Things also to think of:

Above-ground Swimming Pools and Cost
Ignoring the $400 inflatable pool sold by mass merchandisers, the most common pool type is the vinyl above ground pool with 4,300,000 units installed compared to 3,600,000 in-ground pools by the end of 2003! These pools are made of a vinyl plastic "bag" hung on a steel wire frame. Typical sizes are 15 foot diameter circles or 9 X 18 feet rectangles with a flat depth of 4 feet (i.e., a 4 foot wall). Pool entry is via a latter over the 4 foot wall or, in about 30% of the cases, a raised wood deck around the pool edge. Top-of-the-line pools are 16 X 32 feet and cost about $7,200 installed, but not including the wood deck. These high-end models can be installed on a terraced hillside with a surrounding deck which gives the illusion of an in-ground pool. These pools are small (usually less than 10,000 gallons) compared to typical in-ground pools (20,000 gallons) and have a minimal filtration system with no automatic sanitation. On the other hand, over 5 million above-ground filters are sold each year!

In-ground Swimming Pool Construction Materials and Cost
There are three basic pool construction materials: vinyl lined (or package) pools versus Fiberglas pools versus concrete (usually called shotcrete or Gunite) pools. It's difficult to get a long term comparison of these materials because pool contractors build with only one type of material and, therefore, have a financial interest in promoting their particular building method. And no one, like the National Spa and Pool Institute (NSPI), maintains long term records on wear and tear. But there are hints on how these materials wear and age.

Vinyl
The least expensive material is vinyl. Vinyl lined pools are just that, a thin sheet of vinyl holds in the water. The vinyl is measured in "mils" or thousands of an inch. For reference, a trash bag is about 1 mil. Vinyl liners range from 7 mils to 30 mils with the average around 20 mils in thickness. Vinyl lined pools are built with a series of straight and curved 4 foot (1.3 meter) high panels. After the panels are set into the excavation, the pool pit floor is further scooped out for greater depth. This scooping is usually in the form of a short ledge from the bottom of the panel and an angled slope (sometimes referred to as the hopper). The pool pit earth floor is smoothed and covered with a thin 2" cement and Vermiculite mix or PoolCrete (cement with polymer additive). Next, the vinyl sheet is hung from the top of the panels and laid down on the floor. As the pool fills, water stretches the vinyl to give a good looking flat finish to the floor and walls.

Because the panels are just set into the excavation (sometimes with some ready mix concrete poured around the base of the panels) a vinyl pool is filled with water before the pool is backfilled with earth. The pool water pushes out equally on the panels and provides resistance to the earth as it is push up against the panels and compressed. This is a little tricky, since the contractor has to be careful not to get dirt or wet deck concrete into the pool in the last stages of construction.

Unfortunately, vinyl liners are fragile. They can be punctured or separated from the substrate. For example, some vinyl pool contractors use sand instead of a thin coat of concrete as a foundation and in some cases tree roots or nut grass grow through the liner! Also, sharp objects like the end of a pool pole or your pet clawing the pool side can puncture the vinyl and cause it to leak. Once the liner leaks, or if ground water builds up, the liner forms bubbles and drifts away from the wall or subfloor. Long term leaks wash out dirt behind the pool walls and may wash away sand on the floor creating a sinkhole. If the wall panels are made from steel, leaks will corrode the walls requiring repair before a replacement liner can be installed.

The liner is also vulnerable to time, pool water, sunlight, and chlorine. Over a period of years the plastic solvent (plasticizer) migrates out of the vinyl and pool water (especially low pH water) dissolves the calcium out of the vinyl making the liner brittle, which leads to cracking and leaks in the corners or other high stress areas. When draining a pool, the older the vinyl liner, the greater the risk that the liner will not stretch back to its original size or shape without ripping. Sunlight usually fades the faux tile patterns around the water line on the sunny side of the pool. Finally, accidental high concentrations of chlorine bleach vinyl liners. For example, if a chlorine puck (trichloroisocyanurate) is dropped into the pool it would bleach the liner in as little as 6 hours.

