Causes of Excessive Deflections in Reinforced Concrete Slabs
Madeh Izat Hamakareem
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Deflection of reinforced concrete slabs is acceptable to a certain limit which are specified by applicable Codes such as ACI, IS Codes and Euro Codes. These codes to specify the amount of deflection which is acceptable.
When the deflection limitation is exceeded, then it will probably lead to several problems for example aesthetic issues, occupants will not be comfortable to life in the building, cracks may develop in partitions, and occasionally cause malfunctioning of doors and windows.
Excessive deflections in reinforced concrete slabs possibly results in neighboring balcony to deflect inward. This is seriously problematic and unacceptable because rain water fallen on the balcony will move inside the building and can damage the building content. This is one of the crucial factors that make the prevention of excessive deflection of reinforced concrete slab substantially significant.
There are various causes that lead to excessive deflections in reinforced concrete slabs, and it is possible that one single factor or combination of them result in the deflection that is over limitations.
The best way to avoid excessive deflection is to understand the cause of the problems which will be explained in the following sections and only then specify proper solutions.
Fig.1: Deflections in Reinforced Concrete Slab
Causes of Excessive Deflections in Reinforced Concrete Slabs
Following are the different causes responsible for excessive deflections in RCC slabs:
Design of reinforced concrete slab
Material properties used in the reinforced concrete slab
By and large, both reinforced concrete slab thickness and reinforcement ratio of the slab are common causes that may lead to excessive deflection.
Selecting small slab thickness can be appropriate in terms of strength but in most cases lead to excessive deflection and due to this issue applicable Codes such as ACI Code recommends minimum slab thickness to contain deflection within acceptable limit.
Similarly, if the computed reinforcement ratio is small, then it will yield pretty soon and consequently flexural stiffness of the slab decrease substantially and finally the deflection of the slab will be quite large.
Fig.2: Details of Reinforced Concrete Slabs
Material Properties Used in Reinforced Concrete Slabs
It is reported that, if abnormal shrinkage and creep is experienced by materials employed in the slab construction, it will be one of the factor which cause enormous deflection.
Sometimes large shrinkage is generated when high strength concrete is used and this is likely to share part of large deflection.
It should be said that alkali aggregate reactions also imposed detrimental influence on the deflection of reinforced concrete slab because it leads to initiate cracks which eventually decline flexural stiffness and hence increase deflection.
Construction of Reinforced Concrete Slabs
There are several construction aspects which may cause excessive deflection if it is constructed in a wrong way.
For example, when large panels and cantilevers are not cambered properly during construction then they will suffer large deflections.
If curing is not carry out adequately, the slab will not be able to gain required strength and consequently deflect greatly.
It is likely that top reinforcement in slab during construction is forced down by labors. This decreases the effective depth and consequently the improvement of flexural stiffness provided by reinforcement continuity at supports will be declined substantially.
RCC Slab at its early life may crack largely and its stiffness decreases largely because of construction loads imposed by storage materials or propping. If a slab is supported by props during construction and the prop is fixed on a plate with insufficient area to prevent ground settlement, then the movement of prop toward the ground produce deflection in the slab.
It is worth motioning that deflection due to the decrease of effective depth of top reinforcement is considerably usual, and it is reported that decreasing effective depth from 20cm to 17cm can reduce flexural stiffness by larger than 20%.
Therefore, it is significant to pay attention to the construction loads as it may affect the deflection of the element substantially if it is not dealt with properly.
Change in Function of Reinforced Concrete Slabs
Reinforced concrete slab is usually designed for specified purpose and the function for which the strength and serviceability of the elements is checked is known.
However, when the function of the slab is changed and the imposed loads are increased specifically permanent loads, then the slab can greatly deflect.
Environmental conditions of Reinforced Concrete Slabs
