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2015SummerShotcreteEMag

Nozzleman Knowledge Supervisors should be aware that the risk of heat- related illness is most acute as the team members acclimate to the new conditions. Addressing this topic in preconstruction safety training and onsite safety talks can help to keep it at the forefront of the employee’s minds. More frequent rest breaks and ensuring sufficient hydration can help to reduce the risks. In addition, the crew members should also be trained to recognize the signs of heat illness and know what to do in the event it occurs in a worker. High concrete temperature is next on the list of conditions that can affect the quality of the Fig. 1: Nighttime shotcrete placement finished product. The temperature of freshly mixed concrete is largely dependent on the con- quickly as possible. As shotcrete placement tends crete ready mix supplier, and the handling of the to be slower than cast-in-place, controlling tem- ingredients that go in to the concrete prior to perature takes on heightened importance. Good arriving on the jobsite. An important step preplanning by the contractor can go a long way in towards keeping mixed concrete’s temperature facilitating this by ensuring that there is proper traffic down is by lowering the temperature of the control and adequate signage for the drivers to find materials used in its production. Acceptable the pump, as well as allowing enough space for them measures to aid in this may include shading of to back up and start discharging their load. In addi- the aggregate stockpiles or sprinkling the coarse tion, the placing crew needs to be ready, and of a aggregate with water for evaporative cooling. sufficient size for the shotcrete work that is to be Chilled water and ice may also be used to replace performed. Next, all forms and grade where the a portion of the required mixture water. Concrete concrete is to be placed should be premoistened and can also be cooled by using liquid nitrogen. kept cool to give the benefits of evaporative cooling. Many contractors find it useful to use retarding Bear in mind that on projects when applying shot- admixtures as well, to delay hydration and sub- crete to hot surfaces, such as slope paving, the initial sequent rapid setting caused by high tempera- temperature of the concrete being shot may matter tures and allow more workable time. less. High ambient temperature and substrate tem- In the Spring 2015 issue of Shotcrete maga- perature will quickly bring the placed concrete zine, the “Nozzleman Knowledge” article temperature in thinner shotcrete sections to equilib- described the many factors to keep in mind when rium so that the initial concrete temperature will evaluating a ready mix supplier. It is important to have little impact. In the case of slope paving of find one that is able to service your project, and large areas, there are three things you can do in hot provide a mixture that has worked well in the past weather. First, continuously wet the subgrade. This under similar conditions. By varying the amount is done so no free water or erosion damages the of gypsum in the cement to affect the set time, slope. Secondly, place your shotcrete promptly. If a the cement industry works to control the propor- nozzle finish is specified, apply your curing material tions of the mixture to account for local and (liquid membrane curing compound or water) as seasonal fluctuation. Prior test results from other soon as possible. A hand sprayer will not suffice; projects, or preconstruction testing, as well as the you will need a power sprayer or a pressurized proportions of the concrete mixture will need to pneumatic tank to apply a liberal amount of cure. be considered when working in extreme heat. This This prompt, liberal (twice the manufacturer’s should be addressed in preconstruction meetings specification if using a curing compound) applica- and submittals. It is also helpful to note the loca- tion will have a significant impact on the reduction tion of the material supplier’s batch plants, as well on the formation of plastic shrinkage cracks. If as the number of trucks that will be available on finishing is required, an evaporation retardant will shooting days, which will make a difference in definitely reduce moisture loss. Again, after fin- the amount of workable time and spacing between ishing, cure the work as soon as possible. truck deliveries of the batched concrete. Shotcreting in hot weather can have advan- Once the concrete is batched, it needs to be tages. Shotcrete contractors in warmer parts of mixed and transported to the project location so the country seldom use accelerators, where in that it can be discharged, placed, and finished as colder climates they are often a necessity. Also, Shotcrete • Summer 2015 33


2015SummerShotcreteEMag
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