Mix Design Review

Review Questions

The weight of a cubic foot of water is62.4 cu ft

Gallons of water in a cubic foot62.4 cf

Review Questions

Submit a mix design to NCDOT for review on form 312U

A process in which an aggregate is separated into its various sizes by passing it through a nest of sieves isgradation

Review Questions

The minimum number of revolutions at mixing speed required for concrete batched into a transit mixer is70

Solid volume of cement is.48

Review Questions

The water cement ratio needed to hydrate cement is.250

The smallest seive opening through which the entire amount is permitted to pass isNominal Maximum size

Review Questions

The pH of mixing water used in concrete should be between 4.5-8.5

Definitions

Cement Yield – the volume of concrete produced from one bag of cement for one cubic yard.

Water to cement ratio – the number of pounds of water from all sources per pound of cement.

Definitions

Unit weight – the number of pounds of concrete per cubic foot of volume.Specific gravity – ratio of the weight of a given volume of material to the weight of a given volume of water.Cement factor – number of bags or pounds of cement in a cubic yard.

Definitions

Absolute or solid volume – volume of a material in a voidless state.Durability – resistance of concrete to deterioration caused by weathering, chemicals, and abrasives.Consistency – a condition of concrete which relates to its cohesion, wetness, or flow. Slump is a good measure of consistency.

Cementitious Materials - Cement

Portland cement – bonding agent Portland cement vary by color, fineness of grind, and chemical composition.Type I – general all purposeType II – low heat of hydrationType III – high early strength

Cementitious Materials- Cement

1 bag = 94 poundsA 6 bag mix = 564 lb1 barrel = 4 bags = 376 poundsSpecific gravity of portland cement = 3.15

Cementitious Materials - Fly Ash

Flyash – a by product from a coal powered power plantA good flyash is fine with a low carbon contentSubstitute flyash for cement on a 1.2 to 1 ratio up to a 20% replacement of ash for cement

Cementitious Materials

Slag and silica fume – by products of different furnacesSubstitute them for cement on a 1 to 1 ratio up to a 50% replacement of cement

Aggregates

Abrasion resistance – ability to resist wearing or deterioration. Resistance is measured by the L.A. Abrasion testGradation – relative amounts of aggregate particles of consecutively larger and smaller sizes

Aggregates

Well-graded – a uniform percentage of material retained on each standard sieve where the gradation change is uniform from coarse to fineSieve analysis – a process to separate an aggregate into its various sizes by passing a sample through screens with different sized openings

Aggregates

The purpose of sieve analysis is to determine the distribution of the separately sized particlesSaturated surface dry – SSD is a condition where an aggregate will neither absorb nor contribute water to the mix

Fine Aggregate

Sand gives workability to the mix and fills the voids between the coarse aggregate particles.

Workability – a property which is measured in terms of ease of placing, handling, and finishing.

Fine Aggregate

Fineness modulus – an index to the fineness or coarseness of an aggregate: the range for medium sand is 2.50 to 2.80.

Coarse Aggregate

Stone is a filler material and aids in strengthening the concrete mix.Dry Rodded unit weight – the weight per unit volume of dry aggregate compacted in a container by rodding in 3 layers.Maximum size- the sieve size on which the largest particles are permitted to pass.

Coarse Aggregate

Nominal maximum size- the smallest sieve opening through which the entire sample will pass.Rounded aggregate is from a pit.Angular aggregate is from a quarry.The size and shape of the coarse aggregate can affect the strength of the mix.

Angular shaped aggregate is generally stronger than rounded shaped aggregate.

Water

Water causes a chemical reaction with the cement called hydration.

Bleed water – the excess water that accumulates on the surface of plastic concrete when aggregates settle.

Water

For a concrete with a given cement factor, an increase in total mixing water will cause the strength to decrease.Specific gravity of water = 1.The ph of mixing water is 4.5 to 8.5.

