The following factor usually affect deflection in structural members (especially concrete)
Tensile Strength
This property is important because the slab will crack when the tensile stress in the extreme fibre is exceeded. In Eurocode, the concrete tensile strength, fctm is a mean value used for deflection calculations and which increases with compressive strength. The effective tensile strength of concrete depends on the degree of restraint to shrinkage movements which in turn depends on wall layout.
Creep
This is the time-dependent increase in compressive strain in a concrete element under constant compressive stress. Creep is usually considered in the design by modifying the elastic modulus using a creep coefficient, which depends on the age of loading, size of the member, and ambient conditions. In the assessment of creep, the cement strength is taken into consideration. Generally, Class R should be assumed. Class N should be assumed where the ground granulated blast furnace slag (ggbs) content exceeds 35% of the cement combination or where fly ash (pfa) exceeds 20% of the cement combination. Class S may be assumed where ggbs exceeds 65% or where pfa exceeds 35%.
Elastic Modulus
The elastic modulus is influenced by the aggregate type, workmanship, and curing conditions. The effective elastic modulus under sustained loading will be reduced over time due to the effect of creep. The long-term elastic modulus often used in the calculation should be taken as;
Ec,lt = Ec28/ (1 + φ)
Where,
Ec28 = 28-day tangent modulus = 1.05 Ecm where Ecm is the 28-day secant modulus (Table 3.1 (Strength and Deformation Characteristics for Concrete) of EC 2 gives recommended values-see attached)
φ = the creep factor
Loading Sequence
The loading sequence of the suspended slab will influence the point at which the slab will crack and is used in calculating creep. The loading sequence generally depends on the construction method. After construction, slabs that have experienced temporary loading at the stage of construction will be permanently loaded with finishes and partitions, and variable actions. EC 2 recommends the use of a quasi-permanent combination for deflection calculation associated with the loading sequences.
Cracking
A slab undergoing deflection and a visible deflection must have exceeded its cracking capacity. The point at which cracking occurs is determined by the moments induced in the slab and the tensile strength of the concrete, which increases with age. The critical situation usually occurs when the slab is struck or when the load of the slab above is applied. Once the slab cracks, its stiffness is permanently reduced. Hence, it becomes necessary to find the critical loading stage at which cracking first occurs.
The critical loading stage corresponds with the minimum value of k,
Where,
k = fctm/ (w√0.5)
Where,
w = the serviceability loading applied up to that stage
fctm = the concrete tensile strength at that stage
Shrinkage Curvature
Shrinkage depends on the water/cement ratio, relative humidity, and the size and shape of the member. The effect of shrinkage in an asymmetrically reinforced section is to induce a curvature that can lead to significant deflection in shallow members. This effect should be considered in the deflection calculations.
Source
Bond, A.J., Harrison, T., Narayanan, R.S.; Brooker, O., Moss, R.M., Webster, R. and Harris, A.J. (2006). How to design concrete structures using Eurocode 2. The Concrete centre, UK
