There are many engineering, material and construction variables that eventually determine the actual in place capacity of a concrete structure. In concrete construction, one important variable is the compressive strength of the concrete to be used. Proper mix design, selection of materials, manufacturing QC, placement QC and testing methods can ensure the desired concrete compressive strength and thus the anticipated structural performance.

For a contractor or ready mix supplier, there are few things during a construction project that can match the anxiety of receiving compressive strength test results that indicate the material is not reaching the specified strength level.

Why is the compressive strength of reinforced concrete important and measured via cylinder breaks during construction?

The answer to this questions lies within one of the most basic concepts of reinforced concrete behavior. When a beam, slab or column is loaded, some parts of the element go into compression and some parts go into tension. In designing the structure, the engineer will determine both how much compression and tension in "pounds per square inch" (psi) each element will see when loaded. Calculating these forces then allows the engineer to properly select the concrete design mix compression strength and the amount of rebar to meet the loads. Slabs/Beams (bending controlled behavior) vary in concept from columns (compression controlled behavior).

• For Beams and Slabs - When a load is applied to a slab or beam, the element will deflect downward. In order for this to happen, the bottom of the slab or beam must "stretch" - or become longer. Any material that becomes longer must go into Tension. However, concrete itself is actually very weak in tension which is why an engineer places reinforcing steel in an element wherever tension exists - like at the bottom of a slab or beam to bond to the concrete and carry the tension loads.

On the other hand, in order to deflect under load the top side of the beam or slab must "squash" - or get shorter. Any material that gets shorter must go into Compression.  This is why the designer must calculate these compressive forces and specify a concrete that will have enough compressive strength so that it does not literally explode like a cylinder test when loaded.   (SEE FIG 1)

Figure 1 - Concrete is designed to take compression forces and rebar tension forces.

• For Columns - When a vertical load is applied to a column, compressive forces are created. Again, much like a testing a concrete cylinder, the specified compressive strength of the concrete must be enough so that the column does not explode and fail under the anticipated loads.

What Variables Determine the Final Compressive Strength?

Figure 2 - Graph indicates the higher the w/c ratio the lower the compressive strength. (From Design and Control of Concrete Mixtures, 13th Edition, Portland Cement Association, 1988, p. 6.)

There are several variables in a concrete mix design that will determine the final compressive strength that include:

• Water to Cement Ratio (w/c) - Roughly 50% of the water in a good mix design is actually all that is needed to properly hydrate the cement in concrete and form a hard mass! The extra 50% of the water is needed for ease of placement because without it the concrete would not be flowable enough to properly fill the forms and be void free. This "water of convenience" needs to eventually evaporate out of the concrete and as it does it will leave very small empty spaces behind.  If the prescribed volume of water per the design w/c ratio is used, the porosity of the final product will not effect the compressive strength. However, if more water is added to the mix at the plant, in route to the site or at the jobsite, the concrete will end up with a lower compressive strength. FIG 2 shows the relationship between w/c ratio and compressive strength of concrete. What is very clear is that as you add water, a proportionate amount of compressive strength is lost.  This is based on the fact that the excess water that remains in the concrete after full hydration will increase the porosity (empty spaces) within the concrete beyond the normal amount. An extreme example of the effects of these empty spaces can be described by the difference in compressive strength between a piece of concrete and a piece of coral. This is why the Rule of Thumb to NEVER add water to a specified mix design without prior review and approval is well known (but not always followed) as it will negatively affect the compressive strength.
• Different types and amount of cement used - The amount of cement in a design mix (more = higher strength) and which cements are used will yield different final properties. Use of alternate cements should be reviewed and approved prior to changes.
• Chemical composition and gradation (size) of the aggregate - Additionally the type and shape of the aggregate (round vs elongated) will affect compressive strength.
• Type and amount of ad mixtures used - The type, combination and amount of admixtures can affect final strengths. For example, air entraining admixtures purposely create even more porosity (empty spaces) to increase the freeze thaw resistance of concrete. If too much is used, the excessive amount of spaces will affect the compressive strength.
• Curing - Curing of concrete (moist or sealing) reduces the rate at which the excess water in the mix evaporates. The benefit of curing is that the amount of small or large cracks in concrete is limited and this allows the development of higher compressive strengths.
• Temperature during placement - The temperature during placement, as well as changes in temperature will effect the compressive strength of concrete.

Typical Strength Gain of Concrete vs Time After Placement- How long should it take to reach the design spec?

