OUTCOMES OF 3-YEAR CONCRETE FOLLOW UP FOR THE LUSAIL RAIL TRANSIT SYSTEM IN DOHA (QATAR)

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1 OUTCOMES OF 3-YEAR CONCRETE FOLLOW UP FOR THE LUSAIL RAIL TRANSIT SYSTEM IN DOHA (QATAR) Lionel LINGER, Laurent BOUTILLON VINCI Construction Grands Projets, 5 cours Ferdinand de Lesseps, Rueil-Malmaison cedex, France Clerencio RABULAN, Philippe TAVERNIER QDVC (Qatari Diar Vinci Construction), Tornado Tower, 33rd floor, Doha, Qatar Abstract More than m 3 of concrete has already been poured for the LRT project in Doha during the last past three years. For the completion of this underground structure, C40/50 concrete mix has been designed with a triple blend cementitious material (GGBS/OPC/MS), natural washed sand, limestone coarse aggregates and a low W/B ratio close to Polypropylene fibers (2 kg/m 3 ) have been introduced in several concrete structural elements for fire resistance purpose. Routine control tests at both 28 and 56 days have been regularly performed on several durability indicators (chloride migration coefficient, rapid chloride permeability, water penetration, water absorption). This article summarizes and discusses the achieved results corresponding to more than 80 samples during a 3-year period which allow, then, some statistical analysis. From the observed trends, it can be stated that high durability performance concrete has been produced and placed until now. It is noted that some testing protocol are leading to highly dispersed results. Finally, it is also observed that Polypropylene fibers have no major impact to the overall concrete durability. Keywords: Durability indicators, chloride-induced corrosion, triple blend cementitious materials, polypropylene fibres, permeability. 1 Introduction QDVC (Qatari Diar Vinci Construction) is building the first railway transportation system in Qatar in the new city of Lusail, North Doha, which will accommodate people with job for people and around visitors per day. This is a true challenge to build a light metro in a town not yet existing; a challenge of anticipation of the future needs of the city. The project consists of the following: 4 tramway operating lines 15.3 km of double track at grade 6.7 km of single track at grade (and additional 8 km of track at depot area) 6.4 km of underground double track and 1.0 km of single track in cut and cover tunnel 26 stations at grade 8 underground stations and 1 pseudo underground 1 viaduct crossing over the North Road and the regional railway including the elevated Al- Khor interconnection station 1 depot, maintenance/storage and operations centre facility plus test track 34 tramway vehicles utilizing catenary free and overhead catenary technology So far the signed contract s phases represent less than 20% of this amount including the works currently completed.

2 2 Durability challenge Fig. 1: LRT project s overview Premature deterioration of reinforced concrete structures has been a feature of construction in the Middle East in general and in the Arabian Peninsula area in particular for the last few decades due to the harsh prevailing environment. Problems have been experienced in structures after as little as 5 to 15 years due to chlorideinduced corrosion of reinforcing steel which is the most important deterioration mechanism for concerned concrete structures as well as other mechanisms such as sulfate-attack. It is also recognized that workmanship, concrete mix designs and the implementation of a very strict quality control are key factors in achieving design durability provisions. Deterioration mechanism Chloride induced steel reinforcement corrosion Carbonation induced steel reinforcement corrosion Sulfate attack Delayed ettringite formation (DEF) Alkali aggregate reaction (AAR) Table 1: Durability risk assessment Assessment and mitigation measures Select adequate concrete mix design (relevant cementitious material type, low W/C ratio, low concrete water permeability and chlorides ions diffusivity) associated with relevant steel reinforcement concrete covers. Implement a proper concrete placing and curing. Under the prevailing climatic conditions, measures to avoid chloride induced corrosion (dense and compact concrete type) with appropriate steel reinforcement covers will automatically be sufficient to avoid carbonation induced corrosion. The risk of sulfate attack of the concrete can be countered through the selection of relevant cementitious material types and low W/C ratio (< 0.40) offering sufficient physic and chemical sulfate resistance in the environment. Implement additional protective measures (waterproofing membrane for underground structures, coating for airborne salts exposed structures). Risk of DEF is impeded through imposing a temperature requirement of maximum 75 C during hardening of the concrete mix design including high amount of mineral additives ( 50 % GGBS). Risk of alkali aggregate reaction is eliminated by appropriate selection of non-reactive fine and coarse aggregates. The use of high amount of mineral additives (> 50 % GGBS) mitigates also AAR risk.

