PROPORTIONING CONCRETE MIXES

Transcription

1 TOKYO INSTITUTE OF TECHNOLOGY Department of Civil Engineering ATCE ADVANCED TOPICS IN CIVIL ENGINEERING Professor Kamran M. Nemati PROPORTIONING CONCRETE MIXES

2 ATCE Advanced topics in Civil Engineering Proportioning Concrete Mixes Page of 6 Tokyo Institute of Technology Department of Civil Engineering Professor Kamran M. Nemati First Semester 2006 PROPORTIONING CONCRETE MIXES I. INTRODUCTION In designing concrete one attempts to optimize the cost for a given level of strength and durability. This translates into trying to achieve the lowest possible water and cement contents with the largest fraction of coarse aggregate (CA) which in turn has the maximum quantity of fine aggregate (FA) filling the interstitial spaces. This leads the designer into specifying: ) as stiff mixes as practical under the proposed concrete mixing and casting conditions; 2) maximum permissible size of coarse aggregate; and ) adequately sized and properly proportioned FA and CA. II. TRIAL METHOD OF PROPORTIONING This is the simplest approach to mix design. It is as follows: ) Select an appropriate w/c ratio. [Strength & Durability /(w/c)] 2) Make a small trial mix with the selected w/c ratio & obtain the desired wet consistency (i.e., slump & workability). ) It often proves useful to make several trial batches to achieve the most economical mix with the desired properties. The ACI Standard 2. is a Recommended Practice for Selecting Proportions for Concrete. The procedure is as follows: Step. Choice of slump. If slump is not specified, a value appropriate for the work can be selected from Table 9- which is reproduced from the text book below *, (note that the table numbers are given from the text book rather than the ACI standard). * P.K. Mehta and P.J.M. Monteiro, Concrete Microstructure, Properties, and Materials, McGraw Hill, rd Edition, 2006.

3 ATCE Advanced topics in Civil Engineering Proportioning Concrete Mixes Page 2 of 6 TABLE 9- - RECOMMENDED SLUMPS FOR VARIOUS TYPES OF CONSTRUCTION Slump (in.) Types of Construction Maximum* Minimum Reinforced foundation walls and footings Plain footings, caissons, and substructure walls Beams and reinforced walls Building columns Pavements and slabs Mass concrete * May be increased -in. for methods of consolidation other than vibration. Step 2. Choice of maximum size of aggregate. Large maximum sizes of aggregates produce less voids than smaller sizes. Hence, concretes with the larger-sized aggregates require less mortar per unit volume of concrete, and of coarse it is the mortar which contains the most expensive ingredient, cement. Thus the ACI method is based on the principle that the MAXIMUM SIZE OF AGGREGATE SHOULD BE THE LARGEST AVAILABLE SO LONG IT IS CONSISTENT WITH THE DIMENSIONS OF THE STRUCTURE. In practice the dimensions of the forms or the spacing of the rebars controls the maximum CA size. ACI 2. states that the maximum CA size should not exceed: ) one-fifth of the narrowest dimension between sides of forms, 2) one-third the depth of slabs, ) /4-ths of the minimum clear spacing between individual reinforcing bars, bundles of bars, or pre-tensioning strands. Special Note: When high strength concrete is desired, best results may be obtained with reduced maximum sizes of aggregate since these produce higher strengths at a given w/c ratio. Step. Estimation of mixing water and air content. The ACI Method uses past experience to give a first estimate for the quantity of water per unit volume of concrete required to produce a given slump.

4 ATCE Advanced topics in Civil Engineering Proportioning Concrete Mixes Page of 6 In general the quantity of water per unit volume of concrete required to produce a given slump is dependent on the maximum CA size, the shape and grading of both CA and FA, as well as the amount of entrained air. It is interesting to note that the water content is not greatly affected by the amount of cement in the concrete mix proportions. The approximate amount of water required for average aggregates is given in Table 9-2. TABLE APPROXIMATE MIXING WATER AND AIR CONTENT REQUIREMENTS FOR DIFFERENT SLUMPS AND MAXIMUM SIZES OF AGGREGATES Water, lb./yd of concrete for indicated maximum sizes of aggregate Slump, in. /8 in.* ½ in.* ¾ in* in.* ½ in.* 2 in.* in.* 6 in.* Non-air-entrained concrete to 2 to 4 6 to 7 More than 7* Approximate amount of entrapped air in non-airentrained concrete, percent Air-entrained concrete to 2 to 4 6 to 7 More than 7* Recommended average total air content, percent for level of exposure: Mild exposure Moderate exposure Severe exposure **.5** 4.5** **.0** 4.0** * These quantities of mixing water are for use in computing cement factors for trial batches. They are maxima for reasonably well-shaped angular coarse aggregates graded within limits of accepted specifications. The slump values for concrete containing aggregate larger than ½" are based on slump tests made after removal of particles > ½" by wet-screening. (Note that the two footnotes were not reproduced in the textbook.)

