Concrete Mixing Design - Lab Report Example

CONCRETE MIXING DESIGN Number: This report is about creation of different concrete in the laboratory. The three concretes made included normal concrete, air entrained concrete and silica fume added concrete. The ASTM standard was used in mixing the concretes as described in the experimental methods whereby coarse aggregate, fine aggregate, cement and water accordingly. The concrete mixtures were proportioned with water cement of different ratios using a target slump of 3 to 4 inches. batch quantities for 1 ft3 and the batch quantities for moisture were also determined. Contents Abstract 2 Contents 3 INTRODUCTION 4 Problem Statement 12 Experimental Methods 13 Results 17 Conclusion 30 Appendix A 32 References 38 INTRODUCTION Concrete is a solid with void spaces which may contain gas (such as air) or liquid (such as water). The properties of concrete usually depend on the mix proportions as well as the placing and curing methods. It is a common practice for designers, when determining structural dimensions, to either specify or assume a specific strength or modulus of elasticity of the concrete. It is the responsibility of the materials engineer to ensure that the concrete is proportioned, mixed, placed and cured in a proper way so as to bear the properties that the designer specified. The properties of the concrete mix in both the solid and plastic states are affected by its proportioning. The materials engineer is often concerned with the workability as well as the finishing characteristics of the concrete during the plastic state. Some of the properties of hardened concrete which are important to material engineer include the strength, durability, porosity and modulus of elasticity. Generally, strength is the controlling design factor. Concrete strength often refers to the average (mean) compressive strength of three tests unless stated otherwise. Each of the three tests is the average (mean) result of two separate cylinders which are tested in compression after curing for duration of 28 days. According to Kosmatka et al. (2008), the following three qualities were specified by the PCA as requirements for concrete mixtures which are properly proportioned: i. workability of the freshly mixed concrete must be acceptable ii. durability, strength, as well as uniform appearance of the hardened concrete iii. economy The materials engineer needs to determine the proportions of water, cement, fine and coarse aggregates, together with the use of admixtures so as top achieve the characteristics listed above. Over the years, there has been development of various mix design methods which range from an arbitrary volume method of cement, sand: and coarse aggregate in the ratio of 1:2:3 respectively to the weight and absolute volume methods which has been put in place by the American Concrete Institute’s Committee 211. Weight method, which uses a known or assumed unit of weight concrete, provides techniques which are relatively simple in order to estimate mix proportions. The absolute volume method employs the specific gravity for each of the ingredients so as to calculate the unit volume that each will occupy within a unit volume of concrete. The weight method is less accurate than the absolute volume method. The difference in the mix design process for the absolute volume and weight methods only exist in the manner in which the quantity of fine aggregates is determined. According to Kosmatka et al. (2008), the basic steps which are required in order to determine mix design proportions for both absolute volume and weight methods are as follows: 1. The strength requirements is first evluated. 2. The water-cement or water–cementitious materials ratio that is required is datermined. 3. Coarse aggregate requirements are evaluated in terms of: i. maximum aggregate size of the coarse aggregate ii. quantity of the coarse aggregate 4. Air entrainment requirements are determined. 5. Workability requirements of the plastic concrete evaluated. 6. The water content which is required for the mix is estimated. 7. Cementing materials content and type needed are determined. 8. The need and application rate of admixtures is evaluated. 9. Fine aggregate requirements are evaluated. 10. Moisture corrections are determined. 