Factors Affecting Slump

It is rare for a lab mixture to be entirely suitable under the actual conditions that concrete is exposed to in the field. Adjustment to the water quantity and admixtures is something that’s usually needed. These corrections are not by default because of poor quality control or material variations. Differences in slump between concrete trucks can be the result of other factors:

• Batching tolerances

even with the great technology that we are using today, batching plant cannot fully assure 100% similar batches. There are always batching tolerances that affect slump at the end. Tolerances for the individual batching of each mix ingredient are:

• Cementitious materials – 1% (5 pounds of cement per cubic yard)
• Water (by volume or weight) – 1% (2.5 pounds or 1⁄3 gallon of water per cubic yard)
• Aggregates – 2% (60 pounds of aggregate per cubic yard)
water – to – cement ratio (W/C) can vary by 0.01. However, if uniform mixing is maintained, batching tolerances should not change the slump by >0.5 inch.

ASTM C 94 specifies that total mixing water which affects both the slump and wate-to-cement ratio, to be calculated within ±3%. The total mixing water includes any free water in the aggregates, water added on site, any residual mixer washing water. In addition, if the ±3% condition is met, the water content can change by around 1 gallon, W/C can change by 0.02 and slump can vary by 1 inch per cubic yard of concrete.

Another note is that most concrete plants work on the safe side when it comes to mixing. This means they work on the dry side (makes water a little short when mixing). In fact, this allows any necessary adjustment for the slump in field. Check the batch information to know how much water was added to that concrete mixture and how much you can add for slump corrections.

• Aggregate moisture content

the measurement and correction for the moisture of fine aggregate can considerably cause slump variation. For example, for sand content of 1,200 pounds per cubic yard, 1% change in its moisture would cause a change of water by 1.5 gallons in cubic yard.

Many concrete contractors do their calculations of sand moisture in the morning; as the temperature increases during the day and aggregate dries more water is absorbed causing the slump reduction.

Concrete producers should use an accurate and calibrated meter for measuring the sand moisture. However, with sophisticated equipment, it is also difficult to prevent slump variations due to the changes of sand moisture content.

• Concrete temperature

temperature is one of the factors that can affect the concrete slump, even for a slight change in temperature. Temperature rise decreases slump since it accelerates the cement hydration, making it consumes more water at a faster rate.

For example, a 10°F increase in concrete temperature requires one extra gallon of water to maintain the same slump.

• Mixing

Mixing itself can also change the concrete slump. For a uniform concrete batch, ASTM C 94 specifies the number of revolutions to be 70 – 100 at mixing speed. Also, the volume of mixed concrete shouldn’t exceed 63% of the total drum volume. Once the concrete truck gets to the site, rotate the drum another 30 revolutions to decrease any segregation that might have happened.

• Delivery, waiting, and unloading times

Prolonged delivery, waiting, and unloading times can cause a decrease in slump. The concrete producer should have concrete within the required range of slump for 30 minutes starting from the arrival site or after the initial slump adjustment, whichever comes later.

Adding Water to Increase Slump

ASTM C 94 gives some instructions on how to adjust/correct the concrete slump if it was less than a specified when concrete truck arrives to the site.

Adding water must not increase W/C above the allowed limit given by specifications. When you add the water to increase slump, it must be added to the whole batch. Don’t select a certain part of the batch such as the middle or the end and put water in that part only. It’s impossible to accurately guess the amount of concrete remaining in the truck. The quantity of water required for a partial batch of concrete to be within the maximum W/C cannot be measured. In addition, you can’t add water to increase slump to bypass the 1.5 hours or 300 revolution criteria of ASTM C 94.

Thus, make sure to add water correctly. Add water to the batch at the head section of the drum, or by there will injection into the head and discharge section of the drum. Don’t use a hose to spray additional mix water into the discharge end of the drum. This technique doesn’t appropriately distribute the water throughout the mixture and the quantity of water added is only a guess.

ASTM C 94 specifies extra 30 revolutions when water is added to the truck. Note that water cannot be edited if the combined the revolutions from long delivery distance or waiting time is higher than 270.

Other Ways to Increase Slump

concrete properties can you change when additional water is added to the mixture. Adding 1 gallon of water. Cubic yard will lead to increase of slump by 1 inch, decreasing compressive strength by 150 to 200 psi, higher shrinkage by 10%.

Water is not the sole way for correcting slump. You may consider using a
water reducer or superplasticizer in field to increase slump without adding extra water and putting yourself at the risk of compromising the concrete quality.

In cases where the used the concrete is having the maximum allowed W/C, the use of superplasticizer’s can only be the acceptable way for increasing the slump.

Decreasing Slump

on the other hand, sometimes slump requires a reduction. For example, the slump
of the first 0.25 cubic yard of concrete is 4 inches, but it’s for a slipform paver that requires maximum slump of 2 inches.

Concrete workers usually add a couple of extra sacks of cement to the batch trying to dry up the mix. This leads to a reduction in the slump but the process is time-consuming and not very effective. Another good solution is to use an air detrainer to decrease the amount of air in the mixture. Reducing air content by 1% reduces slump by 0.25 inch.

An effective technique for slump reduction is the addition of silica which has the ability to dry up the concrete mixture, but it will darken the concrete a little bit which might not be acceptable in some cases.

Testing Slump

Many contractors test slump twice: initially checking and once for acceptance criteria. The initiator measurement of the slump is useful to determine how much extra water is needed to correct the slump, if any.

References: ASTM C 94, Adjusting slump in the field by B. A. Suprenant