Problem No.1 Using the following data for an unreinforced PCC Pavement slab: Des

Question

Problem No.1 Using the following data for an unreinforced PCC Pavement slab: Design strength fc = 4,500 psi Slab thickness = 12" Standard deviation of obtained from 20 samples 200 psi Ignore any exposure requirement Use air entrained concrete Fineness modulus of fine aggregate 2.65 Maximum aggregate size-2" and nominal maximum aggregate size- Bulk oven-dry specific gravity of coarse aggregate = 2.6 Oven-dry rodded density of coarse aggregate = 125 pcf Find the followings: a. Required compressive strength b. w/c ratio c. Coarse aggregate amount (Ib/yd3) d. If the w/c ratio is 10% reduced, will the quantity of coarse aggregate increase, decrease or remain the same? Explain your answer.

Explanation / Answer

a) Required compressive strength = fc' + 1.34 x standard deviation = 4500 + 1.34 x 200 = 4768 psi

b) From Table 6.3.4(a) of ACI 211.1-91, for air entrained concrete,

w/c ratio = 0.4 for 5000 psi concrete

w/c ratio = 0.48 for 4000 psi concrete

For 4500 psi concrete, w/c ratio = 0.44

c) From Table 6.3.6 and section 6.3.6 of ACI 211.1-91,

For fineness modulus of 2.6 and max aggregate size of 2 in,

volume of oven dry rodded coarse aggregate per yd3 of concrete = 0.76 x 27 ft3

For fineness modulus of 2.8 and max aggregate size of 2 in,

volume of oven dry rodded coarse aggregate per yd3 of concrete = 0.74 x 27 ft3

Given fineness modulus = 2.65

So, volume of oven dry rodded coarse aggregate per yd3 of concrete for fineness modulus of 2.65 is obtained by linear interpolation.

For fineness modulus of 2.65 and max aggregate size of 2 in,

volume of oven dry rodded coarse aggregate per yd3 of concrete = (0.74 + (0.76-0.74)/4) x 27 ft3

                                                                                                        = 0.745 x 27

                                                                                                        = 20.115 ft3

Oven dry rodded density of coarse aggregate = 125 pcf

Amount of coarse aggregate = (20.115 x 125) lb/yd3 = 2514.375 lb/yd3

d) Note below Table 6.3.6 of ACI 211.1-91 states that for less workable concrete, volume of coarse aggregate per unit volume of concrete may be increased by about 10%. Now, when w/c ratio is reduced by 10%, workability is reduced. Due to this the quantity of coarse aggregate will increase.

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