ACI Method of Concrete Mix Design - lceted

ACI Concrete Mix Design | American Concrete Institute (ACI) Method

 

Objective

To design a concrete mix by ACI method for crushing strength of 150 mm cubes at 28 days is 20 MPa and slump is 50 mm.

 

Theory and Scope

The absolute volume procedure as recommended by the ACI mix proportioning method is used for determining the proportions of the ingredients for the concrete mixture. The method is suitable for normal and heavyweight concretes having a maximum 28-day cylinder compressive strength of 45 MPa and a workability (slump)  range of 25 to 100 mm; the values generally used in the applications are listed in Table below. The ACI method presumes that the workability of a mix with a given maximum size of well-graded aggregate (i.e., an aggregate with suitable particle shape and the grading) is dependent upon the water content, the amount of entrained air and certain chemical admixtures, but is largely independent of mix proportions, particularly the amount of cementing material. Therefore, ACI has provided a table relating nominal maximum aggregate size, air entrainment and desired slump to the required mixing water quantity.

 

In the ACI method, the bulk volume of coarse aggregate per unit volume of concrete is estimated for the maximum size of coarse aggregate and fineness modulus of sand. The water-cement ratio is determined as in other methods to satisfy both strength and durability requirements. The air content in concrete is taken into account in calculating the volume of fine aggregate.

 

Apparatus

Sieve sets for finding maximum nominal size and fineness modulus of coarse and fine aggregates respectively;  Weighing balance; Trowels; Tamping bar; Moulds, Universal compression testing machine; Graduated  cylinder; Slump cone apparatus and Buckets.

 

Procedure

Step 1: Perform the sieve analysis of both the fine and coarse aggregates to determine the maximum nominal  size of coarse aggregate and fineness modulus of fine aggregate. Determine the unit weight, specific  gravities, and absorption capacities of both the aggregates.

 

Step 2: If the workability in terms of a slump is not specified for a particular job; select an appropriate value  from the Table below

Slump ranges for specific applications (after ACI, 2000)

Types of Construction	Maximum Slump, mm	Minimum Slump, mm
Reinforced foundation walls and footings	75	25
Plain footings, caissons, and substructure walls	75	25
Beams and reinforced walls	100	25
Building columns	100	25
Pavements and slabs	75	25
Mass concrete	75	25

  

The maximum slump may be increased 25 mm for consolidation by hand, i.e., rodding, etc.

 

Step 3: Estimate the mixing water required for non-air-entrained concretes and entrapped air content from the Table below

 

Approximate requirements of mixing water for non-air-entrained concrete (after ACI 211.1 and  ACI 318)

Slump, mm	Mixing water quantity1, kg/m3
	Specified nominal maximum size of aggregate (after CSA A23.1) (mm)
	10	14	20	28	40	562	802	1502
25 – 50 (Stiff-plastic)	207	199	190	179	166	154	130	113
75 – 100 (Plastic)	228	216	205	193	181	169	145	124
150 – 175 (Flowing)	243	228	216	202	190	178	160	-
Approximate amount of entrapped air, per cent
All	3.0	2.5	2.0	1.5	1.0	0.5	0.3	0.2

The table gives the maximum water content for reasonably well-shaped crushed aggregate.

The slump values are based on the slump made after the removal of particles larger than 40 mm by wet screening.

 

Step 4: Determine the target mean compressive strength of concrete at 28 days, ft by using the

f_t = f_ck + k (=1.65)S

 

where f_ck is the characteristic compressive strength at 28 days, and S is the standard deviation.

 

Step 5:  Determine the water-cement ratio from Table below or Fig. below for the target mean strength computed in Step 4.

 

Relationship between water-cement ratio and compressive strength of concrete

Compressive strength at 28 days, MPa	Water-cement ratio by weight
40	0.42
35	0.47
30	0.54
25	0.61
20	0.69
15	0.79

Maximum permissible water-cement ratios for concrete under severe exposure

Type of Structure	Continuously wet structure exposed to frequent freezing  and thawing	Structure exposed  to seawater or  sulphates
Thin section (railings, curbs, sills, ledges, ornamental work) and sections with less than 25 mm cover over steel	0.45	0.40
All other structures	0.50	0.45

 

Step 6: Calculate cement content from the water content and water-cement ratio determined in Steps 3, 4 and 5, respectively, for the required strength and durability.

 

Step 7: Estimate the coarse aggregate content from the Table below for the maximum nominal size of the coarse aggregate and fineness modulus of sand.

 

Step 8: Determine the content of fine aggregate by subtracting the sum of volumes of the coarse aggregate, cement, water and entrained air from the unit volume of concrete.

 

Step 9: Fix the concrete mix proportions for the first trial mix or trial mix no. 1. Make suitable adjustments for moisture in the aggregates.

Relation between water-cement ratio and compressive strength  of concrete

 

Bulk volume of coarse aggregate per unit volume of concrete for different fineness moduli of fine aggregate (Adapted from ACI 211.1)

Nominal maximum  size of aggregate  (after CSA A23.1),  mm	Bulk volume of oven-dry-rodded coarse aggregate, m3 Fineness modulus of fine aggregate
	2.40	2.60	2.80	3.00
10	0.50	0.48	0.46	0.44
14	0.59	0.57	0.55	0.53
20	0.66	0.64	0.62	0.60
28	0.71	0.69	0.67	0.65
40	0.75	0.73	0.71	0.69
56	0.78	0.76	0.74	0.72
80	0.82	0.80	0.78	0.76
150	0.87	0.85	0.83	0.81

 

Step 10: Measure the workability of the trial mix in terms of slump using only as much water as is needed to reach the desired slump (but not exceeding the permissible w/c ratio). Carefully observe the mix for freedom from segregation and bleeding and its finishing properties. Use the fresh concrete for unit weight, yield and air content.

 

Step 11: Recalculate the mix proportions keeping the free water–cement ratio at the pre-selected value; this will comprise trial mix no. 2. In addition, design two more trial mixes no. 3 and 4 with the water content same as trial mix no. 2 and varying the free water–cement ratio by ±10 per cent of the preselected value.

 

Step 12: Cast three 150 mm cubes for each trial mix and test them after 28 days of moist curing. If required,  a similar number of cubes may be prepared and tested for early strength,.

 

Step 13: Analyse mix nos. 2 to 4 for relevant information, including the relationship between compressive strength and water–cement ratio. Using this information compute the water-cement ratio required for the mean target strength. Recalculate the mix proportions for the changed water–cement ratio taking water content as the same as that determined in trial no. 2.

 

For field trials, produce the concrete by the actual concrete production method used in the field.

 

Observations and Calculations

 

The compressive strength of concrete is……. MPa.  

This mix is suitable/it needs revision.

The mix proportion is ……………..

 

Precautions      

1. For calculating the water-cement ratio, the surface moisture should be added to the water mixed in concrete.

2. In choosing the strength required for a particular purpose, allowance must be made for the inevitable variation in the strength of the test cubes. From the specified minimum strength, the target means strength is estimated according to the degree of control to be exercised, using information obtained from experience.

3. Slump test should be completed within 30 minutes.

4. Contents should be weighed accurately.

5. The inside of the cube should be oiled to prevent the mortar from adhering to the sides of the mould.

6. The ambient temperature at which cubes are prepared should be between 25 and 29°C.

 

Discussion

The method assumes that the workability of a concrete mix is dependent only on the water content in the  mix. The water content decreases with the increase in the maximum nominal size of the coarse aggregate.  The fraction of coarse aggregate itself decreases with the increase of fineness modulus of fine aggregate, i.e.,  the coarser the sand lower will be the bulk volume of dry coarse aggregate required for the mix. However, the  coarse aggregate content increases with the increase in the maximum nominal size of aggregate.

 

REFERENCES

ACI 211.1-91: Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass  Concrete, American Concrete Institute, Farmington Hills, Michigan, 1991.

ACI 211.4R-93: Guide for Selecting Proportions for High-Strength Concrete with Portland Cement and Fly Ash, 1993.

ACI 211.5R-96: Guide for Submittal of Concrete Proportions, American Concrete Institute, 1996.

ACI 211.3R-97: Guide for Selecting Proportions for No-Slump Concrete, 1997.

ACI 211.2-98: Standard Practice for Selecting Proportions for Structural Lightweight Concrete, 1998.

ACI 301-99: Specifications for Structural Concrete, 1999.

ASTM C39: Compressive Strength of Cylindrical Concrete Specimens.

ASTM C617: Capping Cylindrical Concrete Specimens.

ASTM C192: Making and Curing Concrete Test Specimens in the Laboratory.