According to the properties of raw materials and technical requirements for concrete, the consumptions of all the raw materials can be calculated preliminarily by formulas and tables to get the mix proportion for trail mixture.
(1) Determination of the Confected Strength ( A,,,)
Based on the standard value of designed strength ( L,,k ) and the guarantee rate of 95%, the confected strength of concrete can be defined by the formula (5.1 1):
(2) Preliminary Determination of the water-cement Ratio
According to the actual strength of cement that has been tested f, (or the strength grade of cement At ), the aggregate types and the required confected strength of concrete LU,,, the water-cement ratio can be defined by the empirical formula of concrete strength (5.3):
Converted into:
(5.12) And also based on the using conditions of concrete, the maximum water-cement ratio can be found out' in Table 5.13. If the calculated ratio is bigger than the regulated maximum ratio, the prescribed one prevails.
124 Building materials in civil engineering
Table 5.13 The Limits of Maximum Water-cement Ratio and Minimum Cement Consumption
Dry Condition
I
Components for normal rcsidcntial 01
Maximum Water-Cement Ratio
Vithout rcczing ijury
Vith reezing ijury
Minimum Cement Consumption
mid indoor componcnts
@ Outdoor components
@ The components in non- corrosive soil and/or water
0 Outdoor components surcring freezing injury
@ The components in non- corrosive soil and/or water suffering freezing injury (3) High humid indoor components suffering freezing injury Indoor and Outdoor Humid Condition
with Freezing Injury and Dcicer
components surering freczing injury and
(kg/m’]
leinforccc Concrete
160
- Plain
:oncrett
-
00 Leinforcec
Concrete
.65
Conditions Plain
Concrete l o Legulation!
Prestressed Concrete
300
‘restressec Concrete
.60 ollicial
rooms
.70 .60 .60 25 80 300
llumid Condition
.55 50
-
00 .55 300
S O
80
00 300
ldeicer
Note: 1) When part of cement replaced by active ad ctures, the maximum water-cement ratio and minimum cement consumption in this table are the ratio and the consumption bcfore bcing rcplaced.
2) The prcparation of concrctc of C I5 or below cannot be limited by this table.
5 Concrete 125
(3) Determination of Water Consumption of lm3 Concrete ( mwo)
The water consumption can be selected from Table 5.14, according to the slump, the known aggregate types and the maximum particle diameter required by construction.
Table 5.14 The Selection Table of Unit Water Consumption of Concrete (kg/m3) (JG 555-2000)
I Maximum Gravel Particle Size I Maximum Crushed Stone Particle
Note: 1) The water consumptions in this table are the average values of medium sand; if fine sand is used, the water consumption of Im’ concrete can be added by 5-IOkg; as for coarse sand, it can be reduced by 2) Water consumption should be adjusted accordingly when concrete is mixed with various additives or 3) This table does not apply to the concrete whose water-cement ratio is less than 0.4 or more than 0.8 5-IOkg.
admixtures.
and the concrete formed by special moulding process.
(4) Calculation of Unit Cement Consumption of Concrete ( mco)
According to the cement quantity of lm3 concrete and identified water-cement ratio (g) , the cement consumption can be calculated by the following equation:
C m,,
m,, =-xmw, ormcO =-
W WIG (5.13)
Based on the application conditions of structures and requirements of durability, the minimum cement consumption of lm3 concrete can be found out in Table 5.13. Finally, the bigger value of the two results can be determined as the cement quantity of lm3 concrete.
( 5 ) Determination of Sand Percentage ( p,)
The reasonable sand percentage should be determined by slump, cohesion and water retention of concrete mixtures. Generally, the reasonable rate should be found out through experiments or based on the selection experiences of makers. In the absence of experience, the value can be selected from Table 5.15 on the basis of aggregate types and water-cement ratios.
126 Building materials in civil engineering
Water-Cement Ratio ( w/c)
0.40 0.50 0.60 0.70
Table 5.15 Selection Table of Sand Percentage of Concrete (%) (JCJ/T55-2000)
Maximum Gravel Particle Sim Maximum Crushed Stone Particle Size
(mm) (mm)
10 20 40 16 20 40
26-32 25-31 24-30 30-35 29-34 27-32
30-35 29-34 28-33 33-38 32-37 30-35
33-38 32-37 31-36 36-41 35-40 33-38
36-41 35-40 34-39 39-44 38-43 36-41 *
m,To x 100% = ps
I m,o + mgo (5.14)
In this equation: mcp is the assumed apparent density of concrete mixtures (kg/m3); mc,, value can be selected within2350-2450kdm’ in accordance with the apparent density and particle size of aggregates and the strength grades of concrete.
2) Volume Method. Assumed that the volume of concrete mixtures equals to the sum of the absolute volume of all the components and the volume of the air contained in mixtures, the consumption of all the mixing materials can be identified as follows:
In this formula: p, and p, are the densities of cement and water respectively p, and pg are the apparent densities of sand and stone respectively (kg/m3);
c\! is the percentage of air content in concrete; a =I ' i f air-entraining agent is not used.
Solve the above equation (5.14) and can get mso and mso .
Through the above six-step calculation, the total consumptions of water, cement, sand and stone can be calculated to get the preliminary mix proportion.
However, most of the above calculations are obtained by empirical formulas or resources. And the concrete prepared by those data may not meet the actual demands. Thus, the mix proportion should be tested, adjusted and identified.
(dcm3);