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Concrete Design and Production
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| Maximum-size coarse aggregate |
Cement |
Sand (wet) |
Coarse aggregate (wet) |
Water |
| 3/8" |
1 |
2 1/2 |
1 1/2 |
1/2 |
| 1/2" |
1 |
2 1/2 |
2 |
1/2 |
| 3/4" |
1 |
2 1/2 |
2 1/2 |
1/2 |
| 1" |
1 |
2 1/2 |
2 3/4 |
1/2 |
| 1 1/2" |
1 |
2 1/2 |
3 |
1/2 |
These proportions are only a guide and may need adjustments to obtain a workable mix with locally available aggregates (PCA 1988). Packaged, combined, dry concrete ingredients (ASTM C 387) are also available.
Example: Assume that the proportions will be made based on one part being a cubic foot (this is convenient since a 94 lb. bag of cement is 1 cubic foot of bulk material). For a 3/4" maximum coarse aggregate, the mixture would be 1 part portland cement, 2 1/2 parts sand, 2 1/2 parts coarse aggregate, and 1/2 part water. The sum of the parts is 6 ½. In general the final volume of concrete produced will be approximately 2/3 of the sum of all the volumes included in the mixture. Therefore the approximate volume of this concrete mixture is 4.3 cubic feet. There are 27 cubic feet in a cubic yard so 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 3/4" aggregate and use 7 bags of cement, 17.5 cubic feet of sand, and 17.5 cubic feet of ¾ in. 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). This quantity is divided 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 1/2" 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 labor. 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 wheel barrows 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 1/4 yard 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.
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Both 500-mL beakers contain 500 grams 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 2400 kg/m3 (145 lbs/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, and
the water and cement contents, which in turn are influenced by the
maximum size of the aggregate.
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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
7-L (0.25-ft3) air meter container for up to
25-mm (1-in.) nominal max. size aggregate: 14-L (0.5 ft3) container for aggregates up to 50-mm (2-in.). Container should be calibrated at least annually (ASTM C 1077). |
| Nominal maximum |
Air content, percent* |
||
| Severe |
Moderate |
Mild |
|
| <9.5 (3/8) |
9 |
7 |
5 |
* 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. When testing these concretes, however, aggregate
larger than 37.5 mm (1 1/2 in.) is removed by handpicking or sieving
and air
content is determined on the minus 37.5 mm (1 1/2 in.) fraction
of mix. (Tolerance on air content as delivered applies to this value.)
As the maximum aggregate size increases for any given concrete mixture the amount of paste in the overall mixture decreases. For this reason the required air content decreases. In a concrete mixture with a maximum 3/4" 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
so a lower air content will 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 in a range
from 3/8" to 3", however in massive structures aggregates
of up to 6" have been used.
The second variable is exposure: extreme, moderate, and mild. For
extreme exposures the air content ranges from 7 1/2 % for 3/8"
aggregate to 4 1/2 % 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 3 1/2 % 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 4 1/2 % for 3/8" aggregate to 1 1/2 % 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.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 revolutions for the load in question. ACI 301-99, Specifications for Structural Concrete in Buildings, nor 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, also 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.
- Concrete water-cement ratio.
- Sub-base moisture conditions.
- Mineral and chemical admixtures.
- Concrete curing.
- Concrete drying environment.
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 vapor retarder will be required. In most cases if a vapor 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 are a number of different ways to do this and each has its strengths and weaknesses. A vapor retarder placed below a blotter layer (a layer of sand or granular material used to allow moisture to evacuate the slab from both faces) minimizes curling, yet may act as a moisture reservoir to promote higher vapor pressures. A vapor 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. This in turn may make for a weaker finished surface for the floor and will increase the shrinkage rates at the upper surface of the slab promoting curling.
Some designers have adopted the practice of using the vapor 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.
SCMs can be included in concrete, either as an ingredient added at batching, or as a component of a 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. SCMs added directly to concrete are governed by ASTM C 618 (fly ash and natural pozzolans), C 989 (slag), or C 1240 (silica fume), while blended cements are governed by ASTM C 595 or C 1157.
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 a 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 optimization of sulfate content. Sulfate optimization can be particularly important for some fly ashes with high aluminate contents. Although a rare occurrence, some fly ashes can throw off the sulfate balance in fresh concrete, leading to problems with workability and setting. The quality control of blended cements takes one variable out of the concrete batching process.
As can be seen in the figure below, the shrinkage potential of a plain concrete mixture (no reinforcement) typically ranges between 600 millionths to 790 millionths. The spread of shrinkage data for concrete mixtures is similar across the cement types. For small dimension projects that have little restraint in service like countertops, cement type is probably not of critical importance.In general, if Type II is available locally in bags, use it; if not, a Type I or a Type III cement should give close to the same results.
To reduce the potential for cracking, it is probably of greater importance that the concrete be properly cured. Concrete gains strength when it has adequate moisture, temperature, and time. Maintaining the moisture content of the concrete for curing has the added benefit of extending the time at which drying shrinkage takes place. If the concrete is allowed to begin to dry in the first few days after it has been cast, it shrinks, producing tensile stresses that can cause cracking if the concrete still has low strength. However, if the moisture is maintained in the concrete, the shrinkage takes place later in the life of the concrete, after it has developed additional strength. This allows it to better resist the tensile stresses that cause cracking. With this in mind, it may be more important to maintain the moisture in the countertop for a period of 7 days to reduce cracking potential than it is to be overly particular about cement type.
Common recommendations for mass concrete applications also include limiting the maximum internal temperature to 70°C (160°F), and the thermal gradient from the interior to the exterior of the concrete section to 20°C (36°F) (higher limits are possible with low coefficient of thermal expansion aggregates).
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| Flexural strength testing |
- Flexural strength, used for design of pavements (slab-on-grade).
- Compressive strength, used for design of foundations, building elements (walls, columns, slabs), bridges (abutments, columns, decks), etc.
Flexural Strength
Flexural strength increases proportionally with compressive strength (as the compressive strength goes up, so does the flexural strength). This property is used specifically for pavement design the flexural strengths of interest fall in a range of 3.9 MPa (570 psi) to 5.1 MPa (750 psi). These flexural strengths correspond approximately to compressive strengths of 28 MPa (4000 psi) to 48 MPa (7000psi). While concrete can attain much higher flexural strengths, it is not required for pavements, and use of higher strengths would have an adverse effect on the economics of the project with little benefit in performance.
Compressive Strength
 |
| Compressive strength testing |
- Residential and light commercial building projects typically use concrete strengths ranging from 17 MPa (2500 psi) to 34 MPa (5000 psi). Keep in mind that the lower strength concrete is only appropriate for mild environmental exposures, and interior concrete protected from the elements. Severe environmental exposures (freezing and thawing cycles and deicer chemical exposure) require a minimum strength of 4000 psi to assure durability. Local codes commonly provide guidance for the minimum requirements, but in many cases do not address long term durability issues.
- Heavy commercial and special structures (high rise buildings, long span bridges, slabs exposed to heavy abrasion, etc.) typically require concrete strengths of 28 MPa (4000 psi) or more. The actual required strength may be controlled by the structural loading, durability requirements, special property requirements (low permeability, high abrasion resistance, etc.) or a combination of these factors. Concrete design professionals should always be consulted for guidance regarding these important structures.
In inch-pound units, k is typically estimated to be between 9 for rounded gravels and 11 for crushed or angular stone (metric: between 0.7 and 0.8).
It is very common to use a k value of 9 (0.74, metric) for design purposes. Therefore, for a 28 MPa (4000 psi) concrete, the corresponding flexural strength would be 3.9 MPa (570 psi).