SELF COMPACTING CONCRETE FOR LNG TANKS CONSTRUCTION IN TEXAS

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1 SELF COMPACTING CONCRETE FOR LNG TANKS CONSTRUCTION IN TEXAS Olivier Bernabeu and Carl Redon Concrete Section, Saipem, France Abstract Two LNG tanks commissioned by Freeport LNG (Texas) used pumped Self Compacting Concrete to erect 0.8m thick walls about 90m in diameter and 40m in height. SCC technology was chosen to speed up the concreting through heavy steel reinforcement and guarantee a smooth wall surface finish. Absence of vibration and reduction of the work force on top of the formwork also made construction safer. To guarantee the tank service life, durability requirements imposed to pass a chloride permeability less than 2000 Coulombs (ASTM C1202) and to sustain cryogenic thermal shock (-162 C) in case of LNG leakage through the inner tank steel containment. To achieve that durability, reduce thermal shrinkage risks, and also to participate to industrial waste recycling policy, the SCC mix design incorporates a high volume (25% of binder) of pozzolanic class F fly ash worked out with a limited 0.37 water/binder ratio. Correspondingly, 70 MPa range strength levels were obtained at 2 months of concrete age. To ensure self levelling properties (slump flows of 75 cm), polycarboxylate superplasticizer was used in combination with a viscosity admixture guaranteeing that locally available 25mm gravel, unusual in size for SCC, would not segregate. 1. INTRODUCTION 1 Freeport LNG a, has commissioned two LNG b tanks for its import and regazeification sendout facility near Freeport, southern Texas. They are based on a Saipem design and have been constructed in by the Technip-Zachry-Saipem c consortium. Usually this type of tank would have been erected with conventional type of concrete and regular concreting techniques making extensive use of workers. However, depending on the country one is working in, associated direct labor costs have to be considered together with ways to reduce duration of construction. Also considering technical issues linked to concrete placement in high wall lifts in a dense rebar and post-tensioning ducts network, the project management a Freeport LNG : sub-company of the American Conoco-Phillips petroleum group b LNG : Liquefied Natural Gas : Natural gas stored at -162 C in its liquid state c c Technip-Zachry-Saipem : joint venture between Technip (Engineering), Saipem (Engineering) and Zachry (Construction) 939

2 early understood that not only concrete material expense had to be considered in the construction cost balance. Choice was then made to attempt the tanks walls erection with SCC (Self Compacting Concrete). That massive operation for massive structures involved about 20,000 m 3 of pumped SCC. In the following sections, firstly, a description of the main LNG tank design principles and corresponding concrete material requirements are provided. The mix design concepts are then described accordingly, as well as concrete production quality actually achieved. Finally, a point is made on advantages of using SCC concreting technique with respect to construction methods improvements and HSE benefits. 2. REQUIREMENTS FOR FREEPORT LNG TANKS 2.1 Tank design principle The here described LNG tank design is named full containment type. The liquid natural gas is stored at negative 162 C in a steel container tank itself surrounded by layers of insulation materials. An outer containment tank made of concrete, 0.8m thick, about 90m in diameter and 40m in height (Figure 1) encloses the whole. Outer concrete tank dome wall Thermal insulation Inner steel tank LNG (liquid gas) slab Figure 1 LNG full containment tank design principle (left) and illustration of an outer concrete LNG tank wall construction (right) The outer concrete tank can be subdivided in 3 main parts: a base slab, the cylindrical wall and dome roof. The concrete tank wall part this paper deals with is pre-stressed by a network of horizontal and vertical post tensioning tendons running in peripheral and U shaped (from slab to dome) ducts later filled with grout. Besides its structural role as an enclosure of the inner steel tank and as a support of the dome shaped roof, the concrete wall is also designed to act as the last barrier retaining accidental LNG leakage. 2.2 Requirements for the in place concrete material The dual role of the wall imposes not only to specify a strength target but also durability criteria for the SCC to be designed in order to lengthen the tank service life. 940

3 The structural tank design only requires a concrete characteristic strength of 40 MPa. However concrete durability requirements focus on two type of aggressions: - permeability to chlorides as that facility is located on a marine terminal. In the present case, according to ASTM C1202 standard, it was requested that the chloride permeability charge had to pass less than 2000 Coulombs. - cryogenic resistance in case the concrete would have to stop LNG leaks. The acceptance criteria requires that after a quenching of the concrete directly dipped in liquid nitrogen at negative 196 C for at least one hour, the thaw material had to retain at least 80% of its initial strength. 3. DESIGNING THE SCC MIX AND PRODUCTION RESULTS 3.1 Cement, addition and water content to target durability When targeting above defined durability, advantage had to be taken of the fact that the United States of America encourages waste recycling for their abundant production fly ash. Class F fly ash, supplied by Boral, has been incorporated in the mix accounting for 25% of the total binder. It was a pozzolanic complement to an ASTM C150 Type II, sulphate resisting, Holcim cement. Mix was designed with water to binder (fly ash + cement) ratio of 0.37 to further achieve low permeability. Due to long afterwards mechanical and piping works, it often takes about an additional year after concreting completion for the tank to run. Then, deliberate choice was made to let the delayed pozzolanic reaction achieve a low permeable CSH (Hydrated Calcium Silicate) microstructure, rather than focusing attention on 28 days strength achievement Charge in Coulombs Compression strength (Mpa) SCC age (days) Figure 2 SCC Coulombs permeability (left scale) and MPA compression strength (right scale) changes with concrete age Measured ASTM C1202 chloride permeability values are given in Figure 2 with corresponding strength gains as a function of time. One can notice indeed that the 2000 coulombs chloride permeability was not achieved at 28 days. Cement hydration being quite advanced at the end of the first month, enough calcium hydroxide was then available. The fly 941

