General Information

The cover of reinforcement bars is decisive to a great extent for the durability of a structure. Structure durability means today the fulfilment of all conditions of usability across the entire projected period of use, without larger financial expenditures and without greater reduction of the aesthetic values of a structure. The choice of reinforcement cover must be made in the design phase. Structure protection, including corrosion protection according to EC2 should consider 'the mode of use, the projected period of use, the maintenance programme and any influences'.The cover of reinforcement bars is decisive to a great extent for the durability of a structure. Structure durability means today the fulfilment of all conditions of usability across the entire projected period of use, without larger financial expenditures and without greater reduction of the aesthetic values of a structure. The choice of reinforcement cover must be made in the design phase. Structure protection, including corrosion protection according to EC2 should consider 'the mode of use, the projected period of use, the maintenance programme and any influences'. From the above may be inferred that the thickness of the reinforcement cover must at all times be agreed upon with the future user. According to EC2, the thickness of the cover should be selected so as to ensure:

 

  • safe transfer of adhesion forces
  • sufficient concrete packing
  • protection of the reinforcement structure from corrosion.

 

According to these recommendations, the minimum cover thickness should conform to the conditions indicated below:

where:

cmin,b - minimum reinforcement cover thickness due to adhesive properties
cmin,dur - minimum cover thickness due to environment conditions,
∆cdur,γ - cover thickness increase due to safety,
∆cdur,st - cover thickness reduction due to use of stainless steel or other protection of steel from corrosion,
∆cdur,add - cover thickness reduction due to the application of protection of the concrete surface against corrosion.

According to EC2, the minimum cover thickness should conform to the following conditions, considering the proper transfer of forces and assurance of the ability of appropriate packing of the concrete:

cmin,b ≥  ø, jeżeli dg ≤ 32 mm

c min,b ≥  ø + 5 mm, jeżeli dg > 32 mm  

where:

ø – individual bar diameter
dg - maximum aggregate granularity

The minimum reinforcement cover thickness for a structure greatly depends on the environment exposure class. In order for the cover to fulfil its role as a layer protecting the reinforcement bars against corrosion, an analysis of the work conditions of the structure is necessary. Analysing the structure's environment, one should consider in particular aggressive actions (from acid solutions, carbonisation, sulphur salts) or physical influences (abrasion, influences by water or temperature).

The exposure classes used, depending on the ambient conditions influencing the thickness of the cover, are indicated in the following table:

Class designation Environment description Examples of presence of exposure classes
1. No danger from corrosion and aggressive chemical influences
X0
non-reinforced concrete, and not containing other metal components, all environments excluding cases of freezing/thawing or chemical aggression, very dry reinforced concrete types or those containing other metal components concrete inside buildings with very low air humidity
2. Corrosion caused by carbonisation (pertains to cases when reinforced concrete or concrete with other metal components is exposed to air or humidity)
XC1
dry or always wet concrete inside buildings with a low air humidity, concrete constantly submerged in water
XC2
wet, sporadically dry concrete surfaces threatened by long-term exposure to water, in most cases foundations
XC3
moderately humid concrete inside buildings
XC4
cyclically alternating between wet and dry concrete surfaces at risk of contact with water, but not as for exposure class XC2
3. Corrosion caused by chlorates not from sea water (applies to cases, when concrete containing reinforcement bars or other metal components is at the risk of contact with water containing chlorates , including de-icing salts from other sources than seawater)
XD1
moderately humid concrete surfaces at risk of influence of chlorates from the air
XD2
wet, sporadically dry swimming pools, concrete at risk from industrial water containing chlorates
XD3
cyclically alternating between wet and dry bridge elements at risk from spray liquids containing chlorates, road surfaces, parking lots
4. Corrosion caused by seawater-borne chlorates (applies to cases, when reinforced concrete or concrete with other metal components is subjected to seawater-borne chlorates found in water or air)
XS1
subject to salts contained in the air, but no direct contact with seawater structures located on the shore or close to it
XS2
constantly submerged components of sea structures
XS3
inflow, spray and aerosol zone components of sea structures
5. Aggressive freezing and thawing without de-icing media or using de-icing media (applies to cases when wet concrete is subject to increased risk of cyclic freezing/thawing)
XF1
moderate saturation with water without de-icing materials vertical concrete areas at risk of rain and freezing
XF2
moderate saturation with water with de-icing materials vertical concrete areas of road structures at risk of freezing and influence from de-icing materials from the air
XF3
strong saturation with water without de-icing materials horizontal concrete surfaces at risk of rain and freezing
XF4
strong saturation with water without de-icing materials or seawater carriageways of roads and bridges at risk of action by de-icing materials, concrete surfaces directly at risk by aerosols containing de-icing materials and freezing, spray zones in sea structures at risk of freezing
6. Chemical aggression (applies to cases, if concrete at risk of chemical aggression from natural soils or groundwater when wet is at risk of great aggression due to periodic cyclic freezing and thawing)
XA1
low-aggression chemical environment according to table 2 in [N2.8] according to [EC] natural soils and groundwater
XA2
medium-aggression chemical environment according to table 2 in [N2.8] according to [EC] natural soils and groundwater
XA3
strongly aggressive chemical environment according to table 2 in [N2.8] according to [EC] natural soils and groundwater
7. Aggression from abrasion (applies to cases when the concrete surface is as risk of mechanical loads)
XM1
moderate risk of abrasion surfaces and roadways used by vehicles with pneumatic tyres
XM2
strong risk of abrasion surfaces and roadways used by vehicles with full rubber tyres and lift trucks with elastomer tyres or on steel wheels
XM3
extremely strong risk of abrasion surfaces and roadways often used by tracked vehicles, bridge pillars, overflow surfaces, walls of outlets and hydrotechnical shafts, stilling basins

It is worth noting that the concrete may be subject to multiple exposure classes simultaneously. Such a situation requires individual consideration, with the requirements elevated with respect to the influence of the individual factors.

