Design Requirements of Concrete Based on ACI 318-19

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There are several requirements established in ACI 318-19 (Building Code Requirements for Concrete and Commentary) that concrete materials need to meet; otherwise, it would not be suitable for the construction of a structure.

These requirements vary from one structure to another one. They include minimum permissible concrete compressive strength for a particular structure, modulus of rupture, etc.

Contents:

Design Requirements of Concrete Based on ACI 318-19

Design Requirements of Concrete Based on ACI 318-19

1. Compressive Strength

The compressive strength of concrete is specified based on the following criteria.

Based on table 1.
Based on the strength requirements of a structure under consideration.
Based on the durability requirements of the structure. Sometimes, the durability requirements enforce the use of high concrete compressive strength.

Table 1 Minimum Design Concrete Compressive Strength for Different Concrete Structures

Type of application	Minimum concrete compressive strength, MPa
General	17
Foundations for structures assigned to SDC A, B, or C	17
Foundations for Residential and Utility use and occupancy classification with stud bearing wall construction two stories or less assigned to SDC D, E, or F	17
Foundations for structures assigned to SDC D, E, or F other than Residential and Utility use and occupancy classification with stud bearing wall construction two stories or less	21
Special moment frames	21
Structural walls with Grade 420 or 550 reinforcement	21
Structural walls with Grade 690 reinforcement	35
Precast-non-prestressed driven piles Drilled shafts	28
Precast-prestressed driven piles	35
Note: Maximum compressive strength of lightweight concrete in structural walls, moment frames, and their foundations should not be higher than 35MPa, as per ACI 318-19. Disregard this condition if the strength and toughness of elements made with lightweight concrete are equal or greater than that of comparable members made with conventional concrete. Experimental evidence should demonstrate such a result.

Fig. 1: Compressive Strength Test of Concrete

2. Modulus of elasticity

The modulus of elasticity of concrete used to compute deflections, a period of vibration, and drift of structures. It is calculated using equations that are provided by ACI 318-19 Code or testing. There are differences between values obtained from equations and those achieved from testing.

However, the theoretical values from equations are suitable for the majority of structures. Significant differences between equation values and test results are noticed for high strength concrete, lightweight concrete, and concrete mixtures with a low quantity of coarse aggregate such as self-consolidation concrete mixture.

Modulus of elasticity of concrete is computed using the following expressions:

For values of Wc between 1440 and 2560 Kg/m^3:

For Normal-weight concrete:

Where:

Ec: modulus of elasticity

Wc: weight of concrete

Fc’: concrete compressive strength

Finally, modulus of elasticity of concrete evaluated through tests in certain conditions, such as verification of the achievement of specified elastic modulus or when its value used in proportioning of concrete mixture.

3. Modulus of rupture

The modulus of rupture is computed using the following formula:

Where:

Lamda: modification factor which is determined based on table 2 or table 3:

Table 2 Values of ? for Lightweight Concrete Based on Equilibrium Density

Wc, Kg/m^3	Lamda
Equal or smaller than 1600	0.75
Greater than 1600 but equal or lower than 2160	0.0075*Wc equal or lower than 1
Greater than 2160	1

Table 3 Values of ? for Lightweight Concrete Based on the Composition of Aggregates

Concrete	Composition of aggregates	Lamda
All-lightweight	Lightweight fine and coarse aggregates	0.75
Lightweight, fine blend	Lightweight coarse aggregate blended fine aggregate composed of lightweight aggregate and normal-weight aggregate.	0.75 to 0.85
Sand-lightweight	Normal weight aggregate, coarse aggregate is lightweight.	0.85
Sand-lightweight, coarse blend	Normal weight fine aggregate, and blended coarse aggregate composed of lightweight and normal-weight aggregate.	0.85 to 1
Note: Lightweight fine and coarse aggregate are two major types. The first type produced from blast-furnace slag, clay, diatomite, fly ash, shale, slate, pumice, scoria, or tuff. The second type composed mainly of lightweight-cellular and granular inorganic material.