MARINE CONSTRUCTION: UNDERWATER CONCRETE. Tremie method, which is the most common method used for underwater concrete placement. Underwater Concrete - Mix Design and Construction echecs16.info - Download as PDF File .pdf), Text File .txt) or read online. civil. Concrete Placed Underwater – Literature Search & Synthesis Researchers must submit a PDF version of their proposal by. PM (CST) by.
|Language:||English, Spanish, Hindi|
|ePub File Size:||16.37 MB|
|PDF File Size:||15.37 MB|
|Distribution:||Free* [*Register to download]|
PDF | Placement of concrete underwater is necessary in the implementation of most in-shore, and off-shore structures. The pouring of. PDF | On Nov 6, , M. F. Najjar and others published Underwater Concreting by Using Two-Stage (Pre-placed Aggregate) Concrete”. SPECIFICATION M. SPECIFICATIONS FOR UNDERWATER CONCRETE. 1. DESCRIPTION. The work will consist of the mixing of concrete in.
Khayat and Mohammed Sonebi The underwater casting of relatively thin lifts of concrete in water decreases with the shear rate that facilitates pumping and requires the proportioning of highly flowable concrete that can placement. The shear-thinning behavior is affected by the resist water dilution and segregation and spread readily into place. An investigation was carried out to determine the effects of anti- Anti-washout concrete can be highly thixotropic, whereby a washout admixture concentration, water-cementitious materials relatively fast buildup of viscosity can be observed at a given ratio, and binder composition on the washout resistance of highly flowable concrete. Such thixotropy can contribute to stability of Two main types of antiwashout admixtures were used: 1 a pow- freshly cast concrete, including the resistance to water erosion dered welan gum at concentrations of 0. The water-cementitious materials successful underwater placement contradict one another. The concentrations of anti-washout combinations of HRWR and AWA, it is possible to secure admixture have direct impact on washout resistance. For a given high-performance concrete for underwater applications.
In principle, chemical admixtures should not be used to compensate for poor mixture proportions and poor materials quality. Only when the concrete proportions are optimized can chemical admixtures effectively improve the performance of concrete. In summary, the mix design for underwater concrete must be performed with sound technical know-how and thorough considerations for specific project requirements, as any sizable defect due to improper mix design is likely to result in costly repair and less quality structures.
The mix design is essentially an optimization process. The process should be guided by a set of governing variables and an understanding of how each variable affects the concrete performance.
Location of a concrete batch plant is an important consideration in logistics planning and has significant implications in construction cost, risks, and quality control. The batch plant can be established either onshore or offshore, depending on the placement plan and site conditions.
The offshore production option has main advantage of more reliable control of the concrete workability at the point of placement, because the time between concrete batching and placing is relatively short. However, this option could entail a significant investment in the equipment. downloading or leasing such a facility is justifiable only for the largest projects. Other concerns include logistics of materials supply and equipment maintenance. In general, it is difficult and costly to maintain consistent concrete materials quality on barges e.
As materials in storage are consumed, the floating plant will list and trim. The batch scale must be supported in such a way that gives accurate weights despite the barge list and trim. In general, equipment breakdowns are likely and difficult to repair offshore.
In order to ensure continuous placement of underwater concrete, consideration should be given to provision for redundant equipment supplies including the essential accessory items such as barges, tug boat, and lighting , and key standby equipment such as pumps and tremie pipes.
Alternatively, a batch plant may be set up on shore and the concrete is transported to the placement site by transit mixers or hoppers on barges. This often creates logistic problems with regard to the time lapse between concrete mixing and concrete placement. In any circumstance, the concrete mixes must be able to maintain all the required properties such as flowability, cohesiveness, and self- compacting characteristics over the work window. Underwater concrete construction of many bridge foundations frequently consider delivery of concrete on barges, often with a retarding admixture in the concrete, and then re-mixing after arrival at the site.
This method was successfully used to place over 25, cy of tremie concrete during construction of the Braddock Dam. In construction of the Dame Point Bridge, however, the same scheme encountered some difficulties. The concrete mixture was pre-cooled by injection of liquid nitrogen at the mixing plant.
But delays in delivery due to roadway traffic allowed the concrete warm up to near ambient temperature, resulting in extensive thermal cracks in the concrete mass. The lesson learned is that roadway traffic could unexpectedly interrupt concrete delivery. A near-shore plant should be required for this option. Once the concrete batch and mixing plant is selected, the effective mixing time is critical in defining the peak concrete production and placement rates.
It is desirable to determine the mixing time in field mock-up tests taking into account the essential field variables. The mixing time should be such that all the concrete ingredients are fully dispersed and the concrete reaches workable consistency.
The choice of a proper underwater concreting plan for a project has to be ultimately determined by the site conditions, engineering requirements, availability of equipment, and cost. Placement Rate The rate of concrete placement is a critical parameter to the quality of in-place concrete, the form pressure, and the construction planning in general.
High quality of underwater concrete is obtained through a continuous pour at a consistent placement rate.