11. Trial mixes are made and tested. Most concrete supply companies are highly experienced as far as the performance of their materials a variety of applications is concerned. This experience is one of the most dependable methods which is used for selecting mixed proportion so long as it is accompanied by a reliable set of test data on the relationship existing between strength and water–cementitious materials ratio. On the other hand, the understanding of basic principles of mixture design as well as the proper selection of both materials and mixture characteristics is just as necessary as the actual calculation. This implies that guidelines are provided by the PCA procedure which can be adjusted accordingly in order to match the experience that is obtained from the local conditions. Below is a discussion of steps of PCA mix design. 1. Strength Requirements deviations in the in the strength of concrete that is produced by a plant may be brought about by variations in materials as well as batching and mixing of concrete. The variations are generally not taken into consideration by the design engineer when determining the structural members’ size. Thus, if a material with average strength which is equal to that provided by the designer is provided by the material engineer, half of the concrete will consequently be weaker than the length which was specified. This is obviously undesirable. This variance in concrete length is compensated for by the materials engineer by designing the concrete in such a away that it will have an average strength which is greater than the strength which was specified by the structural designer. 2. Water–Cement Ratio Requirements determination of the water-cement ratio is the next step that is needed in producing the required strength. Strength-versus–water–cement ratio curve may be plotted using Historical records, as shown in figure 1 below. Figure 1: Example trial mixture or field data strength curves (Kosmatka et al., 2008). In the absence of Historical data, a curve similar to the one in figure 1 above can be established by the use of three trial batches that are made at water-cement ratios which are different. Table 1 shown below can possibly be employed when estimating for the water-cement ratios for the three trial mixes in the absence of other data. Table 1: Typical Relationship Between Water–Cement Ratio and Compressive Strength of Concrete* The average compressive strength that is required is used together with the strength versus water–cement relationship in order to determine the water–cement ratio that is required for the strength requirements of the project. 3. Coarse Aggregate Requirements determination of the suitable aggregate characteristics for the project is the next step. Large dense graded aggregates are known to generally provide the most economical mix. This is because the large aggregates minimize the quantity of water that is required and, consequently, reduce the quantity of cement needed per cubic meter of mix. For equal workability, angular aggregates require more water than the round aggregates The maximum aggregate size which is allowable is limited by both the dimensions of the structure and capabilities of the construction equipment. The largest maximum aggregate of the largest size which is practical under the ensuing job conditions which satisfies the size limits shown in the table should be used. The normal maximum aggregate should then be used for the rest of the proportioning analysis after the determination of maximum aggregate size. 4. Air Entrainment Requirements the need for air entrainment is then evaluated next. Air entrainment is needed whenever concrete is exposed to either freeze–thaw conditions or deicing salts. Air entrainment is also applied for workability in some situations. The quantity of air that is required varies depending on the exposure conditions and is further affected by the aggregates size. The exposure levels are as defined below: i. Mild exposure—is the Indoor or outdoor service whereby concrete is exposed to neither freezing nor deicing salts. Here, air entrainment may be applied to improve workability. ii. Moderate exposure—freezing exposure occurs, although the concrete is not exposed to free water or moisture for long duration prior to freezing. The concrete is also not exposed to deicing salts. Examples of such concretes are the exterior beams, columns, walls, among others, which are not exposed to wet soil. iii. Severe exposure—is whereby the concrete is exposed to free water, deicing salts or saturation. Examples of such concretes are the pavements, bridge decks, canal linings, curbs, gutters, among others. 