Concrete Design and Production

Q: How much cement is needed for a small concrete mixture?
A: The actual cement required for concrete is affected by the choice of the maximum coarse aggregate size. As the maximum coarse aggregate size is decreased the cement content must increase to provide the required quantity of cement paste to coat all of the aggregate particles.
Example: Assume that the proportions will be made based on parts being 1ft3 (this is convenient since a 94lb bag of cement equals 1ft3 of bulk material). For a ¾" maximum coarse aggregate, the mixture would be 1 part Portland cement, 2½ parts sand, 2½ parts coarse aggregate and ½ part water. The sum of the parts is 6½. In general, the final volume of concrete produced will be approximately two-thirds of the sum of all the volumes included in the mixture. Therefore, the approximate volume of this concrete mixture is 4.3ft3. Since there are 27 cubic feet in a cubic yard, we divide 27 by 4.3 giving 6.2 batches for a cubic yard. Plan on making 7 batches to assure that you will have more than enough material at the start of the job.
In short, use the mixture indicated for the ¾" aggregate and use 7 bags of cement, 17.5 cubic feet of sand, and 17.5 cubic feet of ¾" coarse aggregate; unless you feel comfortable recalculating for a different size coarse aggregate.
The volume calculation for concrete is based on length x width x height to determine the cubic feet of concrete required for the project. EX: 20’ * 12’ * 0.5’ = 120 cubic feet (be sure you always convert all of you measurements into feet units). Divide the obtained quantity by 27 (there are 27 cubic feet in a cubic yard; 120 / 27 = 4.44 cubic yards). A word of caution, an increase in thickness of ½" would increase the required concrete by .37 cubic yards. Be sure that you order a little more material than you need.
Typically it is impractical to use bagged materials or hand mix concrete if the quantity exceeds one to two cubic yards because bagged materials and hand mixing require you to handle the materials several times. A cubic yard of concrete will weigh almost two tons and handling the material three to four times to transport and mix the material requires considerable labour. Hand mixing two cubic yards of concrete is the equivalent of handling 12 to 16 tons of material. As an alternative, ready mixed concrete can be delivered and simply be unloaded from the truck to its final position using the chute on the truck or in some instances the concrete is discharged into wheelbarrows simplifying the concrete placement. Most ready mixed concrete producers have minimum requirements for yardage and short loads may include additional charges to offset delivery costs. Concrete is sold in increments of 1/4yd, so your concrete order for the preceding example would be 4.5 cubic yards. Again, be sure to order a little more than you need since charges for short loads to correct this kind of mistake can be very expensive.

 

Q: What are the unit weights (densities) of cement and concrete?
A: Cement: Cement (finely ground grey or white powder used to bind concrete mixtures) weighs between 830kg/m3 and 1650kg/m3 (52lbs/ft3 and 103lbs/ft3) depending on its handling. The weight of cement that has been pneumatically loaded into a cement silo may be as low as 830kg/m3 (52lbs/ft3), while cement that has been stored for a period of time exposed to vibration may be as heavy as 1650kg/m3 (103lbs/ft3). It is standard practice to consider a 94lb bag of cement to be one cubic foot when freshly packed.
Both 500ml beakers contain 500gm of dry powdered cement. On the left, cement was simply poured into the beaker. On the right, cement was slightly vibrated—imitating consolidation during transport or packing while stored in a silo. The 20% difference in bulk volume demonstrates the need to measure cement by mass instead of volume for batching concrete.
Concrete: Concrete is a mixture of cement, coarse and fine aggregates, water and sometimes supplementary cementing materials and/or chemical admixtures. A normal weight concrete weighs approximately 2400kg/m3 (145lbs/ft3). The unit weight (density) of concrete varies depending on the amount and density of the aggregate, the amount of air that is entrapped or purposely entrained, the water and cement contents, which in turn are influenced by the maximum size of the aggregate.
Fresh concrete is measured in a container of known volume to determine density (unit weight). Scale must be sensitive to 0.3% of anticipated mass of sample and container. Size of container varies according to the size of the aggregate, the 7l (0.25ft3) air meter container for up to 25mm
(1in) Nominal max. Size aggregate: 14l (0.5ft3) container for aggregates up to 50mm (2in).
Container should be calibrated at least annually (ASTM C 1077).

