Expert: Dan Griffin - 3/7/2011

Question
Hey, hope you can help. I'm an expat living in the Philippines and I left my roof pour to the 'experts' (architect and cement company). They poured a 400sqm 5" thick roof slab today when it was obvious it was going to rain.

Long story short - two trucks into the process (about 1hr) along comes the rain - and I mean it was TORRENTIAL rain - the kind you can only get in Asia.

It rained for around 75 minutes and then became a 'sun shower' they busted holes in the outside wall to let the water escape off the already poured cement - and each time they opened a hole it was like a waterfall.

I went home, as I couldn't watch anymore. The architect came over and said that it wasn't 'all bad' that just the top 1/2 of cement has been washed away and that he could cap it later. However I've seen what happens to cement after it sets - (adding to it) - always cracks eventually, never a good bond - never like the pour it should have been. he suggested he might add some wire mesh to it (slapping forehead).

So this was a 15,000usd pour (labor, materials)- what can I do to save it? It's supposed to rain tomorrow as well. Plastic sheeting? nothing?

How can I judge if the entire slab is ruined, or if I can cap it? When can I check? Is there anyway to properly assure a bond? I'm worried about the overall strength now.

Any help is GREATLY appreciated.
;)

Answer
Rice, I really hate it when these things happen.  I've gambled on weather a few times, but it sounds as if your guys didn't listen to the forecast well.

A few facts.  Concrete gets stronger the longer it is kept wet.  Water is great for concrete, you just don't want it applied too soon, and it sure does louse up the finish.  My concern would be the depth of cover over the reinforcing steel and the ultimate structural integrity of the roof slab.  Once the concrete has achieved its initial set, water keeps helping concrete.  Do NOT cover or add any chemicals which would certainly interfere with any cosmetic application of any type of topping cement.

I don't understand the concern over the washed out top if this is the roof, I would assume there will be some type of finish roof installed on top of the slab, be it TPO, hot tar, etc.

I would want the architect/contractor to hire the services of a structural engineer to assure the structural integrity of the roof.  Cosmetics are a minor consideration.  There are some great bonding cements, though if the concrete is sound, I would simply install the finish roofing on the washed out top subject to approval by the roofer.  If this is exposed concrete there are epoxy and other topping cements that have excellent results.  If these are used I would expect arch/cont to provide and guarantee for some substantial time (5 years???)

If I've not understood the issues, please ask again.