Anti-crack Design And Evaluation Of Concrete Used In Functionally Gradient Segment Of Shield Tunnel

Shield construction method has been major technology and solution for river-crossing tunnel, sea-crossing tunnel and urban subway. As the main body of structure in shield-driven tunnel, reinforced concrete segment influences construction quality and servicelife of the tunnel a lot. Degradation of volume stability will lead to cracking, which can accelerate deterioration of durability and generate expensive remedying and maintenance cost. Therefore, the anti-crack design and evaluation of the material has a significant effect on the quality and servicelife of shield tunnel.Funded by the Hi-tech Research and Development Program of China (863 Program) No.2005AA332010 named Material & Structure Design of High Impermeablity Long Service-life Large Dimension Segment of Shield tunnel and its Engineering Application, based on Wuhan Yangtze River Tunnel Engineering, and surrounded the volume stability of functionally gradient segment (abbr. FGS), studies are carried on about volume changes of functional layer and interface layer and the anti-crack performance is synthetically evaluated. The major research results are as follows:1. Accroding to design thought of functionally gradient material, the segment concrete was designed in different function.By functionally gradient desige principle, the emphasis was on protection performance of protective layer and support function of structural layer, and interface weakness zone was reinforced by means of imprinting process. Hence, the function of material and economic cost were both properly taken into consideration.2. Research on volume stability of different functional layer.(1)When quantity of mineral powder was 25%, drying shrinkage of structural layer concrete was declined by 10% and initial cracking time was extended by 7 hours. Drying shrinkage decreased with increase of fly ash, initial cracking time was extended by 7 hours mixing with 20% fly ash. When quantity of Na₂SO₄ was 1.5%, initial cracking time was declined by 15 hours.(2)Both mineral powder and fly ash could decrease temperature rise of structural layer concrete. Mixing with 20-25% mineral powder, the peak value of hydration heat was advanced by 3 hours, while mixing with 25% fly ash, the peak value of hydration heat was delayed by 3 hours. When quantity of Na₂SO₄ was 1% and 3%, the peak value of hydration heat was respectively advanced by 0.6 and 2 hours, with higher temperature rise.(3) Steam curing temperature had a significant effect on the shrinkage-reducing crack-resistant performance of admixtures used in structural layer concrete. Steam curing temperature rising from 45℃to 65℃could decrease cracking sensitivity of concrete using water-reducing agent and UEA expansion agent. Shrinkage-reducing agent was not susceptible to temperature fluctuation. The result of SEM showed that, with rising of steam curing temperature, amount of hydration products increased while amount of Ca(OH)₂ bulk crystals decreased, and structure got compact.(4) Drying and autogenous shrinkage decreased with increase of mineral micro-expansion agent content(at most 59% and 33% respectively). When its quantity was 25%, initial cracking time was 82 hours. Mixing with 10% silica fume, drying and autogenous shrinkage increased by 34% and 32% respectively, initial cracking time was reduced by 23 hours and cracking sensitivity achieved maximum.(5) Drying shrinkage was declined by 27% when quantity of polycarboxylic acid type water-reducer was 0.8%; Early-strength admixture Na₂SO₄ would lead to crystallization expansion, and period of expansion was advanced with amount of Na₂SO₄ increased. There was no obvious effect of triethanolmine on drying shrinkage; Shrinkage-reducing agent could decrease drying shrinkage by 44% and extend initial cracking time to 128 hours at the quantity of 2% while chemical change type crack repair materials decreased drying shrinkage by 13% at the same quantity; Polymer modifier could decrease drying shrinkage and velocity of crack expansion significantly, when ratio of polymer and gel increased to 0.3, initial cracking time reached to 172 hours, decreasing cracking sensitivity significantly.(6) The hybrid of long and short pp-fibers in the proportion of 7:3, intermixing with crack repair materials WP, self-healing effect of crack was the best and strength recovery rate reached to 93%. Self-healing capacity could be estimated by sonic parameters of ultrasonic wave. Ultrasonic velocities and frequency at 90d of the sample intermixing with hybrid fiber and crack repair materials WP were respectively 10.5% and 3% higher than that at 1d. Ultrasonic amplitude took on a downward trend but reduction ranged from 6.5% of black specimen to 2.5%. It was a new evaluation method to apply ultrasonic wave to research on self-healing effect.3. Study on volume changes inconsistency of interface weakness zone, estimation of anti-crack performance and creep.Relationship between ages and parameters involved in anti-cracking performance of interface were as follows:(1)According to compound exponential formula, the regularity of elastic modulusgrowth with ages was E(t) = 41.1×(1 - e^{(-0.442t)0.635}.(2)Conducing regressive analysis of experimental results by ACI C209, theregularity of split strength growth with ages was f_sp = 6.92t /(1.7 + t).(3)Based on experimental results, the average value of poisson ratio was 0.202.(4)The regularity of restrained strain of protective layer and interface layer growthwithageswereε= (1.83-0.42t^-0.39)^11.24 andε= (9.30-6.65t^-0.14)^3.1 respectively.(5)When restrained tensile strain of protective layer and interface layer reached the maximum value, constraint degree were 0.107 and 0.536 respectively. (6)The regularity of protective layer concrete creep growth of with ages wasε_ct=181.8t^0.2.By calculating, the minimum value of protective and interface layer safety factor were 6.56 and 2.97 respectively. Both two values were larger than what was needed in design, therefore, the structure was safe in anti-cracking performance.Due to low activity in early age, fly ash could increase creep of protective layer concrete. It could be decreased significantly by steam curing at high temperature. The creep and creep degree of steam curing structural layer concrete at 360d were 502.3×10^-6 and 32.4×10^-6/MPa respectively, which were declined by 45.1% and 39.8% compared with samples at standard curing. Creep changed obviously at different loading age. The later loading age was, the smaller creep got. Creep degree decreased by 64% at 180d compared with 3d.