Design of Concrete Structures for Durability

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Adequate concrete durability can be obtained in the design phase when a structure is conceived, and structural calculations are made, details are designed, materials and their proportions, as well as possible additional preventative measures, are selected.

Prevention is further materialized as the concrete is prepared, placed, compacted, and cured. It would continue throughout the entire service life of the structure, with programmed inspections, monitoring, and maintenance.

These design and construction measures ensure adequate structural resistance against chloride action, improper construction process, insufficient concrete cover, and inadequate utilization of construction materials such as cement and additives, which are not suitable for specific aggressive conditions.

Durability is the ability of the structure to withstand weathering actions, abrasion, chemical attacks, or any other forms of concrete deterioration.

Design of Concrete Structures for Durability

As durability depends upon permeability. It is essential to control parameters affecting permeability. Therefore, it is necessary to lay down permissible limits for parameters like minimum cementitious material content, maximum water to cement ratio, maximum crack width and extent of cover to reinforcement, etc. depending upon the exposure condition.

1. Optimum Cementitious Materials Content and W/C Ratio

It is necessary to employ optimum cementitious materials content to make concrete durable. Low materials content jeopardize the strength of the structural elements where high cementitious materials would increase the possibility of shrinkage, which commonly reduce concrete durability.

Concerning the W/C ratio, the lower the water to cement ratio, the better to decrease concrete permeability and consequently enhance durability. Table 1 provides recommended minimum cementitious material content and maximum water to cement ratio based on the exposure conditions.

Table 1 Minimum cementitious materials and maximum water-content ratio for PCC, RCC and Prestressed Concrete

Exposure condition	PCC	PCC	PCC	RCC	RCC	RCC	PSC	PSC	PSC
-	Min. grade of mix	Min. cementitious* material content, Kg/m^3	Max w/c ratio	Min. grade of mix	Min. cementitious material content, Kg/m^3	Max w/c ratio	Min. grade of mix	Min. cementitious material content, Kg/m^3	Max w/c ratio
Mild	M-20	300	0.55	M-25	380	0.45	M-35**	400	0.40
Mod	M-25	350	0.50	M-30	400	0.40	M-35**	400	0.40
Severe	M-25	380	0.45	M-35	400	0.40	M-45	430	0.40
Very severe	M-30	400	0.45	M-40	430	0.38	M-50	440	0.35
Extreme	M-30	400	0.40	M-45	430	0.38	M-50	440	0.35

* Min. Cementitious material content is for 20mm maximum size aggregate (MSA)

**Add extra cementitious material:

1. For 10 mm MSA = +20 Kg/m³
2. For 40 mm MSA = - 10 Kg/m³

Note: The maximum content of cementitious material should be 500 Kg/m³.

Table 2 Description of Exposure Conditions

Exposure condition	Description
Mild	Concrete surfaces protected against weather or aggressive conditions
Moderate	Concrete surfaces sheltered from severe rain or freezing while wet, concrete continuously underwater
Severe	Concrete surface exposed to severe rain, alternate wetting, and drying or occasional freezing or severe condensation. Concrete exposed to aggressive subsoil/groundwater or coastal environment
Very severe	Concrete surface exposed to seawater spray, corrosive fumes, severe freezing
Extreme	Concrete surface exposed to abrasive action. Surface of members in tidal zone

2. Minimum Clear Concrete Cover

Concrete cover is responsible for protecting steel bars from detrimental influences of carbonation, chloride, fire, and other external hazards; that is why adequate concrete cover thickness is crucial to maintain concrete durability.

Table 3 presents the minimum recommended values for concrete cover for different structural elements in different exposure conditions.

While designing, it should be ensured that cover does not exceed 2.5 times the diameter of the reinforcing bar. If the concrete cover is more, chicken mesh may be provided in concrete to keep the concrete in position

Table 3 Minimum clear cover for concrete structures under different environmental conditions

Sr. No.	Structures RCC/PSC	Extreme environment	Very severe environment	Severe environment	Mild and moderate environment
1	Slabs	50	50	25	25
2	Beams	60	50	40	35
3	Columns	75	75	50	50
4	Well, piles, and footings	75	75	75	50
5	PSC girder	50	50	50	50
6	PSC girder for HTS cables	75	75	75	50

3. Flexural Crack Width

It is necessary to control the crack width to protect steel rebar against corrosion. The crack width is controlled by reducing shrinkage, and distributing the reinforcement over the zone of maximum concrete tension, uniformly, and using smaller diameter bars.

Cement with low heat of hydration and not too fine should be used. The recommended value of maximum crack width to be considered during design are given in Table 4.

Table 4 Maximum Crack Width to be Considered During Design

Type of structure	Exposed to severe, very severe, and extreme environment	Unexposed to severe, very severe, and extreme environment	Exposed to mild and moderate environment
Bridges	0.10mm	0.20mm	0.20mm
RCC beams, slabs	0.20mm	0.20mm	0.30mm

4. Concrete Placement

Proper placement of concrete is vital to achieve excellent durability. This because proper placement and adequate consolidation of concrete would eliminate voids or rocks pockets that may form in concrete.

5. Proper Curing

The use of an appropriate curing method is essential to achieve adequate durability. This is because it ensures that concrete obtain designated strength.

Moisture and humidity play a significant role during the curing period to keep sufficient hydration of cementitious materials and avoid shrinkage and subsequent cracking

Read more:

Selection of Durable Construction Materials for Concrete Structures

Types of cracks in Concrete Structures

Concrete Durability Requirements Based on ACI-318-19

Concrete Cover Specifications for Reinforcement in Different Codes