CIP 8 - Discrepancies in Yield - Centre Concrete

1 CIP 8 - Discrepancies in YieldWHAT is Concrete Yield ? Concrete Yield is defined as the volume of freshly mixedconcrete from a known quantity of ingredients. Readymixed Concrete is sold on the basis of the volume offresh, unhardened Concrete -in cubic yards (yd3) or cu-bic meters (m3) as discharged from a truck basis for calculating the volume is described in theASTM C 94, Specification for Ready Mixed Con-crete. The volume of freshly mixed and unhardenedconcrete in a given batch is determined by dividing thetotal weight of the materials by the average unit weightor density of the Concrete determined in accordancewith ASTM C 138.

2 Three unit weight tests must bemade, each from a different C 94 notes: It should be understood that thevolume of hardened Concrete may be, or appears tobe, less than expected due to waste and spillage,over-excavation, spreading forms, some loss of en-trained air, or settlement of wet mixtures, none ofwhich are the responsibility of the , the volume of hardened Concrete in place maybe about 2 percent less than its volume in a freshlymixed state due to reduction in air content, settlementand bleeding, decrease in volume of cement and water,and drying Yield complaints concern a perceived or real de-ficiency of Concrete volume.

3 Concerns about yieldshould be evaluated using unit weight measurementsto calculate the Yield . Apparent under-yieldoccurswhen insufficient Concrete is ordered to fill theforms and to account for contingencies discussed be-low. If unit weight and Yield calculations indicate anactual under- Yield it should be Concrete shortages are sometimes causedfor the following reasons:a. Miscalculation of form volume or slab thicknesswhen the actual dimensions exceed the assumed di-mensions by a fraction of an inch. For example, a 1/8-inch (3-mm) error in a 4-inch (100-mm) slab wouldmean a shortage of 3 percent or 1 yd3 in a 32-yd3 (1m3 in a 32-m3) Deflection or distortion of the forms resulting frompressure exerted by the Irregular subgrade, placement over granular fill, andsettlement of subgrade prior to Over the course of a large job, the small amounts ofSample 3 truck mixersRun unit weight on each sampleASTM C 138 - Test for Unit WeightFill unit weight container in 3 layers;Rod each layer 25 times; tap sides with mallet;Strike off with flat plate.

4 Clean outside surfaces and weigh)(mftVolume,Bucket (kg)lb weight,concreteNet )(kg/mlb/ft,Unit )(kg/mlb/ft,3 UW3) UW2(UW1 Unit 3lb/ft Unit Wt., ,ofWeight ,Batch u 33kg/m Unit Wt., ,ofWeight mYield,Batch Sample 3 truck mixersRun unit weight on each sampleASTM C 138 - Test for Unit WeightFill unit weight container in 3 layers;Rod each layer 25 times; tap sides with mallet;Strike off with flat plate; Clean outside surfaces and weigh)(mftVolume,Bucket (kg)lb weight,concreteNet )(kg/mlb/ft,Unit )(kg/mlb/ft,3 UW3) UW2(UW1 Unit 3lb/ft Unit Wt., ,ofWeight ,Batch u 33kg/m Unit Wt., ,ofWeight mYield,Batch WHY do Yield Problems Occur?

5 1979, 1991, AND 2000 Follow These Rules to Avoid Under-Yield1. Measure volume needed accurately. Reevaluate required volume towards the end of the pour and informthe Concrete Estimate waste and potential increased thickness order more than required by at least 4 to 10 To check Yield use the ASTM C 138 unit weight test method on three samples from three different loads Yield is the total batch weight divided by the average unit weight or to Prevent Yield Discrepancies ? Concrete returned each day or used in mud sills orincidental over- Yield can be an indication of a problem if theexcess Concrete is caused by too much air or aggre-gate, or if the forms have not been properly in batched weights of ingredients and aircontent in Concrete , within the permitted tolerances,can result in Discrepancies in prevent or minimize Concrete Yield problems:a.

6 Check Concrete Yield by measuring Concrete unitweight in accordance with ASTM C 138 early inthe job. Repeat these tests if a problem arises. Besure that the scale is accurate, that the unit weightbucket is properly calibrated, that a flat plate is usedfor strike off and that the bucket is cleaned prior toweighing. Concrete Yield in cubic feet (m3) is totalbatch weight in pounds (kg) divided by unit weightin pounds per cubic foot (kg/m3). The total batchweight is the sum of the weights of all ingredientsfrom the batch ticket. As a rough check, the mixertruck can be weighed empty and full.

7 The differ-ence is the total batch Measure formwork accurately. Near the end of largepours, carefully measure the remaining volume sothat the order for the last 2 or 3 trucks can be ad-justed to provide the required quantity of can prevent waiting for an extra 1/2 yd3 afterthe plant has closed or the Concrete trucks have beenscheduled for other jobs. Order sufficient quantityof Concrete to complete the job and reevaluate theamount required towards the end of the pour. Dis-posal of returned Concrete has environmental andeconomic consequences to the Concrete Estimate extra Concrete needed for waste and in-creased placement dimensions over nominal dimen-sions.

8 Include an allowance of 4 to 10 percent overplan dimensions for waste, over-excavation andother causes. Repetitive operations and slip formoperations permit more accurate estimates of theamount of Concrete that will be needed. On the otherhand, sporadic operations involving a combinationof Concrete uses such as slabs, footings, walls, andas incidental fill around pipes, etc., will require a biggerallowance for Construct and brace forms to minimize deflectionor For slabs on grade accurately finish and compactthe subgrade to the proper ASTM C 94, Standard Specification for Ready MixedConcrete, American Society for Testing and Materials,West Conshohocken, ASTM C 138, Standard Test Method for Unit Weight, Yield and Air Content (Gravimetric) of Concrete , Ameri-can Society for Testing and Mixed Concrete , Gaynor, NRMCA Publica-tion 186, NRMCA, Silver Spring, Minus Tolerance on Yield , Malisch, W.

9 R. andSuprenant, B. A., Concrete Producer, May for Variation in Concrete Yield , Suprenant, , The Concrete Journal, March Analysis of Factors Influencing Concrete Pave-ment Cost, by Harold J. Halm, Portland Cement Asso-ciation Skokie, Illinois.