How to support dual folding clothesline?

[Hembi]

Can I use post supports bolted to 100mm thick concrete to hold up the posts for a dual folding clothesline. 
if so, what type. 
The posts that hold up the line are 50x50 galvanised and the line will sit 1900 off the ground. 
I can’t dig through the concrete 

[MitchellMc]

Welcome to the Bunnings Workshop community @Hembi. It's wonderful to have you join us, and many thanks for your question about installing posts for a clothesline.

Would you be able to let our helpful members know what clothesline you'll be installing? That way, they'd be better informed and able to offer solutions. Depending on the size and how many lines your clothesline has, it might alter their recommendations.

Typically folding clotheslines need to be mounted securely to the wall or on posts concreted into the ground. Once the dual clothesline is extended and fully loaded with moist clothes, it supports a great deal of weight. The issue you will have is attaching the support posts to the flat concrete. 

We do have Australian Handyman Supplies 50 x 50mm Internal Fixing Post Brackets, which would bolt to the concrete and hopefully allow you to slip your 50mm post over them and then bolt it in place. This type of fixing method isn't designed to bear weight off to one side, though, as would be the case with your clothesline. I suspect you'd then need to add some solid angle brackets to reinforce the connection. I've created a rendering for you below to illustrate better.

I'm looking forward to hearing more about your project and assisting further.

Mitchell