3 As far as concrete material is concerned, it can be stated that 28 days strength grade requirement is much more dictated by structural matter rather than durability issue, which must mainly be addressed by taking into account relevant requirements dealing with cementitious materials type, dosage and W/C maximal ratio. In order to assess the transfer (water, gas, ions) intrinsic properties of hardened concrete, it is more and more becoming a standard practice worldwide to measure concrete durability indicators, such as water penetration test (BS EN Part 8, formerly DIN 1045), absorption (BS1881 Part 122), Chloride migration coefficient (NORDTEST NT BUILD 492) and/or Rapid Chloride Permeability test (AASHTO T227 or ASTM C1202). 3 Concrete mix designs Two main concrete mix designs are used for the LRT project civil works completion. Table 2: Concrete mix designs C40/50-tripleblend C40/50-PPF-tripleblend GGBS 259 kg/m kg/m kg/m kg/m 3 CEM I kg/m kg/m 3 MS 30 kg/m 3 30 kg/m 3 0/4 Qatar washed natural sand 770 kg/m kg/m 3 4/10 U.A.E crushed limestone 450 kg/m kg/m 3 10/20 U.A.E. crushed limestone 670 kg/m kg/m 3 PPF 2 kg/m 3 Superplasticizer (PCE) 3 l/m 3 5 l/m 3 Free water / Free w/c ratio 140 l/m l/m Compressive strength As far as structural aspects were concerned, a C32/40 concrete grade was strictly necessary. A C40/50 concrete grade has been specified following standards (BS , CIRIA CS 163) recommended values for durability (groundwater aggressiveness) reasons. Globally, compressive (cubes 150x150) strength is, at the present stage, averaging 60 MPa at 7 days and about 70 to 75 MPa at 28 days. From the beginning of the project, it has been observed that C40/50 with polypropylene fibers was achieving lower strength (around 5 MPa) than the reference concrete mix design. An additional strength gain of roughly 10 MPa between 28 days and 56 days has been recorded at the beginning of the project for both concrete mix designs. It has also been observed that concrete strength has significantly been improved since the beginning of project completion. This is linked to a better concrete production process and survey associated with a strict quality control and audits performed by LRT QC team. Finally, the concrete mix design used is corresponding in fact at least to a C50/60 concrete grade. 5 Water Penetration under pressure & Water Absorption Taking into account that many deterioration processes involve the movement of liquid or gas through concrete, if the concrete is less permeable or less absorptive, it should be more durable. A synthesis of achieved results following BS EN Part 8 (formerly DIN 1045) and BS Part 122 testing methods are given here below.

4 Table 3: Concrete water penetration [mm] overall results GLOBAL C40/50 without PPF C40/50 with PPF 28d 56d 90d 28d 56d 90d 28d 56d 90d average 6,7 5,4 4,5 5,7 4,4 2,8 7,8 6,4 6,2 SDT 2,6 2,3 2,8 2,1 1,7 0,3 2,6 2,4 3,4 min Max number of values Fig. 2: Water penetration test records It is confirmed by Table 3 and Fig. 2 that, for the concerned concrete, water permeability is quasi-systematically below 10 mm and is improved toward time (around minus 1.5 mm in average between 28 and 56 days). However, one of the principal difficulties with this testing protocol is that the conditions in the specified test are not necessarily directly comparable with the environment to which the concrete will be exposed in service. For concrete mix including polypropylene fibers, measured average value is about 2 mm higher. This could be explained by possible water channel network created by synthetic fibers into the concrete subjected to water pressure of 500kPa (5bar) applied to an area of one surface of the specimen for a period of 72 hours as per testing protocol. However, the high observed dispersion (coefficient of variation equal to 40 %) confirms statement given in Appendix F of CIRIA C163 saying that the depth of penetration can be difficult to interpret and its measurement is probably accurate only to around 5 mm. It is not easy to conclude that the decreasing trend visually observed graphs (Fig. 2) is due to a concrete improved intrinsic quality because this trend could be linked to the fact that laboratory technicians becomes better trained to this testing protocol after numerous performed tests. This decreasing trend is not confirmed on graphs given in Fig. 3 corresponding to water absorption test. This durability indicator must thus be considered with care and definitively not alone when assessing concrete overall durability. Table 4: Water absorption [%] overall results GLOBAL C40/50 without PPF C40/50 with PPF 28d 56d 28d 56d 28d 56d average 0,9 0,8 0,8 0,8 1,0 0,9 SDT 0,2 0,2 0,2 0,2 0,2 0,2 min. 0,6 0,5 0,0 0,5 0,7 0,6 Max. 1,6 1,4 1,2 1,1 1,6 1,4 number of values

5 Fig. 3: Water absorption test records Water absorption is below a maximum recorded value equal to 1.4 % at 56 days illustrating the high concrete impermeability. Water absorption is slightly decreasing of 0.1 % between 28 days and 56 days for the concerned concrete mix design. Again, for concrete mix including polypropylene fibers, measured average value is about 0.2 % higher. Dispersion of achieved results (coefficient of variation equal to 20 %) is much lower that for water penetration test under pressure and confirms that the indicator is more reliable. Thus, we believe that absorption test should be preferred to water penetration test for routine quality control but could remain not enough sensitive to identify major variations in concrete intrinsic quality and must be then correlated other durability indicator(s). 6 Rapid Chloride Permeability Test & Chloride migration coefficient A synthesis of achieved results following ASTM C1202 and Nordtest NT Build 492 testing methods are given here below (one single result corresponding to 911C at 28 days and 699C at 56 days has been discarded from this analysis). Table 5: Rapid Chloride Penetration Test [Coulombs] overall results GLOBAL C40/50 without PPF C40/50 with PPF 28d 56d 90d 28d 56d 90d 28d 56d 90d average SDT min Max number of values Fig. 4: RCPT test overall records