5. Workability Requirements determination of the workability requirements for the project is the next step in the mix design. By definition, Workability is ease of placing, consolidating, and finishing of the freshly mixed concrete. Although it is necessary for the concrete to be workable, it should neither segregate nor excessively bleed, that is the movement of water to the upper surface of concrete. The slump test (see figure 2 below) is an indicator of workability when similar mixtures are being evaluated. Figure 2: Slump test apparatus. The slump test include the filling a truncated cone with concrete, after which the cone is removed, then finally the distance the concrete slumps is measured (ASTM C143). The slump may be increased by the addition of adding water, water reducer, air-entrainer, superplasticizer, or by the use of round aggregates. The recommendations for the slump of concrete that is used in various types of projects are shown in table 2. Table 2: Recommended Slumps for Various Types of Construction* 6. Water Content Requirements The required water content for a given slump is based on the nominal maximum size as well as maximum shape of the aggregates. It also depends on whether an air entrainer is used. 7. Cementing Materials Content Requirements Since the water-cement ratio or water– cementitious materials ratio and the quantity of water required have been estimated, the quantity of cementing materials needed for the mix is then determined by dividing the weight of water by the water–cement ratio. PCA recommendations include minimum cement content for the concrete which is exposed to severe freeze–thaw, sulfate exposures, and deicers, and should not be less than for concrete that is placed under water. In addition, Table 3 illustrates the minimum cement requirements for proper abrasion resistance, placing, finishing, and durability in flatwork, e.g. slabs. Table 3: Minimum Requirements of Cementing Materials for Concrete Used in Flatwork* 8. Admixture Requirements In case one or several admixtures are used when adding a specific quality in the concrete; it is advisable to consider their quantities in the mix proportioning. Manufacturers of admixtures provide specific information on the amount of admixture that is needed to achieve the desired results. 9. Fine Aggregate Requirements Now that water, cement, as well as dry coarse aggregate weights per cubic meter are already known in addition to the estimated volume of air, the quantity of the dry fine aggregate required is the only remaining factor. Table 4 is used by the weight mix design method in order to estimate the total weight of a concrete which is freshly mixed for various nominal maximum aggregate sizes. Table 4: Estimate of Weight of Freshly Mixed Concrete To determine the weight of the fine aggregates, the weight of the other ingredients is subtracted from the total weight. The component weight as well as the specific gravity is used in determining the volumes of the cement, water, and coarse aggregate in the absolute volume method of the mix design. These volumes, together with the volume of the air, are subtracted from the unit volume of concrete in order to determine the required volume of the fine aggregate. The resulting volume of the fine aggregate is finally converted to a weight by the use of the unit weight. The bulk SSD specific gravity of the aggregates is generally used for the weight–volume conversions of fine as well as coarse aggregates. 10. Moisture Corrections the mix designs often assume that water that is used to hydrate the cement is free water in excess of the moisture content of aggregates at SSD condition. Thus, the last step in the mix design process requires adjustment of the weight of water as well as aggregates in order to account for the aggregates’ existing moisture content. If the aggregates’ moisture content of the is greater than the SSD moisture content, then it is necessary to reduce the weight of mixing water by an amount equal to the free weight of moisture on the aggregates. But if the moisture content is found to be below the SSD moisture content, it is necessary to increase the mixing water. There is also the need to adjust the weights of both coarse and fine aggregates that were estimated in step 3 and step 9 to account for the moisture in the aggregates that was absorbed because they initially assumed dry conditions. 