Gunite and Shotcrete Construction Method
Concrete pools are built by pneumatically applying a cement-and-sand mixture over a reinforced steel rebar framework excavated to the pool’s designed shape. The two application methods — gunite (dry-mix) and shotcrete (wet-mix) — produce structurally identical shells. Gunite mixes dry Portland cement and sand at the nozzle, giving the applicator precise control over water content. Shotcrete pre-mixes all components before pumping, allowing faster application on large pools.
The finished shell cures for 28 days to reach its rated compressive strength of 3,500 to 4,500 psi. During curing, the shell must remain continuously wet to prevent surface cracking. After curing, a finish coat is applied over the raw concrete — the finish determines the pool’s texture, color, chemical interaction, and lifespan.
Concrete pools are the most common pool type in the Charleston metro area. The construction method allows unlimited custom shapes, built-in features (benches, tanning ledges, raised walls), and integration with the Lowcountry’s variable soil conditions. Concrete adapts to the high water table environment through proper drainage engineering and hydrostatic relief valves that prevent the empty shell from floating during seasonal groundwater elevation.
| Construction Phase | Duration | Key Specification |
|---|---|---|
| Excavation | 2-3 days | Shape and depth per engineering plan |
| Steel rebar | 2-4 days | #3 rebar on 12-inch centers, 3 inches of cover |
| Plumbing and electrical | 2-3 days | Returns, skimmers, main drain, lights |
| Gunite / shotcrete application | 1-2 days | 6 to 8 inches shell thickness |
| Curing | 28 days | Continuous wet cure, no chemical exposure |
| Finish application | 1-2 days | Plaster, pebble, or quartz aggregate |
Surface Finishes and Calcium Hardness Requirements
The finish coat determines how the pool interacts with water chemistry. All concrete pool finishes contain calcium in their matrix — the plaster is literally made of calcium hydroxide mixed with marble dust or quartz aggregate. This calcium-rich composition creates a direct relationship between water chemistry and surface longevity.
Plaster surfaces require proper calcium levels — the PHTA mandates 200 to 400 ppm calcium hardness for concrete and plaster pools. Water below 200 ppm extracts calcium from the plaster through a process called leaching, producing rough white patches and progressive pitting. Charleston Water System delivers fill water at only 40 to 60 ppm calcium, creating an immediate 140 to 160 ppm deficit that requires 12 to 15 pounds of calcium chloride per 10,000 gallons at initial fill.
| Finish Type | Composition | Lifespan | Resurfacing Cost | Texture |
|---|---|---|---|---|
| White marcite plaster | Portland cement + marble dust | 8-12 years | $4,500-$6,500 | Smooth, prone to staining |
| Colored plaster | Marcite + pigment oxides | 8-12 years | $5,000-$7,000 | Smooth, hides some staining |
| Pebble finish (PebbleTec) | Cement + natural pebble aggregate | 15-20 years | $8,000-$12,000 | Textured, durable |
| Quartz aggregate | Cement + quartz crystals | 12-18 years | $6,000-$8,000 | Semi-smooth, reflective |
Maintenance Profile and Resurfacing
Concrete pools require more chemical attention than vinyl or fiberglass alternatives because the calcium-based finish actively participates in water chemistry. The porous plaster surface harbors algae more readily than smooth gelcoat or vinyl, requiring consistent chlorine residual and regular brushing — weekly brushing with a stainless steel bristle brush prevents algae from embedding in surface pores.
Scale deposits on pebble and plaster finishes appear when calcium hardness exceeds 400 ppm or pH rises above 7.8. The calcium-rich finish provides nucleation points that accelerate scale bonding compared to non-calcium surfaces.
Acid wash for plaster resurfacing is the standard renovation method — a 50/50 solution of muriatic acid and water strips the top layer of stained or etched plaster, exposing fresh material underneath. A full acid wash removes approximately 1/16 inch of plaster per application, limiting the procedure to 2 to 3 applications before complete resurfacing becomes necessary. Plaster resurfacing costs in the Charleston area range from $6,000 to $8,000 for standard marcite plaster on a 15,000-gallon pool.
Professional gunite pool maintenance includes weekly EDTA titration calcium testing, pH monitoring, and surface inspection to catch leaching or scaling before visible damage occurs.
Related Pool Care Concepts
Concrete pool longevity depends on calcium hardness management — the plaster finish contains calcium that water will extract if dissolved calcium levels fall below 200 ppm. Excess calcium produces visible pool scaling on tile lines and within the porous finish texture. Muriatic acid serves dual roles in concrete pools — lowering pH during routine maintenance and stripping stained plaster during acid wash renovation. Charleston’s soft municipal water makes calcium supplementation the single most critical maintenance task for every concrete pool in the tri-county area.