 

Q: On what basis is air content for concrete specified?
A: Air content is specified on the basis of 2 variables: maximum coarse aggregate size and exposure environment.

 


Nominal Maximum size aggregate mm (in)

AIR CONTENT PERCENT*

Severe Exposure**

Moderate Exposure+

Mid Exposure++

< 9.5 (3/8)

9

7

5

9.5 (3/8)

7 ½

6

4 ½

12.5 (1/2)

7

5 ½

4

19.0 (3/4)

6

5

3 ½

25.0 (1)

6

4 ½

3

37.5 (1 ½)

5 ½

4 ½

2 ½

50 (2) ++

5

4

2

75 (3) ++

4 ½

3 ½

1 ½

 

* Project specifications often allow the air content of the concrete to be within -1 to +2 percentage points of the table target values
** Concrete exposed to wet-freeze-thaw conditions, deicers, or other aggressive agents
† Concrete exposed to freezing but not continually moist, and not in contact with deicers or aggressive chemicals
†† Concrete not exposed to freezing conditions, deicers, or aggressive agents.
‡ These air contents apply to the total mix, as for the preceding aggregate sizes. However, when testing, aggregates larger than 37.5mm (1½in) are removed by handpicking, sieving or air
Content is determined on the minus 37.5mm (1½in) fraction of mix. (Tolerance on air content as delivered applies to this value.)
As the maximum aggregate size for any given concrete mixture increases the amount of paste in the overall mixture decreases, in turn, decreasing the required air content. In a concrete mixture with a maximum ¾" aggregate the required target air content for a severe exposure would be 6%; this would equate to a paste to air content of about 9%, which is the approximate change of volume that water undergoes when it freezes hard.
As the aggregate size increases, the quantity of paste decreases, allowing lower air content to maintain the same relationship. If the maximum aggregate size were decreased, the overall paste content would increase and require an increased air content to maintain the 9% air to paste relationship. Depending on the structure, a wide variety of maximum coarse aggregates may apply. Typically aggregates range from 3/8" to 3"; however, in massive structures aggregates up to 6" have been used.
The second variable is exposure: extreme, moderate, and mild. For extreme exposures the air content ranges from 7½% for 3/8" aggregate to 4½% for 3/8" aggregate. Extreme exposure is defined as “an environment in which concrete is exposed to freeze-thaw conditions, de-icers, or other aggressive agents”.
The air content range for moderate exposure is from 6% for 3/8" aggregate to 3½% for 3" aggregate.
Moderate exposure is defined as “an environment in which concrete is exposed to freezing but will not be continually moist, not exposed to water for long periods before freezing and will not be in contact with deicers or aggressive chemicals”.
The air content range for mild exposure is from 4½% for 3/8" aggregate to 1½% for 3" aggregate. Mild exposure is defined as “an environment in which concrete is not exposed to freezing, deicers or aggressive agents”.
The quotes are taken from ACI 201.1R, Guide to Durable Concrete. See Table for total target air content for concrete

Q: Can we add air-entraining admixture at the jobsite?
A: ASTM C 94, Standard Specification for Ready Mixed Concrete, section 7.3 provides a method for field adjustment of air content when the tested value falls outside of the acceptable tolerance range (plus or minus 1.5%). Air entraining admixture may be added to achieve the proper air content, followed by a minimum 30 drum revolutions at mixing speed. At no time are the total revolutions of the drum to exceed 300, for the load in question. Neither ACI 301-99, Specifications for Structural Concrete in Buildings or AASHTO M 157, Standard Specification for Ready Mixed Concrete address this issue.
The Canadian Standards Publication A23.1-00, Concrete Materials and Methods of Concrete Construction, refers to a procedure for field control of air content. Section 18.4.3.4 says that: “The air content of the concrete shall, if necessary, be brought up to the specified range by the addition of an air-entraining admixture in the field. Mixing shall follow to ensure proper dispersion. The air content shall be retested. ...the amount of admixture added shall be recorded on the delivery slip.”
As an added control check, it might be advisable to measure air content at the point of discharge into the forms. This is recommended in Section 2.10 of ACI 212.3R-91, Chemical Admixtures for Concrete.

