Concrete for Water Retaining Structure
Author: Aritra Pal; Entry No. 2013CEC8314
1.0 Introduction:
The Aim of this study is to select a better choice of water tight concrete for the water proofing construction of water retaining structures like tanks, reservoirs, swimming pools etc. This paper includes the study of early age & later age properties of concretes with Chemical Admixtures & Supplementary cementecious materials, from the aspect of water tightness & durability.
2.0 Principle of Water Tightness:
• Concrete should be dense & durable
• Concrete must have the capacity to withstand hydrostatic pressure without water seepage.
• Concrete generally contains voids and cracking can be happened by the external loading or internal thermal and shrinkage strain, structural movement etc. These voids and cracks can become the path for water seepage.
• For cracks due to external loading nothing can be done with concrete properties but for internal shrinkage strain good mix design is required with limited cement content, proper placing & curing.
• Dense and durable concrete can be achieved by low water to cement ratio & for the proper placing & curing workability of the concrete mix is one of the important factors.
• For high strength, low permeability, high durability & workability use of admixture is recommended.
3.0 Some Properties that have to be taken care:
• Permeability: Concrete is a mixture of cement & aggregate. Aggregates are generally dense materials so permeability of the cement paste determines the water tightness of the concrete. The gel pores in the cement paste are very small, generally they do not affect the density, but the capillary pores with the micro & macro cracks can form the passage for the water. Capi...
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... the voids and the micro cracks thus they can reduce the permeability & increase the water tightness.
• Pozollanic reaction will convert the high dense C-S-H with large cavities to relatively low density C-S-H with smaller voids
• This properties can increase the strength and durability of the concrete.
Permeability:
• Due to the filler effect of the SCM permeability of the concrete will be decreased effectively.
Durability:
• Steel Corrosion: The low permeability of the concrete will prevent the chloride penetration and thus reduce the chance of corrosion in the reinforcement steel.
Due to the impermeability in the concrete carbon-di-oxide gas cannot penetrate into the concrete and the risk of carbonation will be minimized thereby the attack in steel by carbonation will be reduced.
• Acidic Environment: Due to the presence of large amount of calcium hydroxide
The function of waterproof membrane is to resist hydrostatic pressure exerted by moisture in the liquid state. Other than that, their frictional forces which develop at their interface decelerate due to the movement of concrete slab that related to the sub-base layer. Thus, without waterproof membrane it wills easier the water to trap in the structure and causes of flat roof waterproofing
This process results in concrete with increased strength and decreased permeability. Curing is also a key player in mitigating cracks, which can severely affect durability.
According to major supplies, “Use of synthetic fibers for reinforcing concrete is continuously, increasing. The increase has been considerable since 1980, but slowed somewhat in 1990, a year of substantial construction cutbacks. Apparently the construction community believes there are advantages in the use of synthetic fibers in concrete.” (Schupack) Synthetic fibers are used to improve crack control in concrete. Some reports say that synthetic fiber reinforced concrete (SFRC) would replace welded wire fabric in many slab-on-grade applications. But in reality if the welded wire fabric is placed properly it controls crack width better than the synthetic fiber reinforced concrete. In a case study of the use of synthetic fibers in reinforced concrete, the following conclusions were obtained. No matter what concrete placing job is being done, there is no substitute for good concrete practices. The use of a low fiber volume mix will help control plastic shrinkage cracks and bleeding, but not give good cracking control once the crack forms. The synthetic fibers running through a crack have a poor bond providing no shear friction. Impact and toughness tests on synthetic fiber reinforced concrete imply less edge spalling will occur. The American Concrete Institute (ACI) conducted two studies on polypropylene fiber reinforced concrete, one dealing with plastic shrinkage cracking and the other on permeability characteristics. Plastic shrinkage cracking occurs when the surface water on the concrete evaporates faster than the bleed water reaches the surface of the concrete. It was determined by the plastic shrinking cracking study that polypropylene fibers helped reduce the total plastic shrinkage crack area on test panels. Also determined is that the screeding rate affects the total crack area in polypropylene reinforced concrete, while finishing operations showed no significant effects. This study also suggests the use of longer fibers (about 0.75in.) will produce less crack area. “Permeability plays an important role in long-term durability of concrete materials. Permeability of concrete generally refers to the rate at which particular aggressive substances (water, sulfates, chloride ions, etc.) can flow through the concrete.” (Soroushian) As discussed in the plastic shrinkage study that polypropylene fibers reduce cracking. Less cracking in the concrete surface that surface would be less permeable. In the permeability study, they concentrated on the effects of chloride and the permeability of the concrete. The results of this study concluded polypropylene fibers had little effects on chloride permeability of concrete. The polypropylene fibers only help reduce plastic shrinkage cracks.
