Tension Stiffening In Concrete

This paper presents the results of an experimental study on the tension stiffening response of fusion bonded epoxy coated bars embedded in self compacting concrete (SCC). 18 numbers of direct tension specimens recommended by RILEM standards are used with modified specimen lengths of 40 and 50 times the diameter of the bar. From the study it is concluded that fusion bonded epoxy coating on bars has a negative effect on the tension stiffening of SCC. The reduction ranges from 9.4 % to 32% .The tension stiffening effect improves to an extent of 20-30 % as bar diameter is reduced from 12 mm to 10 mm. Spacing of cracks is 7-18% more if fusion bonded epoxy coated bars are used in SCC.

KEYWORD

Self compacting Concrete, Fusion bonded Epoxy coated

In other words, the reduction in strain of reinforcement due to tensile stresses in concrete between cracks is termed as “Tension Stiffening”. Before cracking, the concrete tensile stress increases with load. When the stress in the concrete first reaches the tensile strength at a particular section, cracking occurs, Figure 1(a). When cracking occurs, the stress in the concrete at the crack, drops to zero. The concrete stress increases with distance from the crack, due to the steel-concrete bond, until at some distance from the crack, the concrete stress is no longer affected by the crack, as shown in Figure 1(b). Slip at the concrete-steel interface in the region of significant bond stress causes the crack to open. The concrete stress increases with distance from the crack, due to the steel-concrete bond, offering some resistance to the applied load, which is the ‘Tension Stiffening’ effect in

Avg Force In Concrete(kN) 0 11.3 22.6 29.5 31.8 Avg Force In Steel(kN) 0 0 0 4.4 13.4

480 COATED Applied Load(kN) 0 11.3 22.6 33.9 45.2 Avg Strain(X10^-4) 0 0 0 1 4 TS Strain(X10^-4) 0 12 21 31 38 Avg Force In Concrete(kN) 0 11.3 22.6 31.7 36.3 Avg Force In Steel(kN) 0 0 0 2.2 8.9

Table 5 Measured strain and load values for 10 mm (0.39inch) bar

Specimen length(mm) Type of rod PARAMETER

500 UNCOATED Applied Load(kN) 0 7.8 15.7 23.5 31.4 Avg Strain(X10^-4) 0 0 0 0 2 TS Strain(X10^-4) 0 8 18 28 42 Avg Force In Concrete(kN) 0 7.8 15.7 23.5 28.26 Avg Force In Steel(kN) 0 0 0 0 3.14

500 COATED Applied Load(kN) 0 7.8 15.7 23.5 31.4 Avg Strain(X10^-4) 0 0 0 2 6 TS Strain(X10^-4) 0 8 18 26 38 Avg Force In Concrete(kN) 0 7.8 15.7 20.4 21.98 Avg Force In Steel(kN) 0 0 0 3.14 9.42

400 UNCOATED Applied Load(kN) 0 7.8 15.7 23.5 31.4 Avg Strain(X10^-4) 0 0 0 0 1 TS Strain(X10^-4) 0 8 18 28 43 Avg Force In Concrete(kN) 0 7.8 15.7 23.5 29.8 Avg Force In Steel(kN) 0 0 0 0 1.6

400 COATED Applied Load(kN) 0 7.8 15.7 23.5 31.4 Avg Strain(X10^-4) 0 0 0 0 3 TS Strain(X10^-4) 0 8 18 28 39 Avg Force In Concrete(kN) 0 7.8 15..7 23.5 26.7 Avg Force In Steel(kN) 0 0 0 0