In the year 2018, we made a successful publication on Analysis and Design of Swimming Pools and Underground Water Tanks. In this post, we are going to briefly review some construction aspects of water retaining structures with emphasis on the specification of concrete to be used for construction of water retaining structures. To download the textbook on design of swimming pools, with Staad Pro video tutorials and excel spreadsheet for calculation of crack widths, click HERE.
After carrying out the structural analysis and design of an underground water retaining structure, the next step is to ensure that the construction is properly executed such that the water tightness and strength of the element will not be compromised. A good design and a bad construction is as good as a failed project. The recommendations given in the following sections can be followed to ensure good results in construction of water retaining structures.
For construction of water retaining/excluding structures class N (normal hardening) cement is recommended, and should be used ahead of Class R (rapid hardening) due to shrinkage issues. In general, the concrete should be specified in accordance with BS EN 206 and BS 8500 Parts 1 and 2. For water tanks, all materials in contact with potable water will need to comply with specific regulations, and should be non-toxic. This is why all admixtures that will be used must be approved.
Concrete for water retaining structures must have low permeability. Water tightness is referred to the ability of concrete to hold back or retain water without visible leakage (Kosmatka et al, 2003). It is common knowledge that permeability of concrete is related to the water/cement ratio, because it is the mix design factor that is directly related to the permeability of the hardened cement paste. After hydration reaction is completed, the remaining water leaves the concrete slowly, thereby leaving pores which reduce the strength of the concrete and the durability.
It is widely believed that water/cement ratio of 0.2 (about 10 litres of water to 50 kg bag of cement) is needed to complete hydration reaction, while the rest is to improve workability of the concrete. But according to Mather et al (2006), for a given volume of cement to hydrate completely, the quantity of original mixing water required is 1.2 times the solid volume of the cement. The reason for this is that water should be available to fill up the 30% pore space that must be present after the hydration reaction. Ultimately, the authors opined that not all the cement will hydrate if the water/cement ratio is less than 0.4, even though only half of the water will go into chemical combination. Research carried out by Kim et al (2014) showed that porosity in concrete increased by 150% when the water/cement ratio was increased from 0.45 to 0.6.
According to Powers et al (1954) cited by Kosmatka et al (2003), the permeability of mature hardened cement paste kept continuously moist ranges from 0.1 x 10-12 to 120 x 10-12 cm/sec for water-cement ratios ranging from 0.3 to 0.7 (see Figure 2). Also, in a leakage test conducted by Portland Cement Association on cement mortar disks of 25 mm thickness subjected to water pressure of 140 kN/m2; the disks made with water/cement ratio of 0.5 or less showed no sign of leakage after being moist cured for seven days. However, the disks made with water/cement ratio of 0.8 showed leakage after being cured for the same period of time. Research has also shown that concrete cured in water is less permeable than concrete that hardened in air, therefore it is recommended that immediately after removing the formwork of tank walls, the tank should be flooded with water. This also helps in autogenous healing of cracks.
Furthermore, permeability and gradation of the aggregates, the quality of aggregate/cement paste interface, and ratio between cement paste and aggregates affects the overall permeability of concrete. Workability is usually an issue when we try to keep the water/cement ratio low. The best way out is to use water reducing admixtures to make the concrete workable. This is a better option than using water proofing admixtures and high water/cement ratio. Waterproofing admixtures reduce absorption and water permeability by acting on the capillary structure of the cement paste. They will not significantly reduce water penetrating through cracks or through poorly compacted concrete which are two of the more common reasons for water leakage in concrete structures.
Table 1 gives the recommended concrete material requirements for water retaining structures. A concrete specified and prepared as given in Table 1 should not give problems for water retaining structures provided it is well placed and compacted.
Table 1: Recommendation for concrete to be used water retaining structure
In effect, cracking can only be controlled by structural design based on the structural arrangement adopted: introducing movement joints, or by reinforcing the structure properly to limit the crack widths. Otherwise, Type A water protection should be specified. For water retaining structures, the recommended minimum thickness for water tightness is 250 mm, unless the hydrostatic pressure in the tank walls is very low.
Kim Yun-Yong , Kwang-Myung Lee, Jin-Wook Bang, and Seung-Jun Kwon (2014): Effect of W/C Ratio on Durability and Porosity in Cement Mortar with Constant Cement Amount. Advances in Materials Science and Engineering Volume 2014, Article ID 273460, 11 pages http://dx.doi.org/10.1155/2014/273460
Kosmatka, S. H., Kerkhoff B., and Panarese, W. C. (2003): Design and Control of Concrete Mixtures, EB001, 14th edition, Portland Cement Association, Skokie, Illinois, USA
Mather, B. and Hime, W. G. (2002): Amount of Water Required For Complete Hydration of Portland Cement. American Concrete Institute (ACI) Volume: 2 Issue Number: 6 ISSN: 0162-4075 pp 56 -58 http://worldcat.org/oclc/4163061
Powers, T. C.; Copeland, L. E.; Hayes, J. C.; and Mann, H. M (1954): Permeability of Portland Cement Pastes, Research Department Bulletin RX053, Portland Cement Association, http://www.portcement.org/pdf_files/RX053.pdf
Whiting, D. (1989): “Permeability of Selected Concretes,” Permeability of Concrete, SP108, American Concrete Institute, Farmington Hills, Michigan, pp 195 -222.