Checking the opening of cracks and stress limits, will not be required for concrete elements. When the tensile flexural stress exceeds the effective tensile strength fcteff, then a check for pre-stressed concrete is required. This should normally occur when sections do not have tendons near the tension face or there is no exposure to chlorides, so decompression doesn’t have to be checked. The analysis using uncracked section will be done in sections controlled for decompression and have tendons near the surface tension. As the cracked section analysis is done by computer, it solves the complexity of the calculations. However, this does not mean that pre-stressed cannot be calculated directly from a set of equations as with crack width checks for RC.
Part A: Structural Safety Part B: Fire Safety Part C: Resistance to contaminants and moisture
Because of the existence of some misunderstanding between the concrete manufacturer and the architect/design team, in regards to which party should take the responsibilities to implement the water proofing details and installation. Since the Elliot Lake Inquiry has not come to a conclusion of that matter (Dec.16th, 2013), the responsibility of the waterproofing detailing is not yet clear. These leaks allowed water to penetrate through the topping and let the iron access the frame of the building. Until then, the corroded building’s frame getting worse and yet the roof top parking deck allowed water to penetrate the building until three decades. The corrosion that eventually so severe leads to the failure of the weld on a connection, resulting in the collapse.
In this report, we will introduce and illustrate on precast concrete, pre-stressed concrete, ready-mix concrete, reinforced concrete, terrazzo and Urbanite in details.
In this paper we have studied compressive strength of concrete by replacing natural aggregate with recycled aggregate by 25%, 50% and 75%.This mixture formed in the sample of cubes of dimension 150x150x150mm3.
...n most part if steel were to be used in a Type I structure, it would be to reinforce the concrete because concrete is strong in compressive strength but weak in tensil strength. If there is any exposed steel, it is fire proofed with a fire retardant. Mainly the fire hazards that firefighters have to face in these structures are usually because of human error during the construction of the buildings or when occupants are occupying the building. In a construction sites welding, cutting, plumber’s torches, electrical wiring, and heaters all pose a fire hazard. The other risk of this type of building would be of a collapse of the concrete walls or floors during construction.
As concrete is plastic material in Green State it has to kept in enclosure till its harden. This enclosure is known as ‘ Formwork or Shuttering’.
Besides reducing the usage of cement in concrete, there are several other advantages of using fly ash as supplementary cementitious materials. First of all, it acts as a water reducer. Water can be reduced up to 20%, depending on the quality of fly ash as well as the amount of cement replaced and therefore, act as a superplasticizing admixture when used in high-volume (Mehta, 2004). Besides that, using fly ash makes concrete more workable. This is because the concrete gets more cohesive and the occurrence of costly segregation decreases when less amount of water is needed for the same slump (Bremseth,
Compaction was done as per IS 516: 1959. The moulds were filled with concrete in three layers and after every layer compaction was done with the tamping rod by tamping 25 times. After complete filling moulds were compacted using vibration machine to ensure a symmetrical distribution of concrete.
Mortars consist of finely ground refractory materials which are then mixed with water to form a paste. They are used for laying and bonding shaped refractory products such as bricks. They are normally applied by trowelling [2]. The term Mortar is used to indicate a paste prepared by adding required quantity of water to a mixture of binding material like cement or lime and fine aggregates like sand. The above two components of mortar, namely, the binding material and fine aggregate are sometimes referred to as the matrix and adulterant respectively. The matrix binds the particles of the adulterant and as such, the durability, quality and strength of mortar will mainly depend on the quality and quantity of the matrix. The combined effect of the two components of mortar is that the mass is able to bind the bricks or stones firmly [1-2].
There are two mechanisms which are mainly responsible for this form of healing (1) Due to the continued hydration of unhydrated cement grains and (2) Precipitation of calcium carbonate (CaCO3). Therefore Autogenous self-healing is achieved by hydration reaction of cementitous products present within the matrix. When cracks are formed, the water percolates inside the concrete and reacts with the debris or loose concrete particles and the Portland cement particles remaining unreacted in the cement paste which now reacts to form calcium carbonates and calcium hydroxide resulting in the closure of the crack. Autogenous healing is mostly effective for very narrow cracks. This limitation was addressed by Li and Li (2011), who proposed using engineered cementitous composites (ECC) containing synthetic fibers, such as polypropylene (PP) and polyvinyl alcohol (PVA) for restraining crack width. Moreover, a constant supply of water must be present to support the hydration process. So superabsorbent polymers (SAP) which can store and supply moisture over longer period of time can be used. Their function is to absorb water during wet periods and release it slowly during dry periods. Water is thus made continuously available for the autogenous healing
If concrete mix is designed properly, the use of admixture is not needed. However, to change the properties of concrete the admixture could be most convenient way.