = 1011 kg/m 3 Weight of fine aggregate = 2355 ( ) = 803 kg/m 3

11 60 The fine aggregate content adopted for design = 777 kg Water absorption of coarse aggregates and fine aggregates were not considered for making adjustments in the water content and material quantities since both the materials were used in the dry sunny climate. Estimated quantities of materials per cubic metre of concrete are Cement = 356 kg Fine aggregate = 777 kg Coarse aggregate = 1011 kg Water = 185 L Table 3.9 gives the mix design details. Table 3.9 Mix design details Sl.no. Description Values 1 Characteristic compressive strength (N/mm 2 ) 25 2 Target mean strength of concrete (N/mm 2 ) Degree of quality control Very good 4 Type of exposure Mild 5 Maximum size of aggregate (mm) 20 6 Shape of aggregate Angular 7 Specific gravity of cement Specific gravity of fine aggregate Specific gravity of coarse aggregate Target slump (mm) Water cement ratio Water content per cubic metre of concrete (L) Cement content per cubic metre of concrete (kg) Mix proportion 1:2.18:2.84