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Concrete Construction
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To answer this question, it is first necessary to establish why
wire mesh or fibers would be used in a concrete slab. One of the
main reasons is for crack control. PCA, however, states that cracks
in slabs on ground can be controlled with uniform subgrade support
and proper joint spacing (as a function of slab thickness).
| Is reinforcement necessary? | |
| NO | With uniform support and short joint spacing |
| YES | When long joint spacing is required When joints are unacceptable in floor use |
Although
short joint spacings alleviate the need for reinforcement, wire
mesh will allow for increased distance between joints if correctly
placed in the upper portion of the slab, at least 2 in. below the
surface. In these slabs with longer joint spacings, the purpose
of wire mesh is to hold random intermediate cracks tight.
Plastic fibers should not be expected to replace wire mesh in a slab on ground. However, although not affecting joint spacing, plastic fibers are used to reduce plastic shrinkage cracking.
Plastic shrinkage cracks are those that occur immediately following concrete placement, before the concrete has hardened. Plastic fibers are commonly dosed at 0.1% by volume for slabs on grade; this is equivalent to 0.9 kg/m3 (1.5 lb/yd3). Polypropylene fibers are among the most common for controlling plastic shrinkage cracking. There are many types of fibers in addition to polypropylene, including other plastic materials, such as nylon. Fibers serve different purposes based on the characteristics of the material from which they are made. For instance, steel fibers provide high flexural strengths and impact resistance and are found in heavy-duty industrial floors.
Rule
number ONE is that ALL concrete must be protected from freezing
until it has reached a minimum strength of 3.5 MPa (500 psi), which typically happens
within the first 24 hours. In addition, whenever air temperature
at the time of concrete placement is below 5°C (40°F) and freezing
temperatures within the first 24 hours after placement are expected,
the following general issues should be considered:
(1) Adjustment of construction schedule regarding loads imposed
on the new concrete structure.
(2) Placing and curing temperatures to produce quality concrete.
The exposure of concrete to cold weather will extend the time required for it to gain strength. In structures that will carry large loads at an early age, concrete must be maintained at a minimum of 10°C (50°F) to accommodate stripping of forms and shoring and to permit loading of the structure. In many cases, achieving the required durability will require a protection period of more than 24 hours. This may not be an issue with residential applications where applied loads are typically small and may be applied in small increments over several days or weeks.
In no case should concrete be allowed to freeze during the first 24 hours after it has been placed. Since cement hydration is an exothermic reaction, the concrete mixture produces some heat on its own. Protecting that heat from escaping the system may be all that is required for good concrete quality, while more severe temperatures may require supplemental heat.
1) Moisten subgrade, steel reinforcement, and form work prior to concrete placement.
2) Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.
3) Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature.
4) Use a concrete consistency that allows rapid placement and consolidation.
5) Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.
6) Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set.
7) Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.
8) Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
9) In extreme conditions consider adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement.
With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 35°C (95°F) or more.
Isolation/Expansion Joints: Isolation joints
are used to relieve flexural stresses due to vertical movement of
slab-on-grade applications that adjoin fixed foundation elements
such as columns, building or machinery foundations, etc. Expansion
joints are used primarily to relieve stress due to confinement of
a slab. If the slab is placed adjacent to structures on more than
one face of the slab an expansion joint should be placed to relieve
stress. For example, if a slab were placed between two buildings,
an expansion joint should be placed adjacent to the face of at least
one of the buildings. Confinement on three faces would normally
be handled by placing expansion joint on all three faces, and confinement
on four faces should be isolated on all faces. This allows for thermal
expansion and contraction without inducing stress into the system.
Contraction
(control) joints are placed to control random cracking.
Contraction joints should be placed at
2 times the slab thickness in feet for a maximum aggregate size
of less than ¾”.
For example for a 5” slab with a ¾” coarse aggregate the maximum
joint spacing would be 10’. When the maximum coarse
aggregate size is greater than ¾” the spacing could be increased
to 2 ½” times the thickness. For the prior example this would
increase to 13’.
Applications that require thick slabs of 8" or more and good
load transfer across joints, due to heavy loading, should be limited
to 15' contraction joint spacing to ensure aggregate interlock.
Construction joints are stopping places in the process of construction. Construction-joint types (a) and (b) are also used as contraction joints.
