Page 5 - ICJ Feb Openaccess 2026

POINT OF VIEW

Revising the discharge time limit in ASTM C94/C94M [4] Historically, a significant volume of ready-mixed concrete–
encourages a more flexible dialogue between purchasers and approximately 3 % of total production–has been rejected and
producers. By tailoring limits to project-specific conditions– returned to batching plants due to factors such as excessive
considering mixture type, travel distance, placement method, slump loss, overproduction, or experimental batches. Strict
and ambient conditions–both parties can optimize delivery adherence to the 90-Minute limit has been a primary reason
without compromising concrete quality. Extended delivery times for this waste. Revising project specifications and coordinating
do incur additional costs for materials and truck time; however, closely with design teams regarding delivery times could reduce
when these factors are known, steps can be taken to ensure the concrete rejection rates and enhance sustainability.
concrete is delivered at the required slump.

Since the establishment of the 90-Minute requirement in 1935, 5. EXTENDED DISCHARGE TIME ON THE
ready-mixed concrete technology has evolved dramatically. PROPERTIES OF CONCRETE
Back then, the only remedy for delaying the initial set was to
[9]
add water, which could negatively impact concrete quality. A recent study by Mahgoub et al. investigated the effects
Consequently, concrete was typically produced near or of extended discharge times on both fresh and hardened
on-site to minimize transport time. Today, however, with properties of ready-mixed concrete. In their study, they procured
improved chemical admixtures that can delay setting without ready-mixed concrete from a local supplier and evaluated the
compromising quality, the average haul time has increased properties at 60, 90, 120, and 150 minutes. Fresh concrete
significantly to accommodate urban demands and extreme properties (air content, slump, and temperature) and hardened
weather conditions. In some cases, the 90-Minute limit is properties (compressive strength, freeze-thaw resistance, and
unnecessarily restrictive, leading to the rejection of concrete that surface resistivity) were measured. Their findings showed no
still meets performance criteria; in others, 90 minutes may be significant deterioration in concrete properties even up to
too long for certain mixtures or conditions. 150 minutes, suggesting that the traditional 90-Minute limit is
overly conservative (see Figure1). However, as may be seen in
Modern concrete technology–including hydration-stabilizing Figure 1, there may be a small reduction in compressive and
and workability-retaining admixtures–allows for extended flexural strength in some mixes, when the time of discharge is
delivery times [5,6] . Automated systems can adjust mixtures in above 120 minutes, which may be noted with caution.
transit by adding water and admixtures to maintain a consistent
slump and quality. This flexibility not only reduces waste but also
provides producers with more delivery options. 6. TRANSPORTING CONCRETE FROM A
READY-MIX PLANT
Project specifications should include a detailed discussion on
the schedule and intended use of fresh concrete upon arrival [7,8] . Ready-mix concrete must be delivered within the stipulated
The target slump range is often dictated by placement methods time to maintain its quality and structural integrity. The
(e.g., pumping for walls or tailgate discharge for slabs- pouring choice of transportation method depends on factors such as
concrete straight down the chute from the concrete truck), the location, project size, and environmental conditions. Timely
finishing crew size, and ambient conditions. In larger projects, transportation is crucial for ensuring that the concrete retains its
testing a slab under similar conditions can be an essential step. desired properties. Key logistics factors include:

Compressive strength (MPa) Modulus of Rupture (MPa)

Time-to-discharge (min) Time-to-discharge (min)
Compressive strength vs. time-to-discharge Flexural strength vs. time-to-discharge in hot weather

Figure 1 Compressive and flexural strength of concrete with respect to time of discharge [9]

8 THE INDIAN CONCRETE JOURNAL | FEBRUARY 2026