Engineer’s Corner: Myths and Misconceptions #2

by Reid W. Castrodale, PhD, PE

This is the second in a continuing series of articles in ESCSI Lightweight Design eNews which address common myths or misconceptions about lightweight aggregate or lightweight concrete. These articles specifically address issues related to concrete made using expanded shale, clay, or slate (ESCS) lightweight aggregate that has been manufactured using a rotary kiln process at temperatures around 2000 deg. F. When the particles reach these high temperatures, the material softens and bubbles are formed which remain as pores when the aggregate cools. The pores within the vitrified ceramic aggregate particles give the lightweight aggregate its reduced density and other unique properties.

I will address the following myth in this second article:

Myth #2 – Lightweight concrete floors take much longer to dry than normal weight concrete floors.

Upon first glance, this myth appears to make sense. It is well known that lightweight aggregate has higher absorption than conventional aggregates, and is prewetted prior to placement to maintain slump and allow pumping. The absorbed moisture does not affect the w/cm, but is released from the lightweight aggregate with time to allow continued hydration of the cement. Any excess absorbed moisture escapes from the concrete with time by evaporation. In the past, this was not a significant issue, but with in the introduction of environmental restrictions in the late 1990s that limited volatile organic compounds (VOCs), flooring adhesives became water based, and were therefore more susceptible to degradation from the moisture being released from concrete slabs of any type.

In 2000, Suprenant and Malisch published results for a limited study of relatively small deck specimens built and stored in laboratory conditions that found drying for lightweight concrete to take much longer than for normal weight concrete. This article alarmed many designers and contractors, with the result that they began to avoid the use of lightweight concrete for building slabs to which flooring was to be installed with adhesives. However, the results reported in the article did not seem to accurately represent experience in the field.

In response to the trend to abandon use of lightweight concrete for elevated slabs, ESCSI sponsored a study by Craig (2011) to more closely study the drying of lightweight concrete slabs.

The first phase of the study examined the drying behavior of three deck slabs that were representative of elevated slabs constructed in steel-framed buildings. Two specimens were lightweight concrete and the third was normal weight concrete. All slabs were constructed on metal decking with 2-in. flutes. The lightweight concrete slabs has a maximum thickness of 5.25 in., while the maximum thickness of the normal weight concrete slab was 6.5 in. The slab thicknesses were set to satisfy requirements for Underwriters Laboratories Design No. D916 for a 2-hour fire rated assembly, which also required a concrete density of 107-116 lb/ft3 for lightweight concrete and 147-153 lb/ft3 for normal weight concrete. Measured concrete densities for the mixtures used were 111.7 lb/ft3 and 147.7 lb/ft3 for the lightweight and normal weight concretes, respectively. Both concrete mixtures were proportioned with a w/cm of 0.50. Features of the slab specimens and the quantity of water in each specimen are given in the table below. While the lightweight concrete had more water because of the absorbed water in the lightweight aggregate, the lightweight concrete slabs were thinner, so the total quantity of water in the lightweight concrete (LWC) specimens were only 11.3% more than for the normal weight concrete (NWC) specimen.

Note: Sand moisture content = 4%; normal weight coarse aggregate moisture content = 0.50%; lightweight coarse aggregate moisture content = 18%

The slabs were constructed and stored in protected, but not climate controlled, conditions, similar to a typical building. The three 12-ft-square deck specimens were constructed at the same time and were finished and cured using standard methods, including the use of walk-behind paddle finishing machines followed by covering with polyethylene sheeting for 7 days. Doors on opposite sides of the warehouse in which the slabs were constructed and stored were opened each weekday to expose the slabs to ambient conditions to replicate typical building conditions, while protecting the slabs from receiving additional moisture from rain. The air in the warehouse was not conditioned. Photos of slab specimen finishing from Craig (2011) appear below.

As reported by Craig (2011), slab drying was monitored using two methods: ASTM F1869, “Standard Test Method for Measuring Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride”, which was used to measure the Moisture Vapor Emission Rate (MVER) at the surface of the slabs; and ASTM F2170, “Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes”, which was used to measure the internal relative humidity (RH) of the test slabs. Data for both test methods were collected for a period of about 1 year.

The MVER test results for the three slabs are shown in the figure below.

