I'm drawing a blank figuring out just how many bags of Ready Mix concrete we'd need to replace 6 x 6 timbers that have now rotted through time.. We have a number of walkways that were built by the pervious owner so now we've decided to replace the timbers with a cement border.. We plan on using masonite strips to hold the cement till it hardens , placing them on the weak side ie: grass & sand...

We have 4 strips for a total 230"L x 5"W x 5"H... Cheers. Thanks

Approx 5 bags at 80 lbs

@ dannac, Thanks for the Comeback.. Further to my query on How many bags to do the job... We have a couple of strips from a previous cement job that are approximately 2 inches below the patio surface if you will.. The concern is , people will trip.. Hence the wife would like to build up and smooth out the ripples.. Would we have to spray some kind of additive to get fresh cement to adhere to an existing cemented area or will we be fine?

I'm not following exactly what you plan on doing, but if you plan on adding 2" above an existing slab, I do not think you want to go with bagged concrete.

It will most likely crack, especially if a high traffic area.

There are high strength additives and bagged concrete, but if you are in an area where the winters are hard and freezing the 2" topping probably won't last even if it is high strength. I have seen a lot of cover jobs come apart after a few winters in my area. I can't speak for other places.

Might be a good application for a "crusher run" stone mix. Means different materials in different areas but a well graded stone mix which compacts well.
Similar to: Maryland Materials, Providing Stone Products in Maryland, Delaware and Pennsylvania for 50 Years! (http://www.marylandmaterials.com/products.htm)

You can also find more decorative, colored versions at landscaping outlets or even at big box building suppliers.
Hot mix asphalt also good but not easy to get in small quantities. Cold mix not desirable because it will track on your shoes in hot climates.
But I agree with your spouse, definite safety issue.

If the dimension is not too variable, you could make a "threshold" out of treated lumber and anchor into the existing concrete possibly.

The concern is , people will trip.. Hence the wife would like to build up and smooth out the ripples.. Would we have to spray some kind of additive to get fresh cement to adhere to an existing cemented area or will we be fine?

@smearcase... Thanks for the Comeback... Yes we could have incorporated Pressure Treated wood as a Threshold to part of job but believe it or not the wood timbers were replacing became extremely water logged in a short period of time, some of the handy work was left by the previous owners.... lol.. Management requests more cement than wood.. lol.. Most of the area concerned is less travelled if you will and interlocking brick on both sides and packed tight... I did apply a bonding agent that seemed to quicken the drying process as well, it will be 72 hrs by the end of the day we've had the forms in place... Cheers. Thanks..