6 Table 6: Chloride migration coefficient [m²/s] overall results GLOBAL C40/50 without PPF C40/50 with PPF 28d 56d 90d 28d 56d 90d 28d 56d 90d average 1,10 0,79 0,70 0,96 0,70 0,79 1,24 0,87 0,60 SDT 0,39 0,25 0,13 0,33 0,23 0,44 0,23 min. 0,37 0,24 0,60 0,00 0,24 0,79 0,37 0,44 0,60 Max. 2,44 1,63 0,79 1,56 1,19 0,79 2,44 1,63 0,60 number of values Fig. 5: Chloride migration coefficient test overall records Fig. 6 & 7: D RCM statistical repartition and correlation between 56d and 28d The following trends can be derived from the graphs given in Fig. 4, 5, 6 and 7: - It is not observed significant difference between mixes designed with and without polypropylene fibers. Both concrete mix design are achieving very low RCPT values (global average < 200 Coulombs at 28 days) and Chloride migration coefficient records (global average < m²/s at 28 days). As expected with a GGBS/OPC/MS triple blend, the chloride permeability/diffusivity is significantly decreasing between 28 days to 56 days; - Observed dispersion of RCPT results is very high (σ = 80C for an average value of 182 C, corresponding to COV=45 % at 28 days). For D RCM, dispersion is lower but remains quite important (σ= m²/s for an average value of m²/s, then COV=35 %); - RCPT values are following seasonal variations (Fig. 4) looking like a sinusoidal curve (achieved values are systematically noticeably lower during winter periods), which could be explained by a slower cement hydration process during colder period. This is less visually obvious on Fig. 5 corresponding to D RCM ; - From the Fig. 6 it can be derived that D RCM achieved results toward time, when excluding the three highest values, are basically following a statistical normal law (µ= m²/s; σ= m²/s);

7 - An ageing factor a equal to 0.48 can be derived from average D RCM values achieved at 28 and 56 days. This value is very close to the mean value [0.45] proposed for blast furnace slag cement concrete in table B2-2 of fib bulletin n 34. However plotted values on Fig. 7 illustrate some dispersion around this average value; - Finally the graph shown on Fig 8 illustrates that it is quite difficult to establish a relevant correlation factor between D RCM and RCPT, especially when concrete is achieving very good behavior (low values) against chloride ions penetration. Fig. 8: D RCM versus RCPT 8 Conclusions In order to enable the LRT tunnel and underground structures meeting the required service life of 100 years in the prevailing exposure conditions, concrete mixes has been designed to achieve high intrinsic durability properties. In addition, polypropylene monofilament fibers have been introduced for parts of structures potentially subjected to accidental fire. The collected data dealing with durability indicators corresponding to more than 80 samples during a 3-year period leads to the following observations and conclusions: - The use of mix design including triple blend cementitious material (GGBS/OPC/MS) and very low water to binder ratio leads, as expected, to very high durable concrete as far as chloride ions diffusivity and water permeability are concerned; - The introduction of polypropylene fibers slightly impact indicators (especially water permeability under pressure and at a less extent RCPT and D RCM ) and does not jeopardize concrete durability (D RCM < m²/s average value at 56 days for C40/50 PPF); - Absorption test should be preferred to water penetration test for routine quality control because less subjected to dispersion; - RCPT and D RCM recorded values are subjected to seasonal variations; - Collected data allows to determine relevant input data for modeling as far as similar concrete mix design would be used for another project; - Designing a high durable concrete leads to the achievement of higher mechanical properties (compressive strength and most probably linked properties such as tensile strength, elasticity modulus ) significantly higher than those strictly required for design purpose. This must be taken into consideration at design stage, especially for early-thermal crack assessment, because it could lead to potential adverse effect such as higher cracks opening due to higher short term shrinkage (autogenous and thermal) associated to higher concrete tensile strength.

8 Finally, even if concrete and material considerations are definitively major issues, it must kept in mind that quality control and workmanship remains key factors in achieving design durability provisions. Contractors must be able to provide an educated, capable and motivated work force for the ongoing and new coming challenging project. Means must be implemented provide relevant skills training to improve quality. 9 References ASTM C1202: Standard Test Method for Electrical Indication of Concrete s Ability to Resist Chloride Ion Penetration British version of European Standards [BS EN], including notably BS EN 206-1, BS EN , BRE SD1 relative to concrete materials, BS EN 1990:2002 (Eurocode - Basis of Structural Design) BS EN 12390: Testing hardened concrete - Part 8: Depth of penetration of water under pressure BS1881: Testing concrete - Part 122: Method for determination of water absorption CIRIA Guide CS163 - Guide to the design of concrete structures in the Arabian Peninsula, 2008 fib bulletin 34: Model code for service life design NORDTEST NT BUILD 492, Concrete, Mortar and cement-based repair materials: Chloride migration coefficient from non-steady-state migration experiments. Qatar Construction Specifications (QCS 2010)