The process for producing monolithic exposed-aggregate finishes is as follows:
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1) Place the concrete containing the chosen aggregate in a normal manner in which you fill the forms with the material and rod the surface with a straight edge (typically a straight 2" X 4" board) with a sawing motion, back and forth across the form from side to side. Then close the surface as usual. |
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2) Spray the surface with retarder. This can be obtained at any contractors supply house. Retarder does typically contain sugars but the formulations that are designed for use with concrete are strongly recommended for a more consistent performance. The retarder will slow the set characteristics of the discrete surface layer allowing the interior to harden while the exterior remains soft. |
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3) The third phase is the difficult part. When the concrete has become hard enough to carry your weight without displacing the aggregate the surface is washed with a hose and scrub brush to remove the top layer of cement paste. Care must be taken not to displace the aggregate and not to expose the aggregate too deeply (this can cause the aggregate to lose bond and/or be displaced). Caution: A too long delay in this part of the process can create extreme difficulty in the removal of the top paste layer. The retarder slows the set of the top surface but does not stop it completely. After the desired surface has been achieved the slab should be sealed with a clear sealer and curing compound (also found at your local contractor supply house). |
Bagged cement stored on a job site should be stacked closely together on pallets to reduce air circulation and to raise the cement above any moisture sources; it should also be covered with tarpaulins or other water-proof covers for further protection from moisture.
If possible, store cement inside a structure to further reduce the potential for moisture exposure, but never place the cement directly next to an exterior wall. The relative humidity within the structure should be reduced as much as possible.
Construction practices have a significant effect on the air content of concrete. These include load volume versus drum capacity, haul time, mixing or agitating speed, total revolutions of the drum before discharge, retempering, pumping and conveyors (any high energy process during placement including large drops), internal vibration, temperature, and finishing practices. Air content of concrete is also affected by the materials used in the concrete mixture and the batching process. A good reference on this topic is PCA’s Manual on Control of Air Content in Concrete.
When the temperature of freshly mixed concrete exceeds 77°F (25°C) there may be a number of effects on the fresh and hardened properties of concrete.
One of the effects on fresh concrete properties is accelerated setting, which leads to a shorter time period for transporting the concrete to the job site, and a shorter window for placement, consolidation, and finishing of the material.
The higher temperature also leads to a higher water demand to maintain the concrete at the specified slump, which can tempt the contractor to add water to retemper the mixture leading to lower than expected compressive strength of the hardened material. In addition mixtures at high temperatures may require higher dosages of air-entraining admixtures to produce the required air content for durability in severe climates.
The effect on hardened concrete properties are a high early strength but low ultimate strength compared to mixtures placed with a lower temperature. In addition, if no precautions are taken, there is an increased potential for plastic shrinkage cracking during the finishing operations, and increased potential for cracking due to volume changes caused by drying shrinkage and thermal effects.
Aluminum is an amphoteric material, meaning it will react chemically with either an acid or an alkali. When portland cement is hydrating it releases free calcium hydroxide which is a very strong alkali. The pH of a fresh concrete mix can be more than 13. Consequently any material that reacts chemically with an alkali should be carefully considered if it is to be embedded in or placed adjacent to fresh concrete.
Aluminum posts and flashings are commonly used adjacent to or embedded in concrete that will be dry in service. While there is a reaction between the aluminum and the fresh concrete, the reaction slows or stops as the concrete hardens and dries. In addition it is common for the aluminum to be coated with a protective barrier to prevent the alkalinity of the concrete from reaching the aluminum surface.
When this material is to be used in an exterior application such as a balcony the risks of problems increases considerably for two reasons. First the concrete is exposed to multiple wet cycles that will transport alkalinity in the concrete to the surface of the aluminum which continues to corrode the aluminum. Secondly, steel reinforced concrete used as the structural component of the balcony would produce a galvanic cell (a galvanic cell is created when dissimilar metals, each having a different thermodynamic instability, are placed in contact with each other) leading to an accelerated rate of corrosion.
In general placing aluminum in or next to concrete for exterior applications should be avoided due to the increased risk of corrosion and the accompanying concrete cracking issues caused by the increased volume of the metal as it corrodes.
Note that chloride based accelerators should never be used in concrete that contains any steel reinforcement or metal embeds.