🕑 Reading time: 1 minuteThere are several factors that lead to develop cracks in different elements of reinforced buildings. These cracks might lead to failure of the building in terms of serviceability design. In this article, principle techniques which can be used to mitigate cracks in the reinforced buildings are discussed and explained.
Measures to Mitigate Cracks in Reinforced Concrete Structures
- Planning the layout of restraining members
- Structural separation
- Closure strips, joints, and favorable pouring sequences
- Released connections; Wall/ slab release, slab-column release, wall joints,
- Addition improved layout of mild reinforcement
- Addition or improved layout of tendons
Planning the Layout of Restraining MembersSelecting good position for columns and walls during building architectural planning is the substantially effective technique of restraint-cracks prevention. Equal number of walls with the same length, could be placed to decrease tendency of crack development by permitting the slab to move freely in the direction of planned point of zero movement, such arrangements can be seen in Figure-1.
Fig.1: Favorable Arrangement of Restraining CracksHowever, if the walls and layouts are arranged in such a way that prevents the free movement of walls, then it would make situations which cause to initiate cracks as shown in Figure-2.
Fig.2: Unfavorable Arrangement of Restraining Walls
Structural Separation to Mitigate Cracks in Reinforced Concrete StructuresSlabs with irregular geometry in are especially susceptible to cracking. An example of such slab is provided in Figure-3. The figure shows a structural separation between larger post-tensioned rectangular slab and smaller square slab. The width of structural separation usually ranges from 13 to 26 mm. The difference between expansion joint and structural separation is that, the function of the latter is for limited time which might be two or three months, whereas the former, which its function is to make rooms for temperature produced movements, should work and continue its function for the entire life span of the structure.
Fig.3: Separation between Large and Small Slabs that Create Irregularities
Closure Strips, Joints and Favorable Pouring SequencesA closure strip is a temporary space between two regions of a slab which are constructed and post tensioned differently. These two-separated post tensioned slab parts are permitted to experience shortening independent to each other. The width of the closure strips as shown in Figure-4 is based on the distance needed to install stressing jack between two slabs, and is commonly between 76- 91 cm. The space between two regions of the slab is filled and consolidated with non-shrink concrete, commonly after a period of one to two months. The time required to keep the closure strip open is dictated by the extent of shortening deemed needed before the two slabs are jointed together. The continuity between the two slab portions is provided by reinforcement that extended on each concrete slab side into the closure strips. The amount of steel reinforcement embedded in the closure strip is computed based on the bending moment and shear forces at the position of the closure strip; when the whole slab is considered in continuum. It is recommended that, stressing ends of tendons, which are stopped in the closure strip, must be cut, sealed, and grouted and this is applied for corrosion protection purposes. Based on experience, closure concrete is poured after calculating shortening on each side of the closure strip which is around 6.35 mm.
Fig.4: Closure Strip Width Between Two Regions of a SlabConstruction joints as illustrated in Figure-5 are introduced in the slab between two concrete placements and their positions are specified in advance. The construction joints provide separation for short time between two regions of the slab for crack control purposes in addition to divide large size slabs into smaller ones to manage construction easily.
Fig.5: Construction Joint; (A) Without Stressing, (B) With Intermediate StressingThe construction joints provided in Figure-5 is different from cold joints because not only does the position of construction joint is specified by designer but also it is employed to control cracks. However, cold joints are formed when concrete batch is finished and time interval of three to seven days usually occurs between first and second concrete placement. Intermediate stressing is used for long tendons where large stress loss occurs. Based on performance experience of post tensioned slabs, numbers of guidelines for introducing closure strips are provided. For example, closure strips or structural separation are not required if slab length is smaller than 76 m and supporting wall is located favorably. One centrally positioned closure strip is provided when slab length is greater than 76 m and less than 114 m, and for slab length larger than 114 m it is recommended to provide structural separation.
Released Connections: Wall/ Slab Release, Slab-Column Release, Wall JointsReleased joints are detailed and constructed to allow restricted movement of the slab relative to its support. When structural separation, favorable layout of supporting structural members, construction separation and closure strips are not applied adequately then using released connections are substantially influential for tackling cracks. It can be classified into three types namely:
- Wall / slab release,
- Slab-column release, and
- Wall joints:
Wall / Slab ReleaseThere are various types of this joint which are shown in and slippage material is applied in all kind to ease slips. Joint releases with ties are most efficient type but its utilization is limited because walls are commonly required to transfer shears force and gravity loads at wall-slab interface. Figure-6 shows various types of wall-slab release for exterior walls and terminating slabs, nonetheless all types can be used for intermediate slabs and interior walls with some modifications.
Fig.6: Wall-Slab Release Types
Slab-Column ReleaseIt is possible to design columns to resist lateral forces conducive to lateral displacement between their ends with no distress indications or might be released to make rooms for relative displacements at slab-column joint. Figure-7 illustrates hinged construction at base ends of column and detailing of the joints.
Fig.7: Hinged Construction at Base of End Columns; (A) Elevation View, (B) Joint Detailing
Wall JointsThese are vertical joints between adjacent walls by which displacements of slabs and beams supported by the wall are accommodated. They are considerably influential in mitigating cracks in beams, slabs, and supported walls as well. Figure-8 illustrates plan of rectangular slab supported by interior columns and perimeter walls.
Fig.8: Wall Joints; (A) Plan Showing All Joints and Closure Strips, (B) Plan Showing Arrangements of Different Wall-Slab Joints
Addition or Improved Layout of Mild ReinforcementDespite all crack mitigation measures provided in the above sections, it is required to install extra mild reinforcement at potential distress positions to tackle crack developments. For example, at slab and its supporting walls in which proper release joints to mitigate cracks cannot be provided because of shear transfer requirements which are specified in the design, as shown in Figure-9.
Fig.9: Crack Mitigating Reinforcement next to Shear Walls; (A) Interior Shear Wall. (B) Exterior Shear WallIt is demonstrated that, placing reinforcement as shown in Figure 10 in slabs parallel to shear walls over distance of nearly 3m vertical to the wall is substantially effective. The reinforcement ratio is 0.0015 multiply by slab cross sectional area over one third of the transverse span. The spacing between bars is 1.5 times the thickness and installed alternately at the top and bottom.
Fig.10: Reinforcements at the Corner of the Slab
Addition or Improved Layout of TendonsIt is advised to install tendons to apply additional compressions in areas where maximum losses are anticipated. Overlapping and dead ending tendons arranged in Figure-11 and Figure-12 are provided for that purpose. Strand layout detailing around openings and discontinuities are extremely significant.
Fig.11: Tendon Arrangement for Mitigating Cracks in Mid Spans
Fig.12: Tendon Arrangement to Compensate Restraining Effects of Transverse Wall
Fig.13: Tendon Arrangement at an Interior Opening