Concrete Mix Design

Concrete Mix Design

1. Requirements: 

Assume a reinforced concrete building designed with a specific grade, with bearing walls located in definite location. Design the concrete mix in accordance to the following characteristics:

1. Specified concrete (grade/class): C 16/20.

2. Characteristics of concrete element

a) type of structural member:4N- structural walls

b) minimum section (dimension) of concrete element. b = 200 mm.

c) concrete cover of reinforcing bars. c == 15 mm.

d) minimum spacing of reinforcing bars. D = 75mm.

3. Conditions of exposure and environment conditions: Marine Environment (3N+1).

4. Work conditions: Normal conditions, using giant panels.

5. Transport and placement of concrete: Transit mix trucks and concrete pumps (2N)

6. Moisture content in aggregates:

- F.A. (fine aggregates) (0-7 mm): u_s == 2%;

- C.A. (coarse aggregates) (7 - 71 mm): u_g = 1%.

7. Grade of homogeneity: II

B. Quality mix proportions of constituents

1. Consistency of concrete:

From annex 1, in accordance with the type of structural member (structural wall), transport equipment (transit mix truck), technology of concrete placing (pump) and type of concrete (reinforced), results from item no. 3 the recommended consistency T3/T4 (slump 100 20 mm).

2. Minimum cement content:

From annex 2., in accordance with the environment conditions (Marine Environment), result the conditions of exposure 4b1. From annex 3, results the minimum cement content of 400 kg/m³, according to the type of concrete (reinforced) and the conditions of exposure (4.b.1).

3. Aggregates:

a) Type of aggregates: In accordance with the class of concrete (C 20/16) we will use

natural riverbed aggregates with the relative density r_ag 2,7 kg/dm³ (see annexes 13) T3/T4=200.

b) Maximum size of aggregate:

Depends on the following elements: type of concrete member (structural walls), minimum length of section being formed (b = 200 mm), minimum distance between bars (D == 75 mm), transport and placement of concrete and minimum concrete cover of reinforcing bars (c = 15 mm):

Damx< 1.3xc=19.5mm

d max < 1/4 x b = 1/4 x 200 = 50 mm ;

d max < D - 5 mm = 75 - 5 == 70 mm ;

pumpable dmax =31 mm.

According to section 2.3., because the conditions of exposure (marine environment), the concrete cover of reinforcing bars conditions the maximum size of aggregates as being 20mm( with a min air entrained of 4%). All these conditions have to be simultaneously respected, results dmax <19,5mm. We chose dmax = 20mm.

c) Gradation:

From table I-4 results the grading zone (I) according to the minimum cement content

400 kg/dm³. Knowing the grading zone (I) and the maximum size of aggregate 31mm, from

annex 5 results the following upper and lower grading limits:

sieve 0 - 0,2mm recommended 10-15% we chose 13%;

sieve 0,2 - 1mm recommended 30 - 40% we chose 34%;

sieve 1-3 mm recommended 50 - 60% we chose 54%;

sieve 3-7 mm recommended 70 - 80% we chose 76%;

sieve 7-20 mm recommended 95 - 100% we chose 100%;

4. Cement:

According to environment conditions (marine environment) results the type of cement form table I-11, conditioned by the type and section of concrete element (structural wall) and concrete class (C 20/16). Recommended cement type I - 32,5.

5. Water-cement maximum ratio:

From table I-3, according to the type of concrete (reinforced) and the exposure conditions (4.b.1) results the water-cement maximum ratio (A/C maximum = 0,45).

C. Quantity mix proportions of constituents

1. Water:

The estimated water quantity needed (A) is determined according to the class of concrete (C 20/16) and the consistency (T3/T4).

From table I-13 results:

A=200l/m³

This quantity will be adjusted with a coefficient (c= 1) determined by the maximum aggregate size 31mm, as follows:

A = A x c == 200 x 1.05 = 210 l/m³

2. Water-cement ratio:

From annex 14, according to the concrete grade (C 20/16) and cement grade (32,5), concrete homogeneity grade II, results the water: cement ratio A/C == 0,60. The value will be adjusted with a coefficient for crushed aggregates if it is necessary and the resulting value of A/C will be compared with the maximum value calculated of A/C, determined at section 2.5. From the two values the minimum one will be chosen A/C minim == 0,45.

3. Cement:

The cement quantity is calculated as follows: [kg/m³] > minimum cement contents 400 kg/m³ (see section B.2.). From the two values the maximum one will be chosen C = 467 kg/m³.

4. Aggregates:

The total amount of dry aggregates will be calculated as follows:

Ag = r_ag x (1000 - C/rc A/r_a - r) [kg/m³]

Ag ⁼ 2.7 x (1000 - 467/3 - 210/1 - 20) = 1658 [kg/m³]

Where:

r_ag = relative density of aggregates (2.7 kg/dm³);

r_c = relative density of cement (3.0 kg/dm³);

p = void parameter (approx. 20 dm³/m³), when using air-entrained plasticizers, (see annex 13).

5. Gradation of aggregate:

According to the lower and upper limits of gradation recommended (see section 2.3.c), it can be determined the right amount of each grade of fine and coarse aggregate (this determination can be made by plotting the cumulative percent passing by mass through each sieve, see annex 5). The amount of aggregate for each grade is found as follows:

Were:

Ag = amount of aggregates (kg);

pi = percent passing by mass through sieve 'i';

pi-l= percent passing by mass through sieve 'i-1';

First correction (iteration):

sieve mm; 13/100 x kg/m³

sieve 1mm; (34-13)/100 x 1658 = 348 kg/m³ ;

sieve 3mm; (54-34)/100 x 1658= 331 kg/m³;

sieve -7mm; (76-54)/100 x 1658 = 365 kg/m³;

sieve 20mm; (100-76)/100 x 1658 = 398 kg/m³ ;

Total: 1658 kg/m³

compared with the calculated value of 1658kg/m³

6. Adjustment of water quantity:

The right adjustment of water will be found according to the exact free moisture of the aggregates, as follows:

Where:

Agi = amount of aggregate form sieve 'i' (kg);

ui = free moisture of sieve 'i' (%);

n = total numbers of sieves.

A* = A' - DA (l/m³)

The free amount of moisture form fine aggregates (2%), is calculated as follows:

The free amount of moisture form coarse aggregates (1%), is calculated as follows:

The total amount of free moisture is calculated as follows:

DA=DAn +DAp =25,2+3,98=29,18 l/m³

Adjusted amount of water: A* = A' - AA = 200 - 27,43 = 180,82 l/m³

7. Adjustment of total amount of aggregates by sieve sizes:

The of total amount of aggregates by sieve sizes, is found as follows:

Where:   A_gi - amount of aggregate form sieve i (kg);

ui = free moisture of sieve 'i' (%).

sieve 0 - 0,2 mm;   216 x (1+ 2/100) =220 kg/m³;

sieve 0,2 - 1mm;   348 x (1+ 2/100) = 355 kg/m³;

sieve 1 - 3mm; 331 x (1+ 2/100) =338 kg/m³ ;

sieve 3 -7mm;  365 x (1+ 2/100 ) =372 kg/m³

sieve 7 - 16mm;   398 x (1+ 1/100) =402 kg/m³;

8. Adjustment of total amount of aggregates:

The of total amount of aggregates, is found as follows:

(kg/m³)

Where:

Ag*_i = adjusted amount of aggregate form sieve 'i' (kg);

8. Total weight of one cubic meter of concrete:

1687+467+210=2364 kg/m3