4 ash pozzolanic reaction was to become significant in the second month allowing to further densify the concrete microstructure. At a point, about 56 days of age, the 2000 chloride permeability was then reached. Chloride permeability level as low as 1000 Coulombs, synonym of low permeability concrete according to ASTM C1202 standard (the only standard available so far to quantify permeability on a relative ranking scale) was reached at 90 days of age. As often noticed when durability is a main focus, making use of pozzolans to design a low permeable material, generally results in strength levels which more than likely overcome the specifications. 60, 70 and above 75 MPa values were measured at 28, 56 and 90 days of SCC age. Only as a quick recurrent comment: it is interesting to notice once again the gap between up-the scale strength necessary to ensure durability and the basic strength level required by the mechanical designer. In parallel cryogenic properties revealed to be excellent with more than 87% strength retention. Moreover advantage had to be taken of high content of low cementitious class F fly ash to decrease the heat of hydration. That aspect is of prime importance for thick structures where thermal cracking and DEF (delayed ettringite formation) risks are greater due to the heat mass effect. Beside cost considerations, that was a reason to limit the total binder content to a maximum of 460 kg/m 3 of concrete. Moreover in hot weather days part of the water was incorporated in the mixer as flaked ice. 3.2 Targeting a mix rheology To ensure self placing, SCC usually contains higher volumes of fine particles. With that respect it was decided to force the coarse to fine aggregate mass ratio to 1. However, due to cost considerations, when pouring 20,000 m 3 of SCC, aggregate have to be sourced as locally as possible. The ASTM C33 grade #57 coarse aggregate available could contain up to 5% of gravel as large as 25 to 30 mm in size. In the present case, use of a viscosity modifying agent then become absolutely necessary to avoid segregation of large gravel and compensate for the above noticed limited fine particle amount. However SCC mix rheology criteria were obviously designed to ensure self placing in a 30 C climate. Batching plants were located on the job site and truck delivery never exceeded 10 minutes. Ensuring a 40 minutes workability retention time was sufficient to guarantee that the continuous concrete pouring would lead to ~½ m/h concrete rise in the forms without any risk of cold joint occurrence. In combination with the viscosity agent provided, the Euclid Chemical Company also delivered a polycarboxylate type superplasticiser to reduce the water to binder ratio down to The corresponding slump flow spread was 75 cm with no visible water segregation at the edges of the flow and 83% L Box fill rate was measured (Figure 3). The risk of static segregation was tested directly by mean of in-situ mock-up (see hereafter). Actually the slump flow workability measurement method appeared to be simple and reliable enough to be practiced by the QC operators to control the overall wall concreting. 942

5 5th International RILEM Symposium on Self-Compacting Concrete Figure 3 SCC slump flow test (left pictures) and L Box test (right pictures) 4. CONSTRUCTION BENEFITS 4.1 A wall mock-up to investigate construction feasibility When starting massive construction that size, one cannot rely solely on lab tests results. A mock-up was set up in actual site conditions (hot weather, actual batching plant, truck transit time and pouring pumps, ). Figure 4 illustrates three views of 4.5m high, 10m long and 0.8m wide mock-up under construction, 60m boom pump truck used, and completed trial wall. Figure 4 Mock-up wall and SCC pumping equipment This construction test did validate that pressure at the bottom of the formwork would not exceed ½ the hydrostatic pressure and that SCC workability was suitable to proceed to full scale tank erection. It also revealed that the desired smooth wall surface finish with no significant bubbling was met. Last but not least, this test revealed how important it is to program sequences of concrete boom placing to avoid dynamic segregation of coarser mix constituents that could occur if the mix was let to run more than 10 m away from pouring point. The mock-up turned out to be an excellent training field for concreting operators not necessarily familiar at the time with that new kind of material. 4.2 Decreasing the duration of construction and HSE improvements Instead of processing with regular vibration of ordinary concrete which requires very skilled and numerous work force and results in a lot of noise emission, a crew of only 9 men 943

6 on the formworks could complete 4.5m high SCC lifts instead of regular 3.2m concrete lifts within 10 hours against 12 to 15 hours usually (with at least 25 men on the formworks). Figure 5 The two SCC Freeport LNG tank walls under construction It was then clear that foreseeing about 30% time shortening on the wall construction planning due to the use of higher forms, and a possible 60% reduction in concreting work force, made SCC a very attractive material for wall concreting (Figure 5). 5. CONCLUSION Freeport LNG tanks provide a good example that nowadays SCC mix design is not only able to provide interesting self-placing properties but also high strength and durability levels (here chloride permeability less than 2000 Coulombs and cryogenic resistance) without necessarily using extremely high cement and addition content nor limited aggregate size (here up to mm). Again it is to be reminded that production was here massive (20,000 m 3 ) for massive storage tank structures. But more important is that material engineers get fully involved in overall building cost considerations to better promote that SCC mix design expenses are very advantageously balanced by SCC technical properties, by construction planning improvement as well as work force reduction and HSE benefits. REFERENCES [1] EFNARC, Specification and Guidelines for Self-compacting Concrete, [2] AFGC, Interim Recommendations, [3] SCC Production System for Civil Engineering, final report of task 8.3, Brite-Euram Contract N BRPR-CT [4] SCC Form-system and Surface Quality, final report of task 7, Brite-Euram Contract N BRPR- CT [5] SCC Productivity and Economy, final report of task 7, Brite-Euram Contract N BRPR-CT