An interruption of concrete placement for a period to the concrete set time will result in a cold joint. Given the difficulties with underwater preparation of cold joints, cold joints generally degrade the quality of in-place underwater concrete. Thus, it is essential that concrete be continuously produced and delivered to the placement point at the required placement rate. It is also essential that the necessary quantities of materials can be supplied to the batch plant at the required rate.
The logistical planning should include provision for alternative or redundant supplies, provision of all the accessory items, and standby key equipment. Before chemical admixtures were widely used in concrete construction, underwater concrete mixtures generally had stiffer consistencies than the concretes used today.
Consequently, concrete placed at a slow rate often had very uneven, steep surfaces and a non-homogeneous distribution of concrete mass. In practice, it was found that the rapid placement of concrete resulted in marked improvement in the tremie concrete quality. The technique of the rapid placement takes advantage of the dynamic energy of flowing concrete to overcome the lack of concrete flowability. With a highly flowable concrete mixture, the kinetic energy of the fast flowing concrete is not always required for placement of good quality concrete.
Nevertheless, a smooth and continuous tremie placement is still essential for good quality concrete. In general, a placement rate of 0. Placement Sequence Planning a tremie placement sequence must be based upon the size and geometry of the placement area, the available concrete production and delivery capabilities, and the concrete mixture properties.
There are two basic schemes to sequence a tremie placement. The first scheme is to feed concrete into several tremie pipes at about the same time. Thus, the concrete rises everywhere at approximately the same rate. In this case, the maximum concrete flow distance is approximately one half of the tremie spacing. This placement scheme is suitable for tremie placement in small areas. For relatively large concrete placement, however, this scheme demands very high, and sometimes impractical, concrete production capacity.
Furthermore, cold joints can potentially form between two adjacent tremie pours as laitance accumulates at the boundaries. A practical method is to divide a large area into several smaller areas. Both walls of steel sheet piles and walls of precast concrete have been used.
Within each confined area, the simultaneous placement scheme can be applied. The second placement scheme is the advancing slope method.
In the scheme, the placement starts at one location and progressively proceeds to cover the entire area. Only when the concrete at the tremie location arises to the required elevation and an adjacent tremie has immersed in concrete by at least 0. The tremie concrete flows out with an advancing slope. The surface slope of tremie concrete usually ranges from 1: Thus, the tremie placement advances from one end of the placement area to the other end, following an advancing slope of the tremie concrete.
The main advantage of this method is that it imposes less demand on the concrete production capability than the first scheme. In addition, the scheme facilitates the removal of laitance. As the placement progresses from one side to another, most of the laitance is pushed to the front edge of the advancing slope and eventually collected at one end of the form. Then, the top of the hardened concrete can be jetted off and the suspended laitance be removed by air lifting or eduction.
This method eliminates the potential cold joints between adjacent tremie pours.
In some large-scale concrete placements, a combination of the simultaneous placement and the advancing slope scheme is the most appropriate, i. The objective of this approach is to achieve optimum balance between the required placement rate and the concrete production capability.
Placement Method Underwater concreting is currently carried out by five basic placement methods. They are a the tremie method, b the pump method, c the hydrovalve method, d the buckets or skip method, and e the preplaced aggregates method. Among them, the most common placement methods are the tremie method and pump method.
These two methods function in fundamentally different manners. While tremie placement deposits concrete solely by gravity feed in an open-to-atmosphere system, the pump method utilizes surges of pump pressure to deliver concrete in a closed system. As a result, the technical requirements and inherent risks with the two methods are substantially different. The tremie method is a way of placing underwater concrete by means of gravity flow. The tremie system basically consists of a rigid pipe suspended vertically through the water and a hopper fixed on top of the pipe to receive concrete.
Any concrete added above the hydraulic balance point will cause concrete to flow. The more concrete added above the point, the faster the concrete flow rate. Thus, the concrete flow rate can be reliably controlled by the speed in which concrete is fed to the hopper. The tremie method has proven to be the most reliable way of placing high quality concrete.
Its main advantage is that tremie concrete can be deposited in a continuous and controlled flow speed with little turbulence. If the concrete is carelessly dumped into the tremie, it defeats the purpose of the tremie method.
In order to minimize cement washout and laitance, the placement operation should cause as little disturbance to the concrete underwater as possible. Most of the disturbance occurs during starting and restarting of the placement, or due to loss of the seal, or by dragging the tremie horizontally while embedded in the concrete underwater. This requires that the tremie pipe be embedded in fresh concrete to a minimum depth of 0.
Vertical movement of the tremie pipe should be limited to that absolutely necessary. Horizontal movement of embedded tremie pipes should be generally prohibited. There are two basic techniques to start tremie placement — the dry method and wet method. While the dry method utilizes an end cap to seals off a tremie pipe from the water entry, the wet method utilizes a moving plug to prevent the concrete from mixing with water.
