Ch8: Proportioning Concrete Mixes Concrete Technology Ch8: Proportioning Concrete Mixes Lecture 15 Eng: Eyad Haddad

Concrete mix design: Concrete mix design means the process of proportioning the components of concrete. Determining of concrete proportions by Empirical proportioning methods. (by experience) Calculation methods. Expressing proportions: It is used to express the granular materials such as ( cement, coarse aggregates, fine aggregates) as ratios of weigh or volume.

Expressing proportions: Example: when we said concrete with ratios of Cement Sand Aggregate 1 2 4 It mean that this concrete consist of 1 particle of cement , 2 particles of sand and 4 particles of aggregates. Materials can be expressed by ratios between cement and aggregate (cement/agg ratio) Example: 1 : 6 it means that we use one particle of cement to 6 particles of aggregates.

cement sand aggregates Expressing proportions: Materials can be expressed as how it filled the unit volume determine by weigh. Example: cement sand aggregates 300kg 0.4m3 0.8m3 The cumulative of these quantities are 1 m3 after mixing with water. Cement can be expressed by number of cement pockets 6pockets 0.4m3 0.8m3

Aggregate-paste relationship: Concrete is contain of cement paste (active) and aggregates (inert) The strength of concrete is depended on strength of paste because of the strength of aggregate is very high to strength of cement paste. The crack of compressive strength test appeared on the aggregates by using high strength concrete.

Absolute volume method. Mix design methods: Empirical method. Trial method. Absolute volume method.

High strength concrete Moderate strength concrete 1. Empirical method: This method is determined the proportions of concrete by experience, this method is suitable for small jobs. It applied the components of concrete (cement, sand and aggregates) as ratios of total weight or volume. Water content can be determined according to workability of concrete. aggregates cement sand Type of concrete 3 1 = 2 High strength concrete 6 4 Moderate strength concrete 8 5 Low strength concrete

Disadvantages of empirical method: w/c ratio is not bordered. The above proportions me exceed of 1 m3 reached to 1.2 m3. Sand/aggregates ratio is constant 1 : 2 . The compressive strength of concrete is un accurate.

2. Trial method: This method is dependent of knowing the w/c ratio. Steps of trial method: Have 2.5 kg of cement (5% of the cement pocket weigh). w/c ratio is determined by experience or from curves or tables. Mixing the cement and water to produce cement paste. Preparing quantities of sand and SSD aggregates with MSA must be less than (b/5, 3S/4, and t/3). Mixing the cement paste and aggregate with water to produce stander concrete . Weigh the quantities of retrained proportions to weigh the using quantities. Calculate the quantities by weigh and volume.

3. Absolute volume method: It supposed that absolute volume of concrete is equal to cumulative absolute volume of concrete ingredients. (absolute volume of cement, sand, aggregate and water) as following: Where: C = the weigh of Cement by kg used for 1 m3 of concrete. S = the weigh of Sand by kg used for 1 m3 of concrete. G = the weigh of aggregates by kg used for 1 m3 of concrete. W = the weigh of Water by kg used for 1 m3 of concrete. Gc, Gs, Gg = relative density of cement, sand, and aggregate. Note: 1 m3 of concrete = 1000 liters.

3. Absolute volume method: (Cont.) In this method we must determined the flowing according to the required instruction of strength and workability for fresh concrete: Water content required to 1 m3 of concrete. w/c ratio (by weight) or water quantity required for 1m3 of concrete fine agg./coarse agg. Ratio. Relative density of cement, fine and coarse aggregate. Note: the above data are determining by experiences, practices, laboratory tests. It mean that C, w/c ratio, G/S, Gc, Gs, Gg must be determined before, then use the above equation.

3. Absolute volume method: (Cont.) Example: Determine the proportions of fresh concrete with the following properties. The concrete is in the plastic state, compressive strength of hardened concrete after 28 days is 240 kg/cm2, the percentage passing of mix aggregates from the sieve No.3/16 is 40%. Cement relative density Gc=3.15, aggregate relative density (Agg. and sand) =2.65, unit volume of aggregate (sand and agg.) = 1700kg/cm2. Solution: a) determine fine aggregate (sand)/coarse aggregate: Quantity passed from sieve no. 3/16 is sand and retrained quantity is aggregate. So sand % = 40%, and aggregate% = 60%. Note: these percentages are supposed by experience, and we can suppose it direct as 1:2 (33%sand, 66% aggregate).

Example Solution: (Cont.) b) cement quantity is determined according to compressive strength after 28 days. cement quantity / 1 m3 = compressive strength after 28 d + (50-100)kg/cm2 so cement quantity = 240 + 60 = 300 kg/cm2. c) water content per 1 m3 is determined according to (cement content in concrete, MSA, and workability). This quantity can be supposed direct by experience or from the following Table 5-1

w/c ratio for cement content (kg) per one m3 of fresh concrete Example Solution: (Cont.) C) water content per 1 m3 is determined according to (cement content in concrete, MSA, and workability). This quantity can be supposed direct by experience or from the following Table 5-1 w/c ratio for cement content (kg) per one m3 of fresh concrete MAS(mm) 400 350 300 250 200 0.40 0.475 0.50 0.60 0.70 10 0.385 0.425 0.45 0.55 0.65 20 0.370 0.48 0.61 40 Use w/c ratio = 0.50. water content = cement content * w/c ratio = 300 kg * 0.5 = 150 liter.

Example Solution: (Cont.) d) aggrecates Weigh = 60/40 of sand weigh = 1.5 sand. Aggregate weigh = 1.5 * S = 1.5 * 800 = 1200 kg.

Cement sand aggregates water Example (conclusion) Concrete components ratios by weigh: Cement sand aggregates water 300kg 800kg 1200kg 150kg 1 : 2.67 : 4 : 0.5 Concrete components ratios by volume: 300kg/50 800kg/1700 1200kg/1700 150liter 6 pockets : 0.47m3 : 0.71m3 : 150liter