11. Trial mixes Once the required amount of each ingredient has been computed, it is necessary to mix a trial batch in order to check the mix design. Three (3) cylinders are made, cured for duration of 28 days, and then tested for compressive strength. The air content as well as slump of fresh concrete is also measured. If it is found that the slump, air content, or compressive strength has not met the required standards, the mixture is adjusted accordingly and other trial mixes are made until the required standards are met by the design. Other trial batches could be made by varying slightly the material quantities so as to determine the mix which is the most workable and economical. Problem Statement Different situations and designs call for different types of concretes to be used. This report shows the creation of three different concretes. The first concrete to be created is the normal concrete, the second concrete being the air entrained concrete and finally the third concrete is the one in which silica fume has been added. Influences of different factors on the concretes are also investigated. Experimental Methods i. Mixing and curing procedures Purpose To the purpose of this report is to determine how to make and cure concrete cylindrical and beam specimens. This practice provides standardized requirements for making and curing portland cement concrete test specimens. Specimens can be used to determine strength for mix design, quality control, and quality assurance. Apparatus ■ Cylindrical molds made of steel or another nonabsorbent and nonreactive material. The standard specimen size used to determine the compressive strength of concrete is 152 mm (6 in.) diameter by 304 mm (12 in.) high for a maximum aggregate size up to 50 mm (2 in.). Smaller specimens, such as 102 mm (4 in.) diameter by 203 mm (8 in.) high, are sometimes used, but they are not ASTM standards. ■ Beam molds made of steel or another nonabsorbent, nonreactive material. Several mold dimensions can be used to make beam specimens with a square cross section and a span three times the depth. The standard ASTM inside mold dimensions are 152 by 152 mm (6 by 6 in.) in cross section and a length of not less than 508 mm (20 in.), for a maximum aggregate size up to 50 mm (2 in.). ■ Tamping rod with a length of 0.6 m (24 in.), diameter of 16 mm (5/8 in.), and rounded ends ■ Moist cabinet or room with not less than 95% relative humidity and 23 ± 1.7°C (73 ± 3°F) temperature or a large container filled with lime-saturated water for curing. ■ Miscellaneous items including vibrator (optional), scoop, and trowel. Test Procedure Figure 3: making concrete cylinders The required amount of coarse aggregate, fine aggregate, portland cement, and water was weighed. The materials in the mixer were mixed for 3 to 5 min. slump, air content, and temperature of concrete was checked. For cylindrical specimens, concrete was placed into the mold using a scoop or trowel. the cylinder in three equal layers was filled, and each layer was rodded 25 times. the outside of the cylinder was tapped10 to 15 times after each layer was rodded. the top was striked off and the surface was smoothen. It should be noted that vibrators can also be used to consolidate the concrete instead of rodding. Vibration is optional if the slump is between 25 mm to 75 mm (1 in. to 3 in.) and is required if the slump is less than 25 mm (1 in.) Cover the mold was covered with wet cloth to prevent evaporation. the molds were removed after 16 hours to 32 hours. the specimen was cured in a moist cabinet or room at a relative humidity of not less than 95% and a temperature of 23 ± 1.7°C (73 ± 3°F) or by submersion in lime-saturated water at the same temperature (Figure 4). Figure 4: Curing concrete cylinders in lime-saturated water. Precautions 1. Segregation must be avoided. Over vibration may cause segregation. 