Usually, it is common to test concrete to determine if it has reached its specified design strength at 28 days.  However, concrete designed for slower strength gain (60 to 90 days) may assure better durability because the rate of strength gain can affect the amount of voids in the concrete and limit the amount of micro and macro cracking.  Unfortunately, in many cases, construction schedules make the 60-90 day target not practical.

As a general rule, concrete should meet the required strength at the number of days in the design specification (typically 28 days).  A seven day test can be used as an early warning signal as the resulting strength should be approximately 75% of the specified 28 day strength.

Causes of Low Strength Concrete - What could the cause be? How do I investigate?

When this problem occurs, the cause based on the variables above can be rooted with the designer of the mix, the ready mix/admixture supplier or the contractor who placed the material or took the test samples. A root cause analysis should closely look at all of the functions that each one of these parties performed to rule out or discover possible causes. There are several possible causes of low strength concrete related to one or more of the variables listed above including:

• Inadequate original concrete mix design - due to error by designer or mistake at the ready mix plant during manufacturing.
• Adding excess water - while at the ready mix plant, while transporting concrete to site or on site to make the concrete more flowable or workable (finishing).
• Admixtures - problems related to wrong type, wrong combination, proportioning or excessive admixtures.
• Aggregates - issues related to the aggregates specified, selected  or substituted that affect the strength.
• Hot or cold weather placement - Hot and dry weather, as well as temperatures below 40º F can negatively affect strength of concrete unless special precautions are taken.
• Placement - improper consolidation or vibration of the concrete in the forms creating voids or segregation.
• Inadequate curing - this will negatively affect the rate of strength gain of concrete as well as cause shrinkage resulting in cracking.  It is important to begin curing procedures as soon as it is practical.
• Testing methods - making, curing or storing test cylinders improperly can give false results that do not correlate with the actual strength of the in place concrete. Strict adherence to ACI or ASTM guidelines should be assured - especially when unconventional mixes like Self Consolidating Concrete (SCC) are used. In some rare cases where there is no confidence in the test cylinders results due to errors, designers may permit cylinders to be cored directly from the in place concrete and tested. This coring option, as in the case of a column, may reduce the capacity of the element or damage it in a way that can not be restored. The effects of this process should be well thought out prior to coring. It may also be a good practice to take extra test cylinders in case an anomaly or discrepancy in test results occurs.

Options for Strengthening a Concrete with Low Compressive Strength- How do I fix it?

Options for strengthening a column, beam, shear wall or slab are dependant on both engineering and constructability issues. Obviously, the final product must meet the desired in place structural capacity. However, considering that the structure is already in place, carrying loads, there may be many obstructions and finishes already in place and repairs are usually in a critical path, the constructability is typically the more challenging of the two issues. Constructability issues should focus on:

• What can I physically construct with the current obstructions and not negatively affect access for parking, egress, finishes, headroom, etc?
• What repair options will be acceptable from an aesthetic point of view for the given environment?
• What repair options make economic or timeframe sense vs. just demolishing the element and recasting?

Another option is total removal and replacement. This option may not be viable due to time delays, access, cost of shoring during removal or potential damage to surrounding elements during demolition. Other remedial options include:

Columns

By enlarging 1, 2, 3 or all 4 sides of the column with a fully bonded concrete and rebar, the column is strengthened by physically increasing the size and bearing area. In some cases, the minimum enlargement of 4" may not be viable due to space constraints or clearance issues. (SEE FIG 3)

Figure 3 - Formwork and enlarged concrete column installed for low concrete breaks.

A thin laminated ultra high strength carbon fiber fabric (FRP) can be used to wrap the column horizontally and bonded to the concrete. This will act as additional ties and add significant confinement to the column. By confining the column with this material, it will effectively increase its load bearing capacity (compressive strength). The wrap will usually add less that ¼" to each face and not affect any clearances or finishes.(SEE FIG 4)

Figure 4 - Column wrapped with carbon FRP sheets strengthened due to low concrete strengths.

Beams, Slabs and Shear Walls

Adding additional bonded reinforced concrete section to beams, slabs and shear walls can increase their bending or shear capacity deficiencies created by low compressive strength concrete. Again, due to increasing the size of the member, enlargement may not be viable due to space constraints or clearance issues. (SEE FIG 5)

Figure 5 - Beam Enlargement

 

Externally applied carbon FRP sheets can be bonded to the bottom, top or sides of the concrete to increase bending or shear deficiencies. (SEE FIG 6)

Figure 6 - FRP sheets applied to bottom side of a slab for strengthening.