Air Content

Entrained air is added to a concrete mix to improve it’s resistance to freezing and thawing.Entrained air also improves workability.The air content measured at the site for structural or incidental concrete will be from 4.5 to 7.5 %.

Air Content

To much air entraining agent will result in to many air voids which will reduce the strength of the concrete mix.The volume occupied by 6% air is 1.62 cubic feet.You can calculate the air content by the unit weight method.

Air Content

Increasing the cement will cause the air content to decrease.Increasing the temperature of the mix causes the air content to decrease.

Retarding Admixtures

Retarders are chemicals that are added to a concrete mix to slow down the set and allow more time to place and finish the concrete.To much retarder will cause a longer delay in the initial set of the concrete. It will have little affect on the long term (28 day) strength of the concrete.

Retarding Admixtures

According to NCDOT table 1000-2, using a retarder will allow about 45 extra minutes of placement time.

Mix Design ReviewProblem Solving

How many cubic yards did this mix produce?

•3055 Lb Cement

•5660 Lb. Wet Sand ( 5390 Lb. SSD), at 5.0% Free Moisture

•10,205 lb. Wet Stone ( 10,095 Lb SSD) at 1.1% Free Moisture

•150 Gal. Metered Water

•6% Design Air

•20 Gal. Water added at jobsite

•5.8% air by pressure meter

•147.65 pcf field unit weight

Solution: Find the Total Weight of All Materials in the Mix and Divide by the Field Unit Wt.

3055 + 5660 + 10205 + ( 150 gal x 8.33) +(20 gal x 8.33) = 3055 + 5660 + 10205 + 1250 + 167 = 20,337 lbs.20,337 lbs / 147.65 pcf = 137.74 cu.Ft.137.74 cu.Ft / 27 = 5.1 cu yds.Design air not a factor because the air is taken into account in the field unit weight.

Find Yield, Cement Factor, and the Water Cement Ratio for This Same Mix

Cement Yield: 3055 lb cem / 94 = 32.50 bags

137.74 cu.ft. / 32.50 bags = 4.24CF: 27 / 4.24 = 6.37 bags

6.37 x 94 = 599 lb cementW/C: (5660- 5390) + (10,205 – 10,095) + (150 gal x 8.33) + (20 gal x 8.33) = 1797 lb1797 lbs water / 3055 lbs cem = .588

Given: 1050 lb SSD sandSpecific Gravity = 2.61Total Moisture = 6.4%Absorbed Moisture = 0.6 %

Find wet weight of sand.6.4% - 0.6% = 5.8 % 5.8 / 100 = .058..058 x 1050 lb = 61 lb free water.1050 lb + 61 lb = 1111 lb batch sand.Find dry weight of sand..006 x 1050 lb SSD = 6 lb absorbed water.1050 lb SSD – 6 lb = 1044 lb dry sand.

Find absolute volume of SSD sand(same problem)

1050 lb SSD = 6.45 cu.ft. 2.61 x 62.4

Fly Ash Mix Problem

Given: Class B non vib mix 15% fly ash replacement. Angular aggregate is used.Minimum cement from table is 545 lbs.What is the new cement and cementitious wt?.15 x 545 lb = 82 lb cement replaced.82 lb x 1.2 = 98 lb ash for cement.New cement is 545 – 82 = 463 lbs.463 lb cem + 98 lb ash= 561 lbs cemetitious

Water(same problem)

What is the new water?W/C = .545 for angular aggregate..545 x 561 = 306 lb of water306 lb / 8.33 = 36.7 gallons.

Find the % solids and voids in a coarse aggregate.

Given: Dry rodded unit weight = 95 pcfGiven: Specific gravity = 2.792.79 x 62.4 = 174.10 pcf.(95 pcf / 174.10 pcf) x 100 = 55% Solid.100 –55 = 45% voids

Trial Mix

Class A, vib,AE, rounded aggregate, 3.5 inch slump, 37.5 gallons water. How much cement is needed?Maximum W/C = .48837.5 gal x 8.33 = 312 lb water312 lb water / .488 = 639 lb. cement