To experience a miracle just add graphene to concrete during the production process. Graphene concrete is twice as strong, four times as water resistant, with a smaller carbon footprint compared to the conventional processes. Graphene reduces the amount of materials needed in concrete production by nearly 50 percent and reduces carbon emissions by 446 kg per ton. Center for Graphene Science, University of Exeter, United Kingdom. See: http://www.exeter.ac.uk/news/featurednews/title_654766_en.html
It probably will not be commercially available for fixing the terrace this summer.
I live in Norway in a house built in 1963. At some point in the 1960s or 1970s, a large concrete slab was poured with a carport underneath half of it, and an outdoor storage age under the other half. This slab was at first intended to be the site of a house extension. However, the extension built in the 1980s, was in a different location. Instead the slab was used as a deck or terrace. Because of a sway in the concrete, half of the slab – that over the carport – was removed in the 1990s, and replaced with a wooden structure. Now it is time to rehabilitate the rest of the slab so that the storage location underneath can be used more extensively (2018), with wooden terrace boards covering the concrete (2019). The concrete needs to be waterproofed, or sealed!
A concrete building is probably more waterproof than any other common type of structure to begin with, and only cracks, joints, or window and door openings require attention. Some people have suggesting beginning by approaching the problem from three different ways. First, grind rough, uneven concrete so that a waterproofing membrane or slurry has an even surface to adhere to. Second, fill expansion joints or larger cracks up to 6 mm, with polyurethane caulking. Third, fill joints larger than 6 mm, with a concrete patch, which must be completely dried before proceeding further.
Some sources suggest waterproofing only walls with soil on one side and habitable space on the other, but extending this waterproofing to adjacent surfaces by at least 300 mm. Others disagree, and suggest that all concrete surfaces (in wet climates, especially) should be treated.
A cast in place concrete roof typically uses roofing cement and fiber reinforced roll roofing to prevent water intrusion. The challenge with this project is that the roof was never built as a roof, but as a foundation. Other sources comment that if a structure lacks sufficient slope to allow water drainage a tar or synthetic or seamless rubber waterproofing membrane has to be used.
Another popular comment, is that waterproofing won’t work without adequate drainage. This may require a perimeter footing drain or other drain pipe system. It may even require a sump pump.
Sweep the surface to remove dirt and debris.
Test to see if sealer has been previously applied, by pouring a large cup of water onto the concrete. If the water beads up and stays on the surface, it’s been previously sealed. If sealer has been previously used, it will have to be removed using an acid-based chemical stripper, requiring protective clothing, gloves and eyewear. Less toxic and more environmentally friendly products made from soy or citrus are also available, but take longer to work. Using a 25 mm nap roller spread a thick coat of the stripper onto the concrete. Wait. Scrape using a long-handled scraper to remove the sludge. Discard sludge.
If necessary, clean the surface with a stiff brush.
Scrub the floor with TSP (trisodium phosphate) and water to remove loose material, oil and dirt. Let dry.
Sealing/ waterproofing the concrete. Select one (or more) of the following:
1. Concrete repair sealant fills cracks and pitting using a putty knife.
2. Liquid membranes, polymer-based coatings that can be sprayed, troweled, or rolled onto concrete directly, are quick to apply and low cost, but give uneven coverage.
3. Self-adhering sheet membranes are large, rubberized asphalt membranes that are peeled and place directly onto the concrete. They provide even thickness, but are more expensive. They are extremely sticky that is impossible to un-stick it once it is laid. Pay attention to overlap, as improper installation can result in leakage. Cut lap joints properly. Two people are needed to install.
4. EIFS, Exterior Insulated Finish Systems, offer a durable, attractive and simple coating to exterior concrete walls, providing insulation and waterproofing. For a stucco-like finish, an EIFS finish coat can be applied directly to the concrete, filling voids and minor irregularities, and creating a good moisture resistant surface. It is applied with a trowel, and comes in 20 liter buckets premixed and tinted. Float with a Styrofoam block or rubber float to create a uniform surface and texture. Other EIFS products may be sprayed, brushed, or rolled on with a paint roller.
5. Cementitious waterproofing is easy to mix and apply. Mix with an acrylic additive for a better bond, and apply with a long-handled brush. It lacks elasticity, making it prone to cracking over time.
6. Sodium bentonite is a green approach to waterproofing. It is able to cover smooth as well as coarser surfaces.
7. Concrete sealer needs to be applied in dry conditions, because it won’t adhere to damp concrete. Temperature above 10°C during application and three day drying period. Apply two thin coats to ensure a smooth and even finish. Apply the first coat. Wait at least two hours before applying the second, at right angles to your first coat. Do not step on or otherwise use the sealant covered surface until it is completely dry. This can take up to three days.
Note: The information above has been obtained from various sources (online, written, oral) to aid in the solution of a particular construction problem that I face. Your problem is probably not identical, and you too will have to sort through a maze of opinion to find appropriate solutions.
I have not tested any of these proposed solutions, and cannot verify any claims about them. Risks associated with the proposed work have not been assessed, but construction of any sort can be a hazardous activity. Anyone following these instructions does so at their own risk. People who have experience with these methods are invited to share them in a comment.