The plug fits tightly inside a tremie pipe. As concrete is fed into the tremie, the plug slides down under weight of the concrete and push out water in a piston-like action. Although good quality concrete has been placed by both tremie method and pump method, past experience and further studies found that the quality of tremie concrete is generally superior to that of pumped concrete especially in deep water.
There are two main reasons for this phenomenon. The first reason is related to the concrete flow rate. When pumping concrete down directly to its deposit area, the pump pressure surges plus self-weight of the concrete are at times much greater than the hydrostatic balance head outside the pump line.
Thus, the concrete exits the pump line at an uncontrollably high speed, causing significant disturbance to the concrete that has already been deposited. Secondly, a pump system is closed to the atmosphere. If concrete is being pumped down into deep water, concrete may fall at a rate faster than the pump output.
As a result, a vacuum will be created in the pump line. The vacuum pressure so created will suck away the cement paste from aggregates, causing segregation of the concrete. Nevertheless, the pump method is an excellent way of placing underwater grout or concrete containing only pea gravels. In this case, a pump line has a small diameter and the ratio of the skin friction to concrete volume is high.
The skin friction slows down the speed of concrete fall. It is a good practice to install an air vent over the top bend of the pump line so that the pump system is open the atmosphere and the grout flow is controlled by the hydrostatic equilibrium. The latter consists Fig. Because of the viscous nature of the concrete containing an AWA, washout. For each concrete, the ranges of The repeatabilities were investigated for concrete the concentrations of AWA and SP used are indicated.
Generally, the standard devi- Contml 1 - 2 - 3 6 - 7 - 8 ations of slump and slump flow were Mix 15 found to be lower than 10 and 25 mm, respectively, as indicated by the Nature of test Rep. The dispersion of measure- FineA. However, the coeffi- Rep. This reduction can be 3 0.
On average, the The results of workability tests are shown in Fig. A reduction of the 6 0. The 0. The dosages of 1. Workability of the mix was assessed The effect of the AWA and SP variations on the loss by the slump and the slump flow tests, and the washout was of mass by washout is shown in Figs. The results of the workability and washout tests are given in Table 4. Otherwise, in case of a simultaneous Control 0. These mixes also showed the 17 2. The AWA 3 : Loss of mass by washout using a big basket and big tube appeared to increase viscosity and reduce washout, while the SP enhanced the workability.
For a tively. This sharp tion in the SP alone caused a decrease in values of these increase of the loss of mass by washout was less affected properties. In the case of the AWA content of 0. The increase in dosage of AWA reduced slump and slump flow as shown in Fig. However, the increase 0 0. For example, adding 0. When the AWA dosage increased from 0. Following this investigation, the comparison The losses of mass by washout measured using the CRD between the washout tests CRD C61 and MC-1 can be C61 basket and the big basket and tube were compared to summarised as follows: the loss of mass by washout obtained with the MC-1 test.
CRD C The - The final test results of the MC-1 and the CRD C61 results of the loss of mass by washout measured by the are quantitative; however, an additional visual assessment standard plunge test CRD C61 using a tube of mm of the washout is also available when using the MC-1 diameter and the big tube of mm diameter were test.
This difference can be explained that of the CRD C61 test for the same concrete. The by the fact that the big basket was made of mesh with MC-1 is a more severe test.
The 4. The loss of mass using the MC-1 test was Based on the comparative study between the loss of higher than all other tests The coefficient washout obtained using the MC-1 test is greater than of correlation between the loss of mass by washout after that of the CRD C61 This of results obtained using the MC-1 apparatus is consid- relationship is valid where the loss of mass by washout is ered acceptable.
This result is in line with results of previous research . The concrete was mixed for 3 min. Slump flow values greater than lowed by 1 min. The HRWR demand of mixtures prepared The consistency was evaluated using the noted slump and slump flow tests 1 min. The slump flow corresponded to the mean base diam- Table 3—Results of mixtures with 0. Cumulative loss in mass is 1.
Table 5 gives these results for mixtures prepared with 0. With 0. This leads to the in- Fig. As expected, the increase in AWA dosage crease in free water content that reduces the ability of the necessitated greater addition of HRWR to maintain a paste to retain water and suspended solid particles and fines. For example, the increase of welan gum For any given consistency, the increase in welan gum dos- from 0.
For example, for the gum and 0. Further increases in welan gum to 0.
Similar data are reported in Fig. Therefore, the critical consis- tency beyond which sharp increases in washout loss occur can be extended with further additions of AWA. This is due to the increase in viscosity and water-retention capacity of the paste. Table 4—Results of mixtures with 0. For example, for concrete out resistance.
For example, for a high slump flow of mm, with 0. Table 5—Results of mixtures with 0. High-range water-reducing admixture.
The high substitution of cement by to the enhancement of packing density of the binder and granulated blast-furnace slag capable of retaining less water the improvement of cohesiveness of the paste resulting from than cement, silica fume, or fly ash can result in a greater greater retention of free water.
This reduces the tendency of volume of free water and higher risk of water dilution.