5 ATCE Advanced topics in Civil Engineering Proportioning Concrete Mixes Page 4 of 6 Step 4. Selection of water/cement ratio. The required water/cement ratio is determined by strength, durability and finishability. The appropriate value is chosen from prior testing of a given system of cement and aggregate or a value is chosen from Table 9- and/or Table 9-4. TABLE 9- - RELATIONSHIP BETWEEN WATER/CEMENT RATIO AND COMPRESSIVE STRENGTH OF CONCRETE Compressive strength at 28 days, psi Water/cement ratio, by weight Non-air-entrained Air-entrained concrete concrete TABLE MAXIMUM PERMISSIBLE WATER/CEMENT RATIOS FOR CONCRETE IN SEVERE EXPOSURES Type of Structure Structure wet continuously or fequently exposed to freezing & thawing* Structure exposed to seawater Thin sections (railings, curbs, sills, ledges, ornamental work) & sections with less than -inch cover over steel All other structures * Concrete should also be air-entrained.

6 ATCE Advanced topics in Civil Engineering Proportioning Concrete Mixes Page 5 of 6 Step 5. Calculation of cement content. The amount of cement is fixed by the determinations made in Steps and 4 above. Step 6. Estimation of coarse aggregate content. The ACI Method uses the principle that The same maximum size and grading will produce concrete of satisfactory workability when a given volume of coarse aggregate, on a dry-rodded basis, is used per unit volume of concrete. This has been incorporated in the ACI standard in Table 9-5. The most economical concrete will have as much as possible space occupied by CA since it will require no cement in the space filled by CA. TABLE VOLUME OF COARSE AGGREGATE PER UNIT OF VOLUME OF CONCRETE Volume of dry-rodded coarse aggregate* per unit volume of concrete for Maximum size different fineness moduli of sand of aggregate (in.) /8 /2 /4 ½ * Volumes are based on aggregates in dry-rodded condition as described in ASTM C29 Unit Weight of Aggregate. These volumes are selected from empirical relationships to produce concrete with a degree of workability suitable for usual reinforced construction. For less workable concrete such as required for concrete pavement construction they may be increased about 0 percent. For more workable concrete, such as may sometimes be required when placement is to be by pumping, they may be reduced up to 0 percent. The ACI method is based on large numbers of experiments which have shown that for properly graded materials, the finer the sand and the larger the size of the particles in the CA, the more volume of CA can be used to produce a concrete of satisfactory workability.

7 ATCE Advanced topics in Civil Engineering Proportioning Concrete Mixes Page 6 of 6 In practice Table 9-5 can be used to estimate the volume of CA, on a dryrodded unit volume basis, which can be used in a mix. Thus for a one cubic yard mix one multiplies the number in Table 9-5 by 27 to obtain the volume fraction, on a dry-rodded unit weight basis, in the mix. This volume must then be converted into a weight fraction by using the dry-rodded unit weight of the CA being used in the design. DO NOT USE THE REGULAR BULK SPECIFIC GRAVITY. This dry weight is then used in the next step. Step 7. Estimation of Fine Aggregate Content. At the completion of Step 6, all ingredients of the concrete have been estimated except the fine aggregate. Its quantity can be determined by difference if the absolute volume displaced by the known ingredients-, (i.e., water, air, cement, and coarse aggregate), is subtracted from the unit volume of concrete to obtain the required volume of fine aggregate. Then once the volumes are know the weights of each ingredient can be calculated from the specific gravities. A second, less exact method of estimating the fine aggregate content is discussed in the text book under the weight method. In this course we will only use the more exact absolute volume method. At this stage of the process there is a complete description of the mix proportions, but the moisture content is assumed to be SSD (saturatedsurface dry) for the FA and air-dry for the CA. Step 8. Adjustments for Aggregate Moisture. Aggregate quantities actually to be weighed out for the concrete must allow for moisture in the aggregates. Usually the air-dry condition for the CA is close enough for use in laboratory, but the FA is often 2% or % above or below SSD. This means that a correction must be made before a laboratory batch of concrete is made. Step 9. Trial Batch Adjustments. The ACI method is written on the basis that a trial batch of concrete will be prepared in the laboratory, and adjusted to give the desired slump, freedom from segregation, finishability, unit weight, air content and strength.