How to conditionally concatenate in Excel 2016

Hi all,

I am using Excel 2016 and I want to concatenate values based on their value.

Basically, imagine a column which contains all the values in different rows. Each row of that column has one value. Some values are positive and some are negative.
I want to concatenate two values and drag this formula down. However, if one of these two values is negative then I dont want the concatenation. In other words, I want to find the next positive value to apply the formula.

Can anyone please help me?

I have attached an example. Column A includes the values and Column B includes what I want.

Thanks in advance,
Marios