Design & Control of Concrete Mixtures Whatsfc??? fc is the specified compressive strength.Unless otherwise specified, 28-day strength test is always utilized. Abbreviations SG---Specific Gravity wt. or lbs ---Weight cy and/or cyd---Cubic Yard(s) cf---Cubic Feet pcf ---Pounds per Cubic Feet DRUW---Dry Rodded Unit Weight fl. oz---Fluid Ounce cwt---100 lbs of cement, or hundred weight of cement w/cm---Water-to-Cementitious Ratio Constants 1 gallon of water = 8.33 lbs. Specific Gravity of water = 1.00 Unit Weight of water = 62.4 pcf 1 cubic yard (cy) = 27 cubic feet (cf) 1 bag or sack of cement = 94 lbs. Some Useful Conversions: 1 gallon (gal)= 128 fluid ounces (fl. oz.) 1 fl. oz. = 29.6 milliliters (mL) 1 cubic feet (cf) = 7.48 gals 1 gals of water= 8.33 lbs. 1 cubic yard (cy)= 27 cf Absolute Volume Method of Concrete Mixture Proportioning Proportioning Concrete Mixtures Our objective is to produce a concrete mixture that will meet the specifications and performance requirements for its intended use. Typical Mix Design Considerations Strength Placeability Durability Finishability Available Materials Appearance Impermeability Abrasion Resistance Economics Proportioning Procedure Job specifications may dictate some or all of the following Maximum w/cm Minimum cement content Air content Slump Maximum aggregate size Strength Others Definitions of Basic Terms Absolute VolumeMethod - A more accurate method than weight proportioning methods, it involves use of specific gravity (SG) values for all the ingredients to calculate the absolute volume each will occupy in a unit volume of concrete. The absolute volume is computed from a materials weight and its SG as follows: Abs. Vol.=(Weight of Material) (SG x Unit Wt. H2O*) (* The density of water within the normal ambient temperature range is 62.4 pcf) Definitions Continued Specific Gravity (SG) - A dimensionless ratio of a substances density to some standard reference density. For liquids and solids, the reference density is pure water. SG of Water = 1.0 SG of Portland Cement = 3.15 (3.15 times heavier than water) A material that weighs twice as much as an equal volume of water has a SG of2.0 SG of normal-weight aggregates ranges between 2.4 and 2.9 Definitions Continued Nominal Aggregate Size The smallest sieve opening through which the entire amount of aggregate is permitted to pass. Saturated Surface Dry (SSD) Aggregates are generally reported in either SSD or Bulk Dry (Oven Dry) condition, with SSD most often used Difference is the absorption Water Unit Weight of Stone? SG of water = 1.0 Unit Weight of water (1 cf) = 62.4 lbs. How much does 1 cf of stone weigh if the Specific Gravity (SG) is 2.80 and there are no voids? Water Stone Unit Wt.Stone = SGstone X Unit Wt.Water Unit Wt.Stone = 2.80 X 62.4 pcf Unit Wt.Stone = 174.7 pcf Definitions Continued Dry Rodded Unit Weight (DRUW) - The maximum weight of coarse aggregate in a 1 cf bucket as determined by ASTM C 29. Fineness Modulus (FM) - is an index of the fineness of an aggregate - the higher the FM, the coarser the aggregate. FM (ASTM C 125)is obtained by adding the cumulative percentages by weight retained on each of a specified series of sieves and dividing the sum by 100. (Specified sieves: #100, #50, #30, #16, #8, #4, 3/8, , 1.5, 3 and 6) Overall Concept Concrete is sold and proportioned by volume (ASTM C 94) Concrete designed per cubic yard (yd3) 1 yd3 = 27 cf Concrete Design Methods Sack (Bag) Mixes (Weight) Cement contentPerformance Mixes (Field Data or Trial Mixes) Strength and W/CM ratio Mix Proportioning (Absolute Volume) Verify concrete ingredients - visual Verify slump at plant (job site) Communicate slump adjustment Delivery Ticket Accuracy The Sack or Bag Method Proportioning by Weight 1 bag of cement = 94 lbs. Specified number of bags and water per bag Example: 6-1/2 bags with 5.2 gals of water/bag, or 611 lbs of cement , 282 lbs of water W/CM ratio of 0.46 Performance Mixes Proportioning by Field Data Strength Data & Standard Deviations (S) Test record must equal or exceed the required avg. compressive strength, fcr; fcr=fc + 1.34S fcr = fc +2.33S-500 (psi) Minimum of 30 consecutive tests; If only 15 to 29 are available, use modification factor for correction. Performance Mixes Proportioning by Lab Trial Mixes When field test records are not available, 3 mix designs @ 3 different w/cm ratios or cementitious contents. This is referred to as a 3-point curve. Absolute Volume Method Sample Mix Absolute Volume Method Water CementCoarse Aggregates (Limerocks) Fine Aggregates (Sand) Air Design Method Design for SSD weights; SSD condition is neither absorbing water nor contributing water to the concrete mixture. Example: Abs. Vol. =Weight of Material (SG X Unit Wt. of Water) Abs. Vol. =1700 lbs of #57 (2.5 SG #57 Limestone X 62.4 pcf) Abs. Vol. =10.9 cf of #57 Limestone Water Required Information The SG of the coarse and fine aggregates. The SG of mineral admixture to be used.(E.g.fly ash, slag, etc.) The SG of cement = 3.15 Is there a specification ? Minimum cement content? Maximum w/cm ratio? Example Mix Information Specification requirements: Minimum cement contentof 658 lbs. Water-Cementitious Ratio of 0.41 Allowable Replacement of fly ash 18-22 % pound per pound of cement. Air Content 2.0 - 4.0 % Calculating for Water & Portland Wt. of Water/yd3 = (Wt. of cement X w/cm ratio) Wt. of Water/yd3 = 658 lbs. X 0.41 = 270 lbs./yd3 Convert Wt. to Gal. (1 gal. = 8.33 lbs.) Gal./yd3 = 270 lbs. / 8.33 lbs./gal. = 32.4 gal./yd3 Wt. of Cement & Fly Ash/ yd3 : Wt. of Fly Ash/ yd3 = 658 lbs. X 20% Replacement Wt. of Fly Ash/ yd3 = 132 lbs. Wt. of Cement = 658 lbs. 132 lbs. = 526 lbs. Calculating for Air Content Cement526 lbs. Fly ash132lbs. Water270lbs. (32.4 gals) Specification allows 2.0 - 4.0%;Target = 3.0%! 1.0% of entrained/entrapped air represents 0.27 cubic feet of volume Calculating for Air Content Example : 3.0 %x 27 cubic feet = 0.81 cf Many times mix designs over 4,000 psi will not have any entrained air, however you should still design for entrapped air. Typical Entrapped Air Content Large Rock Mix (#57 & # 67 Stone)= 1.5 % Small Rock Mix (# 89 Stone) = 2.5 % Calculate Volumes of Known Materials What is the SG of the fly ash and aggregates in this mix. These SGs will vary from source to source.The SG of mineral admixtures (fly ash, slag, etc.) can be found on the mill certification. The aggregate SG is the SSD Bulk Specific Gravity and can be obtained from the mine. Specific Gravity We will use the following specific gravities for the fly ash, coarse and fine aggregates Fly ash2.40 Coarse aggregate2.50 Fine aggregate2.63 Calculate Volumes of Known Materials Abs. Vol. =Weight of Material (SG X Unit Wt. of Water) MaterialEquationAbs. Vol. (cf) Cement 526/(3.15 X 62.4) 2.68 Fly Ash 132/ (2.40 X 62.4) 0.88 Water 270/(1.0 X 62.4) 4.33 Air 27 X 0.03 0.81 Total8.70 The Paste The paste