Can you elaborate on the intent of your question? Unreinforced concrete cannot be allowed to develop shear cracking so, in a sense, the angle of that cracking is moot. Pre-cracking, I'd think that you'd be closer to a Mohr's circle / elastic medium situation.
Thanks for Replying my query. To elaborate the question it is 15MPa blinding concrete temporary back fill for a lift core
For safety i am just providing safe working load at time of construction of that lift core. So we need to evaluate the mass concrete footing with no reinforcement.Once lift core is constructed there wont be any issues.
I would say 15MPa blinding has ZERO shear resistance.
Blinding concrete/leanmix/trenchfill, call it what you want, is usually a dry loose mix which is compacted upon placement. Its fine under uniform axial load when confined, but don’t try to shear it.
I wasn't sure whether the OP was referring to shear strength (which is defined for plain concrete under US code) or the angle at which the cracking would take place.
I'm not familiar with the term "blinding concrete", but would recommend that you work with the supplier to test your proposed mix to ensure that your design is satisfactory in accordance with your local code for plain concrete. Note that structural (f'c ~30MPa or greater) ready-mix grouts that can be pumped / tremied into place are available that do not require compaction/vibration - we routinely use these for cast-in-place bored (augercast) piles.
We have also used formed plain concrete for retaining wall facing foundations / level pads for permanent soldier pile walls.
Thanks for the reply all of them. I am checking only for axial loads and nominating a safe distance from the edge to avoid any shear failure during construction. As per Tom reply used that formula in AS3600 and worked out the distance to be 300mm from edge based on loadings.
This check is for lift core which deep in 15m from surface level & the 15mpa concrete is only 3m for the lift core height. So not checked for bending, as it is bearing fully on ground.
I wold second MIStructE's opinion that blinding should not provide any shear resistance. These concrete mixes usually lack the large aggregate that provides shear resistance due to interlock.
jdonville: Compressive strength is in direct relationship with the proportion of coarse aggregate in it; the size of coarse aggregate affects the compressive strength through cement-aggregate bonding and aggregate-aggregate interlock. With the amount of cement holding constant, the lack of coarse aggregate tends to produce a product inferior (lower fc') to that with coarse aggregate at optimum. Doesn't Vc relate to fc'?
We batch f'c 4ksi @ 28 days neat (i.e., no agg) cement regularly. Usually not for RC applications, but the design f'c is still 4ksi. I've considered speccing it for jobs with really difficult access, like pit underpinning an existing footer in the interior of an existing structure.