Q: What is meant by ‘saturated surface dry condition’ of aggregates?
A: Saturated Surface Dry describes the condition of the aggregate in which the pores in each particle of the aggregate particle are filled with water and no excess water is on the particle surface. This allows the absorption and the specific gravity of the aggregate to be measured. Moisture content of aggregate is described by four categories:



The following are examples of how the moisture content of an aggregate affects the mixture proportioning of concrete:
Example 1: If an aggregate had zero moisture content and an absorption value of 2%, then the water added to the concrete mixture would be increased by 2% of the weight of the aggregate to achieve the desired water cement ratio.

Example 2: If an aggregate had a 6% moisture content and an absorption value of 2% then the water for the concrete mixture would be decreased by 6% – 2% = 4% times the weight of the aggregate to maintain the desired water cement ratio.


Q: How do I design for moisture sensitive floor coverings?
A: The choice of slab design for moisture sensitive floor coverings should include the following considerations:

  • Concrete water-cement ratio
  • Sub-base moisture conditions
  • Mineral and chemical admixtures
  • Concrete curing
  • Concrete drying environment

For slab construction of this type, the water cement ratio should be maintained at 0.4. The sub-base moisture conditions should be considered to determine if a vapour retarder will be required or if granular cushion will be sufficient to resist the entrance of moisture into the slab. A water reducer may be considered for use with a low water-cement ratio concrete to promote workability and to aid in ease of consolidation.
The curing practice may be altered to accommodate an early dried condition (three day moist cure). Proper ventilation and low relative humidity environment are recommended for the drying conditions.
Moisture related problems are unique with every slab that is placed. First you will need to consider the ground water and drainage conditions for each site. This information will determine if a vapour retarder will be required. In most cases if a vapour retarder is not required, a 28 day air drying of the slab should prove to be adequate as preparation for placement of the floor covering.
Should retarder be required, things get a little more complicated. There exist numerous ways to approach this, each with its strengths and weaknesses. A vapour retarder placed below a blotter layer (a layer of sand or granular material used to allow moisture to evacuate the slab from both faces) minimises curling, yet may act as a moisture reservoir to promote higher vapour pressures. A vapour retarder in direct contact with the bottom of the slab does not provide this reservoir, but forces the convenience water from the initial placement to evacuate through the top of the slab only. This may substantially change the water cement ratio in the upper surface of the slab, which in turn may make for a weaker finished floor surface and will increase the shrinkage rates at the upper surface of the slab promoting curling.
Some designers have adopted the practice of using the vapour retarder at the bottom contact surface of the slab, a low water cement ratio with water reducers to control the workability of the concrete mixture and a mat of steel in the upper half of the slab to restrain shrinkage and with that to control curling.

Q. What are supplementary cementitious materials (SCMs) and how do they differ from blended cements?
A. More than half of ready-mixed concrete contains fly ash, ground granulated blast furnace slag, silica fume, metakaolin or other pozzolanic materials. These materials are collectively referred to as supplementary cementitious materials (SCMs).
SCMs can be included in concrete, either as an ingredient added at batching, or as a component of blended cement or both (Figure 1). SCMs can be added during batching along with Portland cement. SCMs can also be added to concretes made with blended cements. ASTM C 618 (fly ash and natural pozzolans), C 989 (slag), or C 1240 (silica fume) govern SCMs added directly to concrete, while ASTM C 595 or C 1157 governs blended cements.



Since the benefits of SCMs arise from their physical and chemical characteristics, it might be assumed that similar performance in concrete is achieved, for example, by adding a fly ash at a ready-mixed concrete batch plant or through use of blended cement made with fly ash. Although good concrete performance can be achieved through both techniques, blended cements provide an advantage in that they can be produced with the same quality control techniques as Portland cements, including control of fineness and optimisation of sulphate content. Sulphate optimisation can be particularly important for some fly ashes with high aluminates contents. Although a rare occurrence, some fly ashes can throw off the sulphate balance in fresh concrete, leading to problems with workability and setting. The quality control of blended cements takes a variable out of the concrete batching process.

 

Q: What is workability of concrete?

A: The ease of placing, compacting and finishing of concrete in the desired manner is its workability. Normally the workability is measured through slump; higher the slump, higher the workability. Low slump leads to difficulties in placing of concrete. Therefore it is required to decide the workability based on experience and site conditions.