In 1942, J.M. Crom Sr. first wrapped high-strength wire in a continuous spiral on the exterior of cylindrical concrete tanks. The “wrapping” method tensioned the prestressing wire before it was placed on the wall, and thus avoided prestressing friction losses. After placement, the wrapped prestressed reinforcement was encased in shotcrete to fully bond each individual wire to the underlying tank wall. This shotcrete cover coat provided mechanical and corrosion protection for the critical prestressed reinforcement on the prestressed concrete tanks (PCT). Note that the shotcrete cover coat method is the industry standard. It is used by all wire- and strand- wound prestressed concrete tank builders in the United States and abroad.
Since 1953, CROM has built over 4100 PCTs. CROM has used shotcrete to build both the structural reinforced core wall and cover coat sections of tank walls in nearly all of our tanks. The outstanding longevity and performance of CROM tanks is a testament to the high quality and durability of properly applied shotcrete. These tanks have truly passed the test of time. Investigation of tanks built in 1957 show no appreciable wear or degradation and are still fully finctional.
The term shotcrete is used interchangeably to describe both a material and a process. In the prestressed tank industry, wet mix shotcrete is the standard method or process used to apply a high cement content and low water/cement ratio concrete material at high velocity onto tank walls. The material used for the wet mix is typically provided by local ready-mix concrete vendors. Using an approved mix design, all ingredients are prepared at the ready-mix plant under strict quality controls and then delivered to the site.
CROM’s standard mix design includes a high cementitious material content (approximately 9 bags of cement with up to 25% fly ash replacement) and a low water/cement ratio (0.42 is the typical maximum) along with typical water reducing and air entraining admixtures. Most CROM shotcrete mixes are grout mixes that contain only well graded sand for aggregate. Pea rock (#7 or #89 size aggregate) may be included in some mix designs for selected applications. When tested for compressive strength, these standard shotcrete mixes typically exceed the specified 4,000 or 4,500 psi design requirements for most PCT designs. The high material strength, in addition to distribution of entrained and entrapped air inherent to the velocity of application, makes for excellent long term durability. Shotcrete mixes are significantly higher in cementitious materials than needed for strength due to the application process.
Concrete pumping equipment moves the mix through a relatively small concrete hose to the nozzle where compressed air is added to propel the material from the nozzle at high velocity. The nozzleman manipulates the nozzle to control the final discharge direction and placement of the material.
Fairly low slump material (typically from 2” to 4”) is pumped through variable lengths of a small diameter concrete hose to the shotcrete nozzle. Note that pumping of concrete is only possible when sufficient cementitious paste is present in the mix to have a sufficiently workable material. The paste acts as a lubricant that allows the concrete material to flow inside hoses and deform through reducers. Typical concrete pump outlets are 5” in diameter and the pumped concrete mix must travel through a series of reducers and then lengths of smaller hose until reaching the 2” nozzle at the discharge.
This pumping aspect of the shotcrete process is actually a very good quality control check of the concrete mix delivered by the vendor. If the cementitious content of a load was too low, the material would not be pumpable. Poor material would never make it to the nozzle and could not be incorporated into the structure being built.
High quality shotcrete is the result of a well-trained team using appropriate materials and equipment in the proper manner to build a structure that has a design suitable for construction with shotcrete. Knowledgeable craftsmen are required to produce the high quality product that results from proper coordination and control of the many variables in the shotcrete process.