BUILDING MATERIAL ENGINEERING

CONCRETE MIX DESIGN

Cement Concrete Mix Design means, determination of the proportion of the concrete ingredients i.e. Cement, Water, Fine Aggregate, Coarse Aggregate which would produce concrete possessing specified properties such as work ability, strength and durability with maximum overall economy.

METHODS OF CONCRETE MIX DESIGN

1.I.S. Method

2.British Method

3.A.C.I. Method etc.

These Methods are based on two basic assumptions Compressible Strength of Concrete is governed by its Water-Cement Ratio Work ability of Concrete is governed by its Water Content

DATA REQUIRED FOR CONCRETE MIX DESIGN


Grade of Concrete
eg: RCC-M30-A20

Slump required in mm
eg: 25 – 75 mm

Degree of Site Control
eg: Good

Type of Exposure
eg: Moderate

Grade of Cement
eg: OPC 43 Grade

DESIGNING AND PROPORTIONING NORMAL CONCRETE MIXTURES

(ACI 211)



A Properly Proportioned Concrete Mix Should Posses These Qualities.

Acceptable Workability of Freshly Mixed Concrete

Durability, Strength and Uniform Appearance of the Hardened Concrete

Economy
ACI 211


SLUMP



Normally specified

Choice based type of construction shown on Table 9-6


WATER CONTENT


Approximation based on slump, maximum size of the coarse aggregate, and type of concrete (air-entrained, or non air-entrained)- see Table 9-5

Air contents for AE concrete are normally specified but can also be determined by exposure conditions based on Table 9-5



WATER-TO-CEMENT RATIO


Concrete strength is specified by the design engineer.

Based on specified concrete strength and type of concrete (AE or non AE)– see Table 9-3.

W/C ratio may be modified due to certain environmental conditions–see Tables 9-1 and 9-2.

Concrete strength may also be modified due to special conditions-see Table 9-11

CEMENTITIOUS CONTENT


Cement Content = Recommended water content (Table 9-5) / the recommended water-to-cement ratio (Table 9-3 or 9-1 or 9-2)

Cementitious Material = Portland cement, fly ash, blast furnace slag, and silica fume.

COARSE AND FINE AGGREGATE CONTENTS

How much does 1 cy of Portland cement concrete (PCC) weigh?

What is the volume of 1 cy of PCC?

COARSE AGGREGATE

Coarse aggregate size specified

Determination of VOLUME of CA recommended per unit volume of concrete

Volume based on maximum size of CA and fineness modulus of sand – Table 9-4

Weight of CA = volume of CA x dry rodded unit weight of CA x 27 cf.


VOLUME FORMULAS


Vol. of water = wt. of water/62.4 pcf

Volume of Portland cement = wt. of PC/(3.15 x 62.4)

Vol. of CA = wt. of CA/(SG(ca) x 62.4)

Volume of Air = (%Air content x 100)/27


FINE AGGREGATE


Fine Aggregate Content = 27 cu. ft. - Volume of water - Volume of Cement - Volume of Coarse Aggregate –Volume of Air

Weight of FA = volume of FA x specific gravity of FA x 62.4


DRY WEIGHTS VS. ACTUAL WEIGHTS


Batch weights calculated from volumes assume dry CA and dry FA.

Actual weight of CA and FA are NOT dry

Adjustment based on moisture contents for both CA and FA.

Don’t forget about correcting for water content also.


TRIAL BATCH


Slump should be within ¾-inch of maximum design value.

Air content should be within 0.5% of maximum design value.

Avg. required comprehensiveness strength (psi)
@ 28 days must be equal to or greater than:

f’(cr) = f’(c) + 1.34S
f’(cr) = f’(c) + 2.33S -500