is composed of Portland cement, fly-ash, water, and entrapped air or entrained air. The paste volume = 8.70 cubic feet. This means the rest of the volume (68%) will be made up of our coarse and fine aggregates. Calculate Volume of Aggregates First, determine the available volume for aggregates: You know there is 27.0 cf in a cubic yard You know there is 8.70 cf of paste 27 cf - 8.70 cf= 18.30 cf for Aggregates Calculating for the Aggregates Now we must determine our Rock-to-Sand Ratio or our Coarse-to-Fine Ratio. This will be determine by many factors Gradation Fineness Modulus (FM) Method of Placement Workability Needs Strength Requirements Typical Rock-to-Sand Ratios These ratios can vary. The biggest influencer is the FM of the fine aggregate The higher the FM or coarser the fine aggregate, the higher the percent of fine aggregate. Type of MixRock-to-Sand Ratio Pre-stress65:35 Flatwork60:40 6 Boom Pump60:40 2 Pump Mix45:55 High-Strength65:35 Calculating for the Aggregates We will use a rock-to-sand ratio of 60:40 Remember you have 18.30 cubic feet of aggregates Coarse Aggregate: 18.30 cf X 60% = 10.98 cf Fine Aggregate: 18.3 cf X 40% = 7.32 cf Specific Gravity We will use the following specific gravities for the fly ash, coarse and fine aggregates Fly ash2.40 Coarse aggregate2.50 Fine aggregate2.63 Remember! Calculating for the Aggregates Determine the weight of coarse and fine aggregates: Abs. Vol. =Weight of Material (SG X Unit Wt. of Water) Wt. of Matl = Abs. Vol. X (SG X Unit Wt. of Water) Wt. of Coarse = 10.98 X (2.50 X 62.4) = 1713 lbs. Wt. of Fine = 7.32 X (2.63 X 62.4) = 1201 lbs. The Mix Design Cement526lbs2.68cf Fly Ash132 lbs0.88cf Water270 lbs4.33cf Entrained Air3.0%0.81cf Coarse Agg.1713lbs 10.98 cf Fine Agg. 1201 lbs 7.32cf 3842 lbs27.00 cf Theoretical Unit Weight = Total Wt/cfTheoretical Unit Weight = 3842/27 =142.3 pcf Verification Remember a cubic yard of concrete has 27 cf Your mix design should add up to 27. Also, remember the SG of your fly ash, coarse aggregate, and fine aggregate can change from source to source. Make sure you verify prior to doing your calculation. Adjustment for Aggregate Moisture Remember the aggregates are designed at the SSD condition. We must take a moisture content and determine the free moisture. This can be oven dry, or towel dry method. If you utilize the oven dry method you must account for the absorption of the aggregate. Moisture Conditions Total Moisture = Free (surface) Moisture + Absorbed Moisture Determining Moisture Content Oven or Hot Plate (Most accurate method) Results in total moisture, need to adjust for absorption performed on fine or coarse aggregateChapman Flask Determines free moisture volumetrically Calculate or use chart, need to know specific gravity (dependent on sand cleanliness) Speedy Moisture Meter Determines fine aggregate free moisture, based on gas produced from reaction Use manufacturers adjustment value (less accurate) Towel Dry Performed on coarse aggregate to determine free moisture by weight. (dependent on aggregate cleanliness and less accurate) Moisture Corrections Coarse Aggregates (CA) Free Moisture of CA = 1.5% 1701 lbs X 0.015=26 lbs H2O Fine Aggregates Free Moisture of FA = 4.2% 1201 lbs X 0.042 = 50 lbs H2O Batch water must be adjusted for the free water in the aggregates Adjusted Batch Water = Batch water CA Free water FA Free water Adjusted Batch Water = 270 lbs 26 lbs 50 lbs = 194 lbs Any Questions