Portal: General

The proper encasement of reinforcement in shotcrete is a criti-cal issue with respect to the quality and the durability of a shotcrete application. One simply has to refer to the Shotcrete Nozzleman Training Course offered by the ASA (see Shotcrete Magazine, Vol. 1, No. 4) or the recently available Nozzleman Certification Program of ACI to confirm this statement. Although many concerns have been raised, mostly by owners, regarding the presence of voids around reinforcement and their potential effects on structural per-formance, there is, unfortunately, little technical data available to back up the shotcrete industry on this problem.
Several years of experience with a certification program for shotcrete nozzlemen, as well as early research (Studebaker, 1939) suggest that the best approach is to apply dry-mix shotcrete1 at its wettest stable consistency, which is defined as œthe consistency at which the moisture content is the maximum, the maximum being determined by the stability of the fresh gunite (shotcrete). How-ever, observations on many job sites and training of nozzlemen show that many apply shotcrete with a relatively dry (stiff) shooting con-sistency, which may adversely affect rebar encapsulation as well as increase rebound.
The Industrial Chair on Shotcrete and Concrete Repairs of Laval University (City of Quebec, Canada) has initiated a thorough inves-tigation into the Evaluation of reinforcement encasement quality and its effect on shotcrete quality. This research program is financed by the partners of the Industrial Chair, the Concrete Research Council of ACI, as well as by the American Shotcrete Association (ASA). It has several objectives, including:

Scaffolding, in my opinion, could be listed as the #1 item on a top ten safety checklist. I believe it becomes 50% more diffi cult to work safely and properly on scaffolding than on the ground. Shotcreting using the dry-mix process is much easier to handle because of the weight of the hose, and

Shotcrete has been an important part of the construction industry for more than 90 years. As a specialty concrete technique, it is basically another means for the placement of concrete with its own peculiarities and characteristics. In the early years after its introduction by the Cement Gun Company of Allentown, Pa., in 1910, relatively little testing was done, prima-rily because the technique had limited use. What testing was performed was done to pro-mote the technique”to show its efficacy for specific applica-tions or to exhibit its superior-ity over other existing concrete technologies. The tests in-cluded were for material and design criteria and properties such as compressive, tensile, and flexural strengths, bond, permeability, shrinkage, and soundness. These tests were based on American Society for Testing and Materials (ASTM) concrete tests adapted for the shotcrete process. When inter-est in concrete durability began to heighten after World War II, freeze-thaw tests were also in-troduced. As in conventional concrete, compressive strength has been the defining property of shotcrete testing. However, if other properties are required for a particular application, they can be arranged at the time of specification. The main dif-ference in the tests is in the preparation of samples, which is usually different because of the unique nature of the shotcrete process.
While interest in shotcrete was limited prior to the 1940s, an upsurge developed, espe-cially in the wet-mix process, in the 1950s. In 1990, ASTM decided that the technology had grown sufficiently enough that a new ASTM subcommittee on shotcrete, ASTM C09.46, should be organized. This would complement the existing American Concrete Institute Committee 506 on Shotcreting. ASTM Subcommittee C09.46, Shotcreting, would absorb the

A crane operator, carpenter, baseball player or a shotcrete crew? Probably everyone listed should use them. On a scale of 1 to 10, the shotcrete crew would rate a 10 with need. œWho on the crew should wear them? I asked one of my superintendents, she said, œThat™s easy, everyone on the crew. The nozzleman and air lance man for obvious reasons, but also the pump operator and potman to protect themselves from dust and shotcrete overspray and rebound. The finishers also need eye protection from constantly working vertical concrete surfaces. The laborers also need protection from rebound, dust and overspray, and the occa-sional burst hose or coupling!
A shotcrete operation requires several high-pressure pieces of equipment, all of which, unfortunately, can burst. The dry-mix

Shrinkage is a potential problem in shotcrete construction that can lead to restrained shrinkage cracking.

Cheap Insurance: Most of us in the shotcrete industry have had a plugged hose creating either a dust storm with the dry process or concrete over-spray everywhere with the wet process. When a hose is plugged, it has been my experience that everybody on the crew starts yelling and running toward the operator. Communication between the nozzleman and the gun/pump operator is one of the most important and critical links on the job for productivity, and most of all, safety for everyone. Unfortunately, because of different job site layouts, continual rotation of people, and equipment movement on and around the job site, good communication is often diffi cult to achieve or maintain.

Shotcrete has been widely used by the Ministere de Transports du Quebec for restoration and rehabilitation. Shotcrete has been widely used by the Ministere de Transports du Quebec for restoration and rehabilitation.

Quebec city possesses a historic arc.a in which many concrete structures need repair. For economic as well as practka] reasons. since some of these structures are curved. shotcrete is the logical solution when a repair is considered. In order to provide attractive looking finishes for the re­ paired structures, ii was decided to see if it was possible to obtain a better finish ap­pearance than the usual wood trowel shotcrete finish.

The placement of high strength wet-mix shotcrete is sometimes complicated by the compromise required between obtaining suitable pumpability and shootability.

There is probably nothing as frustrating to even the most tolerant wet-mix shotcrete placing contractor or crew as problems related to pumping. While the following comments are spe-cifically related to shotcrete placement, many of them are also applicable to normal concrete pumping applications. The rheology of shotcrete mixtures is affected by the pumping pro-cess, which typically involves repetitively compressing a shotcrete mixture and forcing the mixture to the point of final placement under pressure.