Construction Measures & Materials to Reduce Deflection of Concrete Beams and Slabs
Madeh Izat Hamakareem
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Deflections of concrete beams and slabs are affected by construction measures and materials used. In most situations, size of the reinforced concrete sections is controlled by deflections.
To achieve more economical sections, certain measures can be employed includes construction techniques and material selection techniques.
In this article construction and material selection measures which can be used to reduce concrete beam and slab deflection are explored in the following sections.
1. Construction Measures to Reduce Deflection of Beams and Slabs
Cure the concrete to allow it achieve strength
It is claimed that the deflection response of concrete members are specified by concrete strength at initial loading rather than final strength of concrete elements.
It is recommended to use concrete that obtain high strength at early stages when constructed members are to be loaded in no time after construction. Additionally, the deflection of cracked members is much higher than the same element in uncracked condition.
Cure the concrete to decrease creep and shrinkage
By and large, proper curing will affect and decrease long term deflection, not short term deflection. Moreover, curing effects on long term deflection components (shrinkage and creep) are similar.
The larger influence of curing is observed in reinforced concrete beams and slabs that are subjected to high shrinkage for example structures in aired atmosphere, members with thin flanges, and restrained members.
Control shoring and reshoring procedures
It is indicated that the load of shoring on floors in multi-story building could be as much as two times the self-weight of concrete slab. Since dead load of the slab is larger than superimposed design load in many cases, the slab could be overstressed because of shoring loads and consequently uncracked state, which assumed based on design loads, will no longer be a valid assumption.
That is why the slab flexural stiffness is decreased by one third of the flexural stiffness value that computed based on assumed design loads. Moreover, it is possible that shore loads are applied to the slab before the design strength is obtained by concrete and this produce cracks because of low modulus of rupture.
Furthermore, based on experience, it is claimed that apparent deflection is changing broadly between slabs even if the same construction and design is employed.
This variation might be due to number of factors for example timing and method of striping forms were not applied uniformly and construction loads were not imposed uniformly. Lastly, the soil beneath shoring support must be checked to avoid settlement under recently poured concrete, because settling of soil under shoring support lead to sagging of forms which is not desired.
Postpone beginning of loading
This technique gives concrete enough time to obtained design strength and not only does it increase modulus of elasticity but also modulus of rupture is rose. Moreover, the more modulus of rupture the fewer amounts of cracks is developed.
An increase in modulus of elasticity leads to increase flexural stiffness. Lastly, by postponing beginning of loading, creep deflection is decreased.
Install deflection sensitive equipments at later times
In this case the incremental deflection which is happen when deflection sensitive equipment is installed until it is removed or deflection reaches its final value, is reduced. ACI 318-11 section 220.127.116.11 provides method of calculating additional long term deflection for delaying partition installation.
Place deflection sensitive equipments or elements to prevent deflection issues
It is recommended to locate devices such as printing presses, scientific equipments, and other equipments that must be level in the middle of the span because modifying slopes resulted from deflection is minimal in this location.
Moreover, place vibration sensitive equipments close to supports because vibration amplitude is very small in this location.
Provide architectural details to accommodate the expected deflection
Partitions which abut the column might exhibit deflection influences such as separating from the column horizontally near the top. So, it is advised that architectural detailing provide rooms for such movements.
Similarly, doors, windows, partitions and nonstructural elements which located under or supported by deflecting concrete element should be equipped with slip joints for accommodating anticipated deflections of concrete elements which are located below or above nonstructural members.
Build camber into the floor slab
Cambering will not affect the deflection value after the deflection takes place. Therefore, to get the best result out of building camber into the slab, firstly, the deflection should be computed accurately and overestimation must be prevented, secondly, specifying the cambering pattern, thirdly, results should be monitored during construction.
Make sure that top steel reinforcements are not displaced downward
Generally, downward displacement of reinforcement decrease member strength. Regarding deflection, uncracked members is slightly influenced by displacement of top reinforcement, but cracked members especially cantilever beams are extremely sensitive and affected by steel downward movement.
In addition, continuous beams deflections are increased when bar displacement is occurred at negative moment positions and redistribution of moment is happened.
2. Materials Selection to Reduce Deflection of Beams and Slabs
Choose materials that increase rupture and modulus of elasticity or decrease shrinkage and creep
Cement, aggregates, silica fumes, and admixtures are those materials that affect properties such as modulus of elasticity and rupture. These properties in return influence the deflection of concrete element.
Use a mix design that lead to reduce creep and shrinkage or increase the elastic modulus and creep
Shrinkage can be decreased when lower water to cement ratio is applied and long term deflection can be reducing. Moreover, proportions of the mixture can be modified to get better mixture.
Use a concrete with higher modulus of elasticity
As per ACI Code procedures, the stiffness of uncracked elements is increased proportionally to the modulus of elasticity. In contrary, the influence of elastic modulus on member’s stiffness is low and fully cracked members is not affected.
Use concrete with a higher modulus of rupture
Increasing stiffness, which in return decrease deflection, is depend on Modulus of rupture combine, reinforcement ratio, and applied load magnitude affect.