Design of 3d Printable Concrete

Abstract

3D concrete printing is the most revolutionary idea in the field of concrete technology to get a breakthrough instead of diversified constraints. it promises to be highly advantageous in terms of time and economical effeciency, accuracy, less manual labour etc. it mainly works on the extrusion principle in which 2d layers are stacked through a printing nozzle to form a 3d framework. in this paper we are going to discuss the several constraints of machine and mix design for the printable concrete.

Introduction

This paper most importantly focuses on the mix design required for the printability of the concrete to get optimum results. The concrete mix must be able to be at par such that it fulfills the following criteria:

1. Maximize compressive strength yet maximize workability.
2. Maximize flowability yet maximize buildability upon pouring.
3. Maximize speed of concrete setting yet maintain appropriate setting time such the subsequent layers get bonded to each other.

Some of the goals conflict with each other so proper balance must be maintained.

Mechanical performance of extruded concrete materials

Extrusion, a high pressure process which changes the strength and stiffness of concrete-due to high pressure extrusion air content is reduced and compressive strength increased. -stiffness is increased by removal of entrained air.

The five most important features of the mix design to fulfill the condition of printability are as follows:

• flowability
• buildabilty
• extrudabilty
• compressive strength
• open time.

The mix must be solid enough to hold itself and liquid enough to bond with each layer such that they do not seem to be different entities. open time is what we call the change of concrete flowabilty with time. and finally the most important feature compressive strength which determines how much load our structure is going to withstand. The MSA (maximum size of aggregate) must be appprox 2 mm in diameter as the nozzle diameter is generally 2 cm. different dry constituents such as cement type 1 and sand are also mixed. superplastisizers such as viscocrete is added such that it increses the workability without changing the w/c ratio. A proper balance between all the constituents should be maintained to get an optimum mix.

How does a 3D printer work?

Concrete 3 d printers work on machine language that controls the print head until the complete 3 d model is produced. -it works on material extrusion. -they operate using x, y and z axis. -first it needs specially designed rails and a horizontal metal beam for setting up the nozzle and the robotic arm. -normal concrete is connected to the printer’s nozzle via a big hose.

Machine design: the machine consists of 3 major components: the tank and pumping process, the printing nozzle and the motion control system. The tank initially contains the concrete which is pumped to the printing nozzle which is given directional feature in x-y-z axis with the help of motion control system.

Motion along the axis: There is a vertical element which supports and contols the mobile tank and nozzle. the motion along the z axis is controlled by the vertical element either hydraulically or manually. the motion along x–axis is supervised by a rotating drill which has adjustable speed.

Nozzle design: the nozzle diameter is directly proportional to the flowability of the concrete mix. the maximum aggregate size and w/c ratio are critical in determinng the nozzle diameter. it consits of two types of trowels which function to straighten the concrete as it passes by.

Tank and pump design: it mainly functions as a storage house for concrete and a media to transport it to the printing nozzle. it works on the pressure mechanism and is quite vital as it determine the speed of conceret by which it gets poured out of the printing nozzle.

MSA and w/c ratio are again the critical properties for the pumpability of the concrete to the nozzle.

Several tests can be conducted to ensure target criteria are met:

• Compressive strength: the target strength of the concrete is determined using concrete cube /cylinder test due to quick setting(within a few minutes) of concrete after printing compressive strength plays a vital role as layers are printed instead of a 3d form in its entirety.
• Extrudability: it is measured as the maximum distance over which the mix/paste can be printed without blocking the nozzle care must be taken so that the paste does not contain any seperations or cracks.
• Flowability: it is measured using the slump flow test (i. e. measurement of diameter at different time intervals of a spreaded concrete.
• Open set time: flowabilty is measured at different time intervals using slump flow test and is vital because concrete is not poured in one go(conventional method).
• Buildability: this is measured by the maximum number of layers that can be stacked/glued without facing a collapse.

Results and conclusion

Machine: a balanced relationship between extrusion rate and machine velocity must be obtained such that appropriate criteria for printing is met. The extrusion rate was calculated to be 0. 09 litres/sec and machine velocity 18. 56 cm/sec such that both are compatible to each other.

Concrete mix: during the testing of the extrudability criteria of the concrete low w/c ratio and low amount of sand gave better extrudabilty. Finally the most optimum ratio was found to be 1. 27(finer aggregate to cement ratio) and 2(finer aggregate to sand ratio. The minimum water to cement ratio was found to be 0. 48 for the extrudability and the optimal nozzle diameter was calculated to be 2 cm due to high blockage caused due to smaller sized nozzles.

The buildability depends on the flowability as follows: As per the graph it is clear that as the flowability increases buildabilty decreases. thus the use of superplasticizers must be optimised according To the results obtained from the graph. It was observed that the flow rate should strictly lie between 1. 0 -1. 2 cm/sec to meet the desirable buildability.

In addition to that adding of accelarator above an optimum amount causes an adverse efect on the extrudability as it quickly sets the paste. use of pozzolanic materials in extruded concrete material -due to their pozzolanic effect and fine particle size they positively influence the extruded concrete material. -in case of fly ash, the well rounded particles are beneficial for extruded concrete.