 

Minimum values for cover thickness cmin,dur (mm) required due to durability of reinforcement steel.

Structure
class
Exposure class
X0
XC1
XC2/XC3
XC4
XD1/XS1
XD2/XS2
XD3/XS3
S1
10
10
10
15
20
25
30
S2
10
10
15
20
25
30
35
S3
10
10
20
25
30
35
40
S4
10
15
25
30
35
40
45
S5
15
20
30
35
40
45
50
S6
20
25
35
40
45
50
55

Including the influence of abrasion, the cover thickness cmin must be increased by the wearing layer ∆cdev. Its minimum value is:

  • 5 mm – wear class XM1 (moderate abrasion, i. e. on industrial premises, where vehicles with pneumatic rubber tyres are used),
  • 10 mm – wear class XM1 (heavy abrasion, i. e. on industrial premises, where vehicles with pneumatic rubber tyres or full rubber tyres are used),
  • 15 mm – wear class XM3 (extreme abrasion on parts of industrial premises, where lift trucks on plastic wheels or tracked vehicles are used).

According to EC2, an increase of cover thickness is required in general for buildings, at ∆cdev=10 mm. The thickness cnom = cmin + ∆cdev, is measured from the final metal bar, meaning, from the tie wire. The EC2 assumes a reduction of the ∆cdev value; if during manufacture a quality control system is used, then the value of ∆cdev may be lower than the assumed value of 10 mm, but hither than 5 mm. If precise tools are used during the inspection, and if flawed products may be rejected, then the value ∆cdev may be omitted.

An increase of the reinforcement cover thickness must be carried out always in a situation when the concrete surface is uneven (wavy) or if it has recesses due to its aesthetic values (i. e. washed concrete). In such a case, it should conform to the expected recesses. Cover for reinforcement laid out on a concrete subsurface (lean concrete) should be 40 mm. However, if spacers are laid out on the soil directly, then the cover, with which the reinforcement bars should be provided, amounts to 75 mm.

In order to ensure appropriate reinforcement cover, more and more spacers made of artificial materials are used, however, industrially they may also be manufactured of concrete mortar (partially reinforced with fibreglass) or steel. Irrespective of the spacer material, they should conform to the following requirements:

  • they have to bear the given loads, but at the same time they should be characterised by low springback, which ensures that when the formwork is removed, the concrete shall not be torn away by the spacer,
  • their shape must be be insensitive to temperature changes (i. e. accelerated maturing)
  • concrete spacers used in locations where influences by humidity are present, as well as periodic freezing and thawing, must exhibit great resistance to low temperature and low water penetration,
  • spacers used with concrete at risk of aggressive chemical influences should conform to appropriate requirements, and at the same time limit the depth of water penetration over the spacer surface,
  • spacers for concrete components, the area of which is exposed, should be characterised by a low support footprint, which should make them invisible in the ready component,
  • during spacing of reinforcement on a soft surface (i. e. mineral wool), spacers with a large footprint are to be used.

 

The advantages of spacers made of concrete and fibreglass-modified concrete is their great resistance to loads, and at the same time the lack of deformations. A further advantage of concrete spacers is their great fire resistance, and if they are produced from the right kind of concrete, they are characterised by excellent resistance to frost and low water permeability. Spacers equipped with tie wire are ideal for spacing vertical reinforcement bars.

Spacers made of synthetic material are also very resistant to loads. They are characterised by low deformations, and numerous clamps make them very stable during concreting. The material, from which they are manufactured, is to a great extent resistant against most chemicals, is impenetrable to water and frost-resistant. Thanks to proper covering of the spacer, despite it being manufactured from an easily melting material, one obtains fire resistance for the entire structure.

Steel spacers may only and exclusively serve the spacing of reinforcement bars for the stretched zone and reinforcement bars of the compressed zone. Such spacers, due to the type of material they are made of (steel) must be fully covered with concrete, ale the rules for covering these spacers are the same as for reinforcement bars.

Before choosing spacers, please acquaint yourself with their properties, the conditions, in which the structure will function, and find out the load. The rules of arranging spacers according to Betomax foresee that in case of use of point spacers in beams and pillars, at each edge one needs to place at least two such spacers, and if the height exceeds one metre, three such components are required, as seen in fig. 1 and fig. 2.


Fig. 1. Arranging point spacers in beams

 

s = 0,5 m dla ømin ≤ 10mm
s = 1,0 m dla 12 mm ≤ ømin ≤ 20 mm
s = 1,25 m dla 20 mm < ømin

 

Fig. 2. Arranging point spacers in pillars

 

If point spacers in slabs are used, their spacing may not exceed s ≤ 500 mm. Should the diameter of the reinforcement bars be at least 16 mm, then the spacing of the support structure may be increased to s = 700 mm. In case of using a linear support structure, the support structure spacing may not exceed s ≤ 500 mm with a bar diameter of 6 mm, for bars with a diameter of 8-14 mm - s ≤ 700 mm, and for bars with a diameter of 16 mm assumed is a spacing of s = 1000 mm.

Fig. 3. Arrangement of spacers in slabs.

 

 

Fig. 4. Arrangement of spacers in shields.

Spacer components from Betomax, if properly selected and used, fully conform to the above tasks.

Sample arrangement of spacers in a reinforced concrete element.