Class Notes (836,587)
Canada (509,861)
Lecture

L02- Concrete Mix Design Procedure.pdf

9 Pages
151 Views
Unlock Document

Department
Civil Engineering
Course
CIVENG 3J04
Professor
Ghani Razaqpur
Semester
Fall

Description
MIX DESIGN PROCEDURE The purpose of mix design is to determine the proportion of the various ingredients of concrete. The basic ingredients are cement, water, coarse and fine aggregates and air. Concrete mixes are produced to satisfy specified requirements of: workability, strength, durability and economy. The mix design procedure described herein results in a trail mix, which may need further modifications/adjustments in the proportions of the ingredients in order to meet the specifications. A basic mix design involves the following steps: Step1: Decide on the required slump (a workability requirement) * Based on experience, or select from Table 1. Table 1: Recommended slump for various types of concrete Types of Construction Max Slump (mm) Min Slump (mm) RC footings, caissons, foundation 75 25 and substructure walls Plain footings, caissons and 75 25 substructure walls RC beams, columns, and walls 100 25 Pavement and slabs 75 25 Mass concrete 50 25 Step 2: Selection of maximum aggregate size (dmax) * Choose the maximum allowable size; it reduces the total void space, and requires less mortar per unit volume of concrete. Here are some limitations: dmax (narrowest dimension of members) / 5 dmax (depth of slab) / 3 dmax 3 x (clear spacing between individual bars or wires) / 4 Step 3: Estimation of the mixing water and air content: Use Table 2. 1 Table 2: Approximate mixing water and air-content requirements for different slumps and nominal maximum sizes of aggregates Slump Water (kg/m ) of concrete for indicated nominal maximum sizes of (mm) aggregate (mm) 10 14 20 28 40 56 80 150 Non-air-entrained concrete 25-50 207 199 190 179 166 154 130 113 75-100 228 216 205 193 181 169 145 124 150-175 243 228 216 202 190 178 160 - Approx. 3 2.5 2 1.5 1 0.5 0.3 0.2 air content (%) Air-entrained concrete 25-50 181 175 168 160 150 142 122 107 75-100 202 193 184 175 165 157 133 119 150-175 216 205 197 184 174 166 154 - Recommended average total air-content Mild 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 exposure Moderate 6.0 5.5 5.0 4.5 4.5 4.0 3.5 3.0 exposure Extreme 7.5 7.0 6.0 6.0 5.5 5.0 5.5 4.0 exposure Note that air entrained concrete is normally used for "moderate" and "severe" exposure conditions. Moderate exposure: in a cold climate, the concrete will be exposed only occasionally to moisture prior to freezing, and where no deicing salts are used (e.g. some exterior walls, beams and slabs that are not in direct contact with soil). Severe exposure: in a cold climate, the concrete may be in continuous contact with moisture prior to freezing, and where deicing salts are used (e.g. bridge decks, pavements, sidewalks, parking garages). Step4: Selection of water-cement ratio (w/c): Use Table 3(a) or (b), whichever is appropriate Step 5: Calculation of the cement content w c w/c 2 Table 3 (a): Approximate relationship between water-cement ratio and compressive strength of concrete Compressive strength Water-cement ratio (by mass) at 28 days, MPa Non-air- Air-entrained entrained concrete concrete 45 0.38 0.30 40 0.42 0.34 35 0.47 0.39 30 0.54 0.45 25 0.61 0.52 20 0.69 0.60 15 0.79 0.70 Note: (1) The strength values are estimated average for concrete containing not more than 2% air for non-air-entrained concrete and 6% total air content for air-entrained concrete. For a constant w/c the strength of concrete is reduced as the air content is increased. (2) The relationships in this table assume a nominal maximum aggregate size of about 19-25 mm. For a given source of aggregate, strength produced at a given w/c ratio will increase as the nominal maximum size of aggregates decreases. Table 3(b): Maximum permissible w/c ratios for concrete in severe exposures Type of structure Structure wet continuously Structure exposed to or frequently and exposed sea water or sulfates to freezing and thawing (Note: if cement Type 20 or (Note: concrete should be air-Type 50 is used these values entraine) may be increased by 0.05) Thin sections (railings, curbs, sills, ledges, ornamental work) and 0.45 0.40 sections with less than 5 mm cover over steel All other structures 0.50 0.45 Step 6: Estimation of the coarse aggregate content based on the dmax and the fineness modulus of sand (FM. ) Use Table 4. Step 7: Estimation of the fine aggregate content So far all the other ingredients of concrete mix have been estimated except for the fine aggregate. 3 Table 4: Bulk volume of coarse aggregate per unit of volume of concrete Nominal Volume of dry- rodded coarse aggregate per unit volume of maximum concrete for different fineness moduli of sand or fine aggregate size of 2.40 2.60 2.80 3.00 aggregate (mm) 10 0.50 0.48 0.46 0.44 14 0.59 0.57 0.55 0.53 20 0.66 0.64 0.62 0.60 28 0.71 0.69 0.67 0.65 40 0.75 0.73 0.71 0.69 56 0.78 0.76 0.74 0.72 80 0.82 0.80 0.78 0.76 150 0.87 0.85 0.83 0.81 Note: In the preliminary mix design it is assumed that the aggregates are oven-dry. However, the final mix must take into account the actual moisture content of the aggregates, i.e. both absorbed and surface moisture . This is achieved by adjusting the proportions of the trial mix after estimating the quantities based on dry mix. Step 7: Estimation of the fine aggregate content So far, the proportions of all the ingredients of concrete have been estimated except the fine aggregate, which can be determined as follows. i) Weight basis method: 3 FA U (CA C W ) , for unit volume (1 m ) Where U = weight of fresh concrete per unit volume (kg/m ) FA = weight of fine aggregates per unit volume (kg/m ) 3 CA = weight of coarse aggregates per unit vo3ume (kg/m ) C = weight of cement per unit volume (kg/3 ) W = weight of water per unit volume (kg/m ) For finding U, the best way is previous experience. In the absence of such experience: 1) Use standard tables (such as Table 5) 2) Use theoretical calculations, which involve aggregate specific gravity, G, and density, 4 U Agg C W agg Agg G agg w w agg Agg Gagg aggw A U G aggagg w C W G aggw 1 V c Vw C W 100 C W V c , Vw , w 1000kg / m Gc w w G agg U 10G agg100 A C 1 W G agg 1 Gc where A = air content in percent G = specific gravity of cement (generally 3.15) c G agg weighted specific gravity of aggregates ii) Absolute
More Less

Related notes for CIVENG 3J04

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit