Page 40 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

EXPERIMENTAL INVESTIGATION

The primary aim of this investigation was to provide
answers to the following questions:

(i) How do the web openings affect the
strength of prestressed concrete beams?

(ii) How does the serviceability characteristics
e.g., cracking, deflection etc., get affected
as a consequence of these openings?

Experiments were conducted on 9 prestressed
Fig 3 Test set-up with third-point loading of beams
concrete T-beams, the details of which are given
in Table 1. Two openings were provided in each Each test beam consisted of 5 numbers of 7-mm
beam; in some cases the openings were provided high tensile steel wires of an ultimate tensile
2
in the shear span whereas in 3 beams the openings strength of about 180kg/mm . Concrete of a
nominal mix of M350 was designed and a mix of
were in the region of constant moment. The results
1:1.42:2.74 by weight with a water-cement ratio
of tests on these beams with openings were
of 0.41 was arrived at. The beams were cast in
compared with those of a similar solid beam. wooden forms Fig. 1. An immersion type vibrator
was used to vibrate the concrete. Rectangular
openings were provided symmetrically with
respect to the midspan. Special care was taken to
concrete around these openings. A span of 4.2m
was chosen to accommodate these openings and
without changing the load points.

A “draped-tendon” profile was given. 6mm mild
steel was used for stirrups and for flange steel.
Additional stirrups and hooks were provided
around and below the openings. Some openings
were trimmed with mild steel. The details
of reinforcement in the beam are shown in
Fig. 2.

Each test beam was gradually tested to failure
under point loads applied at each third-point Fig
3. A 15-t hydraulic jack with a least count of 0.1-
t was used for this purpose. The ultimate load
was reached with the application of 8 to 10 load
increments. At the end of each load increment, the
following measurements were made:
Fig 2 Reinforcement details

The Indian Concrete Journal | November 2018 41