2. In placing the final layer, the operator should attempt to add an amount of concrete that will exactly fill the mold after compaction. Do not add nonrepresentative concrete to an under-filled mold. 3. Avoid overfilling by more than 6 mm (1/4 in.). Report ■ Record mix design weights, slump, temperature of the mix, and air content ■ Specimen type, number of specimens, dimensions, and any deviations from the standard preparation procedure ■ Include this information with the report on the strength of the concrete ii. Testing procedures (refer to ASTM specifications where appropriate) Testing of Concrete Masonry Units Purpose The purpose of this report was to test concrete masonry units for compressive strength, absorption, moisture content, and density. The test produces compressive strength, absorption, moisture content, and density data for the control and specification of concrete masonry units. These data are important for the safety and proper performance of masonry structures. Apparatus ■ Testing machine ■ Steel-bearing blocks and plates ■ Balance Test Procedure for Compressive Strength 1. Three representative units are needed for testing within 72 hours after delivery to the laboratory, during which time they are stored continuously in air at a temperature of 24 ± 8°C (75 ± 15°F) and a relative humidity of less than 80%. 2. The test is performed on either a full-sized unit or a part of a unit prepared by saw cutting. A part of a unit is used if the capacity or size of the testing machine does not allow the testing of a full-sized unit. 3. Measure the length, width, and height of the specimen. 4. Cap the bearing surfaces of the unit, using either sulfur and granular materials or gypsum plaster. 5. Position the test specimen with its centroid aligned vertically with the center of thrust of the spherically seated steel-bearing block of the testing machine (Figure A.48). 6. Apply the load up to one-half the expected maximum load at any convenient rate, after which apply the load at a uniform rate of travel of the moving head, so that the test is completed between one and two minutes. 7. Record the maximum compressive load in newtons (pounds) as Pmax. Test Procedure for Absorption Three representative full-sized units were needed for absorption testing. The specimen were weighed immediately after sampling and the weight were recorded as the received Weight (Wr). the specimen were immersed in water in water at a temperature of 15°C to 26°C (60°F to 80°F) for 24 hours. Specimen were weighed while suspended by a metal wire and completely submerged in water; the weight was recorded as the immersed weight (Wi). the specimen was removed from the water and allowed to drain for 1 min by placing it on a 9.5-mm (3/8-in.) or coarse wire mesh and removing visible surface water with a damp cloth. the specimen was weighed and r the weight was recorded as the saturated weight (Ws). the specimen was dried in a ventilated oven at 100°C to 115°C (212°F to 239°F) for not less than 24 hours and until two successive weights at intervals of 2 hours showed a difference of not greater than 0.2%. the weight was recorded as the oven-dried weight (Wd). . Results The cement, coarse aggregate, fine aggregate and the silica fume from the laboratory are described below: Cement: Type I-II SG=3.15 Coarse aggregate: ¾” top size Dry Rodded Unit Weight = 89.9 lb/ft3 BSG=2.54 Moisture content=0.7% Absorption=2.2% The fine aggregate will be a blend of 70% Fine Aggregate no1 and 30% Fine Aggregate no2 Fine Aggregate No1: BSG=2.63 Fineness modulus=1.86 Moisture content=4.7% Absorption=1.1% Fine Aggregate No2: BSG=2.56 Fineness modulus=3.14 Moisture content=1.9% Absorption=2.0% The calculations below are as a result of the Proportioning a concrete mixture with a water cement ratio of 0.6 using a target slamp of 3 to 4 inches. The batch quantities for 1 ft3 were determined. the batch quantities for moisture were also determined. w/c=0.6 water content=340lb/yd3 Coarse Aggregate should fill 65% of volume Air will take 2% of the volume Volume constituents 1267.1(0.7)=886.97 Mix design Water 340 Cement 567 Coarse Aggregate 1641 Fine Aggregate No. 1 887 Fine Aggregate No. 2 380 Batch quantities: (1 ) Water 12.59 Cement 21 Coarse Aggregate 60.78 Fine Aggregate No. 1 32.85 Fine Aggregate No. 2 14.07 Coarse