Some liners develop pink splotches caused by bacterial organisms that excrete a pink dye. The dye is highly soluble in the plasticizer and is carried through the entire thickness of the vinyl liner. Even if the dye is bleached from the surface of the liner, it reappears as residual dye migrates back to the surface with movement of the plasticizer. If moisture is present behind the liner, bacterial organisms can also be behind the liner which means the liner must be replaced. If there is a constant source of moisture behind the liner, the problem will reappear after the liner has been replaced. In these cases it is necessary to install a moisture poof membrane to isolate the vinyl liner.

A typical liner costs about $4,000 installed and, assuming none of the above problems, lasts about 6 years. If the above problems do arise, homeowners report that the liner is usually replaced each swimming season until the root of the problem is resolved. With vinyl liner pools being vulnerable to rising groundwater, pool contractors are usually cautious about where to locate the pool. They typically only consider a well drained, flat, and previously undisturbed area. Needless to say, if a vinyl lined swimming pool has an elevated water table when it's drained, it will lead to structural damage.

Fiberglass
Next up in cost is Fiberglass. Fiberglass is a sprayed-on resin material that provides an extremely smooth glossy surface. Moreover it's strong and can be made into any shape so it is used to make boats, automobile bodies, as well as swimming pool shells. Fiberglass is said to last longer than vinyl, but when it does age, it does so poorly. If the shinny exterior gel coat cracks, water can leak into the layers and cause blistering. As with vinyl, too low of pH in the pool water will eat away at the gel coat and lead to excessive surface roughness. And over time, the sunlight will oxidize (create a chalky film) and dull the cell coat. Finally, discoloration makes it difficult or impossible to patch a Fiberglass pool, so repairs usually involve the whole structure surface.

Installation is unique in that a crane is used to hoist the pool from the truck, over the house, to the excavation. Once the pool in in the excavation and the plumbing connected, it is filled with water while sand is backfilled against the pool shell. Because of the oxidization problem, new Fiberglas pools are normally covered with a concrete (deck rather than leaving the top edge exposed) and use waterline tiles similar to concrete pools.

A trend in Fiberglass pool construction is that the pool contractors offer a long term warranty that seem to last a lot longer than pool contractor businesses. Of course, when a pool contractor goes out of business, the warranty expires.

Concrete
Last up is a Gunite or shotcrete pool. Shotcrete is the generic term used by the American Shotcrete Association while Gunite is a trade name for concrete blown into place. Sounds mysterious, but it's simply a mixture of water, portland cement, calcium chloride (for rapid mixing and curing), mason's (beach) sand, very small pebbles, and extra fly ash. This "grout mix" is pumped through a 3 inch rubber hose to a nozzle where compressed air is introduced and blows the fine mix onto a surface where it sticks and quickly stiffens. In terms of strength and durability, concrete pools are over engineered for the problem of holding water. As you probably know, concrete continues to cure and strengthen with age. The only real maintenance in a concrete pool is that after about 15 years most of the plaster (white concrete) will have dissolved into the water and have to be replaced. The plaster job for this 20' X 40' case study pool was $3,600.

Concrete dissolves!?! Yup, what pool people call "plaster" is a mixture of water white portland cement, extra calcium (for rapid curing) and marble dust. Marble dust is a mixture of powdered marble and limestone. This means that "plaster" is made mainly from Calcium Phosphate and Calcium Carbonate and has a pH of about 12.5, while "balanced" pool water has a pH around 7.5. So over the years, the mostly-calcium pool surface slowly dissolves into the water much like a marble statue dissolves when exposed to acid rain pollution.

In summary, sunlight appears to be the key aging factor for Fiberglas and vinyl pools. Vinyl pool liners are also vulnerable to minor mechanical problems. But sunlight has little effect concrete pools making the rate of plaster erosion its key aging factor.