Characteristic/Target Strength Relationships
Detailed physical observation is difficult for concrete placed underwater. Thus, while UWC specimens cast in the dry, much greater variability can be expected in an underwater structure. Allowance is made for such variations by increasing the standard deviation and thus the margin between characteristic strength and target strength. But this extent of the increase is difficult to estimate and needs to take account of detail placing techniques, resistance against washout/segregation, and flow/self-compaction qualities. And it is also suggested to increase the partial safety factor for materials at the structural design stage for such special cases.
Fig : Underwater Concreting
Materials For Underwater Concreting
As detailed physical inspection is challenging to perform so, we try to achieve Self-compacting properties in the concrete, so it becomes necessary to selected aggregates and grading, which have high resistance to segregation and bleeding and have high cohesion.1. Coarse aggregates
The use of rounded aggregates is preferred as it increases the cohesion and reduces the tendency of segregation and bleeding for a given sand friction and cement content. Rounded aggregates achieves more dense packing and have reduced water demand for a given degree of workability. However, sometimes strength and abrasion resistance are particularly significant parameters in some underwater applications, and because of this reason, we need to select crushed rock aggregates. If this is the case, then care must be taken in the overall grading of the aggregate.2.Fine aggregates (sand)
There should be a significant proportion (At least 15-20%) of fine aggregates of less than 300 µm size over and above standard FA. This is necessary to enhance the cohesive properties of concrete placed underwater. When suitable sands are unavailable, it is essential to increase the cement content of mixes significantly, or add pulverized fuel ash or ground granulated blast furnace slag.3. Grading
The aggregate grading requirements are very similar to those needed for concrete pump mixes as underwater concrete needs good flow and self-compacting properties, and sufficient cohesion to resist segregation and bleeding. Pump mix requirements include the above properties plus the need for the cement paste and / or mortar phase to form a lubricating film on the pipe walls. While this latter requirement is not essential for underwater concrete mixes, it is common practice to have relatively high cement contents to improve cohesion, compensate for segregation effects, and allow for the inevitable losses of cement due to 'washout'. Various findings show continuous grading curves have to give the best results. Generally, 20 mm maximum size aggregate is most satisfactory with a sand content of at least 40% of the total aggregate. To achieve a cohesive mix, the relative proportions of coarse aggregate and sand need to be adjusted to minimize the total voids in the mix. This will in turn depend on the shape of the various particles. If required, a 'void meter' can be used to optimize the proportions. This approach should be used if crushed rock aggregates are used.4. Cement
The presence of sulfates in groundwater can pose several major threats as the cement content is high in the design to counteract this sulfate resisting portland cement can be used. However, concrete in the splash zone and above is furthermore vulnerable as not only does sulfate attack occur, but also pressure is exerted by salt crystals formed in the pores of the concrete at locations where evaporation has taken place. The use of cement replacement materials such as fly ash and/or the addition of silica fume can considerably reduce the porosity of concrete and thus its susceptibility to sulfate attack and chloride crystallization. These Cement replacement materials also act as an alternative method of reducing thermal effects and provides additional benefits. ACI committee 201.2R recommends that W/C ratios should not exceed 0.45 in conditions of severe and very severe exposure to sulfates.5. Anti-washout admixtures
Anti-washout admixtures are used to reduce the risk of washout and segregation with the tremie methods of placement, improves self-compaction/ flow properties, and enable methods of placement that are faster and less sensitive to operational difficulties. Materials such as natural and synthetic polymers can be used to improve the cohesion but it is necessary to check that the selected materials are compatible with cement hydrates. Some of them can cause severe retardation of the hydration process and limit the use of superplasticizers. Typical commercial underwater concrete admixtures reduce washout from 20-25% down to about 10%. However, when Portland cement is used with 30% PFA or 50% GGFS replacement, washout is reduced still further for the same admixture dose. So in concluding remarks regarding the Mix consideration for UWC we can say that our aim should be focused such that the following parameters are achieved.- Specified strength and durability
- Self-Compaction
- Cohesive (segregation-resistance)
- Washout-resistance (the degree depending on the method of placement)