CEE598: Structural Damage Evaluation and Strengthening

Problem Set - 2

1. A parking ramp, 8” thick deck with 2” cover to reinforcing steel is to be built in Minneapolis, MN. The volumetric steel reinforcement ratio is 1.2%. The structure is to be designed for a 75 year service life. The water-to-cement ratio of the concrete is 0.45. The 28 day diffusion coefficient D28

is 1.05 x 10-11 m2/s. Use the default “m” value in Life365. The parking ramp is exposed to 0.80 wt% chlorides. At the level of reinforcing steel, a threshold chloride concentration of 0.05 wt% is needed to initiate corrosion. The uniform chloride concentration in the concrete (C0) is 0.01%.

a) Plot the chloride concentration profile with depth (for the top 3” of this ramp) at an age of 45 years. Determine the chloride concentration at the level of the reinforcing steel. Is it enough to cause corrosion?

b) The use of a corrosion inhibiting admixture will not change the diffusion coefficient, but will change the threshold chloride concentration to 0.24wt% chlorides. Plot the concentration profile in this case, for the top 3” of the ramp at an age of 45 years. What is the inference?

c) The use of 3% silica fume changes the diffusion coefficient to 6.4 x 10-12 m2/s. Plot the concentration profile in this case, for the top 3” of the ramp at an age of 45 years. Compare and contrast options (a), (b), and (c)

d) Determine the time to initiation in years, for the above cases. For case (a), cost of concrete is $70/yd3, for case (b), $87/yd3 when 3 gal/yd3 of the admixture is used, and for case (c), $79/yd3 when 3% silica fume is used. Black steel costs $0.45/lb. Considering a discount rate of 3%, a repair area of 10% of the ramp, a repair interval of 10 years, and a repair cost of $25 per square feet, determine the total life cycle cost. Tabulate the initial cost, time to initiation, time to 1st repair (initiation + propagation time) in years, and the total life cycle cost per square feet for this ramp. Compare and contrast the scenarios.

2. For a 12” thick bridge deck, with 3” of cover to reinforcing steel, and 1.2% steel volume, determine

potential options that have the lowest cost for the following criteria. You can select the options in Life 365 for steel or concrete protection. State them clearly, and determine the lowest cost options. (i) At the level of reinforcing steel, the concentration of chlorides is 1% after 20 years (ii) Design life of the bridge is 100 years (iii) 3% discount rate (iv) Water-to-cementing materials ratio 0.32 (v) Location – New York, NY

3. Use Problem 3 from your Problem Set-1. The concrete mixture used in that case was Concrete A. An

alternate concrete mixture (Concrete B) made using 75% OPC, 20% fly ash and 5% silica fume, at a water-to-cementing material ratio of 0.36 is expected to be used for the same bridge deck. This changes the 28 day diffusion coefficient to 5 x 10-13 m2/s. The other parameters (m, and the threshold chloride concentration) remain the same. Use Life365 to plot the depth dependent concentration profile of chlorides in the concretes A and B. Also plot the concentration of chloride ions as a function of time at the depth of the reinforcing steel. Determine the service lives of both these concretes. Which of the concretes exhibit a higher service life? Briefly substantiate your observations.

Compare the value for the chloride concentration at the level of the reinforcing steel obtained from part (b) of Problem 3 in Problem Set-1 to the one obtained from the corresponding plot generated from Life365 for Concrete A. Why are these values different?