The slab-on-ground can be constructed by using for shrinkage compensating concrete. The design consideration is provided in accordance for a concrete slab made from cement that conform to ASTM C 845.
The design of concrete slab with such an intention of shrinkage compensation have variations in design when compared with the conventional concrete slab construction. It is more conforming to the specification given in ASTM C 595.
There is the process of contraction or shrinkage to the concrete when it dries up and it undergoes expansion when it gets wet. This process of volume change with the change in moisture content is a property inherent in hydraulic cement concrete.
A more detailed explanation of this phenomenon is explained in ACI 224R. The hydraulic cement concrete is subjected to volume changes even during the temperature changes.
What is Shrinkage Compensation Concrete?
The shrinkage compensating concrete can be defined as an expansive concrete that undergoes expansion by an amount equal or greater than the volume change anticipated for drying shrinkage.
This means the concrete will be properly restrained by means of reinforcement or any other source. The use of shrinkage compensating concrete helps in minimizing the cracks due to drying shrinkage.
The main intention of such a provision is through the reduction of expansion strains even if there exist drying shrinkage. This is in the ideal condition where exists a residual compressive stress in the sample that helps in reducing the shrinkage cracking and the curling problems.
Portland Cement and Blended Concrete in Shrinkage Compensation
The shortening effects due to shrinkage in Portland cement and the blended cement concrete can be reduced by restraining them by reinforcement and friction provided in between the ground and the concrete slab.
This effect of shortening occurs at an early stage and occurs by friction restrain stress in the concrete. This stress is greater than its early tensile strength and finally results in the cracking of the concrete slab.
The more the drying shrinkage, more is the widening of the cracks. This is a more pronounced maintenance problem. If the crack width exceeds more than 0.9mm then the transfer of the loads becomes ineffective.
The provision of closer joint spacing or additional distributed reinforcement or post-tensioning helps in limiting the cracks created due to shrinkage restraint.
Difference between Shrinkage-Compensating Concrete and Conventional Concrete
The difference between the shrinkage compensating concrete and conventional concrete is more varied based on the design arrangement and the load or stress transferring principle.
The main objective of shrinkage compensating concrete is to limit the problems of cracking and curling observed in slab-on-ground. This special concrete is made of cement conforming to ASTM C 845 instead of ASTM C 150 or C 595.
Hence there is a variation in the volume change characteristics of shrinkage compensating concrete when compared with the conventional concrete slab.
The shrinkage compensating concrete undergoes two stages of volume changes initially. First one is expansion during the curing stage which results in the increase in volume. Next, it undergoes drying shrinkage.
The drying shrinkage behavior and characteristics are similar to that are observed in conventional concrete. The factors that will influence the drying shrinkage in both the shrinkage and conventional concrete types are same.
The factors that will influence the drying shrinkage of these concrete include:
- The Concrete Mixtures
- Water Content
- Type and gradation of the aggregate
- The cement contents
The water content in the concrete greatly influences the rate of expansion during the curing period as well as the rate of contraction during the drying shrinkage.
The graph shown in figure-1 illustrates the length-change characteristics of both the Portland cement concrete and shrinkage compensating concrete tested based on ASTM C 878 (ACI 223).
Fig.1: Length Change Characteristics of Shrinkage Compensated Concrete and Portland Cement Concrete (As per ASTM C 878 (ACI 223)
The expansion occurred in the shrinkage compensating concrete is restrained internally by means of bonded reinforcement. These bonded reinforcements are in tension. This forms a source of expansive strain that results in compression. This compression is relieved by the stress due to drying shrinkage and some amount of creep present.
The main objective to be attained in the design of shrinkage compensated concrete is to have the more restrained expansion that is greater than the long-term resulting shrinkage. This is shown in the figure-2. This arrangement makes the concrete to finally remain in compression.
The minimum concrete expansion in slabs on the ground is as per the recommendation is given in ASTM C 878, which is 0.03%.
Fig.2. The effect of reinforcement on the shrinkage and the expansion in concrete at an age of 250 days. As per American Concrete Institute 1980.