Data in the figure show that the two lightweight concrete slabs dried in essentially the same manner over the monitoring period. Also, the lightweight concrete slabs had moisture readings that were greater than the normal weight concrete slab, about 2 lbs higher initially, but after a few months they were consistently only 1 lb or less greater than the normal weight concrete slab. The usual MVER limit of 3 lb is indicated in the figure. All slabs reached this limit, but only once for the two lightweight concrete slabs and twice for the normal weight concrete slab, when ambient conditions were very dry. After those events, the relative humidity in all three slabs, moved back above the limit as they followed the increasing ambient humidity.

Relative humidity test results for the three slabs are shown in the figure below.

Data in this figure show that the relative humidity of all three slabs tracked together over the period, following variations in ambient conditions. The accepted relative humidity limit for placement of flooring is 75%, which is indicated on the figure. In this case, none of the slabs even approached the limit.

The second phase of the study was conducted with smaller specimens that were constructed and stored in climate-controlled conditions. The concrete mixtures were similar to the first study, using the same lightweight aggregate. For this study, the absorption of the lightweight aggregate at batching was 23.6%, which was higher than for the first study. Both concrete mixtures were proportioned with a w/cm of 0.50. Maximum concrete thicknesses for slabs were the same as for the initial study: 5.25 in. for the lightweight concrete and 6.5 in. for the normal weight concrete.

After the specimens for this phase of the study were subjected to drying for 7 months at 70 F and 50% relative humidity, neither slab had reached the target MVER of 3 lb. The normal weight concrete slab reached 4.0 lb and the lightweight concrete reached 6.4 lb. The relative humidity also did not drop below 80% for either concrete even after more than 9 months of drying, as shown in the table below, which presents data from Craig (2011).

OBSERVATIONS: Based on the findings of the ESCSI study by Craig (2011), the following observations can be made regarding the use of lightweight or normal weight concrete for elevated slabs on metal decking in buildings.

1. In both phases of the ESCSI study, the lightweight concrete slabs had higher MVER and relative humidity readings compared to the normal weight concrete slab.
2. When exposed to ambient conditions (phase 1 of the ESCSI study), there was little difference in the MVER or relative humidity readings between the lightweight and normal weight concrete slabs; neither type of concrete reached a consistent state below the industry limits for installing adhesive-attached flooring.
3. When exposed to climate controlled conditions of 70 F and 50% relative humidity (phase 2 of the ESCSI study), neither type of concrete reached the industry limits for installing adhesive-attached flooring.
4. It appears possible that the slabs tested in phase 1 of the ESCSI study could have reached and maintained moisture levels below the industry moisture limits if they had been placed in a climate controlled enclosed space to maintain the temperature and reduce the relative humidity.

CONCLUSION: Based on the results reported in the ESCSI study which conducted direct comparisons between lightweight and normal weight concrete slabs, Myth #2 is not true. The test results reveal that while lightweight concrete does have a higher moisture content than the normal weight concrete, the differences are small (not large as the myth asserts), and both types of concrete will likely not reach the industry accepted limits when it is time for installation of adhesive-attached floor coverings. Therefore, it is expected that both lightweight and normal weight concretes will require flooring adhesives capable of working on a higher moisture substrate or some mitigation technique to address the potential for moisture contents in concrete slabs greater than the industry limits.

Therefore, as concluded by Craig and Wolfe (2012) after their discussion of the ESCSI test program, the “benefits of lightweight concrete shouldn’t be dismissed solely on the premise that switching to normal weight concrete will solve the concrete drying issue.”

Further discussion of this topic and strategies for mitigating high moisture content for both lightweight and normal weight concrete are discussed in Wolfe and Ries (2017). Finally, additional discussion of the benefits of using lightweight concrete for slabs on metal decking in buildings, which generally offset any concerns with drying, can be found in an article by Martin, et al. (2013).

Suprenant, B. A., and Malisch, W. R. (2000) “Long Wait for Lightweight,” Concrete Construction, November 2000.

Craig, P. A. (2011) Concrete Floor Drying Study for the Expanded Shale, Clay and Slate Institute. Expanded Shale, Clay and Slate Institute, Chicago, IL. 13 pp.

Craig, P. A., and Wolfe, W. H. (2012) “Another Look at the Drying of Lightweight Concrete,” Concrete International, 34(1), 53-58.

Wolfe, W. H. and Ries, J. P. (2017) “Concrete on Metal Deck,” STRUCTURE, 24(9), 9-11.

Martin, D. P., Zimmer, A. S., Bolduc, M. J., and Hopps, E. R. (2013) “Is Lightweight Concrete All Wet?” STRUCTURE, 20(1), 22-26.