What is 3D Concrete Curing?

3D Concrete curing is often misunderstood as concrete drying but in fact refers to the chemical process that allows concrete to gain its designed strength and durability. Drying is just the evaporation of water from the concrete over time. Curing is a more complex process that continues long after the concrete dries. Let’s take a closer look.

What Happens During 3D Concrete Curing?

After concrete is extruded through the print nozzle, a series of chemical reactions called hydration begins that harden and strengthen the concrete. In hydration, water reacts with the cement to form binders that glue together the aggregates and make concrete rigid:

• Cement is largely composed of compounds called C3S and C2S that react with water and harden over time[1].
• When water is added to cement, these compounds dissolve and react, forming calcium silicate hydrate (CSH) gel[2].
• CSH gel acts as a binder that grows and spreads through the concrete, gluing together fine and coarse aggregates.
• Calcium hydroxide is also produced, which reacts further with remaining compounds and contributes to strength[3].
• As hydration continues, the concrete is transformed from a plastic state to hardened solid over time.

This complex process begins immediately when water is added to the cement but continues for a long time. Full hydration and maximum strength may take over 28 days. But concrete gets stronger gradually as these reactions proceed over weeks to years.

Why Proper Curing is Essential for succesfully 3D Printing Concrete

Proper curing is vital for full hydration reactions to produce strong and durable concrete:

• Hydration needs sufficient moisture in concrete to proceed. Drying out too early stops hydration[4].
• Heat accelerates hydration while cold temperatures slow it down or stop it.
• Time is needed for the slow pozzolanic reactions to take place completely.
• Adequate curing ensures the designed properties are achieved in practice.
• Poor curing leads to weak and brittle concrete prone to cracking and early failure.

Managing the Curing Timeline of 3D Printed Concrete

It takes about 28 to 90 days for full curing but concrete gets stronger gradually:

• Early strength (3-7 days) – 40% of full strength
• 12-14 days – 60% of strength
• 28 days – 80-90% of full strength[2]
• 90 days – 100% strength and fully cured

While each lower layer has to be strong enough to support the weight of subsequent layers to be printed on top, the initial 3-7 day curing period is most critical for strength development. Some curing should continue for 28 days and full curing takes about 90 days. Curing time for the concrete to be able to support its own weight is often the limiting factor for the printing speed of 3D Printed Concrete.

Factors Affecting Curing Time of 3D Printed Concrete

Several factors impact hydration rate and curing time:

• Water Content – Enough water must be available for continued hydration. Higher w/c ratio provides more water for curing.
• Cement Content – More cement provides more compounds to hydrate and increases rate of curing[1].
• Curing Temperature – Heat accelerates curing while cold weather delays or stops hydration.
• Curing Method – Retaining moisture in concrete promotes continued hydration and proper curing[4].
• Concrete Mix – Ingredients like slag cement and admixtures slow or accelerate curing[2].
• Type of Concrete – Special concrete mixes tailor curing rate for early strength or improved durability.

Let’s discuss how each affects the curing process and timeline in more detail.

Water Content Effects on Curing 3D Concrete

Water content impacts curing:

• Just enough water for full cement hydration is required, more delays curing.
• A higher water-cement (w/c) ratio above 0.4 provides excess water and longer curing.
• All the added mixing water should react during curing, little free water should remain later.
• Excess water beyond cement needs evaporates rather than promoting curing.

Cement Content and Curing

Higher cement contents speed up curing:

• More cement provides more substances to hydrate and harden.
• Accelerates the formation of strength giving CSH gel and calcium hydroxide.
• Higher cement contents produce high early strength suitable for fast formwork removal.
• More cement also raises the heat of hydration, benefiting curing at lower temperatures.

Curing Temperature Effects

Curing temperature impacts hydration:

• Heat accelerates curing reactions and strength gains.
• Optimal temperatures for curing range between 50-100°F.
• Curing slows below 50°F and stops below 40°F. Prevent concrete freezing.
• Concrete poured in hot weather needs protection to reduce evaporation.
• The first 3 days are most crucial, avoid too high or too low temperatures.

Methods of Curing 3D Concrete

Special curing methods prolong hydration:

• Ponding, misting and fogging supply extra moisture for ongoing curing.
• Wet coverings like burlap and paper retain moisture in concrete.
• Plastic sheets and curing blankets minimize moisture loss.
• Concrete sealers reduce evaporation and promote curing.
• Formwork left in place protects concrete from drying out.
• Longer curing times produce stronger and more durable concrete.

Effects of Concrete Mix Design

Concrete ingredients impact the curing process:

• Fly ash and slag cement increase long term strength by providing more compounds to hydrate.
• Supplementary materials slow early curing but enhance ultimate strength.
• Accelerator admixtures speed up the hydration reactions and produce early strength faster.

Types of 3D Concrete and Curing Rate

Different concrete types are designed and cured differently:

• Fast setting concrete gains strength rapidly by reducing w/c ratio and adding accelerators.
• High strength concrete also has lower w/c ratios to reduce curing time and increase durability.
• Self consolidating concrete has higher water content and takes longer to cure fully.
• Pervious concrete drains away excess water for faster curing.
• Premixed concrete is made under controlled conditions for proper and predictable curing.
• Concrete with slag cement like slag cement concrete takes longer to cure fully.

With care taken to hydrate concrete properly, the curing process can be managed for sufficient strength development and durability. Understanding curing allows controlling the concrete timeline for specific applications.

References:

[1] https://www.cement.org/docs/default-source/fc_concrete_technology/durability/is536-types-and-causes-of-concrete-deterioration.pdf

[2] https://www.nrmca.org/aboutconcrete/cips/31p.pdf

[3] https://www.cement.org/docs/default-source/fc_concrete_technology/durability/is536-types-and-causes-of-concrete-deterioration.pdf

[4] https://www.thespruce.com/how-to-cure-concrete-longer-makes-it-stronger-845589