Micro-cracking and crack growth in notched concrete and mortar beams
Date
2015-02-05
Authors
Gill, Laurence Mark
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Abstract
This dissertation addresses the question of the fracture
behaviour of notched concrete and mortar beams.
The major purpose of the work was to study the development
of the micro-cracked zone and identify the point
at which main crack growth began, and thus to characterise
concrete and mortar at the start of main crack
growth.
Notched concrete and mortur beams of width 100 mm,
depth either 200 mm or 300 nun, and with a span/depth
ratio of three, were tested. Measurements of midspan
deflection, midspan load, surface displacements across
the fracturing section and ultrasonic pulse transit
time were made. Ordinary Portland cement and mineral
aggregates were used for the concrete and mortar
beams.
The J'■integral, surface displacements across the fracturing
section and ultrasonic pulse transit time measurements
were used to detect the onset of main crack
growth. It was found that a reduction in the load carrying
capacity of concrete and mortar is possible due
to micro-cracking only. The value of the J-integral
at the start of main crack growth was found to be essentially
the same for concrete and mortar. The value
of the J-integral at the start of micro-cracking was
■ <
found to be essentially the same for concrete and
mortar, and about 40% of the value of the J-integral
at the start of main crack growth. The value of the
J-integral at the start of micro-cracking and at the
start of main crack growth was found, on average, to
increase for an increase in beam depth.
Surface displacements across the fracturing section
showed the tension zone at the start of main crack
growth to be approximately twice the size of the compression
zone for both concrete and mortar. The'size
of the micro-cracked zone, as determined from surface
displacements across the fracturing section, was found
to be 42% of the residual ligament depth for concrete,
and 41% of the residual ligament depth for mortar.
Scatter in the results was found to be considerable,
thus meaning that only general trends could be identified
Description
A dissertation submitted to the Faculty of Engineering, University of the
Wttwatersrand, Johannesburg, in fulfilment of th e degree of Master of
science in Engineering
Johannesburg 1988