Thickness and Reinforcement of Slab-on-Ground for Shrinkage Compensation
The determination of the thickness of slabs on the ground made from shrinkage compensating concrete is similar to that used by the conventional concrete. In the case of reinforcement, the following considerations are specifically featured:
Reinforcement for Restrain
The shrinkage compensation can be developed by the provision of an elastic type of restraint by having an internal reinforcement. Other means of restraint like include the subgrade friction, the internal abutments, the presence of adjacent structural elements make the structure to be more indeterminate. This provides either too less or more restraint.
The subgrade frictional coefficient value ranging from one to two is found to be satisfactory. High compressive stress will be introduced if we go for higher compressive stress in the concrete. This will only have little shrinkage compensation. The design implemented must specify the recommended reinforcement as per ACI 223.
In each direction of shrinkage compensation, the minimum ratio of reinforcement to be provided is 0.0015. This ratio is reinforcement area to the gross concrete area. This minimum ratio has no sort of connection with the yield strength of the reinforcement.
The location of the reinforcement has a great role in the slab and the internal behavior of concrete. As per ACI 223, the reinforcement has to be placed at a depth of 1/3rdfrom the top. This position helps in balancing the restraint provided by the subgrade itself.
When using the similar type of reinforcement, care must be taken while placement. Use of stiffer and wide space between the reinforcement helps in having lower reinforcement percentages. The use of ASTM A 487 deformed wire reinforcement or the ASTM A 615 deformed bars are recommended. Any other bar reinforcement is acceptable like the ASTM A 996 and the A 706.
Having full shrinkage compensation means to have an expansive strain equal to or greater than the restrained stains due to shrinkage. Based on a study conducted by Kesler in 1973 it is recommended to have a maximum reinforcement equal to 0.6%.
At this point, the strain due to the restrained expansion will be equal to the shrinkage strains. In this case, also, the strain is not dependent on the yield strength of the steel reinforcement used.
The concrete can be prevented to have a strain due to shrinkage greater than the restrained expansion by providing the lesser percentage of steel reinforcement.
The required level of expansion strains can be measured from the figure-3 as per ASTM C 878. The figure is a graph showing the relationship between the prism expansion and the reinforcement provided internally, the concrete slab expansion and the volume surface relationships.
Fig.3. The graph between slab expansion and prism expansion for different volume surface ratios and for different reinforcement percentages
The anticipated member shrinkage is determined from the above graph by using the volume surface ratio in the case of different slabs and the different reinforcement percentages.
Full shrinkage compensation is obtained if the resulting slab expansion is greater than the resulting shrinkage strains. This is the required objective. The prism value obtained is the minimum value that must be specified and verified in the lab.
The verification of the value must be carried out in trial mixtures and samples. A per ASTM C 878 the recommended minimum concrete expansion for the slabs on the ground is 0.03 %.
Minimum Restrain Levels
Russell concluded that the restrained expansion must be greater or equal to the restrained shrinkage (1979). This expansion in the concrete is largely dependent on the expansion capability of the concrete mixture. This, in turn, is influenced by the cement factor, the admixture used in the concrete, the level of restraints that are provided both internally and externally and the curing conditions and time.
Hence, the minimum reinforcement required to have complete shrinkage compensation depends also on the potential shrinkage of the slab, the restrained expansion of the prism as per the ASTM C 878.
If the expansion of the slab is greater than the shrinkage strain that has a surface-volume ratio of 6:1, then we will be able to obtain full compensation of shrinkage as per the data are given in American Concrete Institute by Russel in 1980.
Special care must be taken while using low reinforcement ratios. The use of light reinforcement results in the situation of accident depression of the reinforcement layer to the bottom of the slab.
This results in failure reasons like warping and cracking. This problem can be solved using light but stiff reinforcement like large bars and wires which are used in a greater spacing.
The maximum spacing of the reinforcement must be three times the thickness of the slab. If the smooth wire is used, the spacing must be not less than 360mm. Wider spacing is suitable for the easy working with the workers but is not good for design requirement.
The spacing of deformed bars if used is similar to that used for reinforcing bars. If no tests and design calculations are implemented a minimum reinforcement of 0.15% is provided.