Roller Compacted Concrete 5

[mickmac]

I am about to oversee a project involving the placement of RCC in an environment where the temperature remains at a constant 70 degrees fahrenheit. My question is do I need to saw cut expansion joints?

 

[cvg]

there generally is very little shrinkage since the W/C is so low. I have not seen it done, but it might depend on what you are constructing and the details of your project.

 

[Ron]

What do the specifications require? Usually don't need "expansion" joints. You might want to consider control joints, which would be sawcut.

RCC is a bit different than conventional concrete, but the older it gets, the more it tends to work like it.

 

[mickmac]

Thanks for the replies.

My situation is a mine haul road 1000m underground. I've already laid a modified RCC almost 2 years ago with an additive which prevents thermal expansion and renders the concrete 'flexible'. This material had 8% OPC and achieved 32MPa compressive and 10.2 MPa flexural strengths at 28 days. The compressive strength rose to 62MPa at one year.

I am considering there is no need for the additive with an environment which remains constant (temperature and humidity).

 

[cvg]

sawcut joints are for shrinkage crack control. they do little or nothing to control expansion and with your constant temperature, you should not see any appreciable amount of expansion. I would be leary of any "additive" that promises to eliminate thermal expansion and contraction. Same with flexibility. what type of additive are you talking about?

 

[mickmac]

The additive I have used is ConcreCem, produced by PowerCem BV in Holland. I have also had concrete tested beyond the hydrocarbon fire curve(1250C) for 50 minutes. This concrete had no fibres and did not spall explosively nor melt! I have also produced a self levelling cementitious floor screed (compressive strength 20MPa, flexural strength 10MPa) which is not subject to shrinkage nor edge curl. It's all down to the development of acicular strands on a nanoscale which form a mesh like structure in the hydration compounds.

 

[miningman]

Well personally I'd want to know a lot about the stoping system in use at the operation. One project I was involved with called for 80 MPa concrete to be used on the haulage ways. The proposed transport and placement of such a mix caused me nightmares but I had a good understanding of the rock stresses associated with the proposed block caving and I'll guarantee that even 80MPa would be cracking up within 12 months of placement.

Conversely blasthole stoping at moderate depth would not cause any cracking at all. Dony worry about the temperatures , talk to the rock mechanics

 

[mickmac]

Thanks miningman, but the rock mechanics don't seem to be annoying this material even with 50 tonne axles. It's not the compressive strength of this material that matters but the fact that it can withstand repeated stresses with limited micro strains therein. The RCC seems to be behaving like an aspahltic material.

 

[miningman]

Micmac , you misunderstand my post. Its nothing at all to do with the engineering properties of the local rock or concrete, its more a case of how the local in situ streses are redistributed as mining progresses. It's a difficult concept to understand without formal training in mine engineering.

As an example in most coal mines........ low strength rock at relatively shallow depths, its common for the floor to heave upwards by 2 or 4 or 6 feet over a period of a few years at most. Do you think your concrete mix could withstand those kind of stresses?? Similarily block caving can completely collapse underground workings within weeks or months despite the application of steel rockbolts, 100mm of shotcrete AND concrete on the floor.

Let me put it another way, the expression "solid as a rock" is any oxymoron..... in the underground evironment it is moving all the time, the only question is whether its millimeters over decades or several inches / feet in less than a few weeks.

 

[cvg]

unless you have very high tire loads, extremely heavy axle loads, solid rubber or steel wheel vehicles or tracked vehicles, your pavement strength and durability is controlled more by the support given by the subgrade than it is by the strength of the concrete. If the subgrade is cracking / moving a lot due to the drilling and blasting operations or due to subsidence then a rigid pavement will probably flex / shear and then fail. Anything that can increase tensile strength such as your additive might help. A thicker section will also perform better, just costs more.