Aggregate Mc=0.7% Abs=2.2% 1.5% below SSD 0.015(60.78)=0.9117 Reduce by 0.9117lb Coarse Aggregate Content=60.78-0.9117=59.86lb Fine Aggregate No. 1 Mc=4.7% Abs=1.1% 3.6% above SSD 0.036(32.85)=1.183lb Add by 1.183lb Coarse Aggregate Content=32.85+1.183=34.03lb Fine Aggregate No. 2 Mc=1.9% Abs=2.0% 0.1% below SSD 0.001(14.07)=0.01407lb Reduce by 0.01407lb Coarse Aggregate Content=13.22-0.01407=14.06lb Water Add 0.9117lb of water to compensate for coarse Aggregate Reduce 1.183lb of water to compensate for the Fine Aggregate No. 1 Add 0.01407lb of water to compensate for Fine Aggregate No.2 Water content =12.59+0.9117-1.183+0.01407=12.33 lb/ft3 Adjusted Batch quantities Water 12.33 Cement 21 Coarse Aggregate 59.86 Fine Aggregate No. 1 34.03 Fine Aggregate No. 2 14.06 The cement, coarse aggregate, and fine aggregate that was used in the laboratory for the air-entrained concrete are described below (homework No. 5): Cement: Type I-II SG=3.15 Coarse aggregate: ¾” top size Dry Rodded Unit Weight = 89.9 lb/ft3 BSG=2.54 Moisture content=0.7% Absorption=2.2% The fine aggregate will be a blend of 70% Fine Aggregate no1 and 30% Fine Aggregate no2 Fine Aggregate No1: BSG=2.63 Fineness modulus=1.86 Moisture content=4.7% Absorption=1.1% Fine Aggregate No2: BSG=2.56 Fineness modulus=3.14 Moisture content=1.9% Absorption=2.0% The calculations below are as a result of the Proportioning a concrete mixture with a water cement ratio of 0.5 using a target slump of 3 to 4 inches. The assumption made is that the concrete will have moderate exposure to freezing and thawing. The batch quantities for 1 ft3 has been determined, and the batch quantities for moisture have been adjust. w/c=0.5 water content=305lb/yd3 Coarse Aggregate should fill 65% of volume Air will take 5% of the volume Volume constituents 1188.9(0.7)=832.2 Mix design Water 305 Cement 610 Coarse Aggregate 1641 Fine Aggregate No. 1 832 Fine Aggregate No. 2 357 Batch quantities: (1 ) Water 11.3 Cement 22.6 Coarse Aggregate 60.78 Fine Aggregate No. 1 30.81 Fine Aggregate No. 2 13.22 Coarse Aggregate Mc=0.7% Abs=2.2% 1.5% below SSD 0.015(60.78)=0.9117 Reduce by 0.9117lb Coarse Aggregate Content=60.78-0.9117=59.86lb Fine Aggregate No. 1 Mc=4.7% Abs=1.1% 3.6% above SSD 0.036(30.81)=1.1092lb Add by 1.1092lb Coarse Aggregate Content=30.81+1.1092=31.92lb Fine Aggregate No. 2 Mc=1.9% Abs=2.0% 0.1% below SSD 0.001(13.22)=0.01322lb Reduce by 0.01322lb Coarse Aggregate Content=13.22-0.01322=13.21lb Water Add 0.9117lb of water to compensate for coarse Aggregate Reduce 1.1092lb of water to compensate for the Fine Aggregate No. 1 Add 0.01322lb of water to compensate for Fine Aggregate No.2 Water content =11.3+0.9117-1.1092+0.01322=11.12 lb/ft3 Adjusted Batch quantities Water 11.12 Cement 22.6 Coarse Aggregate 59.86 Fine Aggregate No. 1 31.92 Fine Aggregate No. 2 13.21 The cement, coarse aggregate, fine aggregate and the silica fume from the laboratory for the third concrete slab to which silica fume was added are described below (homework No. 6): Cement: Type I-II SG=3.15 Coarse aggregate: ¾” top size Dry Rodded Unit Weight = 89.9 lb/ft3 BSG=2.54 Moisture content=0.7% Absorption=2.2% The fine aggregate will be a blend of 70% Fine Aggregate no1 and 30% Fine Aggregate no2 Fine Aggregate No1: BSG=2.63 Fineness modulus=1.86 Moisture content=4.7% Absorption=1.1% Fine Aggregate No2: BSG=2.56 Fineness modulus=3.14 Moisture content=1.9% Absorption=2.0% Silica Fume: SG=2.20 The calculations below are as a result of the proportioning of a concrete mixture with water-cementitious ratio of 0.4 using a target slump of 3 to 4 inches. 12% replacement of cement with silica fume was used. water content=340lb/yd3 . Air will take 2% of the volume Volume of constituents: 5.45+3.81+0.743+0.54+10.35=20.89 995(0.7)=697 Mix Design: Water 340 Cement 748 Silica fume 102 Coarse Aggregate 1640.8 Fine Aggregate No. 1 691 Fine Aggregate No. 2 296 Conclusion The three different concretes were successfully created and the adjustments were also successfully made as instructed. The desired batch quantities for 1 ft3 were determined, and the batch quantities for moisture were also adjusted. Appendix A References Kosmatka, S. H., B. Kerkhoff, and W. C. Panarese. Design and Control of Concrete Mixtures. 14th ed. Skokie, IL: Portland Cement Association, 2008. Read More