CROM's Shotcrete History & Use
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 also provided corrosion protection for the critical prestressed reinforcement on the prestressed concrete tanks (PCT). Today, a shotcrete cover coat method is the industry standard used ALL wire- and strand-wound prestressed concrete tank builders.
Since 1953, CROM has constructed over 4300 PCTs. CROM uses shotcrete to build both the structural core wall and cover coat. The longevity of CROM tanks is a testament to the durability of properly applied shotcrete; they have truly passed the test of time. Recent investigations of tanks built in 1957 showed no appreciable wear or degradation and remain fully functional today.
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 process used to apply a highly cementitious concrete mix with a low water/cement ratio at high velocities to the wall. The material used for the wet-mix is typically provided by local ready-mix concrete suppliers. Using an approved mix design, all ingredients are prepared at the ready-mix plant under strict quality controls prior to delivery to the site.
CROM’s shotcrete mix designs include over 850 lbs of cementitious material producing a water/cement ratio below the 0.42 maximum allowed by AWWA D110. The mixes are fortified with typical water reducing and air entraining admixtures. The high cementitious content and low water/cement ration applied at high velocity results in a higher density of internal bonds, higher strength, and lower permeability. These attributes also create a material with better durability than standard cast in place concrete. When tested for compressive strength, CROM’s shotcrete mixes typically exceed the specified 4000 or 4500 psi design requirements by 50%.
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.
Note that pumping equipment of concrete is only possible when sufficient cementitious paste is present in the mix. The paste acts as a lubricant that allows the concrete material to flow inside hoses and defore 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.
The pump-ability of a mix is a very good quality control check. If the cementitious content is too low, the material will not pump. Therefore it is very unlikely, poor material could make it to the nozzle and be incorporated into the structure being built.
High quality shotcrete application is only possible with knowledgeable craftsmen using appropriate materials and equipment in the correct manner. Proper coordination and control of the many variables in the shotcrete process are critical for a successful project.