Concrete Mix Design ปฏ ภาคส วนผสมของคอนกร ต. 11 December 2003

Transcription

1 Concrete Mix Design ปฏ ภาคส วนผสมของคอนกร ต 11 December 2003

2 Introduction The selection of concrete proportions involves a balance between reasonable economy and requirements for placeability, strength, durability, density, and appearance.

3 Basic Relationship Concrete proportions must be selected to provide necessary placeability, density, strength, and durability for the particular application. In addition, when mass concrete is being proportioned, consideration must be given to generation of heat.

4 Placeability (การเทได ) Workability (ความสามารถท างานได ) :The property of the concrete that determines its capacity to be placed and consolidated properly and be finished without harmful segregation.

5 Consistency (ความข นเหลว) Consistency: It is the relative mobility of the concrete mixture, and measured in terms of the slump; the greater the slump value the more mobile the mixture.

6 Strength (ก าล งต านทาน) Strength: The capacity of the concrete to resist compression at the age of 28 days.

7 Water-Cement Ratio (อ ตราส วนระหว างน าก บป นซ เมนต ) Water-cement (w/c) or water-cementitious (w/(c+p)) ratio: Defined as the ratio of weight of water to the weight of cement, or the ratio of weight of water to the weight of cement plus added pozzolan. Either of these ratios is used in mix design and considerably controls concrete strength.

8 Durability (ความคงทน) Durability: Concrete must be able to endure severe weather conditions such as freezing and thawing, wetting and drying, heating and cooling, chemicals, deicing agents, and the like. An increase of concrete durability will enhance concrete resistance to severe weather conditions.

9 Density (ความหนาแน น) Density: For certain applications concrete may be used primarily for its weight characteristics. Examples are counterweights, weights for sinking pipelines under water, shielding from radiation, and insulation from sound.

10 Generation of heat (การคายความร อน) Generation of heat: If the temperature rise of the concrete mass is not held to a minimum and the heat is allowed to dissipate at a reasonable rate, or if the concrete is subjected to severe differential or thermal gradient, cracking is likely to occur.

11 Background of data To the extent possible, selection of concrete proportions should be based on test data or experience with the materials actually to be used. Where such background is limited or not available, estimates given in this recommended practice may be employed.

12 The following information for available materials will be useful: Sieve analyses of fine and coarse aggregates. Unit weight of coarse aggregate. Bulk specific gravities and absorption of aggregates. Mixing-water requirements of concrete developed from experience with available aggregates.

13 Relationship between strength and water-cement ratio or ratio of water-to-cement plus other cementitious materials Specific gravity of Portland cement and other cementitious materials, if used Optimum combination of coarse aggregates to meet the maximum density grading for mass concrete Estimate of proportions of mix for preliminary design

14 Procedure The procedure for selection of mix proportions given below is applicable to normal weight concrete. Some or all of the following specifications are required; maximum watercement or water-cementitious material ratio, minimum cement content, air content, slump, maximum size of aggregate, strength, and admixtures

15 Steps in calculating proportions Step 1. Choice of slump (Table A ) Step 2. Choice of nominal maximum size of aggregate Step 3. Estimate of mixing water and air content (Table A ) Step 4. Selection of water-cement ratio (Table A (a)) for durability (Table A (b)

16 Step 5. Calculation of cement content Step 6. Estimation of aggregate content (Table A ) Step 7. Estimation of fine aggregate content Weight Method Absolute Volume Method Step 8. Adjustments for aggregate moisture

17 Step 9. Trial batch adjustments The estimated mixing water to produce the same slump as the trail batch will be equal to the net amount of mixing water used divided by the yield of the trial batch in m 3. If slump of the trail batch was not correct, increase or decrease the reestimated water content by 2 kg/ m 3 of concrete per each increase or decrease of 10 mm in slump desired.

18 To adjust for effect of incorrect air content in a trial batch of air-entrained concrete on slump, reduce or increase the mixing water content of A by 3 kg/ m 3 of concrete for each 1 % by which the air content is to be increased or decreased from that of the trial batch.

19 Example Problem Concrete is required for a portion of a structure which will be below ground level in a location where it will not be exposed to severe weathering or sulfate attack. Structural considerations require it to have an average 28-day compressive strength of 24 MPa with slump of 75 to 100 mm. The coarse aggregate has a nominal maximum size of 37.5 mm and dryrodded mass of Other properties of the ingredients are: cement-type I with specific gravity of 3.15; coarse aggregate-bulk specific gravity 2.68, absorption 0.5%; fine aggregate-bulk specific gravity 2.64, absorption 0.7%, and fineness modulus 2.8.